PELVIC IRRADIATION OF THE OBESE PATIENT - A TREATMENT STRATEGY INVOLVING MEGAVOLTAGE SIMULATION AND INTRATREATMENT SETUP CORRECTIONS

This study involves a fractionated course of external radiation therap y for a 42 year old female weighing 150 kg, diagnosed with stage IIb c ancer of the cervix. The patient could not be simulated in the convent ional sense due to weight restrictions on the simulator couch, and bod y casts or molds were impractical. Using an on-line portal imaging dev ice, treatment fields were established during the first session, and i ntratreatment verification was used before every subsequent treatment to measure and, when necessary, to correct the patient setup. Two cour ses of treatment were prescribed with a total dose of 60 Gy delivered by a four field box technique (A/P, P/A, and two lateral fields). Out of a total of 108 treatment fields monitored, 12 anterior fields and 1 posterior field were corrected (exclusive of the first, or simulation fraction). Without corrections, 10% of the initial setup displacement s would have had displacements greater than 10 mm, 21% greater than 7 mm, and 41% greater than 5 mm. With the application of intratreatment corrections, only 2% of the displacements were greater than 10 mm, 11% were greater than 7 mm, and 32% were greater than 5 mm. It was also f ound that the second field treated in a parallel opposed pair (i.e., a nterior/posterior or left/right lateral) had lower setup displacements and did not require verification or correction. (C) 1996 American Ass ociation of Physicists in Medicine.