Report of AAPM Therapy Physics Committee Task Group 74: In-air output ratio, Sc, for megavoltage photon beams

An interface tool to parametrize treatment plans for the TrueBeam radiotherapy system into TOPAS parameter control files for Monte Carlo simulation

PURPOSE

The Monte Carlo (MC) method, the gold standard method for radiotherapy dose calculations, is underused in clinical research applications mainly due to computational speed limitations. Another reason is the time-consuming and error prone conversion of treatment plan specifications into MC parameters. To address this issue, we developed an interface tool that creates a set of TOPAS parameter control files (PCF) from information exported from a clinical treatment planning system (TPS) for plans delivered by the TrueBeam radiotherapy system.

METHODS

The interface allows the user to input DICOM-RT files, exported from a TPS and containing the plan parameters, and choose different multileaf-collimator models, variance reduction technique parameters, scoring quantities and simulation output formats. Radiation sources are precomputed phase space files obtained from Varian. Based on this information, ready-to-run TOPAS PCF that incorporate the position and angular rotation of the TrueBeam dynamic collimation devices, gantry, couch, and patient according to treatment plan specifications are created.

RESULTS

Dose distributions computed using these PCF were compared against predictions from commercial TPS for different clinical treatment plans and techniques (3D-CRT, IMRT step-and-shoot and VMAT) to evaluate the performance of the interface. The agreement between dose distributions from TOPAS and TPS (>98 % pass ratio in the gamma test) confirmed the correct parametrization of treatment plan specifications into MC PCF.

CONCLUSIONS

This interface tool is expected to widen the use of MC methods in the clinical medical physics field by facilitating the straightforward transfer of treatment plan parameters from commercial TPS into MC PCF.

Towards real-time EPID-based 3D in vivo dosimetry for IMRT with Deep Neural Networks: A feasibility study

We investigate the potential of the Deep Dose Estimate (DDE) neural network to predict 3D dose distributions inside patients with Monte Carlo (MC) accuracy, based on transmitted EPID signals and patient CTs. The network was trained using as input patient CTs and first-order dose approximations (FOD). Accurate dose distributions (ADD) simulated with MC were given as training targets. 83 pelvic CTs were used to simulate ADDs and respective EPID signals for subfields of prostate IMRT plans (gantry at 0∘). FODs were produced as backprojections from the EPID signals. 581 ADD-FOD sets were produced and divided into training and test sets. An additional dataset simulated with gantry at 90∘ (lateral set) was used for evaluating the performance of the DDE at different beam directions. The quality of the FODs and DDE-predicted dose distributions (DDEP) with respect to ADDs, from the test and lateral sets, was evaluated with gamma analysis (3%,2 mm). The passing rates between FODs and ADDs were as low as 46%, while for DDEPs the passing rates were above 97% for the test set. Meaningful improvements were also observed for the lateral set. The high passing rates for DDEPs indicate that the DDE is able to convert FODs into ADDs. Moreover, the trained DDE predicts the dose inside a patient CT within 0.6 s/subfield (GPU), in contrast to 14 h needed for MC (CPU-cluster). 3D in vivo dose distributions due to clinical patient irradiation can be obtained within seconds, with MC-like accuracy, potentially paving the way towards real-time EPID-based in vivo dosimetry.

Dual beam-current transformer design for monitoring and reporting of electron ultra-high dose rate (FLASH) beam parameters

PURPOSE

To investigate the usefulness and effectiveness of a dual beam-current transformer (BCTs) design to monitor and record the beam dosimetry output and energy of pulsed electron FLASH (eFLASH) beams in real-time, and to inform on the usefulness of this design for future eFLASH beam control.

METHODS

Two BCTs are integrated into the head of a FLASH Mobetron system, one located after the primary scattering foil and the other downstream of the secondary scattering foil. The response of the BCTs was evaluated individually to monitor beam output as a function of dose, scattering conditions, and ability to capture physical beam parameters such as pulse width (PW), pulse repetition frequency (PRF), and dose per pulse (DPP), and in combination to determine beam energy using the ratio of the lower-to-upper BCT signal.

RESULTS

A linear relationship was observed between the absorbed dose measured on Gafchromic film and the BCT signals for both the upper and lower BCT (R²  > 0.99). A linear relationship was also observed in the BCT signals as a function of the number of pulses delivered regardless of the PW, DPP, or PRF (R²  > 0.99). The lower-to-upper BCT ratio was found to correlate strongly with the energy of the eFLASH beam due to differential beam attenuation caused by the secondary scattering foil. The BCTs were also able to provide accurate information about the PW, PRF, energy, and DPP for each individual pulse delivered in real-time.

CONCLUSION

The dual BCT system integrated within the FLASH Mobetron was shown to be a reliable monitoring system able to quantify accelerator performance and capture all essential physical beam parameters on a pulse-by-pulse basis, and the ratio between the two BCTs was strongly correlated with beam energy. The fast signal readout and processing enables the BCTs to provide real-time information on beam output and energy and is proposed as a system suitable for accurate beam monitoring and control of eFLASH beams.

AAPM MEDICAL PHYSICS PRACTICE GUIDELINE 5.b: Commissioning and QA of treatment planning dose calculations-Megavoltage photon and electron beams

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines:

Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. While must is the term to be used in the guidelines, if an entity that adopts the guideline has shall as the preferred term, the AAPM considers that must and shall have the same meaning.

Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

Commissioning a secondary dose calculation software for a 0.35 T MR-linac

Secondary external dose calculations for a 0.35 T magnetic resonance image-guided radiation therapy (MRgRT) are needed within the radiation oncology community to follow safety standards set forth within the field. We evaluate the commercially available software, RadCalc, in its ability to accurately perform monitor unit dose calculations within a magnetic field. We also evaluate the potential effects of a 0.35 T magnetic field upon point dose calculations. Monitor unit calculations were evaluated with (wMag) and without (noMag) a magnetic field considerations in RadCalc for the ViewRay MRIdian. The magnetic field is indirectly accounted for by using asymmetric profiles for calculation. The introduction of double-stacked multi-leaf collimator leaves was also included in the monitor unit calculations and a single transmission value was determined. A suite of simple and complex geometries with a variety field arrangements were calculated for each method to demonstrate the effect of the 0.35 T magnetic field on monitor unit calculations. Finally, 25 patient-specific treatment plans were calculated using each method for comparison. All simple geometries calculated in RadCalc were within 2% of treatment planning system (TPS) values for both methods, except for a single noMag off-axis comparison. All complex muilt-leaf collimator (MLC) pattern calculations were within 5%. All complex phantom geometry calculations were within 5% except for a single field within a lung phantom at a distal point. For the patient calculations, the noMag method average percentage difference was 0.09 ± 2.5% and the wMag average percentage difference was 0.08 ± 2.5%. All results were within 5% for the wMag method. We performed monitor unit calculations for a 0.35 T MRgRT system using a commercially available secondary monitor unit dose calculation software and demonstrated minimal impact of the 0.35 T magnetic field on monitor unit dose calculations. This is the first investigation demonstrating successful calculations of dose using RadCalc in the low-field 0.35 T ViewRay MRIdian system.

