IMMOBILIZATION IMPROVES THE REPRODUCIBILITY OF PATIENT POSITIONING DURING 6-FIELD CONFORMAL RADIATION-THERAPY FOR PROSTATE CARCINOMA

Purpose: To determine the magnitude of patient positioning errors asso ciated with six field conformal therapy for carcinoma of the prostate, and to assess the impact of alpha-cradle immobilization on these erro rs. Methods and Materials: The records of 22 patients, treated at two of the treatment facilities within our department, using computed tomo graphy-planned conformal six field therapy for carcinoma of the prosta te, were reviewed. At one facility (UCD), patients were routinely trea ted with immobilization, while at the other (UCSF) no rigid immobiliza tion was used. Portal films of patients treated at both facilities wer e subsequently reviewed, and the deviation of each portal from the sim ulation film was determined (simulation-to-treatment variability). In addition, for each patient, the average deviation of each portal film from the average portal film (treatment-to-treatment variability) was determined. Results: The mean and median simulation-to-treatment varia bility was 0.4 cm for those patients treated with immobilization, vers us 0.6 cm for those treated without immobilization. The 90th percentil e of simulation-to-treatment variability was 0.7 cm for those patients treated with immobilization, versus 1.1 cm for those not immobilized. There was a significant reduction in the number of portals observed w ith errors of greater than or equal to 0.50 cm (132/201 vs. 37/87, 66% vs. 43%; p < 0.001), 0.75 cm (184/201 vs. 59/87, 92% vs. 68%; p < 0.0 01), and 1.0 cm (196/201 vs. 74/87, 98% vs. 85%; p < 0.001) for patien ts treated with immobilization. There was also a significant reduction in the number of patients with treatment-to-treatment variability gre ater than or equal to 0.5 cm (1/10 vs. 8/12; p = 0.01) for patients tr eated with immobilization. Conclusion: The use of immobilization devic es significantly reduces errors in patient positioning, potentially pe rmitting the use of smaller treatment volumes. Immobilization should b e a component of conformal radiation therapy programs for prostate car cinoma.