Room: Exhibit Hall | Forum 4
Purpose: To investigate the heterogeneous effects of compensator shape and patient anatomy on monitor unit calculations for brain, lung, pancreas and prostate cases with Mevion S250 double scattering proton therapy system.
Methods: Using a benchmarked Monte Carlo (MC) simulation toolkits (TOPAS version 3.1 p2), three dimensional patient doses were calculated using CT images and the actual field specific components (aperture and compensator) were exported from the treatment plan (TPS). Patient scatter factors (PSF) and compensator scatter factors (CSF) for each of the treatment fields were extracted for the total of 12 treatment fields with four different target sites. Each MC calculated factor was then compared to corresponding factor calculated in TPS with pencil beam algorithm. For prostate and lung cases, types of materials were qualitatively analyzed based on their Hounsfield Unit values within a structure defined by 50% isodose line for each field; and then plotted against the corresponding PSF value. The inter-quartile range (IQR) of compensator thickness within a small region of interest along the central beam axis was calculated to evaluate the impact of compensator shapes on CSF.
Results: The range of simulated PSF and CSF values for all cases were found to be 0.919-1.023 and 1.000-1.044, respectively. The average differences in CSFs between MC and TPS for all cases were below 0.5% and the average differences in PSFs for the brain, pancreas, prostate and lung were 1.6, 1.0, 0.6 and 1.9% respectively. A linear relation between the quantity of air or bone and the corresponding PSF was found for lung or prostate cases. Likewise, there is a proportional relationship between the IQR value and the CSF for compensators.
Conclusion: Good agreements were found between MC and TPS for CSFs. PSFs agreed better for fields with less tissue heterogeneity.