Report of AAPM Task Group 219 on independent calculation-based dose/MU verification for IMRT

Independent verification of the dose per monitor unit (MU) to deliver the prescribed dose to a patient has been a mainstay of radiation oncology quality assurance (QA). We discuss the role of secondary dose/MU calculation programs as part of a comprehensive QA program. This report provides guidelines on calculation-based dose/MU verification for intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) provided by various modalities. We provide a review of various algorithms for "independent/second check" of monitor unit calculations for IMRT/VMAT. The report makes recommendations on the clinical implementation of secondary dose/MU calculation programs; on commissioning and acceptance of various commercially available secondary dose/MU calculation programs; on benchmark QA and periodic QA; and on clinically reasonable action levels for agreement of secondary dose/MU calculation programs.

Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non-equilibrium conditions

Small-field dosimetry used in advance treatment technologies poses challenges due to loss of lateral charged particle equilibrium (LCPE), occlusion of the primary photon source, and the limited choice of suitable radiation detectors. These challenges greatly influence dosimetric accuracy. Many high-profile radiation incidents have demonstrated a poor understanding of appropriate methodology for small-field dosimetry. These incidents are a cause for concern because the use of small fields in various specialized radiation treatment techniques continues to grow rapidly. Reference and relative dosimetry in small and composite fields are the subject of the International Atomic Energy Agency (IAEA) dosimetry code of practice that has been published as TRS-483 and an AAPM summary publication (IAEA TRS 483; Dosimetry of small static fields used in external beam radiotherapy: An IAEA/AAPM International Code of Practice for reference and relative dose determination, Technical Report Series No. 483; Palmans et al., Med Phys 45(11):e1123, 2018). The charge of AAPM task group 155 (TG-155) is to summarize current knowledge on small-field dosimetry and to provide recommendations of best practices for relative dose determination in small megavoltage photon beams. An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size is provided. Recommendations are included on appropriate detector systems and measurement methodologies. Existing published data on dosimetric parameters in small photon fields (e.g., percentage depth dose, tissue phantom ratio/tissue maximum ratio, off-axis ratios, and field output factors) together with the necessary perturbation corrections for various detectors are reviewed. A discussion on errors and an uncertainty analysis in measurements is provided. The design of beam models in treatment planning systems to simulate small fields necessitates special attention on the influence of the primary beam source and collimating devices in the computation of energy fluence and dose. The general requirements for fluence and dose calculation engines suitable for modeling dose in small fields are reviewed. Implementations in commercial treatment planning systems vary widely, and the aims of this report are to provide insight for the medical physicist and guidance to developers of beams models for radiotherapy treatment planning systems.

Portal dosimetry of small unflattened beams

We developed and validated a dedicated small field back-projection portal dosimetry model for pretreatment andin vivoverification of stereotactic plans entailing small unflattened photon beams. For this purpose an aSi-EPID was commissioned as a small field dosimeter. Small field output factors for 6 MV FFF beams were measured using the PTW microDiamond detector and the Agility 160-leaf MLC from Elekta. The back-projection algorithm developed in our department was modified to better model the small field physics. The feasibility of small field portal dosimetry was validated via absolute point dose differences w.r.t. small static beams, and 5 hypofractionated stereotactic VMAT clinical plans measured with the OCTAVIUS 1000 SRS array dosimeter and computed with the treatment planning system Pinnacle v16.2. Dose reconstructions using the currently clinically applied back-projection model were also computed for comparison. We found that the latter yields underdosage of about -8% for square beams with cross section near 10 mm x 10 mm and about -6% for VMAT treatments with PTV volumes smaller than about 2cm³. With the methods described in this work such errors can be reduced to less than the ±3.0% recommendations for clinical use. Our results indicate that aSi-EPIDs can be used as accurate small field radiation dosimeters, offering advantages over point dose detectors, the correct positioning and orientation of which is challenging for routine clinical QA.

Commissioning and performance evaluation of RadCalc for the Elekta unity MRI-linac

Recent availability of MRI‐guided linear accelerators has introduced a number of clinical challenges, particularly in the context of online plan adaptation. Paramount among these is verification of plan quality prior to patient treatment. Currently, there are no commercial products available for monitor unit verification that fully support the newly FDA cleared Elekta Unity 1.5 T MRI‐linac. In this work, we investigate the accuracy and precision of RadCalc for this purpose, which is a software package that uses a Clarkson integration algorithm for point dose calculation. To this end, 18 IMRT patient plans (186 individual beams) were created and used for RadCalc point dose calculations. In comparison with the primary treatment planning system (Monaco), mean point dose deviations of 0.0 ± 1.0% (n = 18) and 1.7 ± 12.4% (n = 186) were obtained on a per‐plan and per‐beam basis, respectively. The dose plane comparison functionality within RadCalc was found to be highly inaccurate, however, modest improvements could be made by artificially shifting jaws and multi leaf collimator positions to account for the dosimetric shift due to the magnetic field (67.3% vs 96.5% mean 5%/5 mm gamma pass rate).

Modeling scattered radiation from multi-leaf collimators (MLCs) to improve calculation accuracy of in-air output ratio

This study aims to model an extra-focal source for the scattered radiation from multi-leaf collimators (MLCs), namely an MLC scatter source, and to correct in-air output ratio (S_c) calculated using the conventional dual source model (DSM) to achieve better accuracy of point dose calculation. To develop the MLC scatter source, a 6 MV photon beam from a Varian Clinac® iX linear accelerator with millennium 120 MLCs was used. It was assumed that the position for the MLC scatter source was located at the center of the MLC, consisting of line-based and area-based sources to consider the characteristics of the scattered radiation from the MLCs empirically. Based on the measured S_c values for MLC-defined fields, optimal parameters for the line-based and area-based sources were calculated using optimization process. For evaluation of proposed method, measurements were taken for various MLC-defined square and irregular fields. The S_c values calculated using the proposed MLC scatter source and conventional DSM were compared with the measured data. For MLC-defined square fields, the measured S_c values showed better agreement with those calculated using the MLC scatter source (the mean difference was − 0.03%) compared with those calculated using the DSM (the mean difference was 0.18%). For MLC-defined irregular fields, the maximum dose differences between measurements and calculations using the MLC scatter source and DSM were 0.54% and 1.45%, respectively. The developed MLC scatter source could improve the accuracy of S_c calculation for both square and irregular fields defined by MLCs.

