J Fan

Implementation of Monte Carlo Dose calculation for CyberKnife treatment planning

Accurate dose calculation is essential to advanced stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) especially for treatment planning involving heterogeneous patient anatomy. This paper describes the implementation of a fast Monte Carlo dose calculation algorithm in SRS/SRT treatment planning for the CyberKnife? SRS/SRT system. A superposition Monte Carlo algorithm is developed for this application. Photon mean free paths and interaction types for different materials and energies as well as the tracks of secondary electrons are pre-simulated using the MCSIM system. Photon interaction forcing and splitting are applied to the source photons in the patient calculation and the pre-simulated electron tracks are repeated with proper corrections based on the tissue density and electron stopping powers. Electron energy is deposited along the tracks and accumulated in the simulation geometry. Scattered and bremsstrahlung photons are transported, after applying the Russian roulette technique, in the same way as the primary photons. Dose calculations are compared with full Monte Carlo simulations performed using EGS4/MCSIM and the CyberKnife treatment planning system (TPS) for lung, head & neck and liver treatments. Comparisons with full Monte Carlo simulations show excellent agreement (within 0.5%). More than 10% differences in the target dose are found between Monte Carlo simulations and the CyberKnife TPS for SRS/SRT lung treatment while negligible differences are shown in head and neck and liver for the cases investigated. The calculation time using our superposition Monte Carlo algorithm is reduced up to 62 times (46 times on average for 10 typical clinical cases) compared to full Monte Carlo simulations. SRS/SRT dose distributions calculated by simple dose algorithms may be significantly overestimated for small lung target volumes, which can be improved by accurate Monte Carlo dose calculations.

Development of Laser Accelerated Proton Beams for Radiation Therapy

Recent advances in laser technology have made proton (ion) acceleration possible using laser induced plasmas. In this presentation we will review the theoretical and experimental results of laser-proton acceleration for radiotherapy applications. We will report on our work progress in the development of a laser-proton therapy system at Fox Chase Cancer Center. The new proton therapy system is designed as a compact and cost-effective alternative to conventional accelerator based proton systems capable of delivering intensity-modulated proton therapy (IMPT). The specific aims of our research are: (1) target design for laser-proton acceleration, (2) system design for particle/energy selection and beam collimation, and (3) dosimetric studies on the use of laser-accelerated protons for cancer therapy. We have established a 150 TW laser system for preliminary experimental studies. We also patented a compact particle selection and beam collimating system for IMPT beam delivery and a new gantry design to make the whole system compact and easy to operate with adequate shielding considerations. Our Monte Carlo results show that IMPT using laser protons provided superior target coverage and much reduced critical structure dose and integral dose. IMPT is more dosimetrically advantageous than photon IMRT or conventional proton beams.

WE-C-AUD-03: Investigation of Fast Monte Carlo Dose Calculation for CyberKnife SRS/SRT Treatment Planning

Purpose: Advanced stereotactic radiosurgery(SRS) and stereotactic radiotherapy (SRT) treatments require accurate dose calculation for treatment planning especially for treatment sites involving heterogeneous patient anatomy. In this work, we have implemented a fast Monte Carlodose calculation algorithm for SRS/SRT treatment planning with the CyberKnife® system. Methods and Materials: Our system employs a superposition Monte Carlo algorithm.Photon mean free paths and interaction types for different materials and energies as well as the tracks of secondary electrons are pre‐simulated using the EGS4 code system. Photon interaction forcing and splitting are applied to the source photons in a patient calculation and the pre‐simulated tracks are repeated with proper corrections based on the tissue density and electron stopping powers. Electron energy is deposited along the tracks and accumulated in every voxel of the simulation geometry. Scattered and bremsstrahlung photons are transported, after applying the Russian Roulette technique, in the same way as the primary photons.Dose calculations are compared with full Monte Carlo simulations and the CyberKnife treatment planning system (TPS) for lung and head & neck treatments. Results: Comparisons with full Monte Carlo simulations show excellent agreement (within 0.5%). Significant differences in the target dose are found between Monte Carlo simulations and the CyberKnife TPS for SRSlung treatment. The calculation time using our superposition Monte Carlo algorithm is reduced up to 62 times (46 times on average for 10 typical clinical cases) compared to full Monte Carlo simulations. Conclusions: SRS/SRT dose distributions calculated by simple dose algorithms may be significantly overestimated for small lung target volumes, which can be improved by accurate Monte Carlodose calculations. Properly implemented fast Monte Carlo algorithms can improve dosimetric accuracy with little or no compromise to computational efficiency.

