ChangMing Charlie Ma
Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA 19111
Corresponding author: ChangMing Charlie Ma, Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA 19111 Email: [email protected]
Copyright © 2019 Ma CM.
Citation: Ma CM. (2019). Physics and Dosimetric Principles of SRS and SBRT. Mathews J Cancer Sci. 4(2): 22
ABSTRACT
This paper reviews the radiation physics and dosimetric principles of stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Recent radiation therapy physics and technology developments are reviewed followed by a brief description of the equipment available for SRS and SBRT. The treatment planning process and beam orientation optimization for SRS and SBRT are discussed. The dose calculation algorithms for radiation therapy are described including simple correctionbased methods, advanced modelbased algorithms and Monte Carlo simulations. Guidelines for SRS and SBRT treatment planning and plan evaluation are summarized including detailed dose prescription, organ dose tolerance and plan acceptance criteria. Finally, dosimetry measurements for SRS and SBRT are discussed with a focus on small field dosimetry.
Keywords: SRS, SBRT, dose calculation, Monte Carlo, treatment planning, dose prescription, organ dose tolerance, plan acceptance criteria, dosimetry measurement
INTRODUCTION
Radiation Physics and Technology Development
Stereotactic radiosurgery (SRS) is a precise radiation therapy technique, specially developed for the treatment of small tumors and functional abnormalities in the brain [1]. The physics and dosimetric principles of intracranial SRS, viz. highprecision, highdose, focused radiation treatment, have also been applied to the treatment of extracranial tumors. This new treatment procedure is referred to as stereotactic body radiation therapy (SBRT).
Despite their names, SRS and SBRT are nonsurgical procedures that deliver preciselytargeted, highdose radiation in a single or very few fractions, as compared to conventionallyfractionated radiotherapy. SRS and SBRT are made possible with the advancement of medical imaging and radiation therapy technologies that allow maximum dose deposition in the treatment target while minimizing radiation damage to the surrounding normal tissue. The goal is to deliver a lethal dose to the tumor in order to achieve permanent disease control.
Physics and technological developments for SRS and SBRT
The development of SRS and SBRT is based on several radiation physics and technological advancements:
Historically, stereotactic surgery (or stereotaxy) is a minimally invasive form of surgical intervention that employs a threedimensional (3D) reference coordinate system (such as a head frame) to locate tumors or abnormalities in the brain and perform some action on them e.g., biopsy, injection, implantation, stimulation, ablation, lesion and radiosurgery [2]. For extracranial sites, however, there are significant difficulties in establishing a reliable reference coordinate system that maintains a constant spatial relationship to bony structures or soft tissues. Thus, even though body frame systems have been developed and tested clinically for SBRT during its early clinical implementation, stereotaxy has predominantly been achieved using 3D imaging systems on modern SRS and SBRT machines. Advanced imaging modalities including CT, MRI, 2D/3D ultrasound and PET/CT have been used to locate treatment targets in the body. Multimodality imaging is used in treatment planning to delineate the target volume and critical structures and in the delivery process to facilitate the precise positioning of the patient and realtime monitoring of the target during SRS and SBRT treatment sessions.
Although radiosurgery has commonly been referred to as a singlefraction treatment, it is sometimes necessary to perform a multiplefraction stereotactic treatment, which is still ablative in nature. This is mainly for larger tumors because the normal tissue volume receiving high dose of radiation increases proportionally to the tumor size. Fractionated treatments will minimize damage to normal tissues by allowing time for repair, thus allowing for ablative doses to be delivered to the treatment target, while still maintaining a good safety profile. This procedure has been referred to as fractionated stereotactic radiotherapy (SRT), which is typically used to treatment large intracranial lesions in two to five fractions. SBRT generally refers to fractionated stereotactic radiotherapy for extracranial treatments. If the prescription dose is sufficiently high for an ablative treatment it is also referred to as stereotactic ablative body radiotherapy (SABR). For convenience, we will only use SRS for single fraction treatments and SRT/SBRT for multiple fraction treatments in this paper. In the United States, insurance companies set policies to reimburse single fraction SRS, and up to 5 fractions for SRT/SBRT, which were not based on radiation physics, radiobiology or clinical efficacy.
SRS and SBRT equipment
Commonly used SRS and SBRT machines can be classified into three categories based on the radiation sources used:
Treatment planning and optimization for SRS and SBRT
Unlike conventional radiation therapy, which delivers a uniform dose to the entire target volume, the dosimetric requirements for SRS and SBRT are different in two aspects: (1) a small target volume, containing the gross tumor and its close vicinity, is treated with a very high prescription dose per fraction and heterogeneous dose distributions (e.g., hotspots) within the treatment volume are considered to be acceptable [9]; and (2) the normal tissue volume outside the treatment target that receives high doses is minimized to reduce the risks of treatment toxicity, which requires a steep dose falloff away from the target [9].
Definition of Target Volume and Critical Structures
Radiation therapy is a localized treatment. The success of radiation therapy depends on the precise knowledge of the treatment target and the surrounding critical structures. To define the treatment volume(s) for SRS and SBRT, the location and extent of the disease including subclinical and microscopic tumor extension must be characterized. Various imaging modalities, and help and advice from diagnostic specialists are therefore crucial for radiation therapy planning.
Three main volumes have been defined for radiation therapy treatment planning [10]:
For some body sites that are affected by respiratory motion, such as a lung tumor or a liver tumor, an internal margin is added to the CTV to compensate for internal physiologic movement and variations of the tumor in position, shape and size. This is referred to as internal target volume (ITV). Fourdimensional computed tomography (4DCT) is currently being used to obtain the ITV either by delineating the target volumes on CT images at 10 breathing phases or on a single maximum intensity projection (MIP) image. Early studies showed that the ITV could be estimated by combining target volumes drawn on CT images at the end of expiration and that at the end of inspiration (a poor man’s ITV). An isotropic 5mm ITVPTV margin has been used by many institutions for Linacbased lung SBRT with 4D imaging [9]. For most cases, however, this margin should be determined based on the equipment used and the institutional experience with SRS and SBRT.
An important goal of treatment planning is to avoid radiation damage to critical organs and normal structures, known as organs at risk (OARs). OARs are delineated, and in some cases, a safety margin is added around an OAR to account for the random and systemic uncertainties. This is referred to as a planning OAR volume (PRV). It is useful to apply the PRV concept for some critical organs such as the spinal cord in SRS and SBRT planning, in which higherthansafe exposures to a small fraction of the OAR (e.g., a serial organ) may cause a severe clinical manifestation. GTV, CTV, ITV, PTV, OAR and PRV are very important concepts in SRS and SBRT treatment planning, which can affect clinical outcomes.
In principle, the delineation of the GTV and CTV should be independent of the treatment techniques, be it SRS, SBRT or conventional radiation therapy. For the delineation of volumes, and in their use, it is irrelevant whether gamma rays, xrays, protons, or any other radiation is to be employed. It is important not to let the intended treatment modality affect how these volumes are delineated. Consistent target and OAR determination is a key requirement to combine, compare, or retrospectively analyze treatment plans for more than one treatment modality.
