Feasibility of patient specific quality assurance for proton therapy based on independent dose calculation and predicted outcomes.

Abstract

Patient specific quality assurance (PSQA) is required to verify the treatment delivery and the dose calculation by the treatment planning system (TPS). The objective of this work is to demonstrate the feasibility to substitute resource consuming measurement based PSQA (PSQA_M) by independent dose recalculations (PSQA_IDC), and that PSQA_IDC results may be interpreted in a clinically relevant manner using normal tissue complication probability (NTCP) and tumor control probability (TCP) models. A platform for the automatic execution of the two following PSQA_IDC workflows was implemented: (i) using the TPS generated plan and (ii) using treatment delivery log files (log-plan). 30 head and neck cancer (HNC) patients were retrospectively investigated. PSQA_M results were compared with those from the two PSQA_IDC workflows. TCP/NTCP variations between PSQA_IDC and the initial TPS dose distributions were investigated. Additionally, for two example patients that showed low passing PSQA_M results, eight error scenarios were simulated and verified via measurements and log-plan based calculations. For all error scenarios ΔTCP/NTCP values between the nominal and the log-plan dose were assessed. Results of PSQA_M and PSQA_IDC from both implemented workflows agree within 2.7% in terms of gamma pass ratios. The verification of simulated error scenarios shows comparable trends between PSQA_M and PSQA_IDC. Based on the 30 investigated HNC patients, PSQA_IDC observed dose deviations translate into a minor variation in NTCP values. As expected, TCP is critically related to observed dose deviations. We demonstrated a feasibility to substitute PSQA_M with PSQA_IDC. In addition, we showed that PSQA_IDC results can be interpreted in clinically more relevant manner, for instance using TCP/NTCP.