SU‐GG‐T‐143: MLC‐Based CyberKnife Radiotherapy for Prostate Cancer

Purpose: The Cyberknife system allows for real‐time organ motion correction and it enables delivery of a large number of non‐isocentric, non‐coplanar beams. A CyberKnife using multi‐leaf collimators(MLC) may achieve an improved dose advantage while maintaining a short delivery time. The purpose of this study is to evaluate the plan quality of MLC‐based prostate roboticradiotherapy.Method and Materials:Treatment plans were compared between regular IMRT, CyberKnife using the IRIS collimator (CK IRIS) and CyberKnife using a MLC (CK MLC). The IMRT plan and the CK MLC plan were generated using the Eclipse inverse planning system. Various numbers of non‐coplanar fields have been tried for CK MLC plans. With the CK IRIS plan, all 12 possible collimator sizes were included in the optimization process to achieve the best performance. Results: Isodoses for the IMRT plan, the CK MLC plan and the CK IRIS plan are compared in the same mid‐axial plane. All plans provide very conformai and complete converge of the target. However, the 50% isodose clearly demonstrates that the CK MLC plan provides much sharper dose fall off than the IMRT and the CK IRIS plans. The CK IRIS plan shows the worst rectal sparing with the 50% isodose line covering more than half of the rectum. A further comparison of DVHs shows significantly improved sparing of rectum and bladder with the CK MLC plan, when compared to the IMRT plan and the CK IRIS plan. Conclusions: The CyberKnife with MLC can produce superior dose distributions for sparing rectum and bladder and excellent DVHs for the target compared with IMRT and CyberKnife IRIS plans, and produces similar dose heterogeneities as IMRT plans. With significantly less beams, it can deliver a 2Gy/fx treatment in 10 minutes.

TU‐C‐AUD‐04: Laser‐Proton Acceleration for Radiation Therapy

Purpose: Rapid developments in laser technology have facilitated proton (light ion) acceleration using laser‐induced plasmas. In this work, we investigate an experimental system for laser‐accelerated proton therapy.Method and Materials: Our system consists of a commercial 150 TW laser, custom‐made laser‐pulse compression and target chambers, particle selection and beam collimating devices, dosimetry monitoring systems and shielding constructions. We have performed initial laser‐proton acceleration experiments with thin aluminum foils as target materials. The maximum protonenergy was measured using CR‐39 film and a Thomson parabola ion analyzer. We have performed particle‐in‐cell simulations to investigate the optimal laser parameters and target configurations to facilitate laser‐proton acceleration and dosimetric studies. Results: The primary particles resulting from the laser‐target interaction are protons and electrons. Our particle in cell simulation predicted protons of up to 300 MeV and electrons of 20 MeV for a laser intensity of 1021 W/cm2. The maximum number was 1011 and 1012 per pulse for protons and electron, respectively. Our initial testing with a 1018 W/cm2 laser intensity (at 10 TW) produced up to 1 MeV protons with a broad energy spectrum. Conclusion: We have developed an experimental laser‐proton accelerator for radiation therapy applications. Initial experimental studies have demonstrated proton acceleration at low laser power levels. Further studies with laser intensities up to 2 × 1020 W/cm2 are being conducted with different target materials and configurations.