Regarding the PTV margin, AAPM TG101 report stated that at the present time, it is difficult to determine target margins based on clinical results directly [9]. For SRS and SBRT planning, target margins should be determined based on the inhouse immobilization and localization techniques used and the random and systematic treatment uncertainties associated with the disease site for an individual patient. Accordingly, systematic efforts should be made to gain clinical experience and establish margin recipes for future applications.
Treatment plan optimization
The dose delivery techniques for SRS and SBRT can be 2D conventional, 3D conformal radiation therapy (3DCRT), dynamic conformalarc therapy, intensitymodulated radiotherapy (IMRT) and volumetricmodulated arc therapy (VMAT). Modern commercial treatment planning systems (TPS) that can plan for SRS and SBRT have implemented advanced optimization algorithms. Detailed descriptions of optimization algorithms for special delivery techniques, e.g., IMRT and VMAT, are beyond the scope of this review. However, beam orientation is still an important factor for a planner to consider in the SRS and SBRT planning process.
The goal of beam orientation optimization in SRS and SBRT planning is to avoid sensitive organs and to select short beam paths whenever possible. Mechanical constraints and collision risks imposed by the equipment must be considered. Generally, more radiation beams lead to more conformal target dose distribution and more isotropic dose gradient outside of the target volume, especially for centrally located targets. When an SBRT plan contains a sufficient number of beams, the choice of beam orientation becomes insignificant. However, for shallow or irregularly shaped targets, multipleangle IMRT may still be preferable. It is generally desirable to keep the entrance dose as low as possible (e.g., <30% of the accumulative dose) to prevent acute skin reactions. This can be achieved by avoiding overlapping beams and maintaining an isotropic dose falloff. Higher energy photon beams (e.g., 10 MV vs. 6 MV on a Linac) are more suitable for larger patients to reduce peripheral doses, especially for treating abdominal and pelvic targets. However, this difference diminishes when a sufficient number of beam angles is used (the advantages of rotational arc therapy).
Mechanisms for beam orientation optimization to minimize dose deposition in normal tissue have been investigated. Rotational therapy such as VMAT is generally superior to its static field counterparts in producing conformal dose distributions to cover the target, spare critical structures, and reduce treatment times. In many cases, a uniform dose falloff with VMAT is desirable, but in some cases the treatment target is in close proximity to one or more critical structures, a sharper dose falloff may be required in some particular directions, which may be achieved by selecting more perpendicular beam angles. When treating paraspinal tumors with SBRT, for example, the target volume usually contains the vertebral bone and/or attached softtissue tumor growth, which is immediately adjacent to the spinal cord. An isotropic dose falloff around the treatment target, typically achievable with VMAT, may exceed the dose tolerances for the cord. An IMRT plan with 911 carefully selected beam angles may generate a sharper dose falloff of >10%/mm between the spinal cord and the target, adequately delivering >90% of the prescription dose to the PTV while sparing the cord.
Dose calculation
An important physical quantity in radiotherapy dosimetry is Absorbed Dose (D), which is defined as the energy imparted by ionizing radiation per unit mass of medium (unit of D: gray or Gy; 1Gy = 1J/kg). The accuracy of absorbed dose calculation is critical in radiation therapy treatment planning, while dose calculation algorithms are instrumental in the treatment planning process.
There are three types of dose calculation algorithms that are commonly used in TPS:
Correctionbased algorithm
Correctionbased algorithms were widely used in conventional radiation therapy, which involved minimal computation. There are generally two steps in dose calculation using a correctionbased algorithm:
(1) Establishing dose calculation data;
(2) Reconstructing patient dose distribution by applying corrections.
Establishing Dose Data Library
The basic dose data (or dose data library) can be measured in water under some standard conditions including a fixed sourcetosurface distance (SSD) and normal beam incidence. The water phantom has to be large enough in volume to provide full electron equilibrium. The dose measurements under those conditions include:
Another dose quantity used in earlystage radiotherapy is the tissueair ratio (TAR). This is the ratio of the dose to water at a point in water to dose to water at the same point free in air. For beam energies above 4 MV, it becomes impractical to measure TAR (the real difficulty is to determine dose to water free in air). A more practical dose quantity, the tissuephantom ratio (TPR), is used for megavoltage photon beams, which is defined as the ratio of dose to water at a depth in water to that at a reference depth. If the reference depth is set to the depth of the maximum dose in water, this ratio is referred to as tissuemaximum ratio (TMR). TMR is a dose quantity widely used clinically, especially for SAD (sourceaxis distance) treatment techniques.
Theoretically, the dose in a photon field that satisfies the above standard conditions can be calculated using the following equation:
Where, MU is the monitor chamber reading of a Linac (or the beamontime of a cobalt unit) and C is the machine calibration factor, which specifies the dose received at a reference point for a monitor unit (MU) (or a unit of beamontime). This equation is still widely used by medical physicists to perform a “hand calculation” to check dose contribution for a given MU, or the MUs needed for individual beams of a treatment plan for a given prescription dose, using the SAD technique.
However, the situations encountered in clinical radiotherapy often impose limitations on the use of “standard” distribution functions and dose calculation parameters. Therefore, various correction methods have been used under nonstandard conditions.
Reconstructing Dose Distribution
Correction for Irregular Fields
Photon scatter contributions can be estimated using the Clarkson method by summing up or integrating the individual scatter contributions of each decomposed fan beamlet, i.e., a small sector of the irregular field [12]. The dose at a point in an irregular field is calculated from the primary and scattered components using an effective TMR, that is:
where TMR(0,d) is the primary photon component and SMR(d,r) is the scatter maximum ratio at depth d for a fan beamlet with a radius r_{i}, which is calculated as the ratio of the scattered dose at the point to the effective primary dose at the same point at the depth of maximum dose.
Correction for Surface Irregularities
A convenient, yet effective method to make contour corrections to account for beam oblique incidence or surface curvature is the TMR method. Since TMR only depends on the depth and the field size at the depth, not SSD, thus, the surface contour correction factor (CF) will be
Where, r_{P} is the projected field size at P (i.e., at the distance of SSD + d).
The corrected dose D_{c }at P is thus given by:
Where, D is the uncorrected dose.
Correction for Tissue Heterogeneities
Traditionally, the effects of tissue heterogeneities are corrected using semiempirical methods, such as the TAR method. The key to this correction is to employ an effective depth by scaling the physical depth using the relative electron density (with respect to that of water). The physics principle for this method is that, for highenergy photon beams, the dominant mode of interaction is Compton Effect, which results in beam attenuation and photon scattering in any tissue scalable with electron density. Thus, the heterogeneity correction factor is defined as
where r_{d} is the field size projected at P, d the actual depth of P from the surface and d_{e} is the equivalent water depth.
