Validation and clinical implementation of a full Monte Carlo code for scanned proton pencil beams

We present a universal method to model a proton PBS dedicated nozzle by using acceptance and commissioning measurements, together with a full Monte Carlo (MC) code (Topas/Geant4), to address both the halo inherent from the nozzle as well as a simplified implementation of a range shifter . A double Gaussian source spot profile model was implemented to better address the halo due to interaction of protons with components in the nozzle. The phase space parameters (including beam size, angular divergence and energy spread) and protons per MU were extracted and tuned without simulating any components of the nozzle by comparing Topas simulation with a series of commissioning measurements using scintillation screen/CCD camera detector and ionization chambers. The range shifter was simulated as an independent object. The beam model was validated by comprehensive measurements of the size of single spots, field size factors (FSF) and three dimensional dose distributions of Spread Out Bragg Peaks (SOBPs) both without and with the range shifter. Figure 1 shows FSF along beam path in air and in water after the range shifter for energies of 115 and 225 MeV. The excellent agreement between a TOPAS and measurement reflects high accuracy of Topas in halo modeling. To faciltate assessment of clinical treatment plans, the source model was directly implemented into a second fast, PBS dedicated MC code, MCsquare. The difference between FSF of 200x200mm and FSF of 40x40mm can be as large as 15% at air gap 195mm after the range shifter, which indicates that comprehensive modeling of the spot profile is mandatory for TPS commissioning of range shifter. Figure 2 shows a representative head-and-neck case calculated using TOPAS, MCsquare and a commercial treatment planning system (Eclipse 13.7). In conclusion, two different MC codes have been implemented with universal commissioning method for treatment quality assurance

Evaluation of Motion Mitigation using Abdominal Compression in the Clinical Implementation of Pencil Beam Scanning Proton Therapy of Liver Tumors

This study reports a planning preparation workflow that can be used for beam angle selection and the evaluation of the efficacy of abdominal compression (AC) to mitigate motion for liver tumor patients treated with pencil beam scanning proton therapy (PBSPT). Four-dimensional computed tomography scans (4DCT) with and without AC were available from 10 liver tumor patients with fluoroscopy-proven motion reduction by AC. For each scan, the motion amplitudes and the motion-induced variation of water equivalent thickness (ΔWET) in each voxel of the target volume were evaluated during treatment plan preparation.  Optimal proton beam angles were selected after volume analysis of the respective beam-specific planning target volume (BSPTV). M⊥80 and ΔWET80 derived from the 80th percentiles of perpendicular motion amplitude (M⊥) and ΔWET were compared with and without AC. 4D dynamic dose calculation was performed post plan to determine motion criteria for treatment.  AC resulted in reductions in mean Liver-GTV dose, M⊥, ΔWET, and BSPTV volumes and improved dose degradation (ΔD95 and ΔD1) within the CTV. For small motion (M⊥80  10 mm or ΔWET80 > 7 mm), AC and/or some other form of mitigation strategies were required. A workflow for screening patients’ motion characteristics and optimizing beam angle selection was established for PBSPT of liver tumors. Abdominal compression was found to be useful at mitigation of moderate and large motion

Assessing the clinical impact of TPS dose calculation for proton PBS treatment using fast Monte Carlo algorithm

The impact of approximated analytical dose calculation (ADC), often used in TPS, on the proton PBS treatment plan quality was assessed using an open-source fast Monte Carlo (MC) code, MCsquare. Firstly, MCsquare was commissioned and validated using water and tissue-mimicking phantom measurements as well as benchmarked with the general purpose MC application TOPAS for various representative patient cases. Both MC codes enabled to dramatically improve the dose calculation in the IROC lung phantom with respect to ADC. Indeed, the gamma-index analysis (7%/5mm) passing rate increased from below 85%, to over 93%. Figure1 compares the 1-dimensional dose profiles through the center of the PTV between simulation and film measurements. Second, a total of 50 patients were investigated with 10 patients per site (liver, pelvis, brain, H&N and lung) by comparing dose distributions between ADC and MCsquare. Differences were evaluated using DVH indicators, estimations of tumor control probability (TCP) and a gamma-index analysis as shown in Figure2. Generally, the impact of approximated ADC on the plan quality increases with the tissue heterogeneities. ADC overestimated the target doses on average by up to 1.7% for lung patients. The D95 were predicted within 6.5%, while D02 and V90 within 2.9% of the MC dose. Dose differences can result in large TCP differences for lung (<10.5%), head and neck (<7.5%), and small differences for brain (<2.5%), pelvis and liver (<1.5%). Establishment of fast MC calculation can facilitate patient plan reviews at any institution given the accuracy, speed and availability of the open source dedicated MC