MO‐EE‐A3‐01: Comparison of Prostate Rotation and Calypso Beam Rotation for Prostate Margin Evaluation

Purpose: The Beacon rotation angles reported by the Calypso system have not been utilized clinically due to the difficulties in rotating the patient. The actual prostate rotation angle is affected by Beacons migration, prostates shrinkage and deformation due to rectal/bladder filling. This work investigates the actual prostate rotation angles between the planning CT and the treatment CBCT and to compare those reported by the Calypso system. Method and Materials: The Calypso system reports centroid shifts and rotation angles of the 3 implanted Beacons, relative to their locations on the planning CT.CBCT scans were obtained for 9 treatment fractions of 5 patients. The same sets of the Beacons in the planning CT and post‐treatment CBCT were segmented according to their high intensities and aligned according to their centroid positions. An iterative closest point (ICP) method was developed to find the best matching iteratively between two sets of Beacons after rotation and translation. Results: The maximum inter‐Beacon distance varied from 1.1 to 4.7 mm with inter‐Beacon distances from 14.4 to 40.4 mm. The mean Beacon rotation angle reported by the Calypso system in each plane was 4.7±3.8°, which were likely caused by the inter‐Beacon distance change. After the best 3D matching of the Beacons, our method reported a mean rotation angle of 1.5±1.7° for all the fractions. If the projected inter‐Beacon distance was >9mm there was good agreement between our method and the Calypso system. As the inter‐Beacon distance decreased, the uncertainty in the reported rotation angle increased. Conclusion: The actual rotation angles for the prostate were smaller than those for the Beacons reported by the Calypso system. It is recommend to avoid small (<9mm) projected inter‐Beacon distances to reduce the uncertainty in the reported rotation angle.

MO-FG-CAMPUS-JeP3-05: Evaluation of 4D CT-On-Rails Target Localization Methods for Free Breathing Liver Stereotactic Body Radiotherapy (SBRT)

PURPOSEnLiver SBRT patients unable to tolerate breath-hold for radiotherapy are treated free-breathing with image guidance. Target localization using 3D CBCT requires extra margins to accommodate the respiratory motion. The purpose of this study is to evaluate the accuracy and reproducibility of 4D CT-on-rails in target localization for free-breathing liver SBRT.nnnMETHODSnA Siemens SOMATOM CT-on-Rails 4D with Anzai Pressure Belt system was used both as the simulation and the localization CT. Fiducial marker was placed close to the center of the target prior to the simulation. Amplitude based sorting was used in the scan. Eight or sixteen phases of reconstructed CT sets (depends on breathing pattern) can be sent to Velocity to create the maximum intensity projection (MIP) image set. Target ITV and fiducial ITV were drawn based on the MIP image. In patient localization, a 4D scan was taken with the same settings as the sim scan. Images were registered to match fiducial ITVs.nnnRESULTSnTen liver cancer patients treated for 50Gy over 5 fractions, with amplitudes of breathing motion ranging from 4.3-14.5 mm, were analyzed in this study. Results show that the Intra & inter fraction variability in liver motion amplitude significantly less than the baseline inter-fraction shifts in liver position. 90% of amplitude change is less than 3 mm. The differences in the D99 and D95 GTV dose coverage between the 4D CT-on-Rails and the CBCT plan were small (within 5%) for all the selected cases. However, the average PTV volume by using the 4D CT-on-Rails is 37% less than the CBCT PTV volume.nnnCONCLUSIONnSimulation and Registration using 4D CT-on-Rails provides accurate target localization and is unaffected by larger breathing amplitudes as seen with 3D CBCT image registration. Localization with 4D CT-on-Rails can significantly reduce the PTV volume with sufficient tumor.

MO‐G‐213AB‐05: Beam Simulation and Measurement for Energy‐Intensity Modulated Electron Radiotherapy (MERT) with a Computer‐Controlled Electron Multileaf Collimator Device

