Quality assurance for mixed electron-photon beam radiation therapy using treatment log files and MapCHECK

BACKGROUND

Mixed photon-electron beam radiotherapy (MBRT) is a technique that combines the use of both photons and electrons in one single treatment plan to exploit their advantageous and complimentary characteristics. Compared to other photon treatment modalities, it has been shown that the MBRT technique contributes to better target coverage and organ-at-risk (OAR) sparing. However, the use of combined photons and electrons in one delivery makes the technique more complex and a well-established quality assurance (QA) protocol for MBRT is essential.

PURPOSE

To investigate the feasibility of using MapCHECK and log file-dose reconstruction for MBRT plan verification and to recommend a patient-specific quality assurance (PSQA) protocol for MBRT.

METHODS

MBRT plans were robustly optimized for five soft-tissue sarcoma (STS) patients. Each plan comprised step-and-shoot deliveries of a six MV photon beam and a combination of five electron beam energies at an SAD of 100 cm. The plans were delivered to the MapCHECK device with collapsed gantry angle and the 2D dose distributions at the detector depth were measured. To simulate the expected dose distribution delivered to the MapCHECK, a MapCHECK computational phantom was modeled in EGSnrc based on vendor-supplied blueprint information. The dose to the detectors in the model was scored using the DOSXYZnrc user code. The agreement between the measured and the simulated dose distribution was evaluated using 2D gamma analysis with a gamma criterion of 3%/2 mm and a low dose threshold of 10%. One of the plans was selected and delivered with a rotating gantry angle for trajectory log file collection. To evaluate the potential interlinac and intralinac differences, the plan was delivered repeatedly on three linacs. From the collected log files, delivery parameters were retrieved to recalculate the 3D dose distributions in the patient's anatomy with DOSXYZnrc. The recalculated mean dose to the clinical target volume (CTV) and OARs from all deliveries were computed and compared with the planned dose in terms of percentage difference. To validate the accuracy of log file-based QA, the log file-recalculated dose was also compared with film measurement.

RESULTS

The agreement of the total dose distribution between the MapCHECK measurement and simulation showed gamma passing rates of above 97% for all five MBRT plans. In the log file-dose recalculation, the difference between the recalculated and the planned dose to the CTV and OARs was below 1% for all deliveries. No significant inter- or intralinac differences were observed. The log file-dose had a gamma passing rate of 98.6% compared to film measurement.

CONCLUSION

Both the MapCHECK measurements and log file-dose recalculations showed excellent agreement with the expected dose distribution. This study demonstrates the potential of using MapCHECK and log files as MBRT QA tools.

Robust mixed electron-photon radiation therapy planning for soft tissue sarcoma

BACKGROUND

Mixed electron-photon beam radiation therapy (MBRT) is an emerging technique in which external electron and photon beams are simultaneously optimized into a single treatment plan. MBRT exploits the steep dose falloff and high surface dose of electrons while maintaining target conformity by leveraging the sharp penumbra of photons.

PURPOSE

This study investigates the dosimetric benefits of MBRT for soft tissue sarcoma (STS) patients.

MATERIAL AND METHODS

A retrospective cohort of 22 STS of the lower extremity treated with conventional photon-based Volumetric Modulated Arc Therapy (VMAT) were replanned with MBRT. Both VMAT and MBRT treatments were planned on the Varian TrueBeam linac using the Millenium multi-leaf collimator. No electron applicator, cutout or additional collimating devices were used for electron beams of MBRT plans. MBRT plans were optimized to use a combination of 6 MV photons and five electron energies (6, 9, 12, 16, 20 MeV) by a robust column generation algorithm. Electron beams in this study were planned at standard 100 cm source-axis distance (SAD). The dose to the clinical target volume (CTV), bone, normal tissue strip and other organs-at-risk (OARs) were compared using a Wilcoxon signed-rank test.

