Radiation and Cancer Physics

SS 15 - Physics 3 - Treatment Planning

114 - Assessment of Clinical Impact of Analytical Dose Calculation in TPS for Proton PBS Treatment Using Fast Monte Carlo Simulation

Monday, October 22
5:25 PM - 5:35 PM
Location: Room 006

Assessment of Clinical Impact of Analytical Dose Calculation in TPS for Proton PBS Treatment Using Fast Monte Carlo Simulation
S. Huang1, K. Souris2, S. Li1, M. Kang1,3, G. Janssens4, A. Lin1, E. H. Garver1, C. Ainsley1, Y. Xiao1, and L. Lin1; 1Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA, 2Center for Molecular Imaging and Experimental Radiotherapy, Institut de Recherche Expérimentale et Clinique,Université catholique de Louvain, Brussels, Belgium, 3Department of Radiation Oncology, Georgetown University, Washington, DC, 4Advanced Technology Group, Ion Beam Applications SA, Louvain-la-Neuve, Belgium

Purpose/Objective(s): The limited accuracy of analytical dose calculation algorithms (ADC) in commercial TPS will lead to dose degradation for proton treatment plan. Monte Carlo dose calculation (MDC) is generally superior to ADC to model the dose distribution especially for patients with high heterogeneity. The purpose of this study is to provide a validated and fast Monte Carlo (FMC) code, MCsquare, to assess the impact of approximations in ADC on clinical pencil beam scanning (PBS) plans covering various sites.

Materials/Methods: First, FMC was commissioned and validated using water and tissue-mimicking IROC lung phantom measurements as well as benchmarked with the general purpose Monte Carlo TOPAS. Then a comparative analysis between FMC and ADC were performed for a total of 50 patients with 10 patients per site (including liver, pelvis, brain, head and neck, lung). Differences between FMC and ADC were evaluated using dosimetric indices (target Dmean, D95, homogeneity index, V90) based on the dose-volume histogram, a 3D gamma-index (3%/3mm), and estimations of tumor control probability (TCP).

Results: The FMC significantly reduced the calculation time from ~18 hours for TOPAS to 10mins on average with 1E8 protons simulated on our workstation. Both FMC and TOPAS significantly improved the gamma-index (7%/5mm) passing rate between simulation and 2D film measurements using the IROC lung phantom from below the passing threshold of 85% for ADC to over 93%. Comparison between FMC and TOPAS for selected 20 patients covering 5 sites showed less than 1.7% difference for all the dosimetric indices/TCP value and larger than 99% for the 3D gamma-index of the target. Generally, through investigations of the difference between ADC and FMC for 50 patients, we found that the impact of approximations in ADC on the plan quality increases from site liver, pelvis, brain, head and neck, to lung, as the degree of tissue heterogeneity increases (see Table). The median gamma-index passing rate for target volumes decreased from 99.3% for liver to 75.8% for lung. Dose calculation differences between ADC and FMC can result in large TCP differences for lung (≤10.5%) and head and neck (<7.5%), with smaller differences seen for brain (≤2.5%), pelvis and liver (≤1.5%).

Conclusion: Comparison between FMC and ADC for 50 patients indicates that current ADC leads to under-dosage of the target by as much as 4.2%, resulting in differences in TCP of up to 10%. The establishment of the FMC can facilitate patient plan reviews at any institution and provide unbiased comparison in clinical trials given its accuracy, speed and open source availability. More accurate dose calculation algorithms like Monte Carlo simulations are needed for proton therapy especially for high heterogeneous sites, such as head and neck, lung.
Median[Min~Max] Liver Pelvis Brain H&N Lung
D95 (%) 0.3 [-1.3~1.2] -1.3 [-2.2~0.1] -1.9 [-3.5~0.5] -3.5 [-5.4~0.4] -4.3 [-6.5~-0.4]
3D Gamma (%) 99.3 [89.3~100] 99.4 [85.7~100] 95.2 [76.1~99.6] 77.0 [56.7~99.5] 75.8 [40.6~99.4]

Author Disclosure: S. Huang: None. K. Souris: None. S. Li: None. M. Kang: None. G. Janssens: None. A. Lin: Employee; Children's Hospital of Philadelphia. Honoraria; ASTRO. Advisor; Galera Phamaceuticals. C. Ainsley: None. L. Lin: None.

Sheng Huang, PhD

Disclosure:
Employment
University of Pennsylvania: Post-Doc: Employee

Biography:
Sheng Huang, PhD is currently a research fellow and medical physics resident of the department of radiation oncology of Memorial Sloan Cancer Center. He received his certificate of medical physics from University of Pennsylvania in 2016 and his PhD in nuclear technology and application from Peking University in 2014. He subsequently worked as a Post-Doc at University of Pennsylvania. His specific interest is in developing physics framework of full and fast Monte Carlo dose calculation for pencil beam scanning proton therapy treatment. He also have involved in the motion evaluation and interplay effect study for lung and liver patients treated with proton PBS.

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114 - Assessment of Clinical Impact of Analytical Dose Calculation in TPS for Proton PBS Treatment Using Fast Monte Carlo Simulation



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