Purpose: To verify an add‐on computer‐controlled multileaf collimator (eMLC) device on a Varian linac capable of delivering accurate dose for energy‐intensity modulated electron radiotherapy (MERT). Methods: The eMLC has 27 pairs of tungsten leaves (tongue and groove design to reduce intraleaf leakage)with 0.56cm width and 2cm thickness, providing a field size as large as 15 cm × 15 cm defined at 94cm SSD. Measurements were done to determine the appropriate jaw setting for an eMLC shaped field, mainly to reduce the leaf leakage outside the eMLC shaped field. The phase space data were acquired by Monte Carlo(MC) simulations for electron beams of energies 6, 9, 12 and 15 MeV, respectively and used as an input source in MCdose calculations in a phantom. MC calculated PDDs and dose profiles were compared with measurements for large fields (e.g. 10 cm × 10 cm) and small fields (e.g. 3.4 cm × 3.4 cm). The eMLC leakage for various energies was measured both in‐air and in phantom (at dmax) as a ratio of doses with the eMLC closed and completely open. Results: With the jaw position at 0.5 cm beyond the edge of the eMLC shaped field, it was showed to best eliminate the interleaf leakage, especially for high energies, e.g. 15 MeV. The average leaf leakage ranged from 0.3% (6 MeV) to 2.3% (15 MeV), which were consistent with lower in‐phantom values than in‐air values. MC calculated PDDs and dose profiles generally agreed with measurements to within 2mm/2%. Conclusions: This eMLC device is capable of delivering energy and intensity modulated electron beams accurately with acceptable leaf leakage for advanced MERT treatment.

SU‐FF‐T‐244: Impact of the Isocenter Shift as a Function of Couch and Gantry Angles On the Stereotactic Radiosurgery (SRS) Dose

Purpose: The most important component of the pre‐treatment QA for radiosurgery is the verification of the target position in the beam. There are some generally accepted rules for the alignment test, e.g., the positioning differences should be within 1 mm or better. However, the impact on delivered dose of the shift in different directions during gantry and couch rotation may not be the same. Detailed investigations are desired to find out the relationship between the shift functions and the final dose distributions. Method and materials: In this study, the impact on the delivered dose was evaluated by Monte Carlo simulations using an EGS4‐based code MCSIM. The code was modified for arc therapy so that it can be used to do patient dose calculation for any given arc range and couch angle. A two‐step investigation has been carried out in this research. First, several assumed meaningful shift functions of gantry and couch movement were implemented into the Monte Carlo simulation to find out the dose impact from each component. Then actual shift functions based on measurements were used to evaluate the dose change due to the isocenter uncertainties for a real machine. A SRS plan for a braintumor (9 arcs with a 10 mm cone) was used in these simulations. Results and conclusions: Based on the results from the assumed shift functions and the measured shift functions, we found that the isodose line shift is generally less than 0.5 mm on our Trilogy and Primart, which is much smaller than the isocenter uncertainties. Also big differences were mainly found in the high dose region (>90% of the maximum dose). The isodose line shift at the typical dose prescription 2457_4level, e.g., 70% or 80%, has been reduced to 0.2∼0.3 mm, which is comparable to the imaginguncertainties.

SU-F-T-81: Treating Nose Skin Using Energy and Intensity Modulated Electron Beams with Monte Carlo Based Dose Calculation

PURPOSEnTreating nose skin with an electron beam is of a substantial challenge due to uneven nose surfaces and tissue heterogeneity, and consequently could have a great uncertainty of dose accuracy on the target. This work explored the method using Monte Carlo (MC)-based energy and intensity modulated electron radiotherapy (MERT), which would be delivered with a photon MLC in a standard medical linac (Artiste).nnnMETHODSnThe traditional treatment on the nose skin involves the usage of a bolus, often with a single energy electron beam. This work avoided using the bolus, and utilized mixed energies of electron beams. An in-house developed Monte Carlo (MC)-based dose calculation/optimization planning system was employed for treatment planning. Phase space data (6, 9, 12 and 15 MeV) were used as an input source for MC dose calculations for the linac. To reduce the scatter-caused penumbra, a short SSD (61 cm) was used. A clinical case of the nose skin, which was previously treated with a single 9 MeV electron beam, was replanned with the MERT method. The resultant dose distributions were compared with the plan previously clinically used. The dose volume histogram of the MERT plan is calculated to examine the coverage of the planning target volume (PTV) and critical structure doses.nnnRESULTSnThe target coverage and conformality in the MERT plan are improved as compared to the conventional plan. The MERT can provide more sufficient target coverage and less normal tissue dose underneath the nose skin.nnnCONCLUSIONnCompared to the conventional treatment technique, using MERT for the nose skin treatment has shown the dosimetric advantages in the PTV coverage and conformality. In addition, this technique eliminates the necessity of the cutout and bolus, which makes the treatment more efficient and accurate.