Positioning errors and prostate motion during conformal prostate radiotherapy using on-line isocentre set-up verification and implanted prostate markers

Purpose: To evaluate treatment errors from set-up and inter-fraction prosta tic motion with port films and implanted prostate fiducial markers during c onformal radiotherapy for localized prostate cancer. Methods: Errors from isocentre positioning and inter-fraction prostate moti on were investigated in 13 men treated with escalated dose conformal radiot herapy for localized prostate cancer. To limit the effect of inter-fraction prostate motion, patients were planned and treated with an empty rectum an d a comfortably full bladder, and were instructed regarding dietary managem ent, fluid intake and laxative use. Field placement was determined and corr ected with daily on-line portal imaging. A lateral portal film was taken th ree times weekly over the course of therapy. From these films, random and s ystematic placement errors were measured by matching corresponding bony lan dmarks to the simulator film. Superior-inferior and anterior-posterior pros tate motion was measured from the displacement of three gold pins implanted into the prostate before planning. A planning target volume (PTV) was deri ved to account for the measured prostate motion and field placement errors. Results: From 272 port films the random and systematic isocentre positionin g error was 2.2 trim (range 0.2-7.3 mm.) and 1.4 mm (range 0.2-3.3 mm), res pectively. Prostate motion was largest at the base compared to the apex. Ba se: anterior, standard deviation (SD) 2.9 min; superior, SD 2.1 mm. Apex: a nterior, SD 2.1 mm; superior, SD 2.1 mm. The margin of PTV required to give a 99% probability of the gland remaining within the 95% isodose line durin g the course of therapy is superior 5.8 mm, and inferior 5.6 mm. In the ant erior and posterior direction, this margin is 7.2 mm at the base, 6.5 mm at the mid-gland and 6.0 mm at the apex. Conclusions: Systematic set-up errors were small using real-time isocentre placement corrections. Patient instruction to help control variation in bla dder and rectal distension during therapy may explain the observed small SD for prostate motion in this group of patients. Inter-fraction prostate mot ion remained the largest source of treatment error, and observed motion was greatest at the aland base. In the absence of real-time pre-treatment imag ing of prostate position, sequential portal films of implanted prostatic ma rkers should improve quality assurance by confirming or-an position within the treatment field over the course of therapy. (C) 2001 Elsevier Science I reland Ltd. All rights reserved.