| Endometrial cancer patients treated with adjuvant whole pelvic radiation therapy (RT) often experience a variety of acute and chronic sequelae. Most sequelae involve the gastrointestinal (GI) tract (diarrhea, proctitis, small bowel obstruction, fistulae). Intensity modulated whole pelvic radiation therapy (IM-WPRT) can reduce the incidence of toxicities by reducing the volume of normal tissues irradiated. Due to the steep dose gradients present in intensity modulated radiation therapy (IMRT), setup errors can have a greater effect on the delivered dose distribution than in conventional external beam radiation treatments (EBRT).The goal of this research is to reduce the variation of the delivered dose distribution from the planning dose distribution. Our hypothesis is that by measuring and characterizing the setup errors the actual dose delivered to each patient can be estimated. Subsequent analysis of the change in dose delivered to each anatomical structure will allow a method to be developed to account for these setup errors. The resulting delivered dose will more closely reflect the treatment planning dose.This study measured the setup errors of 46 patients treated with IM-WPRT, calculated their systematic and random components and found that the anterior-posterior (AP) setup error was the largest. The effect of the setup errors on the delivered dose was determined by using the static dose distribution shifting method to displace the original dose distribution. The shifted dose distributions were used to construct confidence interval dose volume histograms (CI-DVHs). Regression analysis was performed to determine which component of the setup errors was responsible for the change in volume of tissue irradiated. It was concluded that the most significant correlations were for AP setup error and the change in volume of normal tissues irradiated to the 80% and 100% dose levels.To correct for the setup errors a novel adaptive radiotherapy scheme, position compensated plans (PCP), was proposed which uses additional treatment plans with shifted isocenters. The PCP correction scheme decreases the residual errors, and allows AP setup margin to be reduced. Such an approach reduces the variation in the delivered dose and may decrease the incidence and severity of radiation sequelae in these patients. |
