Variations in Rectal Volumes and Dosimetry Values Including NTCP due to Interfractional Variability When Administering 2D-Based IG-IMRT for Prostate Cancer

We estimated variations in rectal volumes and dosimetry values including NTCP with interfractional motion during prostate IG-IMRT. Rectal volumes, DVH parameters, and NTCPs of 20 patients were analyzed. For this patient population, the median (range) volume on the initial plan for the rectum was 45.6？cc (31.3–82.0), showing on-treatment spread around the initial prediction based on the initial plan. DVH parameters of on-treatment CBCT analyses showed systematic regularity shift from the prediction based on the initial plan. Using the Lyman-Kutcher-Burman model, NTCPs of predicted late rectal bleeding toxicity of rectal grade ≥ 2 (RTOG) and the QUANTEC update rectal toxicity for the prediction based on the initial plan were 0.09% (0.02–0.24) and 0.02% (0.00–0.07), respectively, with NTCPs from on-treatment CBCT analyses being 0.35% (0.01–6.16) and 0.12% (0.00–4.11), respectively. Using the relative seriality model, for grade ≥ 2 bleeding rectal toxicity, NTCP of the prediction based on the initial plan was 0.64% (0.15–1.22) versus 1.48% (0.18–7.66) for on-treatment CBCT analysis. Interfraction variations in rectal volumes occur in all patients due to physiological changes. Thus, rectal assessment during 2D-based IG-IMRT using NTCP models has the potential to provide useful and practical dosimetric verification. 1. Introduction Intensity modulated radiotherapy (IMRT) allows successful delivery of escalated dose radiation to the tumor target with high precision, while simultaneously sparing sensitive adjacent tissues. This irradiation technique is adequate for treatment of prostate cancer, as increasing the dose to the prostate achieves better local control, but there is a risk of increased late toxicity [1]. The success of IMRT is related to the ability to accurately target the prostate during treatment. Several factors influence this ability, such as the accuracy of organ delineation, adequate patient immobilization, and intrafraction and interfraction organ motions. Because the treatment plan is normally based on treatment planning computed tomography (CT) images, these errors could result in the dose distribution delivered being different from that planned [2]. This may result in insufficient dose coverage to the prostate tumor volume, that is, less than that prescribed, and thereby decrease the tumor control rate. On the other hand, if the organ at risk is not at the same position as in the planning CT image, there is a potential of delivering an excessive dose possibly resulting in more serious complications. Delivered doses can only be fully