Interpretation of In-air Output Ratio of Wedged Fields in Different Measurement Conditions

BACKGROUND

The collimator scatter factor (Sc) is one of the most important parameters in monitor unit (MU) calculation. There are several factors that impact Sc values, including head structures, backscatter in dose monitoring chambers, and wedges. The objective of this study was to investigate the variation of Sc with different buildup cap materials, wall thickness of buildup caps, source-to-chamber distances (SCDs), ionization chambers, and wedge angles in 6 MV photon beam.

METHODS

In this study, copper and Perspex buildup caps were made with two different thicknesses for each buildup cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK dosimeter with a sensitive volume of 0.120 cm3 and Farmer-type ion chamber with a sensitive volume of 0.65 cm3. In all measurements, buildup caps and ionization chambers were positioned such as to stand vertically to the beam central axis. It was also investigated the effect of internal wedge with different angles (30° and 60°) different SCDs on Sc.

RESULTS

It was found in large field sizes, Sc values in Perspex buildup cap were higher than copper. Different SCDs and type of ion chamber and wall thickness of buildup caps had no significant influence on Sc values. The presence of wedge influenced Sc values significantly. Variation of Sc in wedged fields compared to open fields had a maximum deviation of 0.9% and 6.8% in 30° and 60° wedge angles, respectively.

CONCLUSION

It was found that the presence of wedges had a significant influence on Sc and increases with wedge angles. As such, it should be taken into account in manual MU calculations.

[Investigation of Estimation Accuracy of Phantom Scatter Factor by Clarkson Method Considering Depth in MLC Irregular Field]

In monitor unit (MU) independent verification by calculation for irregular field (MLC field) using multileaf collimator in X-ray therapy, it has become common to use collimator scatter factor (S_c) and phantom scatter factor (S_p) instead of total scatter factor (S_{c, p}). It is usually expressed as S_{c, p} (A)=S_c (A)×S_p (A), and the field size A is considered but the depth d is not. S_c is data of in-air output, and measure with a mini-phantom at constant depth to remove electron contamination. On the other hand, S_p is obtained from measurement data of S_{c, p} and S_c, and can be expressed as S_{c, p} (d, A)=S_c (constant depth, A)×S_p (d, A) at an arbitrary depth d, thus S_p depends on the depth of S_{c, p}. Therefore, S_p needs to consider depth. In addition, a linear accelerator equipped with the tertiary MLC has two field sizes, that are collimator field by upper and lower collimators and MLC field by tertiary MLC below them. In MU independent verification by calculation, it is often used that the estimated value of S_p obtained by converting MLC field to equivalent square field and referring to data of S_p in square field. To convert the MLC field to equivalent square field, a conversion formula from sector radius r to equivalent square field L by Clarkson's sector integration (Clarkson method) is used. In this study, using 24 types of MLC fields to evaluate estimation accuracy due to the difference of conversion formula in Clarkson method, we estimated value of S_p using r=0.5611L of B-Clarkson method and using r=0.5580L of A-Clarkson method. And the difference with the measured value of S_p obtained by measuring S_{c, p} and S_c in the same MLC fields was compared. While, to evaluate estimation accuracy due to the different depths using these Clarkson methods, the difference between estimated value and measured value of S_p similarly obtained at depth of 5, 10 and 15 cm was compared. As results, estimated value of S_p using A-Clarkson method than using B-Clarkson method was close to measured value, and it was the same trend at depth of 5, 10 and 15 cm. Therefore, it was suggested that estimation accuracy of S_p by A-Clarkson method is higher than B-Clarkson method when verifying beams with different depths in MU independent verification by calculation for MLC field.

[Investigation of the Influence of the Conversion Method to Equivalent Square Field on the Depth Direction of Phantom Scatter Factor]

In X-ray therapy, equivalent square field (side of equivalent square field) is important because it influences the accuracy of independent verification of monitor unit (MU) by calculation. To calculate the side of equivalent square field for rectangular fields, we often use a table of domestic standard measurement method (Day's method), or A/P method calculated by area-perimeter ratio. The sides of equivalent square fields of these methods are assumed to be unchanged by depth and energy, but there are reports that it is not valid. Therefore, the depth dependency of side of equivalent square fields of Day's method, A/P method, and area ratio correction (ARC) method was compared by measuring phantom scatter factors (S_p). From the analysis of S_p measured at different depths, the estimated value of S_p on the equivalent square side of the Day's method and A/P method had a depth dependency that the difference from the measured value was large when the measurement depth was deep. The estimated value of S_p on the equivalent square side of the ARC method had a small difference from the measured value even when the measurement depth was deep, and the depth dependency was small compared with the Day's method and the A/P method. Side of equivalent square field of ARC method had a smaller difference of depth dependency than in the case of Day's method and A/P method. Therefore, in the independent verification of MU for rectangular field, using the equivalent square side of the ARC method is better.

EPID-based in vivo dosimetry using Dosimetry Check™: Overview and clinical experience in a 5-yr study including breast, lung, prostate, and head and neck cancer patients

BACKGROUND

Independent verification of the dose delivered by complex radiotherapy can be performed by electronic portal imaging device (EPID) dosimetry. This paper presents 5-yr EPID in vivo dosimetry (IVD) data obtained using the Dosimetry Check (DC) software on a large cohort including breast, lung, prostate, and head and neck (H&N) cancer patients.

MATERIAL AND METHODS

The difference between in vivo dose measurements obtained by DC and point doses calculated by the Eclipse treatment planning system was obtained on 3795 radiotherapy patients treated with volumetric modulated arc therapy (VMAT) (n = 842) and three-dimensional conformal radiotherapy (3DCRT) (n = 2953) at 6, 10, and 15 MV. In cases where the dose difference exceeded ±10% further inspection and additional phantom measurements were performed.

RESULTS

The mean and standard deviation ( μ ± σ ) of the percentage difference in dose obtained by DC and calculated by Eclipse in VMAT was: 0.19 ± 3.89 % in brain, 1.54 ± 4.87 % in H&N, and 1.23 ± 4.61 % in prostate cancer. In 3DCRT, this was 1.79 ± 3.51 % in brain, - 2.95 ± 5.67 % in breast, - 1.43 ± 4.38 % in bladder, 1.66 ± 4.77 % in H&N, 2.60 ± 5.35% in lung and - 3.62 ± 4.00 % in prostate cancer. A total of 153 plans exceeded the ±10% alert criteria, which included: 88 breast plans accounting for 7.9% of all breast treatments; 28 H&N plans accounting for 4.4% of all H&N treatments; and 12 prostate plans accounting for 3.5% of all prostate treatments. All deviations were found to be as a result of patient-related anatomical deviations and not from procedural errors.