RESULTS

As part of the original VMAT treatment, tissue-equivalent bolus was required in 10 of the 22 patients. MBRT plans did not require bolus by virtue of the higher electron entrance dose. CTV coverage by the prescription dose was found to be clinically equivalent between plans of either modality:V 50Gy $V_{\text{50Gy}}$ (MBRT) = 97.9 ± 0.2% versusV 50Gy $V_{\text{50Gy}}$ (VMAT) = 98.1 ± 0.6% (p=0.34). Evaluating the absolute paired difference between doses to OARs in MBRT and VMAT plans, we observed lowerV 20Gy $V_{\text{20Gy}}$ to normal tissue in MBRT plans by 14.9 ± 3.2% (p < 10 - 6 $p<10^{-6}$ ). Similarly,V 50Gy $V_{\text{50Gy}}$ to bone was found to be decreased by 8.2 ± 4.0% (p < 10 - 3 $p<10^{-3}$ ) of the bone volume.

CONCLUSION

For STS with subcutaneous involvement, MBRT offers statistically significant sparing of OARs without sacrificing target coverage when compared to VMAT. MBRT plans are deliverable on conventional linacs without the use of electron applicators, shortened source-to-surface distance (SSD) or bolus. This study shows that MBRT is a logistically feasible technique with clear dosimetric benefits.

Implementation and experimental validation of a robust hybrid direct aperture optimization approach for mixed-beam radiotherapy

Purpose

The objectives of the work presented in this paper were to (1) implement a robust‐optimization method for deliverable mixed‐beam radiotherapy (MBRT) plans within a previously developed MBRT planning framework; (2) perform an experimental validation of the delivery of robust‐optimized MBRT plans; and (3) compare PTV‐based and robust‐optimized MBRT plans in terms of target dose robustness and organs at risk (OAR) sparing for clinical head and neck and brain patient cases.

Methods

A robust‐optimization method, which accounts for translational setup errors, was implemented within a previously developed treatment planning framework for MBRT. The framework uses a hybrid direct aperture optimization method combining column generation and simulated annealing. A robust plan was developed and then delivered to an anthropomorphic head phantom using the Developer Mode of a TrueBeam linac. Planar dose distributions were measured and compared to the planned dose. Robust‐optimized and PTV‐based plans were developed for three clinical patient cases consisting of two head and neck cases and one brain case. The plans were compared in terms of the robustness to 5 mm shifts of the target volume dose as well as in terms of OAR sparing.

Results

Using a gamma criterion of 3%/2 mm and a dose threshold of 10%, the agreement between film measurements and dose calculations was better than 97.7% for the total plan and better than 95.5% for the electron component of the plan. For the two head and neck patient cases, the average clinical target volume (CTV) dose homogeneity index (V95%–V107%) over all the considered setup error scenarios was on average 19% lower for the PTV‐based plans and it had a larger standard deviation. The robust‐optimized plans achieved, on average, a 20% reduction in the OAR doses compared to the PTV‐based plans. For the brain patient case, the CTV dose homogeneity index was similar for the two plans, while the OAR doses were 22% lower, on average, for the robust‐optimized plan. No clear trend in terms of electron contributions was found across the three patient cases, although robust‐optimized plans tended toward higher electron beam energies.

Conclusions

A framework for robust optimization of deliverable MBRT plans has been developed and validated. PTV‐based MBRT were found to not be robust to setup errors, while the dose delivered by the robust‐optimized plans were clinically acceptable for all considered error scenarios and had better OAR sparing. This study shows that the robust optimization is a promising alternative to conventional PTV margins for MBRT.

Ion chamber and film-based quality assurance of mixed electron-photon radiation therapy

PURPOSE

In previous work, we demonstrated that mixed electron-photon radiation therapy (MBRT) produces treatment plans with improved normal tissue sparing and similar target coverage, when compared to photon-only plans. The purpose of this work was to validate the MBRT delivery process on a Varian TrueBeam accelerator and laying the groundwork for a patient-specific quality assurance (QA) protocol based on ion chamber point measurements and 2D film measurements.

