ACCURACY OF PELVIC RADIOTHERAPY - PROSPECTIVE ANALYSIS OF 90 PATIENTSIN A RANDOMIZED TRIAL OF BLOCKED VERSUS STANDARD RADIOTHERAPY

The aim of this study was to assess the accuracy of pelvic radiotherap y during a trial of blocked radiotherapy at the Royal Marsden Hospital , UK. Prospective evaluation was performed on 90 patients receiving CT planned pelvic radiotherapy using weekly anterior-posterior and later al portal films. Field placement errors (FPEs) were calculated by comp aring field centres of each film with a designated point of interest. Data was evaluated to calculate the overall treatment simulator differ ences, the number of error free treatments, and mean treatment-simulat or position and to evaluate the role of systematic versus random error s. Age, weight, disease site, position of treatment, fractionation, bl ocked versus conventional techniques were assessed for their effect on treatment accuracy. The mean absolute error between treatment and sim ulator films was anterior right-left (ARL) 0.25 cm, anterior superior- inferior (ASI) 0.32 cm, lateral anterior-posterior (LAP) 0.42 cm, and lateral superior-inferior (LSI) 0.28 cm. On average the field centre w as displaced by 0.66 cm (standard deviation, S.D. = 0.34) from that in tended. On each treatment day 29% of anterior films and 45% of lateral films had at least one 0.5 cm error. Overall 59% of treatments had at least one 0.5 cm error and 9% a 1.0 cm error, The field centre was mo re than 0.5 cm from the position intended in 66% of treatments and ove r 1 cm for 14% of treatments. Analysis of variance showed that both ra ndom and systematic errors occurred in all directions, Though random e rrors were of similar magnitude in all direction (variance sigma(2) = 0.06-0.09 cm(2)); systematic errors showed a 4-fold variation being gr eatest in the LAP direction (sigma(2) = 0.19 cm(2)) and least the ARL direction (sigma(2) = 0.048 cm(2)). No factor consistently predicted f or worse outcome in all directions. Hypofractionated treatments were l ess accurate in the LSI direction (P > 0.05). Systematic errors were a ssociated in the ARL direction with hypofractionation (P < 0.01) and, in the LSI direction with weight (P < 0.03) and age (P < 0.05). We con clude that significant random and systematic errors can occur during p elvic radiotherapy especially in the LAP direction. These results sugg est that in the absence of a customised immobilisation device, to cove r 95% of errors, margins of 0.6 cm for RL and SI directions and 0.9 cm for AP direction should be allowed between the planning and clinical target volumes. However, ideally, each centre should determine their o wn margin requirements according to local clinical practice.