CONCLUSIONS

This preliminary data shows that EPID-based IVD with DC may not only be useful in detecting errors but has the potential to be used to establish site-specific dose action levels. The approach is straightforward and has been implemented as a radiographer-led service with no disruption to the patient and no impact on treatment time.

The commissioning and validation of Monaco treatment planning system on an Elekta VersaHD linear accelerator

Accurate beam modeling is essential to help ensure overall accuracy in the radiotherapy process. This study describes our experience with beam model validation of a Monaco treatment planning system on a Versa HD linear accelerator. Data were collected such that Monaco beam models could be generated using three algorithms: collapsed cone (CC) and photon Monte Carlo (MC) for photon beams, and electron Monte Carlo (eMC) for electron beams. Validations are performed on measured percent depth doses (PDDs) and profiles, for open‐field point‐doses in homogenous and heterogeneous media, and for obliquely incident electron beams. Gamma analysis is used to assess the agreement between calculation and measurement for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans, including volumetric modulated arc therapy for stereotactic body radiation therapy (VMAT SBRT). For all relevant conditions, gamma index values below 1 are obtained when comparing Monaco calculated PDDs and profiles with measured data. Point‐doses in a water medium are found to be within 2% agreement of commissioning data in 99.5% and 98.6% of the points computed by MC and CC, respectively. All point‐dose calculations for the eMC algorithm in water are within 4% agreement of measurement, and 92% of measurements are within 3%. In heterogeneous media of air and cortical bone, both CC and MC yielded better than 3% agreement with ion chamber measurements. eMC yielded 3% agreement to measurement downstream of air with oblique beams of up to 27°, 5% agreement distal to bone, and within 4% agreement at extended source to surface distance (SSD) for all electron energies except 6 MeV. The 6‐MeV point of measurement is on a steep dose gradient which may impact the magnitude of discrepancy measured. The average gamma passing rate for IMRT/VMAT plans is 96.9% (±2.1%) and 98.0% (±1.9%) for VMAT SBRT when evaluated using 3%/2 mm criteria. Monaco beam models for the Versa HD linac were successfully commissioned for clinical use.

Collimator scatter factor: Monte Carlo and in-air measurements approaches

BACKGROUND

Linac output as a function of field sizes has a phantom and a head scatter component. This last term can be measured in-air with appropriate build-up ensuring a complete electron equilibrium and the absence of the contaminant electrons. Equilibrium conditions could be achieved using a build-up cap or a mini-phantom. Monte Carlo simulations in a virtual phantom mimicking a mini-phantom were analysed with the aim of better understanding the setup conditions for measuring the collimator scatter factor that is the head scatter component of the linac output factors.

METHODS

Beams of 6 and 15 MV from a TrueBeam, with size from 4 × 4 to 40 × 40 cm² were simulated in cylindrical acrylic phantoms 20 cm long, of different diameters, from 0.5 to 4 cm, with the cylinder axis coincident with the beam central axis. The PRIMO package, based on PENELOPE Monte Carlo code, was used. The phase-space files for a Varian TrueBeam linac, provided by the linac vendor, were used for the linac head simulation. Depth dose curves were analysed, and collimator scatter factors estimated at different depth in the different phantom conditions. Additionally, in-air measurements using acyrilic and brass build-up caps, as well as acrylic mini-phantom were acquired for 6 and 18 MV beams from a Varian Clinac DHX.

RESULTS

The depth dose curves along the cylinders were compared, showing, in each phantom, very similar curves for all analysed field sizes, proving the correctness in estimating the collimator scatter factor in the mini-phantom, provided to position the detector to a sufficient depth to exclude electron contamination. The results were confirmed by the measurements, where the acrylic build-up cap showed to be inadequate to properly estimate the collimator scatter factors, while the mini-phantom and the brass caps gave reasonable measurements.

CONCLUSION

A better understanding of the beam characteristics inside a virtual mini-phantom through the analysis of depth dose curves, showed the critical points of using the acrylic build-up cap, and suggested the use of the mini-phantom for the collimator scatter factor measurements in the medium-large field size range.

A novel approach to EPID-based 3D volumetric dosimetry for IMRT and VMAT QA

Intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) are relatively complex treatment delivery techniques and require quality assurance (QA) procedures. Pre-treatment dosimetric verification represents a fundamental QA procedure in daily clinical routine in radiation therapy. The purpose of this study is to develop an EPID-based approach to reconstruct a 3D dose distribution as imparted to a virtual cylindrical water phantom to be used for plan-specific pre-treatment dosimetric verification for IMRT and VMAT plans. For each depth, the planar 2D dose distributions acquired in air were back-projected and convolved by depth-specific scatter and attenuation kernels. The kernels were obtained by making use of scatter and attenuation models to iteratively estimate the parameters from a set of reference measurements. The derived parameters served as a look-up table for reconstruction of arbitrary measurements. The summation of the reconstructed 3D dose distributions resulted in the integrated 3D dose distribution of the treatment delivery. The accuracy of the proposed approach was validated in clinical IMRT and VMAT plans by means of gamma evaluation, comparing the reconstructed 3D dose distributions with Octavius measurement. The comparison was carried out using (3%, 3 mm) criteria scoring 99% and 96% passing rates for IMRT and VMAT, respectively. An accuracy comparable to the one of the commercial device for 3D volumetric dosimetry was demonstrated. In addition, five IMRT and five VMAT were validated against the 3D dose calculation performed by the TPS in a water phantom using the same passing rate criteria. The median passing rates within the ten treatment plans was 97.3%, whereas the lowest was 95%. Besides, the reconstructed 3D distribution is obtained without predictions relying on forward dose calculation and without external phantom or dosimetric devices. Thus, the approach provides a fully automated, fast and easy QA procedure for plan-specific pre-treatment dosimetric verification.