Another improvement was to predicate scattered dose by scaling the field size using relative electron density. The heterogeneity correction factor based on the “equivalent” tissueair ratio (ETAR) is
where r_{e} is the equivalent field size scaled by the weighted average density of the heterogeneities.
Modelbased algorithm
Deterministic modelbased algorithms such as the convolutionsuperposition method have been developed and implemented clinically in various forms. The common feature of these algorithms is the use of dose kernels to model dose distributions resulting from interactions of primary radiation particles at a point or along a ray line in the dose calculation geometry. Generally, dose kernels at different locations in a human body should not be the same due to the existence of various tissue compositions and densities. For efficiency, however, simplified dose kernels have been used in commercial TPS. Thus, the accuracy of a convolutionsuperposition algorithm depends critically on how the kernel variation is implemented for heterogeneous geometry.
ConvolutionSuperposition with Point Dose Kernels
Mathematically, for a monoenergetic photon field, the dose at a point (a vector that consists of x, y, z component, i.e. ) is given by:
where μ/ρ is the massenergy attenuation coefficient, the primary energy fluence, and the pointdose kernel. The point dose kernel is the dose distribution by primary photon interactions at a point in water (including contributions from both scattered photons and secondary charged particles). The product of massenergy attenuation coefficient and the primary energy fluence has been referred to as TERMA, the total energy released per unit mass.
Since the primary energy fluence and pointdose kernels are functions of energy, the total dose can then be calculated by integrating TERMA with the point dose kernel over the energy spectrum.
The Fast Fourier Transform (FFT) Algorithm
Dose calculation in homogeneous geometry using Eq. 7 can be performed very efficiently with the FFT method. The convolution of two functions with three variables each in the physical space can be transformed into the product of the two functions in the frequency space. The FFT method is similar to correctionbased algorithms in accuracy for heterogeneous geometry because it ignores the spatial variation of the point dose kernel completely.
The Superposition Algorithm
When dose calculation is performed in heterogeneous geometry, the point dose kernels vary spatially and the dose at a point has to be summed up by superposition of the variable point dose kernels, which are typically approximated by stretching or compressing the point dose kernel based on the electron density of the local medium. Mathematically, the dose at a point (i,j,k) is given by
Where, represents the scaled point dose kernel based on local electron density. The point dose kernels can be represented in a Cartesian grid or discretized using collapsed cones [13]; the latter are more computationally efficient (e.g., for kernel tilting and density scaling). It is also more efficient to separate the point dose kernel into a primary kernel for secondary charged particles and a scatter kernel for scattered photons with a different spatial resolution. Due to kernel scaling, the superposition method is more accurate and timeconsuming than the FFT convolution method for heterogeneous geometry.
ConvolutionSuperposition with PencilBeam Dose Kernels
The pencilbeam dose kernel is the dose distribution from a single ray of primary photons (i.e., dose contributions resulting from scattered photons and secondary electrons). The dose distribution can be calculated by
Where, is energyintegrated TERMA at the patient surface, and is the pencil beam dose kernel.
For heterogeneous geometry, the depth coordinate z is scaled by electron density, which means the pencilbeam kernel is stretched or compressed longitudinally. The effect of lateral density variation from the considered ray is ignored or approximated using the ETAR correction (Eq.6). This places the pencilbeam algorithm in the category of correctionbased dose algorithms.
The FiniteSize PencilBeam (FSPB) Algorithm
In FSPB, the dose kernel is a 3D dose distribution resulting from a photon beam of a finite field size that typically corresponds to a beamlet used in IMRT or VMAT. The advantage of FSPB is its high computation efficiency, as the convolutionsuperposition is performed in two dimensions, which is essential to preoptimization dose calculations when hundreds or even thousands of beamlets are needed to achieve an optimal dose distribution. However, due to its approximation in heterogeneity correction, FSPB algorithms may show significant uncertainties at the vicinity of heterogeneities. Therefore, more accurate dose algorithms (e.g., convolutionsuperposition or collapsedcone methods) are often used in the postoptimization dose calculation for advanced treatment techniques.
The Anisotropic Analytical Algorithm (AAA) Algorithm
The AAA algorithm in the Varian Eclipse TPS is an improved pencilbeam algorithm, which uses multiple pencilbeam dose kernels to describe the dose contributions from various radiation sources of a clinical beam. In particular, the pencilbeam dose kernels for the photon sources are further separated into a depth component and a lateral component. The heterogeneity correction is incorporated in the dose summation stage by scaling the depth dose component using the equivalent path length and the lateral component anisotropically according to local electron density. This has significantly improved its accuracy for heterogeneous geometry compared to other pencilbeam algorithms. Varian Eclipse TPS also implemented a photon dose calculation algorithm called Acuros XB (AXB), which is based on the solution of the Linear Boltzmann Transport Equation (LBTE). AXB uses the same source model and commissioning beam data as the AAA algorithm but it is more efficient for RT plans with more beam angles such as VMAT.
Monte Carlo Simulations
The Monte Carlo method is theoretically the most complete and rigorous dosecalculation method, since it simulates the radiation transport and energy deposition of individual particles following the fundamental laws of physics. In fact, it is the only method that takes into account electronic disequilibrium at medium interfaces and in tissue heterogeneities, as well as particle backscattering from dense materials such as teeth, bones and metal prostheses in a patient.
Monte Carlo dose calculation consists of using a computer program to simulate the transport and interaction of individual particles in a patient by random sampling from probability distribution functions that govern the underlying physical processes. The patient’s geometry is reconstructed from CT data with different biological media and mass densities. The dose distribution is calculated by tallying the ionization events that give rise to energy deposition in individual calculation voxels. In order to obtain statistically meaningful dose distributions, a large number (>10^{8}) of radiation particles have to be simulated for a radiation treatment, resulting in long CPU times. This situation has been improved with the availability of fast computers and various variancereduction and efficiencyimprovement techniques such as photon interaction forcing, particle splitting, Russian roulette and electron track repeating.
Accurate Monte Carlo dose calculation requires the precise knowledge of the phasespace information (i.e., the angle, position and energy) of the radiation particles impinging on the patient. This can be achieved by directly simulating the radiation beams from the clinical accelerator or using source models with parameters derived from measurements or Monte Carlo simulated phase space data [14]. Patientspecific beam modifiers such as wedges, blocks and multileaf collimators can be directly simulated in the patient dose calculation to account for their attenuation and scattering effects.
Currently, Monte Carlo dose calculation has been used extensively for assessing existing dose calculation algorithms, investigating novel treatment modalities and treatment techniques prior to their widespread clinical applications, and validating treatment dose delivery in combination with advanced image guidance and in vivo dosimetry measurements [15]. Several commercial treatment planning systems have implemented Monte Carlo algorithms for advanced radiotherapy treatments [16,17]. Monte Carlo algorithms are expected to be the dose engine for the next generation of TPS.