METHODS

MC beam models used to calculate the MBRT dose distributions of each modality (photons/electrons) were validated with a single-angle beam MBRT treatment plan delivered on a slab of Solid Water phantom with a film positioned at a depth of 2 cm. The measured film absorbed dose was compared to the calculated dose. To validate clinical deliveries, a polymethyl methacrylate (PMMA) cylinder was machined and holes were made to fit an ionization chamber. A complex MBRT plan involving a photon arc and three electron delivery angles was created with the aim of reproducing a clinically realistic dose distribution in typical soft tissue sarcoma tumours of the extremities. The treatment plan was delivered on the PMMA cylinder. Point measurements were taken with an Exradin A1SL chamber at two nominal depths: 1.4 cm and 2.1 cm. The plan was also delivered on a second identical phantom with an insert at 2 cm depth, where a film was placed. An existing EGSnrc user-code, SPRRZnrc, was modified to calculate the stopping power ratios between any materials in the same voxelized geometry used for dose calculation purposes. This modified code, called SPRXYZnrc, was used to calculate a correction factor, k MBRT , accounting for the differences in electron fluence spectrum at the measurement point compared to that at reference conditions. The uncertainty associated with neglecting potential ionization chamber fluence perturbation correction factors using this approach was estimated.

RESULTS

The film measurement from the Solid Water phantom treatment plan was in good agreement with the simulated dose distribution, with a gamma pass rate of 96.1% for a 3%/2 mm criteria. For the PMMA phantom delivery, for the same gamma criteria, the pass rate was 97.3%. The ion chamber measurements of the total delivered dose agreed with the MC-simulated dose within 2.1%. The beam quality correction factors amounted to, at most, a 4% correction on the ion chamber measurement. However, individual contribution of low electron energies proved difficult to precisely measure due to their steep dose gradients, with disagreements of up to 28% ± 15% at 2.1 cm depth (6 MeV). Ion chamber measurement procedure of electron beams was achieved in less than 5 min, and the entire validation process including phantom setup was performed in less than 30 min.

CONCLUSION

The agreement between measured and simulated MBRT doses indicates that the dose distributions obtained from the MBRT treatment planning algorithm are realistically achievable. The SPRXYZnrc MC code allowed for convenient calculations of k MBRT simultaneously with the dose distributions, laying the groundwork for patient-specific QA protocol practical for clinical use. Further investigation is needed to establish the accuracy of our ionization chamber correction factors k MBRT calculations at low electron energies.

Technical Note: A dose calculation framework for dynamic electron arc radiotherapy (DEAR) using VirtuaLinac Monte Carlo simulation tool

PURPOSE

Dynamic electron arc radiotherapy (DEAR) is a novel dynamic technique that achieves highly conformal dose through simultaneous couch and gantry motion during delivery. The purpose of this study is to develop a framework integrating a Monte Carlo dose engine (VirtuaLinac) to a treatment planning system (TPS, Eclipse) for DEAR. A quality assurance (QA) procedure is also developed.

METHODS AND MATERIALS

The interfaces include the following: computed tomography image export and conversion for VirtuaLinac; VirtuaLinac computation tasks management through application programming interface (API); and dose matrix processing and evaluation. The framework was validated with both static beam and DEAR plan with a 3 × 3 cm² cutout for both 6 and 9 MeV electrons. Verification plans for DEAR were created on flat phantom and a hybrid dose calculation technique was developed which convolves precalculated small field kernel with the beam trajectory, and the resulting dose was compared with the full VirtuaLinac calculation and film measurement.

RESULTS

Excellent agreement between VirtuaLinac and eMC was observed with three-dimensional γ pass rate of 98% at 1%/1 mm criteria for both 6 and 9 MeV electrons. Film measurement shows two-dimensional (2D) γ passing rate of 99.8 % (6 MeV) and 97.1% (9 MeV) at 2%/2 mm criteria. For DEAR plans the comparison of VirtuaLinac and measurement shows the 2D γ passing rates of 94% at 2%/2 mm for 6 MeV. The dose distributions from hybrid method in phantom are identical to the full VirtuaLinac simulations, but can be done instantly.

CONCLUSIONS

A framework has been developed for DEAR dose calculation using VirtuaLinac Monte Carlo dose engine. The VirtuaLinac calculated dose was validated against measurement. A feasible and practical DEAR QA method has been developed for dose measurement in phantom. The hybrid dose calculation technique is efficient and suitable for DEAR QA purpose.