Two-argument total scatter factor for small fields simultaneously collimated by MLC and jaws: application to stereotactic radiosurgery and radiotherapy

Direct use of the total scatter factor (S _tot) for independent monitor unit (MU) calculations can be a good alternative approach to the traditional separate treatment of head/collimator scatter (S _c) and phantom scatter (S _p), especially for stereotactic small fields under the simultaneous collimation of secondary jaws and tertiary multileaf collimators (MLC). We have carried out the measurement of S _tot in water for field sizes down to 0.5  ×  0.5 cm² on a Varian TrueBeam STx medical linear accelerator (linac) equipped with high definition MLCs. Both the jaw field size (c) and MLC field size (s) significantly impact the linac output factors, especially when c [Formula: see text] s and s is small (e.g. s  <  5 cm). The combined influence of MLC and jaws gives rise to a two-argument dependence of the total scatter factor, S _tot(c,s), which is difficult to functionally decouple. The (c,s) dependence can be conceived as a set of s-dependent functions ('branches') defined on domain [s _min, s _max  =  c] for a given jaw size of c. We have also developed a heuristic model of S _tot to assist the clinical implementation of the measured S _tot data for small field dosimetry. The model has two components: (i) empirical fit formula for the s-dependent branches and (ii) interpolation scheme between the branches. The interpolation scheme preserves the characteristic shape of the measured branches and effectively transforms the measured trapezoidal domain in (c,s) plane to a rectangular domain to facilitate easier two-dimensional interpolation to determine S _tot for arbitrary (c,s) combinations. Both the empirical fit and interpolation showed good agreement with experimental validation data.

Effect of VERO pan-tilt motion on the dose distribution

Tumor tracking is an option for intra-fractional motion management in radiotherapy. The VERO gimbal tracking system creates a unique beam geometry and understanding the effect of the gimbal motion in terms of dose distribution is important to assess the dose deviation from the reference conditions. Beam profiles, output factors (OF) and percentage depth doses (PDD) were measured and evaluated to investigate this effect. In order to find regions affected by the pan-tilt motion, synthesized 2D dose distributions were generated. An evaluation of the 2D dose distribution with the reference position was done using dose difference criteria 1%-4%. The OF and point dose at central axis were measured and compared with the reference position. Furthermore, the PDDs were measured using a special monitoring approach to filtering inaccurate points during the acquisition. Beam profiles evaluation showed that the effect of pan-tilt at inline direction was stronger than at the crossline direction. The maximum average deviation of the full width half maximum (FWHM), flatness, symmetry, penumbra left and right were 0.39 ± 0.25 mm, 0.62 ± 0.50%, 0.76 ± 0.59%, 0.22 ± 0.16 mm, and 0.19 ± 0.15 mm respectively. The ÔF and the measured dose average deviation were <0.5%. The mechanical accuracies during the PDD measurements were 0.28 ± 0.09 mm and 0.21 ± 0.09 mm for pan and tilt and pan or tilt position. The PDD average deviations were 0.58 ± 0.26 % and 0.54 ± 0.25 % for pan-or-tilt and pan-and-tilt position respectively. All the results showed that the deviation at pan and tilt position are higher than pan or tilt. The most influences were observed for the penumbra region and the shift of radiation beam path.

An Analytical Method to Calculate Phantom Scatter Factor for Photon Beam Accelerators

Introduction

One of the important input factors in the commissioning of the radiotherapy treatment planning systems is the phantom scatter factor (Sp) which requires the same collimator opening for all radiation fields. In this study, we have proposed an analytical method to overcome this issue.

Methods

The measurements were performed using Siemens Primus Plus with photon energy 6 MV for field sizes from 5×5cm² to 40×40cm². Phantom scatter factor was measured through the division of total scatter output factors (Scp), and collimator scatter factor (Sc).

Results

The mean percent difference between the measured and calculated Sp was 1.00% and -3.11% for 5×5, 40×40 cm² field size respectively.

Conclusion

This method is applicable especially for small fields used in IMRT which, measuring collimator scatter factor is not reliable due to the lateral electron disequilibrium.

Measured and Monte Carlo simulated electron backscatter to the monitor chamber for the Varian TrueBeam Linac

To accurately simulate therapeutic electron beams using Monte Carlo methods, backscatter from jaws into the monitor chamber must be accounted for via the backscatter factor, S _b. Measured and simulated values of S _b for the TrueBeam are investigated. Two approaches for measuring S _b are presented. Both require service mode operation with the dose and pulse forming networking servos turned off in order to assess changes in dose rate with field size. The first approach samples an instantaneous dose rate, while the second approach times the delivery of a fixed number of monitor units to assess dose rate. Dose rates were measured for 6, 12 and 20 MeV electrons for jaw- or MLC-shaped apertures between [Formula: see text] and [Formula: see text] cm². The measurement techniques resulted in values of S _b that agreed within 0.21% for square and asymmetric fields collimated by the jaws. Measured values of S _b were used to calculate the forward dose component in a virtual monitor chamber using BEAMnrc. Based on this forward component, simulated values of S _b were calculated and compared to measurement and Varian's VirtuaLinac simulations. BEAMnrc results for jaw-shaped fields agreed with measurements and with VirtuaLinac simulations within 0.2%. For MLC-shaped fields, the respective measurement techniques differed by as much as 0.41% and BEAMnrc results differed with measurement by as much as 0.4%, however, all measured and simulated values agreed within experimental uncertainty. Measurement sensitivity was not sufficient to capture the small backscatter effect due to the MLC, and Monte Carlo predicted backscatter from the MLC to be no more than 0.3%. Backscatter from the jaws changed the electron dose rate by up to 2.6%. This reinforces the importance of including a backscatter factor in simulations of electron fields shaped with secondary collimating jaws, but presents the option of ignoring it when jaws are retracted and collimation is done with the MLC.

Deriving detector-specific correction factors for rectangular small fields using a scintillator detector

The goal of this study was to investigate small field output factors (OFs) for flattening filter‐free (FFF) beams on a dedicated stereotactic linear accelerator‐based system. From this data, the collimator exchange effect was quantified, and detector‐specific correction factors were generated. Output factors for 16 jaw‐collimated small fields (from 0.5 to 2 cm) were measured using five different detectors including an ion chamber (CC01), a stereotactic field diode (SFD), a diode detector (Edge), Gafchromic film (EBT3), and a plastic scintillator detector (PSD, W1). Chamber, diodes, and PSD measurements were performed in a Wellhofer water tank, while films were irradiated in solid water at 100 cm source‐to‐surface distance and 10 cm depth. The collimator exchange effect was quantified for rectangular fields. Monte Carlo (MC) simulations of the measured configurations were also performed using the EGSnrc/DOSXYZnrc code. Output factors measured by the PSD and verified against film and MC calculations were chosen as the benchmark measurements. Compared with plastic scintillator detector (PSD), the small volume ion chamber (CC01) underestimated output factors by an average of ‐1.0%±4.9%(max.=‐11.7% for 0.5×0.5cm2 square field). The stereotactic diode (SFD) overestimated output factors by 2.5%±0.4%(max.=3.3% for 0.5×1cm2 rectangular field). The other diode detector (Edge) also overestimated the OFs by an average of 4.2%±0.9%(max.=6.0% for 1×1cm2 square field). Gafchromic film (EBT3) measurements and MC calculations agreed with the scintillator detector measurements within 0.6%±1.8% and 1.2%±1.5%, respectively. Across all the X and Y jaw combinations, the average collimator exchange effect was computed: 1.4%±1.1% (CC01), 5.8%±5.4% (SFD), 5.1%±4.8% (Edge diode), 3.5%±5.0% (Monte Carlo), 3.8%±4.7% (film), and 5.5%±5.1% (PSD). Small field detectors should be used with caution with a clear understanding of their behaviors, especially for FFF beams and small, elongated fields. The scintillator detector exhibited good agreement against Gafchromic film measurements and MC simulations over the range of field sizes studied. The collimator exchange effect was found to be important at these small field sizes. Detector‐specific correction factors were computed using the scintillator measurements as the benchmark.