Recommendations on heterogeneity dose calculations
Heterogeneity correction is important to treatment planning for lung and headandneck cancers, in which the effects of charged particle disequilibrium and lateral electron scattering in lowdensity tissue may result in a significant reduction in the target dose especially at the target boundary. Up to 20% target dose reductions have been observed in lung SBRT cases. Simple correctionbased dose algorithms that do not consider lateral charged particle scattering can produce incorrect dose distributions.
Many commercial TPS available for SRS and SBRT have implemented modelbased dose algorithms that employ dose spread kernels precalculated by Monte Carlo and convolutionsuperposition techniques. Although these modelbased dose algorithms consider the effect of secondary electron transport, only approximate methods are used to account for the heterogeneity effect. For example, the pencilbeam superposition algorithm does not account properly for variations of electron scattering in lowerdensity tissues. For those methods that use point dose spread kernels, ray tracing of the radiological distance from the interaction to the dose calculation point is performed for density scaling, which assumes that charged particles travel in straight lines or assuming no effects from nonstraight paths.
The accuracy of inhomogeneity corrections for SRS and SBRT smallfield dosimetry has been investigated. The Imaging and Radiation Oncology Core (IROC) Houston QA Center (previously RPC) presented dose comparisons of various dose calculation algorithms used by different hospitals participating in the Radiation Therapy Oncology Group (RTOG) lung SBRT (0236) trial using an anthropomorphic thorax phantom. Both correctionbased and modelbased algorithms predicted the dose accurately at the target center. However, significant differences were found in the peripheral regions surrounding the target. AAPM TG 65 report recommended the use of inhomogeneity corrections for treatment planning dose calculations, and the potential pitfalls of several heterogeneity correction algorithms in commercially available TPS were discussed for various clinical scenarios [18]. AAPM Task Group 65 specifically disallows the pencilbeam algorithm for situations, in which the target volume is surrounded by low density tissue such as in lung SBRT.
It is generally accepted that modelbased algorithms such as convolutionsuperposition are adequate for most SRS and SBRT applications, where there exists charged particle disequilibrium at the lungtissue/bonetissue interface or target margin in a lowdensity medium. Dose algorithms perform precise particle transport, e.g., Monte Carlo is favorable for the most demanding clinical situations, and for example, a small tumor is surrounded completely by lowdensity lung tissue. Currently, Monte Carlo dose calculation has been implemented clinically and some commercial systems have included Monte Carlo dose calculation for SRS and SBRT applications, for example, the Accuracy CyberKnife TPS [17] and the Elekta Monaco system [19]. SRS and SBRT treatment planning for lung and head and neck should not use pencilbeam algorithms for dose calculation.
Recommendations on voxel size
The dose calculation voxel size (or grid resolution) in treatment planning dose calculation may affect the dose computation results. Studies show that a 2.5 mm uniform grid results in a 1% dose calculation accuracy in the highdose region for highly modulated IMRT plans [20] while a 4 mm grid resolution deteriorated the dose to 5% [21]. It is reported that a 2.3% dose difference can be seen between 2 mm and 1.5 mm grid resolution, rising to 5.6% for a grid resolution of 4 mm. Thus, 2 mm grids are necessary for SRS and SBRT dose calculation, especially for small dose dimensions and sharp dose falloffs [22]. Since SRS and SBRT dose distributions commonly include highly variable doses inside the target and sharp dose falloffs outside of the target and often utilize sophisticated delivery techniques, it has been recommended to use a uniform grid resolution of 2 mm or smaller. Larger (≥ 3 mm) grid sizes are discouraged for SRS and SBRT dose calculation [9].
Dosimetric Considerations for SRS/SBRT Planning
Target Dose Heterogeneity
For SRS and SBRT the prescription dose is often specified at lower isodose, e.g., 50% for SRS and 60%80% for SBRT, as recommended by the AAPM and ASTRO [9,23]. This is because the dose gradient between 4080% isodose lines is much greater than that between 8595% isodose lines. For conventionally fractionated radiotherapy, the dose prescription is specified at 8595% isodose, which results in better target dose uniformity. The dose falloff outside of the target is usually slow but this is acceptable with conventional dose fractionation. Because of the ablative nature of SRS and SBRT, sharper dose falloffs outside of the treatment target is required to avoid damaging nearby OARs. Thus, the prescription dose is specified to lower isodose lines; the selection of 5080% isodose for dose prescription is a compromise between dose falloff and target dose heterogeneity. This practice results in much higher dose heterogeneity within the treatment target in comparison with the practice in conventional fractionation. High dose heterogeneities within the target volume are acceptable for SRS and SBRT plans if the target volume does not contain functional normal tissue. AAPM TG101 report states that hotspots within the central region of a tumor might offer a special advantage in eradicating radioresistant/hypoxic cells that might be more likely located in that area. Higher doses inside the treatment volumes can be desirable, provided that hotspots are within the target. This recommendation is supported by observations of dose response for tumor control in early clinical SRS and SBRT trials [9].
Dose Gradient
Similar to planning for intracranial SRS, it is very effective to use as many nonoverlapping beams as possible to achieve a sharp dose falloff around a small target in SBRT planning. Unless there are critical structures such as the spinal cord or a rib immediately adjacent to the treatment target, the dose outside the target volume should fall off uniformly. This can be achieved using many beams focusing on the target like the GammKnife system or using rotational delivery like dynamic conformal arc or VMAT on a Linac.
Two major factors affect the dose gradient outside of the treatment target. One is the beam energy; the beam penumbra is broader for higher beam energies because of secondary electron scattering in a medium. This effect is more significant for lower density tissues, such as lung. For SRS and SBRT lung applications, 6 MV photon beams, available on most modern Linacs, can be used, which is a reasonable compromise between the beam penumbra characteristics and depth penetration [9]. Recent developments of imageguided rotational gammaray systems (CybeRay, OUR United RT Group, Beijing, China) have shown great potential for SRS and SBRT [24]. Smallsource cobalt beams have sharp penumbra and rapid depth dose falloff, which are ideal for peripheral lung tumors with partialarc deliveries to spare nearby critical structures and the opposite lung. The other is the leaf geometry of the MLC. Partial transmission through the rounded leaf ends and the tongue and groove structure increases the beam penumbra. In general, finer MLC leaves improve the conformity of the MLC field with respect to the crosssection of the target volume. Because of the finite source size and secondary electron lateral scattering, however, negligible improvements have been found for very small MLC leaf width (3 mm or smaller) except for very small lesions [25]. The 5 mm MLC leaf width available on most modern Linacs is adequate for most SBRT applications. Small tumors may be better treated with small cone collimators, which have narrower beam penumbra than MLC.
Dosimetric Parameters for Plan Evaluation
