Rectum motion and morbidity prediction: Improving correlation between late morbidity and DVH parameters through use of rectum planning organ at risk volumes

Abstract Background and purpose. The rectum is a major dose-limiting organ at risk (OR) in radiotherapy (RT) of prostate cancer. Methods to predict adverse effects in the rectum are therefore important but their precision often limited, not the least by the internal motion of this organ. In this study late rectal morbidity is investigated in relation to the internal motion of the rectum by applying the ‘Planning organ at Risk Volume’ (PRV) concept. Materials and methods: Late rectal morbidity was analysed in 242 prostate cancer patients treated to 70 Gy with conformal RT to either the prostate, the prostate and seminal vesicles or the whole pelvis (initial 50 Gy only). Late rectal morbidity was classified by the late gastro-intestinal (GI) RTOG toxicity scoring system. Cumulative dose-volume histograms (DVHs) were derived for the rectum OR and six rectum PRVs i.e. the OR expanded with six different margins (narrow/intermediate/wide in anterior direction or in both anterior and posterior direction). The difference in rectum dose-volume parameters between patients with Grade 0–1 vs. Grade 2 or higher morbidity was investigated by logistic regression and permutation tests. Results: Late Grade 2 or higher morbidity was observed in 25 of 242 (10%) patients. The logistic regression analysis and the permutation tests reached significance (p ≤ 0.05) for only one dose level of the rectum OR (40 Gy). For the PRVs, several dose levels were found to be significant (p-value range: 0.01–0.046), most pronounced for the PRV with narrow margins of 6 mm anterior and 5 mm posterior with five intermediate (38–42 Gy) and ten high (62–71 Gy) dose levels. Conclusions: The statistical methods applied displayed consistently a small though significant difference in DVH parameters between patients with vs. without Grade 2 or higher late rectal morbidity for intermediate and high dose levels. The difference became most evident when using a PRV with narrow margins.