AN ESTIMATE OF THE MARGIN REQUIRED WHEN DEFINING BLOCKS AROUND THE PROSTATE

The portal films of 54 consecutive patients treated for primary prosta te cancer have been compared to the simulation films. The systematic a nd random uncertainty in the set-up, defined by the couch movement req uired to move the patient to the simulated position, was determined to be 1.6 mm UP (SD 3.3 mm), 0.3 mm RT (SD 2.6 mm) and 1.3 mm IN (SD 2.4 mm). The area of fields defined on simulation films was compared with that on portal films to determine the error in block production which was -0.7 mm (SD = 0.9 mm). Five sources of uncertainty in the radioth erapy have been identified, three occur before and two during the cour se of treatment. A method for combining these uncertainties is propose d and used on the data obtained in this study. This provides estimates of the margin required when drawing blocks so that the minimum dose t o the target is 95% of the prescription in 95% of treatments. The bloc k margins are not uniform and range from 21 mm, when drawing the block outline to the posterior on a lateral film, to 13 mm when drawing lat erally on an anterior film.