Set-up error in supine-positioned patients immobilized with two different modalities during conformal radiotherapy of prostate cancer

Background: Conformal radiotherapy requires reduced margins around the clin ical target volume (CTV) with respect to traditional radiotherapy technique s. Therefore, high set-up accuracy and reproducibility are mandatory. Purpose: To investigate the effectiveness of two different immobilization t echniques during conformal radiotherapy of prostate cancer with small field s. Materials and methods: 52 patients with prostate cancer were treated by con formal three- or four-held techniques with radical or adjuvant intent betwe en November 1996 and March 1998. In total, 539 portal images were collected on a weekly basis for at least the first 4 weeks of the treatment on later al and anterior Is MV X-ray fields. The average number of sessions monitore d per patient was 5.7 (range 4-10). All patients were immobilized with an a lpha-cradle system; 25 of them were immobilized at the pelvis level (group A) and the remaining 27 patients were immobilized in the legs (group B). Th e shifts with respect to the simulation condition were assessed by measurin g the distances between the same bony landmarks and the held edges. The glo bal distributions of cranio-caudal (CC), posterior-anterior (PA) and left-r ight (LR) shifts were considered; for each patient random and systematic er ror components were assessed by following the procedure suggested by Bijhol d et al. (Bijhold J, Lebesque JV, Hart AAM, Vijlbrief RE. Maximising set-up accuracy using portal images as applied to a conformal boost technique for prostatic cancer. Radiother. Oncol. 1992;24:261-271). For each patient the average isocentre (3D) shift was assessed as the quadratic sum of the aver age shifts in the three directions. Results: Group B showed a better accuracy and reproducibility than group A for PA shifts (2.6 versus 4.4 mm, 1 SD), LR shifts (2.4 versus 3.6 mm, 1 SD ) and CC shifts (2.7 versus 3.3 mm, 1 SD). Furthermore, group B showed a ra te of large PA shifts (>5 mm) equal to 4.4% with respect to the 21.6% of gr oup A (P < 0.0001). This value was also better than the corresponding value found in a previously investigated group of 21 non-immobilized patients (I talia C, Fiorino C, Ciocca M, et al. Quality control by portal film analysi s of the conformal radiotherapy of prostate cancer: comparison between two different institutions and treatment techniques (abstract). Radiother. Onco l. 1997;43(Suppl. 2):S16, 16.8%, P = 0.001). For both groups there was no c lear prevalence of one component (systematic or random) with respect to the other. The average isocentre shifts (averaged on all patients) were 3.0 mm (+/-1.4 mm, I SD) for group B and 5.0 mm (+/-2.8 mm, 1 SD) for group A aga inst a value of 4.4 mm (+/-2.4 mm, I SD) for the previously investigated no n-immobilized patient group. Conclusions: Immobilization of the legs with an alpha-cradle system seems t o improve both the accuracy and reproducibility of the positioning of patie nts treated for prostate cancer with respect to alpha-cradle pelvic-abdomen immobilization. Based on these data, we decided to use the legs immobiliza tion system and to reduce the margin around the CTV (from 10 to 8 mm) in th e PA direction. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.