Production of patient-specific electron beam aperture cut-outs using a low-cost, multi-purpose 3D printer

Electron beam collimators for non‐standard field sizes and shapes are typically fabricated using Styrofoam molds to cast the aperture cut‐out. These molds are often produced using a dedicated foam cutter, which may be expensive and only serves a single purpose. An increasing number of radiotherapy departments, however, has a 3D printer on‐site, to create a wide range of custom‐made treatment auxiliaries, such as bolus and dosimetry phantoms. The 3D printer can also be used to produce patient‐specific aperture cut‐outs, as elaborated in this note. Open‐source programming language was used to automatically generate the mold's shape in a generic digital file format readable by 3D printer software. The geometric mold model has the patient's identification number integrated and is to be mounted on a uniquely fitting, reusable positioning device, which can be 3D printed as well. This assembly likewise fits uniquely onto the applicator tray, ensuring correct and error‐free alignment of the mold during casting of the aperture. For dosimetric verification, two aperture cut‐outs were cast, one using a conventionally cut Styrofoam mold and one using a 3D printed mold. Using these cut‐outs, the clinical plan was delivered onto a phantom, for which the transversal dose distributions were measured at 2 cm depth using radiochromic film and compared using gamma‐index analysis. An agreement score of 99.9% between the measured 2D dose distributions was found in the (10%–80%) dose region, using 1% (local) dose‐difference and 1.0 mm distance‐to‐agreement acceptance criteria. The workflow using 3D printing has been clinically implemented and is in routine use at the author's institute for all patient‐specific electron beam aperture cut‐outs. It allows for a standardized, cost‐effective, and operator‐friendly workflow without the need for dedicated equipment. In addition, it offers possibilities to increase safety and quality of the process including patient identification and methods for accurate mold alignment.

Part 2: Dynamic mixed beam radiotherapy (DYMBER): Photon dynamic trajectories combined with modulated electron beams

PURPOSE

The purpose of this study was to develop a treatment technique for dynamic mixed beam radiotherapy (DYMBER) utilizing increased degrees of freedom (DoF) of a conventional treatment unit including different particle types (photons and electrons), intensity and energy modulation and dynamic gantry, table, and collimator rotations.

METHODS

A treatment planning process has been developed to create DYMBER plans combining photon dynamic trajectories (DTs) and step and shoot electron apertures collimated with the photon multileaf collimator (pMLC). A gantry-table path is determined for the photon DTs with minimized overlap of the organs at risk (OARs) with the target. In addition, an associated dynamic collimator rotation is established with minimized area between the pMLC leaves and the target contour. pMLC sequences of photon DTs and electron pMLC apertures are then simultaneously optimized using direct aperture optimization (DAO). Subsequently, the final dose distribution of the electron pMLC apertures is calculated using the Swiss Monte Carlo Plan (SMCP). The pMLC sequences of the photon DTs are then re-optimized with a finer control point resolution and with the final electron dose distribution taken into account. Afterwards, the final photon dose distribution is calculated also using the SMCP and summed together with the one of the electrons. This process is applied for a brain and two head and neck cases. The resulting DYMBER dose distributions are compared to those of dynamic trajectory radiotherapy (DTRT) plans consisting only of photon DTs and clinically applied VMAT plans. Furthermore, the deliverability of the DYMBER plans is verified in terms of dosimetric accuracy, delivery time and collision avoidance. For this purpose, The DYMBER plans are delivered to Gafchromic EBT3 films placed in an anthropomorphic head phantom on a Varian TrueBeam linear accelerator.

RESULTS

For each case, the dose homogeneity in the target is similar or better for DYMBER compared to DTRT and VMAT. Averaged over all three cases, the mean dose to the parallel OARs is 16% and 28% lower, D2% to the serial OARs is 17% and 37% lower and V10% to normal tissue is 12% and 4% lower for the DYMBER plans compared to the DTRT and VMAT plans, respectively. The DYMBER plans are delivered without collision and with a 4-5 min longer delivery time than the VMAT plans. The absolute dose measurements are compared to calculation by gamma analysis using 2% (global)/2 mm criteria with passing rates of at least 99%.

CONCLUSIONS

A treatment technique for DYMBER has been successfully developed and verified for its deliverability. The dosimetric superiority of DYMBER over DTRT and VMAT indicates utilizing increased DoF to be the key to improve brain and head and neck radiation treatments in future.

