title: A Monte Carlo study of the impact of the choice of rectum volume definition on estimates of equivalent uniform doses and the volume parameter.
abstract: Calculations of normal tissue complication probability (NTCP) values for the rectum are difficult because it is a hollow, non-rigid, organ. Finding the true cumulative dose distribution for a number of treatment fractions requires a CT scan before each treatment fraction. This is labour intensive, and several surrogate distributions have therefore been suggested, such as dose wall histograms, dose surface histograms and histograms for the solid rectum, with and without margins. In this study, a Monte Carlo method is used to investigate the relationships between the cumulative dose distributions based on all treatment fractions and the above-mentioned histograms that are based on one CT scan only, in terms of equivalent uniform dose. Furthermore, the effect of a specific choice of histogram on estimates of the volume parameter of the probit NTCP model was investigated. It was found that the solid rectum and the rectum wall histograms (without margins) gave equivalent uniform doses with an expected value close to the values calculated from the cumulative dose distributions in the rectum wall. With the number of patients available in this study the standard deviations of the estimates of the volume parameter were large, and it was not possible to decide which volume gave the best estimates of the volume parameter, but there were distinct differences in the mean values of the values obtained.
authors.affiliation: Section of Oncology, Institute of Medicine, Medical Faculty, University of Bergen, N-5021 Bergen, Norway. ykvi@helse-bergen.no
authors.forename: Yngve
authors.initials: Y
authors.lastname: Kvinnsland
authors.forename: Ludvig Paul
authors.initials: LP
authors.lastname: Muren
authors.forename: Olav
authors.initials: O
authors.lastname: Dahl
mesh_terms: D001822:Body Burden; D003198:Computer Simulation; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D008297:Male; D008954:Models, Biological; D015233:Models, Statistical; D009010:Monte Carlo Method; D009929:Organ Size; D009928:Organ Specificity; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011832:Radiation Injuries / Q000209:etiology / Q000517:prevention & control; D011835:Radiation Protection / Q000379:methods; D011859:Radiography; D011874:Radiometry / Q000379:methods*; D011878:Radiotherapy / Q000009:adverse effects / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012007:Rectum / Q000000981:diagnostic imaging / Q000473:pathology / Q000503:physiopathology* / Q000528:radiation effects*; D015203:Reproducibility of Results; D018570:Risk Assessment / Q000379:methods*; D012307:Risk Factors; D012680:Sensitivity and Specificity