Room: Exhibit Hall
Purpose: To commission the Monte Carlo (MC) tool TOPAS for the double scattering (DS) proton beam nozzle. Plan evaluation of a commercial Treatment Planning System (TPS) was performed using TOPAS thus commissioned.
Methods: The initial proton beam source was modeled based on the phase space information. The DS nozzle model was developed based on data provided by manufacturer and previous measurements. Nozzle set-up and beam current modulation were calculated using vendor-specific ConvAlgo table. The TOPAS DS nozzle model was initially verified in homogeneous water phantom, by comparing the Percent Depth Doses (PDD), Spread-Out Bragg Peaks (SOBP) and lateral profiles with measurements. A test plan of the RPC head phantom (without compensator) was subsequently investigated. Phantom CT, RT structure and RTDOSE files were exported from TPS for patient modeling and scoring partition in TOPAS. The TOPAS calculated doses distribution were then compared with the TPS calculated doses.
Results: TOPAS calculated proton ranges, distal fall offs and entrance doses agreed very well with measured PDD curves. The SOBP for different range and modulation width also agreed well with the measurement data, with the largest difference for the 4 cases being below 3.5%. For the RPC phantom plan, the TOPAS calculated SOBP range and width agreed well with the Eclipse calculation, although dose conformity is sub-optimal due to the small number of histories scored in this region. We plan to increase the total history number to lower the statistical error.
Conclusion: We have successfully developed a Monte Carlo-based DS treatment nozzle model which can be used for commissioning and dose verification of treatment plans in proton therapy. The current study would continue with pencil beam scanning nozzle commissioning and investigations of dose distributions in highly inhomogeneous phantoms.
Funding Support, Disclosures, and Conflict of Interest: Mr. Hongdong Liu is funded by the National Natural Science Foundation of China(11575180) and National Key R&D Program of China(2017YFC0107500).