Many dosimetric parameters are used in SRS and SBRT plan evaluation. Tight target dose conformality is an important requirement for SRS and SBRT. This is usually obtained using multiple, nonoverlapping, and sometimes noncoplanar beams or arcs, spread in a large solid angle with fairly equal weighting to minimize the entrance dose and ultimately the volume of intermediate dose. Another important clinical factor is the entrance dose, which should be kept to a modest level to prevent potential severe skin or chest wall toxicity. This can be achieved by optimize the number of beam directions and the relative beam weights for SRS/SBRT planning. If the beam’s eyeview of each radiation field coincides with the crosssection of the PTV (i.e., with no block margin or a 12mm block margin), less surrounding normal tissues will be exposed to direct radiation fields. The resulting dose distribution can be normalized by specifying the prescription dose to a lower isodose line, e.g., 60% to 80%, to provide 95% PTV coverage. For conventionally fractionated radiotherapy, a 46 mm block margin will be used so that a prescription isodose line of 8595% can be used to cover the PTV with a more homogeneous target dose distribution.
Target dose coverage: The target dose is normalized to prescribe the dose to either 50% isodose for intracranial SRS or 6080% isodose for SBRT to achieve a 95% or higher coverage of the target volume. This results in a D_{max} up to 200% of the prescription dose for SRS or between 130170% of the prescription dose for SBRT, which must be inside the PTV. Another important target dose requirement is at least 99% of the PTV should receive more than 90% of the prescription dose (i.e., allowing a cold spot of <10%).
In assessment and evaluation of SRS and SBRT plans, three major criteria are recommended by ASTRO: conformity index, high dose spillage, and intermediatedose spillage [23]. These concepts have been used in RTOG multicenter SBRT trials. The target dose requirements and OAR constraints as a function of structure volume can be found in the radiotherapy sections of these protocols (e.g., RTOG 0236, 0618, 0813 and 0915, etc).
Conformity index (CI): The conformity index is defined as the ratio of the volume of the isodose shell that receives the prescription dose (TV100%) to the PTV volume (CI = TV100%/PTV). It is recommended that this ratio be kept to less than 1.2 to minimize the volume of tissue receiving an ablative dose. Greater CI (e.g., up to 1.5) may be used for small tumors (<2.5cm).
Highdose spillage: Any areas receiving greater than 105% of the prescription dose, commonly referred to as highdose spillage, are generally confined to the PTV. For difficult cases, normal tissue volume receiving >105% of prescription dose should be kept under 15% of the PTV.
Intermediatedose spillage: Intermediatedose spillage is responsible for most of the toxicity associated with SRS and SBRT if the target volume is adjacent to critical organs or structures. This can be evaluated using one or both the following methods: (1) to keep dose to any point 2cm away from the PTV surface (D2cm) below a limit, and (2) to keep the ratio of 50% isodose volume (TV50%) to the PTV volume (R50% = TV50%/PTV) as low as possible. For example, try to keep R50% under 5 for large targets (>4cm in any direction) or under 8 for small tumors (<2cm in any direction).
Target Dose Prescription and Critical Structure Dose Constraints
SRS and SBRT treatments have garnered great interest in the last decades and various dose/fractionation schemes have been adopted in clinical trials and used for routine clinical treatments. At the time of writing, there have been no sufficient clinical outcome data to determine the best dose prescription/fractionation or precise dose constraints for various treatment sites and critical organs/structures. Presently, singlefraction SRS is primarily used for intracranial tumors and spine lesions while multiplefraction SBRT is often used for other treatment sites. Generally smaller target volumes will allow for fewer fractions of large doses while for large tumor volumes especially when they are adjacent to critical structures more fractions will be needed to reduce normal tissue toxicities. Below are typical target dose prescriptions used in clinical trials and our institution for various body sites.
Brain: For brain metastases, with median minimum peripheral doses of 16.525Gy, a 7095% local control rate can be expected. Up to 5 lesions can be treated at one time. A whole brain irradiation (WBI) of 3040Gy at 2Gy/fraction can be given before or after the SRS treatment, However, recent studies have shown a single SRS treatment without WBI may result in better survival and neurocognitive function. Prescription doses for other diseases: SRS 16.520Gy for AVM; SRS 1520Gy and SRT 4550Gy for pituitary adenoma; SRS 1012.5Gy and SRT 2025Gy for acoustic schwannomas (neuromas); SRS/SRT 1050Gy for meningioma; SRS 1217Gy before or after RT for GBM; SRS/SRT 70Gy for trigeminal neuralgia, 120180Gy for thalamotomy, and 120160Gy for pallidotomy.
Spine: Various prescription doses have been used: for example, UPMC: SRS a single mean dose of 16.31Gy; Georgetown Univ.: SBRT 20.59Gy in 3 fractions; FCCC: SRS 1618Gy and SBRT 2430Gy in 36 fractions.
Lung: Prescription dose for lung SBRT depends on tumor size and location. In our institution we typically prescribe 48Gy in 4 fractions for peripheral tumors >2 cm from the primary bronchial tree; 50Gy in 5 fractions for central tumors or large tumors adjacent to chest wall or rib; 50Gy in 5 fractions for isolated recurrence after conventional RT in a potentially curable patient; and 4050Gy in 5 fractions for oligometastatic disease. The latest NCCN clinical practice guidelines for nonsmall cell lung cancer (version 3.2019, available at NCCN.org) listed SBRT dose prescription: 2534Gy single fractions for peripheral, small (<2cm) tumors, especially >1cm away from the chest wall; 4560Gy in 3 fractions for peripheral tumors >1cm from the chest wall; 4850Gy in 4 fractions for central or peripheral tumors, <45cm, especially >1cm from the chest wall; 5055Gy in 5 fractions for central and peripheral tumors, especially >1cm from chest wall; and 6070Gy in 810 fractions for central tumors.
Liver: Prescription dose for liver SBRT varies drastically in various clinical trials, for example, 1545Gy in 15 fractions for primary and metastatic disease [26]; 3660Gy in 3 fractions for one to three hepatic metastatic lesions [27]; 60Gy in 5 fractions [28]; 4562Gy in 3 fractions for hepatic metastases [29]; and 37Gy in 5 fractions for hepatic metastases [30]. At FCCC, we typically treat hepatic metastases from colorectal cancer (CRC) with 60Gy in 5 fractions, and hepatocellular carcinoma (HCC) with 50Gy in 5 fractions, or 40Gy in 5 fractions (possible 3 fractions of 8Gy, one month break and then 2 fractions of 8Gy).
Prostate: New ASTRO/ASCO/AUA guidelines suggest moderate hypofractionation (2.43.4Gy/day) as an alternative to conventional fractionation (1.82 Gy/day), for example: 60Gy in 20 fractions or 70Gy in 28 fractions. Ultrahypofractionation (≥5Gy/day) may be offered to lowrisk patients as an alternative, but only through clinical trials for intermediateand highrisk patients, for example: 35Gy in 5 fractions or 36.25Gy in 5 fractions (consecutive days). The RTOG trial 11074 prescribes 37Gy in 5 fractions (2 fractions per business week).
Pancreas: SBRT trials have been designed to treat pancreatic cancers to increase the likelihood of surgical resection and therefore prolong patient’s survival. Various dose schemes have been used including 1525Gy in a single fraction [31,32], 2445Gy in 3 fractions [3335] and 2533Gy in 5 fractions [36,37]. At FCCC, we typically prescribe 2537Gy in 5 fractions.
Although there are variations of dose/fractionation schemes for a particular body site, the actual dose/fractionation for an individual patient may be affected by the potential normal tissue toxicities. Tables 1 to 5 list the volumedose constraints and the maximum point dose limits for various organs and normal tissues for singlefraction SRS and multiplefraction (e.g., 3, 4, 5 and 8) SBRT treatment planning. The dose tolerance values in these tables are based on the RTOG protocols and data in the literature [38,39], which have been used in our institution as a planning guide.
Table 1: Volumedose constraints and the maximum point doses (volume <0.035cc) for various tissues/organs for a singlefraction SRS treatment. Circumferential irradiation should be avoided for tubelike organs such as esophagus, bronchus, duodenum, jejunum/ileum, colon and rectum.
Tissue/Organ 
Volume 
Volume Max Dose 
Max Point Dose 
Optic pathway 
<0.2 cc 
8 Gy 
10 Gy 
Cochlea 
9 Gy 

