Rectal dose during radiotherapy: how much is too much?

Full text: The clinical intent of radiotherapy for prostate cancer is to deposit high radiation dose to the prostate and as low as possible to healthy tissue. The rectum is one adjacent structure that is very sensitive to side effects including rectal bleeding, stricture, and ulceration. The dose that the rectum receives is often difficult to predict because its position and size will differ on each treatment day from the original planning CT images. The aim of this work is to use current measured values from the literature on rectal wall motion to mathematically model the dynamic rectal wall. The model is used with a pre calculated dose distribution to evaluate the difference between planned anticipated and actually delivered rectal radiation doses. The dose delivered will depend on the status of the rectum in the preliminary planning CT scan. Deviations from the planned dose were larger if the rectum was empty in the planning CT scan (ΔD = ± 25%) than if it was full (ΔD = ± 15%). If the planning CT scan demonstrated the rectum in the mean treatment position the dose variation is reduced (ΔD = ± 10%). These results support the conclusion that care should be taken to plan treatments using CT images that contain reproducible information