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

The portal films of 54 consecutive patients treated for primary prostate cancer have been compared to the simulation films. The systematic and random uncertainty in the set-up, defined by the couch movement required 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 radiotherapy have been identified, three occur before and two during the course of treatment. A method for combining these uncertainties is proposed and used on the data obtained in this study. This provides estimates of the margin required when drawing blocks so that the minimum dose to the target is 95% of the prescription in 95% of treatments. The block 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 laterally on an anterior film.