Brainstem (not medulla) 
<0.5 cc 
10 Gy 
15 Gy 
Spinal cord and medulla (10cm above and below treatment target) 
<0.35 cc <1.2 cc 
10 Gy 8 Gy 
14 Gy 
Spinal cord subvolume (56mm above and below treatment target) 
<10% 
10 Gy 
14 Gy 
Cauda equina 
<5 cc 
14 Gy 
16 Gy 
Sacral plexus 
<5 cc 
14.4 Gy 
16 Gy 
Esophagus 
<5 cc 
11.9 Gy 
15.4 Gy 
Brachial plexus 
<3 cc 
13.6 Gy 
16.4 Gy 
Heart/pericardium 
<15 cc 
16 Gy 
22 Gy 
Great vessels 
<10 cc 
31 Gy 
37 Gy 
Trachea and large bronchus 
<4 cc 
17.4 Gy 
20.2 Gy 
Bronchus smaller airways 
<0.5 cc 
12.4 Gy 
13.3 Gy 
Lung (right & left) 
1500 cc 
7 Gy 

Lung (right & left) 
1000 cc 
7.6 Gy 
V_{8Gy} <37% 
Rib 
<5 cc 
28 Gy 
33 Gy 
Skin 
<10 cc 
25.5 Gy 
27.5 Gy 
Stomach 
<5 cc 
17.4 Gy 
22 Gy 
Bile duct 
30 Gy 

