Assessment of normal tissue complications following prostate cancer irradiation: Comparison of radiation treatment modalities using NTCP models

Purpose: Normal tissue complication probability (NTCP) of the rectum, bladder, urethra, and femoral heads following several techniques for radiation treatment of prostate cancer were evaluated applying the relative seriality and Lyman models. Methods: Model parameters from literature were used in this evaluation. The treatment techniques included external (standard fractionated, hypofractionated, and dose‐escalated) three‐dimensional conformal radiotherapy (3D‐CRT), low‐dose‐rate (LDR) brachytherapy (I‐125 seeds), and high‐dose‐rate (HDR) brachytherapy (Ir‐192 source). Dose‐volume histograms (DVHs) of the rectum, bladder, and urethra retrieved from corresponding treatment planning systems were converted to biological effective dose‐based and equivalent dose‐based DVHs, respectively, in order to account for differences in radiation treatment modality and fractionation schedule. Results: Results indicated that with hypofractionated 3D‐CRT (20 fractions of 2.75 Gy/fraction delivered five times/week to total dose of 55 Gy), NTCP of the rectum, bladder, and urethra were less than those for standard fractionated 3D‐CRT using a four‐field technique (32 fractions of 2 Gy/fraction delivered five times/week to total dose of 64 Gy) and dose‐escalated 3D‐CRT. Rectal and bladder NTCPs (5.2% and 6.6%, respectively) following the dose‐escalated four‐field 3D‐CRT (2 Gy/fraction to total dose of 74 Gy) were the highest among analyzed treatment techniques. The average NTCP for the rectum and urethra were 0.6% and 24.7% for LDR‐BT and 0.5% and 11.2% for HDR‐BT. Conclusions: Although brachytherapy techniques resulted in delivering larger equivalent doses to normal tissues, the corresponding NTCPs were lower than those of external beam techniques other than the urethra because of much smaller volumes irradiated to higher doses. Among analyzed normal tissues, the femoral heads were found to have the lowest probability of complications as most of their volume was irradiated to lower equivalent doses compared to other tissues.