Anorectal irradiation in pelvic radiotherapy: An assessment using in-vivo dosimetry

The objectives of this study were to measure by in-vivo techniques the radi ation doses received by the anorectum during pelvic radiotherapy and compar e these with doses predicted by a GE TARGET(TM) treatment planning system. Nine patients with cancers of the prostate. bladder, cervix or uterus were planned with computed tomography (CT) using the TARGET system. A Scanditron ix rectal probe containing five n-type photon-detecting diodes was placed i n the anorectum during the planning CT scans. The probe position was standa rdized with the five diodes at 2 cm intervals from the anal verge. The prob e diodes were calibrated for 10 MV photons. Doses were measured for each di ode for two consecutive fractions in the first four patients and for five c onsecutive fractions in the remaining five. Thermoluminescent dosimeters we re used initially to verify diode doses. The TARGET and diode measured dose s were compared. In all patients diodes situated in the target volume were within 7% of pred icted doses. This improved to 2.5% after measurement on five fractions. At the edges of the target volume, wide variability existed between measured a nd predicted doses (measured dose range -68% to +68% of predicted dose). Ou tside the target volume, considerable doses (up to 0.3 Gy per fraction) wer e measured in the anal canal. which were not predicted by TARGET. We conclude that TARGET planned doses are accurate within the confines of t he target volume. The greatest variability was seen at the edges of the tar get volume, where dose can vary by 50% across a 1 cm distance in the anteri or-posterior plane. TARGET does not account for scattered dose beyond the f ield edges and therefore underestimates the dose received by the anal canal .