PACS number(s): 87.56.Fc

Dosimetric properties of equivalent-quality flattening filter-free (FFF) and flattened photon beams of Versa HD linear accelerator

This study presents the basic dosimetric properties of photon beams of a Versa HD linear accelerator (linac), which is capable of delivering flattening filter‐free (FFF) beams with a beam quality equivalent to the corresponding flattened beams based on comprehensive beam data measurement. The analyzed data included the PDDs, profiles, penumbra, out‐of‐field doses, surface doses, output factors, head and phantom scatter factors, and MLC transmissions for both FFF and flattened beams of 6 MV and 10 MV energy from an Elekta Versa HD linac. The 6MVFFF and 10MVFFF beams had an equivalent mean energy to the flattened beams and showed less PDD variations with the field sizes. Compared with their corresponding flattened beams, Dmax was deeper for FFF beams for all field sizes; the ionization ratio variations with the field size were lower for FFF beams; the out‐of‐field doses were lower and the penumbras were sharper for the FFF beams; the off‐axis profile variations with the depths were lesser for the FFF beams. Further, the 6MVFFF and 10MVFFF beams had 35.7% and 40.9% less variations in output factor with the field size, respectively. The collimator exchange effect was reduced in the FFF mode. The head scatter factor showed 59.1% and 73.6% less variations, on average, for the 6MVFFF and 10MVFFF beams, respectively; the variations in the phantom scatter factor were also smaller. The surface doses for all beams increased linearly with the field size. The 6MVFFF and 10MVFFF beams had higher surface doses than the corresponding flattened beams for field sizes of up to 10×10cm2 but had lower surface doses for larger fields. Both FFF beams had lower average MLC transmissions than the flattened beams. The finding that the FFF beams were of equivalent quality to the corresponding flattened beams indicates a significant difference from the data on unmatched FFF beams.

PACS number(s): 87.56.bd, 87.55.Qr

An MLC-based version for the ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

A very simple method to measure the effect of the backscatter from secondary collimators into the beam monitor chambers in linear accelerators equipped with multi-leaf collimators (MLC) is presented here. The backscatter to the monitor chambers from the upper jaws of the secondary collimator was measured on three beam-matched linacs by means of three methods: this new methodology, the ecliptic method, and assessing the variation of the beam-on time per monitor unit with dose rate feedback disabled. This new methodology was used to assess the backscatter characteristics of asymmetric over-traveling jaws. Excellent agreement between the backscatter values measured using the new methodology introduced here and the ones obtained using the other two methods was established. The experimental values reported here differ by less than 1% from published data. The sensitivity of this novel technique allowed differences in backscatter due to the same opening of the jaws, when placed at different positions on the beam path, to be resolved. The introduction of the ecliptic method has made the determination of the backscatter to the monitor chambers an easy procedure. The method presented here for machines equipped with MLCs makes the determination of backscatter to the beam monitor chambers even easier, and suitable to characterize linacs equipped with over-traveling asymmetric secondary collimators. This experimental procedure could be simply implemented to fully characterize the backscatter output factor constituent when detailed dosimetric modeling of the machine's head is required. The methodology proved to be uncomplicated, accurate and suitable for clinical or experimental environments.

Measurement and comparison of head scatter factor for 7 MV unflattened (FFF) and 6 MV flattened photon beam using indigenously designed columnar mini phantom

AIM

To measure and compare the head scatter factor for 7 MV unflattened and 6 MV flattened photon beam using a home-made designed mini phantom.

BACKGROUND

The head scatter factor (Sc) is one of the important parameters for MU calculation. There are multiple factors that influence the Sc values, like accelerator head, flattening filter, primary and secondary collimators.

MATERIALS AND METHODS

A columnar mini phantom was designed as recommended by AAPM Task Group 74 with high and low atomic number material for measurement of head scatter factors at 10 cm and d max dose water equivalent thickness.

RESULTS

The Sc values measured with high-Z are higher than the low-Z mini phantoms observed for both 6MV-FB and 7MV-UFB photon energies. Sc values of 7MV-UFB photon beams were smaller than those of the 6MV-FB photon beams (0.6-2.2% (Primus), 0.2-1.4% (Artiste) and 0.6-3.7% (Clinac iX (2300CD))) for field sizes ranging from 10 cm × 10 cm to 40 cm × 40 cm. The SSD had no influence on head scatter for both flattened and unflattened beams. The presence of wedge filters influences the Sc values. The collimator exchange effects showed that the opening of the upper jaw increases Sc irrespective of FF and FFF.

CONCLUSIONS

There were significant differences in Sc values measured for 6MV-FB and unflattened 7MV-UFB photon beams over the range of field sizes from 10 cm × 10 cm to 40 cm × 04 cm. Different results were obtained for measurements performed with low-Z and high-Z mini phantoms.

A comparison of phantom scatter from flattened and flattening filter free high-energy photon beams

Flattening filter free (FFF) photon beams have different dosimetric properties from those of flattened beams. The aim of this work was to characterize the collimator scatter (Sc) and total scatter (Scp) from 3 FFF beams of differing quality indices and use the resulting mathematical fits to generate phantom scatter (Sp) data. The similarities and differences between Sp of flattened and FFF beams are described. Sc and Scp data were measured for 3 flattened and 3 FFF high-energy photon beams (Varian 6 and 10MV and Elekta 6MV). These data were fitted to logarithmic power law functions with 4 numerical coefficients. The agreement between our experimentally determined flattened beam Sp and published data was within ± 1.2% for all 3 beams investigated and all field sizes from 4 × 4 to 40 × 40cm(2). For the FFF beams, Sp was only within 1% of the same flattened beam published data for field sizes between 6 × 6 and 14 × 14cm(2). Outside this range, the differences were much greater, reaching - 3.2%, - 4.5%, and - 4.3% for the fields of 40 × 40cm(2) for the Varian 6-MV, Varian 10-MV, and Elekta 6-MV FFF beams, respectively. The FFF beam Sp increased more slowly with increasing field size than that of the published and measured flattened beam of a similar reference field size quality index, i.e., there is less Phantom Scatter than that found with flattened beams for a given field size. This difference can be explained when the fluence profiles of the flattened and FFF beams are considered. The FFF beam has greatly reduced fluence off axis, especially as field size increases, compared with the flattened beam profile; hence, less scatter is generated in the phantom reaching the central axis.

