A METHOD OF ANALYZING RECTAL SURFACE-AREA IRRADIATED AND RECTAL COMPLICATIONS IN PROSTATE CONFORMAL RADIOTHERAPY

Purpose: To develop a method of analyzing rectal surface area irradiat ed and rectal complications in prostate conformal radiotherapy. Method s and Materials: Dose-surface histograms of the rectum, which state th e rectal surface area irradiated to any given dose, were calculated fo r a group of 27 patients treated with a four-field box technique to a total (tumor minimum) dose ranging from 68 to 70 Gy, Occurrences of re ctal toxicities as defined by the Radiation Therapy Oncology Group (RT OG) were recorded and examined in terms of dose and rectal surface are a irradiated, For a specified end point of rectal complication, the co mplication probability was analyzed as a function of dose irradiated t o a fixed rectal area, and as a function of area receiving a fixed dos e, Lyman's model of normal tissue complication probability (NTCP) was used to fit the data. Results: The observed occurrences of rectal comp lications appear to depend on the rectal surface area irradiated to a given dose level, The patient distribution of each toxicity grade exhi bits a maximum as a function of percentage surface area irradiated, an d the maximum moves to higher values of percentage surface area as the toxicity grade increases, The dependence of the NTCP for the specifie d end point on dose and percentage surface area irradiated was fitted to Lyman's NTCP model with a set of parameters. The curvature of the N TCP as a function of the surface area suggests that the rectum is a pa rallel structured organ. Conclusions: The described method of analyzin g rectal surface area irradiated yields interesting insight into under standing rectal complications in prostate conformal radiotherapy. Appl ication of the method to a larger patient data set has the potential t o facilitate the construction of a full dose-surface-complication rela tionship, which would be most useful in guiding clinical practice.