Immobilization improves the reproducibility of patient positioning during six-field conformal radiation therapy for prostate carcinoma

Purpose: To determine the magnitude of patient positioning errors associated with six field conformal therapy for carcinoma of the prostate, and to assess the impact of alpha-cradle immobilization on these errors. Methods and Materials: The records of 22 patients, treated at two of the treatment facilities within our department, using computed tomography-planned conformal six field therapy for carcinoma of the prostate, were reviewed. At one facility (UCD), patients were routinely treated with immobilization, while at the other (UCSF) no rigid immobilization was used. Portal films of patients treated at both facilities were subsequently reviewed, and the deviation of each portal from the simulation 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 variability was 0.4 cm for those patients treated with immobilization, versus 0.6 cm for those treated without immobilization. The 90th percentile 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 with errors of ≥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 < 0.001), and 1.0 cm ( 196 201 vs. 74 87, 98% vs. 85%; p < 0.001) for patients treated with immobilization. There was also a significant reduction in the number of patients with treatment-to-treatment variability ≥0.5 cm 1 10 vs. 8 12; p = 0.01) for patients treated with immobilization. Conclusion: The use of immobilization devices significantly reduces errors in patient positioning, potentially permitting the use of smaller treatment volumes. Immobilization should be a component of conformal radiation therapy programs for prostate carcinoma.