Using a 2D detector array for meaningful and efficient linear accelerator beam property validations

Following linear accelerator commissioning, the qualified medical physicist is responsible for monitoring the machine's ongoing performance, detecting and investigating any changes in beam properties, and assessing the impact of unscheduled repairs. In support of these responsibilities, the authors developed a method of using a 2D ionization chamber array to efficiently test and validate important linear accelerator photon beam properties. A team of three physicists identified critical properties of the accelerator and developed constancy tests that were sensitive to each of the properties. The result was a 14‐field test plan. The test plan includes large and small fields at varying depths, a reduced SSD field at shallow depth for sensitivity to extra focal photon and electron components, and analysis of flatness, symmetry, dose, dose profiles, and dose ratios. Constancy tests were repeated five times over a period of six weeks and used to set upper and lower investigation levels at ±3 SDs. Deliberate variations in output, penumbra, and energy were tested to determine the suitability of the proposed method. Measurements were also performed on a similar, but distinct, machine to assess test sensitivity. The results demonstrated upper and lower investigation levels significantly smaller than the comparable TG‐142 annual recommendations, with the exception of the surrogate used for output calibration, which still fell within the TG‐142 monthly recommendation. Subtle changes in output, beam energy, and penumbra were swiftly identified for further investigation. The test set identified the distinct nature of the second accelerator. The beam properties of two photon energies can be validated in approximately 1.5 hrs using this method. The test suite can be used to evaluate the impact of minor repairs, detect changes in machine performance over time, and supplement other machine quality assurance testing.

PACS numbers: 87.56bd, 87.56Fc

Small field in-air output factors: the role of miniphantom design and dosimeter type

PURPOSE

The commissioning of treatment planning systems and beam modeling requires measured input parameters. The measurement of relative output in-air, Sc is particularly difficult for small fields. The purpose of this study was to investigate the influence of miniphantom design and detector selection on measured Sc values for small fields and to validate the measurements against Monte Carlo simulations.

METHODS

Measurements were performed using brass caps (with sidewalls) or tops (no sidewalls) of varying heights and widths. The performance of two unshielded diodes (60012 and SFD), EBT2 radiochromic film, and a fiber optic dosimeter (FOD) were compared for fields defined by MLCs (5-100 mm) and SRS cones (4-30 mm) on a Varian Novalis linear accelerator. Monte Carlo simulations were performed to theoretically predict Sc as measured by the FOD.

RESULTS

For all detectors, Sc agreed to within 1% for fields larger than 10 mm and to within 2.3% for smaller fields. Monte Carlo simulation matched the FOD measurements for all size of cone defined fields to within 0.5%.

CONCLUSIONS

Miniphantom design is the most important variable for reproducible and accurate measurements of the in-air output ratio, S(c), in small photon fields (less than 30 mm). Sidewalls are not required for fields ≤ 30 mm and tops are therefore preferred over the larger caps. Unlike output measurements in water, S(cp), the selection of detector type for Sc is not critical, provided the active dosimeter volume is small relative to the field size.

Comparison of Head Scatter Factor for 6MV and 10MV flattened (FB) and Unflattened (FFF) Photon Beam using indigenously Designed Columnar Mini Phantom

To measure and compare the head scatter factor for flattened (FB) and unflattened (FFF) of 6MV and 10MV photon beam using indigenously designed mini phantom. A columnar mini phantom was designed as recommended by AAPM Task Group 74 with low and high atomic number materials at 10 cm (mini phantom) and at approximately twice the depth of maximum dose water equivalent thickness (brass build-up cap). Scatter in the accelerator (Sc) values of 6MV-FFF photon beams are lesser than that of the 6MV-FB photon beams (0.66-2.8%; Clinac iX, 2300CD) and (0.47-1.74%; True beam) for field sizes ranging from 10 × 10 cm² to 40 × 40 cm². Sc values of 10MV-FFF photon beams are lesser (0.61-2.19%; True beam) than that of the 10MV-FB photons beams for field sizes ranging from 10 × 10 cm² to 40 × 40 cm². The SSD had no influence on head scatter for both flattened and unflattened beams and irrespective of head design of the different linear accelerators. The presence of field shaping device influences the Sc values. The collimator exchange effect reveals that the opening of the upper jaw increases Sc irrespective of FB or FFF photon beams and different linear accelerators, and it is less significant in FFF beams. Sc values of 6MV-FB square field were in good agreement with that of AAPM, TG-74 published data for Varian (Clinac iX, 2300CD) accelerator. Our results confirm that the removal of flattening filter decreases in the head scatter factor compared to flattened beam. This could reduce the out-of-field dose in advanced treatment delivery techniques.

The clinical impact of detector choice for beam scanning

Recently, the developers of Eclipse have recommended the use of ionization chambers for all profile scanning, including for the modeling of VMAT and stereotactic applications. The purpose of this study is to show the clinical impact caused by the choice of detector with respect to its ability to accurately measure dose in the penumbra and tail regions of a scanned profile. Using scan data acquired with several detectors, including an IBA CC13, a PTW 60012, and a Sun Nuclear EDGE Detector, three complete beam models are created, one for each respective detector. Next, using each beam model, dose volumes are retrospectively recalculated from actual anonymous patient plans. These plans include three full‐arc VMAT prostate plans, three left chest wall plans delivered using irregular compensators, two half‐arc VMAT lung plans, three MLC‐collimated static‐field pairs, and two SBRT liver plans. Finally, plans are reweighted to deliver the same number of monitor units, and mean dose‐to‐target volumes and organs at risk are calculated and compared. Penumbra width did not play a role. Dose in the tail region of the profile made the largest difference. By overresponding in the tail region of the profile, the 60012 diode detector scan data affected the beam model in such a way that target doses were reduced by as much as 0.4% (in comparison to CC13 and EDGE data). This overresponse also resulted in an overestimation of dose to peripheral critical structure, whose dose consisted mainly of scatter. This study shows that, for modeling the 6 MV beam of Acuros XB in Eclipse Version 11, the choice to use a CC13 scanning ion chamber or an EDGE Detector was an unimportant choice, providing nearly identical models in the treatment planning system.

