Impact of variable RBE in prostate cancer IMPT planning on normal tissue complications

This study aimed to investigate the variable relative biological effectiveness with respect to the fixed RBE (1.1) on the prostate intensity-modulated-proton-therapy (IMPT), specifically focusing on normal tissue complication probabilities (NTCPs) in organs-at-risk (OARs) surrounding prostate tumors. The primary objective was to compare NTCP values between plans with fixed RBE (1.1) and plans with variable RBEs, considering the potential implications on treatment outcomes and normal tissue toxicity. Twelve prostate cancer patients undergoing simultaneous-integrated-boost treatments on a focal intraprostatic tumor (IPT) were studied. A linear function of the dose-averaged linear-energy-transfer (LET_d) was utilized to generate variable RBEs. IMPT plans with fixed RBE (1.1) and variable RBEs were generated with the same beam and spot parameters. Dosimetric evaluations, including dose-volume histograms (DVHs) with RBE weighted doses, were performed to assess the impact of RBE variations on dose distribution. NTCP calculations were conducted using the Lyman-Kutcher-Burman (LKB) Probit model to quantify the risk of normal tissue complications in the bladder wall and rectal wall. The analysis revealed notable differences in NTCP values between plans with fixed and variable RBEs. The bladder wall and rectal wall showed increased NTCP values in plans with variable RBEs compared to fixed RBE plans. The average NTCP differences were 5.7% for the bladder wall and 4.6% for the rectal wall, with maximum differences reaching 9.5%. These variations were attributed to the doses lying in the steep region, where even slight dose increases resulted in significant NTCP elevations. Incorporating variable RBEs in proton therapy treatment planning leads to higher NTCP values in OARs, indicating an increased risk of normal tissue complications. This highlights the importance of accurately accounting for RBE variations to optimize treatment outcomes and minimize the potential for radiation-induced toxicities in surrounding organs.