Duodenum 
<5 cc <10 cc 
11.2 Gy 9 Gy 
17 Gy 
Liver 
700 cc 
11 Gy 

Renal cortex (right & left) 
200 cc 
9.5 Gy 

Jejunum/ileum 
<30 cc 
12.5 Gy 
22 Gy 
Colon 
<20 cc 
18 Gy 
29.2 Gy 
Rectum 
<3.5 cc <20 cc 
39 Gy 22 Gy 
44.2 Gy 
Ureter 
35 Gy 

Bladder wall 
<15 cc 
12 Gy 
25 Gy 
Penile bulb 
<3 cc 
16 Gy 

Femoral heads 
<10 cc 
15 Gy 

Renal hilum/vascular trunk 
15 cc 
14 Gy 
Table 2: Volumedose constraints and the maximum point doses (volume <0.035cc) for various issues/organs for a 3fraction SRT/SBRT treatment. Circumferential irradiation should be avoided for tubelike organs such as esophagus, bronchus, duodenum, jejunum/ileum, colon and rectum.
Tissue/Organ 
Volume 
Volume Max Dose 
Max Point Dose 
Optic pathway 
<0.2 cc 
15.3 Gy 
17.4 Gy 
Cochlea 
14.4 Gy 

Brainstem (not medulla) 
<0.5 cc 
15.9 Gy 
23.1 Gy 
Spinal cord and medulla (10cm above and below treatment target) 
<0.35 cc <1.2 cc 
15.9 Gy 13 Gy 
22.5 Gy 
Spinal cord subvolume (56mm above and below treatment target) 
<10% 
18 Gy 
22.5 Gy 
Cauda equina 
<5 cc 
21.9 Gy 
25.5 Gy 
Sacral plexus 
<5 cc 
22.5 Gy 
24 Gy 
Esophagus 
<5 cc 
17.7 Gy 
25.2 Gy 
Brachial plexus 
<3 cc 
22 Gy 
26 Gy 
Heart/pericardium 
<15 cc 
24 Gy 
30 Gy 
Great vessels 
<10 cc 
39 Gy 
45 Gy 
Trachea and large bronchus 
<5 cc 
25.8 Gy 
30 Gy 
Bronchus smaller airways 
<0.5 cc 
18.9 Gy 
23.1 Gy 
Lung (right & left) 
1500 cc 
10.5 Gy 

Lung (right & left) 
1000 cc 
11.4 Gy 
V_{11Gy}<37% 
Rib 
<5 cc 
40 Gy 
50 Gy 
Skin 
<10 cc 
31 Gy 
33 Gy 
Stomach 
<5 cc 
22.5 Gy 
30 Gy 
Bile duct 
36 Gy 

Duodenum 
<5 cc <10 cc 
15.6 Gy 12.9 Gy 
22.2 Gy 
Liver 
700 cc 
17.1 Gy 

Renal cortex (right & left) 
200 cc 
15 Gy 

Jejunum/ileum 
<30 cc 
17.4 Gy 
27 Gy 
Colon 
<20 cc 
24 Gy 
34.5 Gy 
Rectum 
<3.5 cc <20 cc 
45 Gy 27.5 Gy 
49.5 Gy 
Ureter 
40 Gy 

Bladder wall 
<15 cc 
17 Gy 
33 Gy 
Penile bulb 
<3 cc 
25 Gy 

Femoral heads 
<10 cc 
24 Gy 

Renal hilum/vascular trunk 
15 cc 
19.5 Gy 
Table 3: Volumedose constraints and the maximum point doses (volume <0.035cc) for various issues/organs for a 4fraction SRT/SBRT treatment. Circumferential irradiation should be avoided for tubelike organs such as esophagus, bronchus, duodenum, jejunum/ileum, colon and rectum.
Tissue/Organ 
Volume 
Volume Max Dose 
Max Point Dose 
Optic pathway 
<0.2 cc 
19.2 Gy 
21.2 Gy 
Cochlea 
18 Gy 

Brainstem (not medulla) 
<0.5 cc 
20.8 Gy 
27.2 Gy 
Spinal cord and medulla (10cm above and below treatment target) 
<0.35 cc <1.2 cc 
18 Gy 14.6 Gy 
25.6 Gy 
Spinal cord subvolume (56mm above and below treatment target) 
<10% 
18 Gy 
25.6 Gy 
Cauda equina 
<5 cc 
26 Gy 
28.8 Gy 
Sacral plexus 
<5 cc 
26 Gy 
28 Gy 
Esophagus 
<5 cc 
18.8 Gy 
30 Gy 
Brachial plexus 
<3 cc 
24.8 Gy 
29.6 Gy 
Heart/pericardium 
<15 cc 
28 Gy 
34 Gy 
Great vessels 
<10 cc 
43 Gy 
49 Gy 
Trachea and large bronchus 
<5 cc 
28.8 Gy 
34.8 Gy 
Bronchus smaller airways 
<0.5 cc 
20 Gy 
28 Gy 
Lung (right & left) 
1500 cc 
11.6 Gy 

Lung (right & left) 
1000 cc 
12.4 Gy 
V_{13Gy}<37% 
Rib 
<5 cc 
43 Gy 
54 Gy 
Skin 
<10 cc 
33.6 Gy 
36 Gy 
Stomach 
<5 cc 
25 Gy 
33.2 Gy 
Bile duct 
38.4 Gy 

Duodenum 
<5 cc <10 cc 
17.2 Gy 14 Gy 
24.4 Gy 
Liver 
700 cc 
19.2 Gy 

Renal cortex (right & left) 
200 cc 
17 Gy 

Jejunum/ileum 
<30 cc 
18.8 Gy 
30 Gy 
Colon 
<20 cc 
26 Gy 
37.2 Gy 
Rectum 
<3.5 cc <20 cc 
47.2 Gy 30 Gy 
52.4 Gy 
Ureter 
43 Gy 

Bladder wall 
<15 cc 
18.5 Gy 
35.6 Gy 
Penile bulb 
<3 cc 
27 Gy 

Femoral heads 
<10 cc 
27 Gy 

Renal hilum/vascular trunk 
15 cc 
21.5 Gy 
Table 4: Volumedose constraints and the maximum point doses (volume <0.035cc) for various issues/organs for a 5fraction SRT/SBRT treatment. Circumferential irradiation should be avoided for tubelike organs such as esophagus, bronchus, duodenum, jejunum/ileum, colon and rectum.
Tissue/Organ 
Volume 
Volume Max Dose 
Max Point Dose 
Optic pathway 
<0.2 cc 
23 Gy 
25 Gy 
Cochlea 
22 Gy 

Brainstem (not medulla) 
<0.5 cc 
23 Gy 
31 Gy 
Spinal cord and medulla (10cm above and below treatment target) 
<0.35 cc <1.2 cc 
22 Gy 15.6 Gy 
28 Gy 
Spinal cord subvolume (56mm above and below treatment target) 
<10% 
22 Gy 
28 Gy 
Cauda equina 
<5 cc 
30 Gy 
31.5 Gy 
Sacral plexus 
<5 cc 
30 Gy 
32 Gy 
Esophagus 
<5 cc 
19.5 Gy 
35 Gy 
Brachial plexus 
<3 cc 
27 Gy 
32.5 Gy 
Heart/pericardium 
<15 cc 
32 Gy 
38 Gy 
Great vessels 
<10 cc 
47 Gy 
53 Gy 
Trachea and large bronchus 
<5 cc 
32 Gy 
40 Gy 
Bronchus smaller airways 
<0.5 cc 
21 Gy 
33 Gy 
Lung (right & left) 
1500 cc 
12.5 Gy 