Latent uncertainties of the precalculated track Monte Carlo method

PURPOSE

While significant progress has been made in speeding up Monte Carlo (MC) dose calculation methods, they remain too time-consuming for the purpose of inverse planning. To achieve clinically usable calculation speeds, a precalculated Monte Carlo (PMC) algorithm for proton and electron transport was developed to run on graphics processing units (GPUs). The algorithm utilizes pregenerated particle track data from conventional MC codes for different materials such as water, bone, and lung to produce dose distributions in voxelized phantoms. While PMC methods have been described in the past, an explicit quantification of the latent uncertainty arising from the limited number of unique tracks in the pregenerated track bank is missing from the paper. With a proper uncertainty analysis, an optimal number of tracks in the pregenerated track bank can be selected for a desired dose calculation uncertainty.

METHODS

Particle tracks were pregenerated for electrons and protons using EGSnrc and geant4 and saved in a database. The PMC algorithm for track selection, rotation, and transport was implemented on the Compute Unified Device Architecture (cuda) 4.0 programming framework. PMC dose distributions were calculated in a variety of media and compared to benchmark dose distributions simulated from the corresponding general-purpose MC codes in the same conditions. A latent uncertainty metric was defined and analysis was performed by varying the pregenerated track bank size and the number of simulated primary particle histories and comparing dose values to a "ground truth" benchmark dose distribution calculated to 0.04% average uncertainty in voxels with dose greater than 20% of Dmax. Efficiency metrics were calculated against benchmark MC codes on a single CPU core with no variance reduction.

RESULTS

Dose distributions generated using PMC and benchmark MC codes were compared and found to be within 2% of each other in voxels with dose values greater than 20% of the maximum dose. In proton calculations, a small (≤ 1 mm) distance-to-agreement error was observed at the Bragg peak. Latent uncertainty was characterized for electrons and found to follow a Poisson distribution with the number of unique tracks per energy. A track bank of 12 energies and 60000 unique tracks per pregenerated energy in water had a size of 2.4 GB and achieved a latent uncertainty of approximately 1% at an optimal efficiency gain over DOSXYZnrc. Larger track banks produced a lower latent uncertainty at the cost of increased memory consumption. Using an NVIDIA GTX 590, efficiency analysis showed a 807 × efficiency increase over DOSXYZnrc for 16 MeV electrons in water and 508 × for 16 MeV electrons in bone.

CONCLUSIONS

The PMC method can calculate dose distributions for electrons and protons to a statistical uncertainty of 1% with a large efficiency gain over conventional MC codes. Before performing clinical dose calculations, models to calculate dose contributions from uncharged particles must be implemented. Following the successful implementation of these models, the PMC method will be evaluated as a candidate for inverse planning of modulated electron radiation therapy and scanned proton beams.

A source model for modulated electron radiation therapy using dynamic jaw movements

PURPOSE

The development of fast and accurate source models (SMs) might be of crucial importance for the future clinical implementation of modulated electron radiation therapy (MERT). In this study, a SM is presented for reconstructing phase-space information of modulated electron beams using a few-leaf electron collimator (FLEC) and the photon jaws.

METHODS

During a FLEC-based delivery, two collimation devices (jaws and FLEC) modulate the electron beam characteristics dynamically. The SM separates the beam into a primary and a scattered component. The primary component is derived by a fast Monte Carlo (MC) transport calculation in air using the EGSnrc/BEAMnrc code. The scattered beam is modeled analytically. The accelerator was decomposed into its individual leaf components and the scattered beam was characterized at various levels of the accelerator. Scattered particles are assigned an energy and position by sampling pre-calculated probability distributions. The direction is estimated by geometrical arguments. Particles were assumed to emerge from tunable virtual sources on the side of each collimator leaf. A leaf-hit algorithm was developed to dynamically reject particles that are incident on any collimating leaf. Electron transport in air between the two collimation levels was calculated based on a MC-modified version of the Fermi-Eyges scattering theory. Correlations between direction and position were observed and taken into account at the final collimation level.

RESULTS

To validate the model, reconstructed phase-space data were compared with the full accelerator MC phase-space data. The model accurately reproduced the beam characteristics and preserved important correlations. Depth and profile dose distributions in water were derived for square, rectangular, and off-axis field sizes and for a range of clinical energies. Discrepancies in the dose distributions and dose output were within 3% in all cases.

CONCLUSIONS

Fast and accurate SMs open the possibility for fast treatment planning in MERT, based on an inverse optimization MC treatment planning scheme.