PACS number: 87.55.kh

Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71

A protocol is presented for the calculation of monitor units (MU) for photon and electron beams, delivered with and without beam modifiers, for constant source-surface distance (SSD) and source-axis distance (SAD) setups. This protocol was written by Task Group 71 of the Therapy Physics Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol defines the nomenclature for the dosimetric quantities used in these calculations, along with instructions for their determination and measurement. Calculations are made using the dose per MU under normalization conditions, D'0, that is determined for each user's photon and electron beams. For electron beams, the depth of normalization is taken to be the depth of maximum dose along the central axis for the same field incident on a water phantom at the same SSD, where D'0 = 1 cGy/MU. For photon beams, this task group recommends that a normalization depth of 10 cm be selected, where an energy-dependent D'0 ≤ 1 cGy/MU is required. This recommendation differs from the more common approach of a normalization depth of dm, with D'0 = 1 cGy/MU, although both systems are acceptable within the current protocol. For photon beams, the formalism includes the use of blocked fields, physical or dynamic wedges, and (static) multileaf collimation. No formalism is provided for intensity modulated radiation therapy calculations, although some general considerations and a review of current calculation techniques are included. For electron beams, the formalism provides for calculations at the standard and extended SSDs using either an effective SSD or an air-gap correction factor. Example tables and problems are included to illustrate the basic concepts within the presented formalism.

A methodological approach to reporting corrected small field relative outputs

PURPOSE

The goal of this work was to set out a methodology for measuring and reporting small field relative output and to assess the application of published correction factors across a population of linear accelerators.

METHODS AND MATERIALS

Measurements were made at 6 MV on five Varian iX accelerators using two PTW T60017 unshielded diodes. Relative output readings and profile measurements were made for nominal square field sizes of side 0.5 to 1.0 cm. The actual in-plane (A) and cross-plane (B) field widths were taken to be the FWHM at the 50% isodose level. An effective field size, defined as √FS eff=A · B, was calculated and is presented as a field size metric. FSeff was used to linearly interpolate between published Monte Carlo (MC) calculated [Formula in text] values to correct for the diode over-response in small fields.

RESULTS

The relative output data reported as a function of the nominal field size were different across the accelerator population by up to nearly 10%. However, using the effective field size for reporting showed that the actual output ratios were consistent across the accelerator population to within the experimental uncertainty of ± 1.0%. Correcting the measured relative output using [Formula in text] at both the nominal and effective field sizes produce output factors that were not identical but differ by much less than the reported experimental and/or MC statistical uncertainties.

CONCLUSIONS

In general, the proposed methodology removes much of the ambiguity in reporting and interpreting small field dosimetric quantities and facilitates a clear dosimetric comparison across a population of linacs.

Quantitative analysis of in-air output ratio

Output factor (Scp) is one of the important factors required to calculate monitor unit (MU), and is divided into two components: phantom scatter factor (Sp) and in-air output ratio (Sc). Generally, Sc for arbitrary fields are calculated using several methods based on Sc determined by the absorbed dose measurement for several square fields. However, there are calculation errors when the treatment field has a large aspect ratio and the opening of upper and lower collimator are exchanged. To determine Sc accurately, scattered photons from the treatment head and backscattered particles into the monitor chamber must be analyzed individually. In this report, a simulation model that agreed well with measured Sc was constructed and dose variation by scattered photons from the treatment head and by backscattered particles into the monitor chamber was analyzed quantitatively. The results showed that the contribution of scattered photons from the primary collimator was larger than that of the flattening filter, and backscattered particles were affected by not only the upper jaw but also the lower jaw. In future work, a new Sc determination algorism based on the result of this report will be proposed.

Study of head scatter factor in 4MV photon beam used in radiotherapy

The 4 MV photon beam offers equal build-up region behavior like Co-60 beam and it plays a major role in head and neck and pediatric radiotherapy. In this study an attempt is made to study the head scatter factor (SC) for 4 MV photon beam using locally designed PMMA and Brass miniphantoms. The SC is measured in combination of PMMA miniphantom with 0.6 cc chamber and Brass miniphantom with 0.6 cc and 0.13 cc chambers. The measured SC is compared with the literature data and it agrees within ± 1.98%. The study reveals that either 0.13 cc or 0.6 cc chamber with PMMA or Brass phantom materials can be used for SC measurements in a 4 MV photon beam. The variation of SSD does not alter the head scatter factor. The collimator exchange effect is found to be within 1, and it is less than that of other linear accelerators. It is also found that the presence of internal wedge has significant contribution to head scatter factor. The Phantom scatter factor is also calculated and it agrees within ±1% with published data.

Evaluation of beam hardening and photon scatter by brass compensator for IMRT

When a brass compensator is set in a treatment beam, beam hardening may take place. This variation of the energy spectrum may affect the accuracy of dose calculation by a treatment planning system and the results of dose measurement of brass compensator intensity modulated radiation therapy (IMRT). In addition, when X-rays pass the compensator, scattered photons are generated within the compensator. Scattered photons may affect the monitor unit (MU) calculation. In this study, to evaluate the variation of dose distribution by the compensator, dose distribution was measured and energy spectrum was simulated using the Monte Carlo method. To investigate the influence of beam hardening for dose measurement using an ionization chamber, the beam quality correction factor was determined. Moreover, to clarify the effect of scattered photons generated within the compensator for the MU calculation, the head scatter factor was measured and energy spectrum analyses were performed. As a result, when X-rays passed the brass compensator, beam hardening occurred and dose distribution was varied. The variation of dose distribution and energy spectrum was larger with decreasing field size. This means that energy spectrum should be reproduced correctly to obtain high accuracy of dose calculation for the compensator IMRT. On the other hand, the influence of beam hardening on k(Q) was insignificant. Furthermore, scattered photons were generated within the compensator, and scattered photons affect the head scatter factor. These results show that scattered photons must be taken into account for MU calculation for brass compensator IMRT.

Correction method for in-air output ratio for output variations occurring with changes in backscattered radiation

PURPOSE

The in-air output ratio (S(c)) for a rectangular field is usually obtained using an equivalent square field formula. However, it is well-known that S(c) obtained using an equivalent square field formula differs slightly from the measured S(c). Though several correction methods have been suggested for the monitor-backscatter effect, the authors propose a more simple correction method for a rectangular field.

METHODS

For rectangular fields and equivalent square fields, the authors assumed that the output variation was the product of six output variations for each backscattering area at the top of the collimator jaws, and the correction factor was the ratio of the output variation for a rectangular field to the output variation for an equivalent square field. The output variation was measured by using a telescope measurement.

RESULTS

The differences between the measured and corrected S(c) ranged from -0.20% to 0.28% for symmetric rectangular fields by applying the correction factor to S(c) obtained using an equivalent square field formula. This correction method is also available for asymmetric rectangular fields.

CONCLUSIONS

The authors propose a method to correct S(c) obtained using an equivalent square field formula, and a method to obtain the output variation for a field defined by collimator jaws.