Lung (right & left) 
1000 cc 
13.5 Gy 
V_{13.5Gy}<37% 
Rib 
<5 cc 
45 Gy 
57 Gy 
Skin 
<10 cc 
36.5 Gy 
38.5 Gy 
Stomach 
<5cc 
26.5 Gy 
35 Gy 
Bile duct 
41 Gy 

Duodenum 
<5 cc <10 cc 
18.5 Gy 14.5 Gy 
26 Gy 
Liver 
700 cc 
21 Gy 

Renal cortex (right & left) 
200 cc 
18 Gy 

Jejunum/ileum 
<30 cc 
20 Gy 
32 Gy 
Colon 
<20 cc 
28.5 Gy 
40 Gy 
Rectum 
<3.5 cc <20 cc 
50 Gy 32.5 Gy 
55 Gy 
Ureter 
45 Gy 

Bladder wall 
<15 cc 
20 Gy 
38 Gy 
Penile bulb 
<3 cc 
30 Gy 

Femoral heads 
<10 cc 
30 Gy 

Renal hilum/vascular trunk 
15 cc 
23 Gy 
Table 5: Volumedose constraints and the maximum point doses (volume <0.035cc) for various issues/organs for an 8fraction SRT/SBRT treatment. Circumferential irradiation should be avoided for tubelike organs such as esophagus, bronchus, duodenum, jejunum/ileum, colon and rectum.
Tissue/Organ 
Volume 
Volume Max Dose 
Max Point Dose 
Optic pathway 
<0.2 cc 
27.2 Gy 
29.6 Gy 
Cochlea 
26.4 Gy 

Brainstem (not medulla) 
<0.5 cc 
27.2 Gy 
37.6 Gy 
Spinal cord and medulla (10cm above and below treatment target) 
<0.35 cc <1.2 cc 
26.4 Gy 18.2 Gy 
33.6 Gy 
Spinal cord subvolume (56mm above and below treatment target) 
<10% 
26.4 Gy 
33.6 Gy 
Cauda equina 
<5 cc 
36 Gy 
38.4 Gy 
Sacral plexus 
<5 cc 
36 Gy 
38.4 Gy 
Esophagus 
<5 cc 
21.6 Gy 
38.4 Gy 
Brachial plexus 
<3 cc 
32.8 Gy 
39.2 Gy 
Heart/pericardium 
<15 cc 
34.4 Gy 
38.4 Gy 
Great vessels 
<10 cc 
55.2 Gy 
38.4 Gy 
Trachea and large bronchus 
<5 cc 
38.4 Gy 
48.8 Gy 
Bronchus smaller airways 
<0.5 cc 
22.4 Gy 
36 Gy 
Lung (right & left) 
1500 cc 
13.6 Gy 

Lung (right & left) 
1000 cc 
15.2 Gy 
V_{15Gy}<37% 
Rib 
<5 cc 
50 Gy 
63 Gy 
Skin 
<10 cc 
43.2 Gy 
45.6 Gy 
Stomach 
<5 cc 
31.2 Gy 
42 Gy 
Bile duct 
48 Gy 

Duodenum 
<5 cc <10 cc 
21 Gy 16 Gy 
30.4 Gy 
Liver 
700 cc 
24 Gy 

Renal cortex (right & left) 
200 cc 
21 Gy 

Jejunum/ileum 
<30 cc 
23.2 Gy 
37 Gy 
Colon 
<20 cc 
33 Gy 
48 Gy 
Rectum 
<3.5 cc <20 cc 
58.4 Gy 37.5 Gy 
63.2 Gy 
Ureter 
53 Gy 

Bladder wall 
<15 cc 
22.4 Gy 
44.8 Gy 
Penile bulb 
<3 cc 
35 

Femoral heads 
<10 cc 
35 Gy 

Renal hilum/vascular trunk 
15 cc 
28 Gy 
After plan optimization, the dose distribution can be renormalized so that the target prescription dose and the total fraction number can be determined based on the maximum critical organ/structure volumedose and the maximum point dose values achievable/acceptable for the plan under consideration. It should be noted that the values in Tables 15 should be used with caution since most of them were extrapolated/converted from the dose tolerances for conventionaldose/fractionation treatments using the linearquadratic model [38,40]. On the other hand, one can derive the volumedose constraints and the maximum point dose limits using the linearquadratic model for fraction numbers not included in Tables 1 to 5.
Dosimetry Measurement
SRS and SBRT utilize small photon fields to achieve the desired, highly modulated and conformal dose distribution. Measurement of small photon beams for SRS and SBRT TPS commissioning and plan validation is complicated due to conditions of charged particle disequilibrium, detector volumeaveraging effects, detectorinterface artifacts, detector perturbation effects, and detector positionorientation effects [9]. For small field dosimetry, the AAPM Task Group 101 recommends the use of an appropriate dosimeter with a spatial resolution of approximately 1 mm or better (stereotactic detectors). Even with stereotactic detectors and careful measurement setup, large (> 10%) measurement discrepancies have been reported for cone factors of very small (<10mm) photon fields [41,42].
AAPM Task Group 101 discussed the issue of small field dosimetry in great detail and provided useful recommendations for beam commissioning and plan validation. For SRS and SBRT beam commissioning, the maximum dimension of the active detector volume should be less than half the full width at half maximum (FWHM) of the smallest photon field in order to determine the central axis depth dose, tissuephantom ratio, output factor and scatter factor. For the lateral dose profile or offaxis ratio measurement, one can unfold the chamber size effect from measured small beam profiles by deconvolving the detectorresponse artifact from each point in the profiles. The detector volume effect only becomes significant when the maximum dimension of the active detector volume is comparable to the half size of the small photon fields.
Because of the small dimensions and rapid falloffs of SRS and SBRT dose distribution, only those adequate detectors should be used, as recommended by the joint IAEAAAPM report [43]. For SRS and SBRT dose measurement, large errors are often associated with small setup errors or measuring point misplacement. Measurements for MLCshaped fields also show more geometric and dosimetric uncertainties than those for fields collimated by circular cones. This is mainly due to the complex MLC leaf geometry (tongue and groove and leaf end) and mechanical uncertainty. For small MLC fields, the leafedge effect depends mainly on the field size and type of MLC and is not related to the depth [44]. For more detailed discussions on small field dosimetry, please refer to the recently published report “An IAEA–AAPM International Code of Practice for Reference and Relative Dose Determination. Technical Report Series No. 483” [43].
REFERENCES