Validation and clinical implementation of a full Monte Carlo code for scanned proton pencil beams

Purpose: To present a methodology for commissioning and validating a full Monte Carlo (MC) code (TOPAS/Geant4) for proton pencil beams utilizing a double Gaussian phase space source model and a simplified range shifter implementation. Application of this source model onto an independent fast MC code (MCsquare), and comparison between MC simulations with analytical treatment planning system (TPS) are investigated. Methods: The phase space parameters and protons per MU were extracted and tuned without simulating any components of the nozzle by comparing TOPAS simulations with a series of commissioning measurements. The beam model was validated by comprehensive measurements of single spots, field size factors (FSF) and three dimensional dose distributions of Spread Out Bragg Peaks (SOBPs) both without and with range shifter. To demonstrate the application, this source model was directly implemented into a fast, dedicated PBS MC code, MCsquare. Clinical treatment cases were compared between TOPAS, MCsquare and our commercial treatment planning system. Results: Based on comprehensive comparison with measurements, TOPAS was validated for all aspects. The difference in field size factors and absolute output at various depths of SOBPs between measurement and simulation were within 2%, indicating an accurate source modeling with and without a range shifter. Comparison of two dimensional dose distributions and DVHs for representative liver case and lung case between MC and analytical calculations (TPS) highlights limitations in the TPS dose calculation in situations of highly heterogeneous geometries. Conclusions: We have proposed a universal method to model a proton PBS dedicated nozzle, with better addressing the halo inherent from nozzle and simplified implementation of a range shifter, using acceptance and commissioning measurements. We compared patient treatments between two MC codes and analytical calculations to show this tool can be implemented clinically to provide an independent dose calculation algorithm for patient specific QA and for benchmarking other dose calculation engines under development

Development of Virtual 4DCT for Image Guided Proton Therapy

This study reports development of virtual 4D-CT workflow that can be used during image guided proton therapy to evaluate patient anatomy changes and if motion mitigation with abdominal compression has been properly setup for liver and lung tumor patients treated with pencil beam scanning proton therapy (PBSPT). A total of ten (5 lung and 5 liver) patients were studied with one planning 4D-CT (p4D-CT) and 2-3 CBCT. CBCT projections were sorted into 8 breathing phases using the Amsterdam Shroud method.. Two different 4D-CBCT reconstruction algorithms were then compared. The MA-ROOSTER method uses a priori knowledge from 4D-CT to improve the 4D-CBCT reconstruction, while the conjugate gradient iterative reconstruction only uses the sorted projections. The velocity fields (Vel4D-CT and Vel4D-CBCT) that characterize the motion between each phase were generated with deformable image registration (DIR). This allowed to create Mid-Position 3D images (MP-CT and MP-CBCT) from p4D-CT and 4D-CBCTs, respectively. The virtual MP-CT (vMP-CT) is then generated by deforming MP-CT on MP-CBCT with DIR. This vMP-CT can then be deformed to each phase using Vel4D-CBCT in order to generate a virtual 4D-CT (v4D-CT). As a result, The conjugate gradient method is found to be sufficient quality and about 5-10x faster than MAROOSTER. For patients with irregular breathing, 4DCBCT is found to be of less artifact therefore more robust than 4DCT. However, streak artifacts can limit left-right and ante-post motion accuracy in the conjugate gradient CBCTs especially for lower contrast liver tumors