A randomized trial evaluating rigid immobilization for pelvic irradiation

Abstract

Background Accuracy and reproducibility of the patient’s position is fundamental to the successful delivery of radiation therapy. In recent years, a number of pelvic immobilization techniques have been developed. Few have been evaluated in randomized trials, and many of these studies have produced contradictory and inconclusive results. Purpose To assess whether the use of rigid immobilization devices improve the accuracy and reproducibility of prostate irradiation to a clinically useful degree. Methods and materials A total of 100 patients receiving radical irradiation for either prostate or bladder cancer were randomized to be treated with or without the use of rigid immobilization (RI) devices. Of these, 96 patients were suitable for analysis. The control group consisted of the patients being simulated and treated in the prone position with no immobilization devices. Patients randomized to the immobilized arm were simulated and treated prone using a customized Uvex cast of the pelvis as well as ankle- and shoulder-stabilizing devices. Weekly orthogonal port films (PFs) were obtained for each patient. Using previously specified bone landmarks, we measured variations in the isocenter position on each PF compared with the simulation film. The assessors were unaware of the treatment assignment. Patient comfort, skin toxicity, and treatment times were recorded. Results The average simulation-to-treatment deviation of the isocenter position was 8.5 mm in the control group and 6.2 mm in the immobilization group ( p < 0.001). In the control arm, 30.9% of port films had isocenter deviations >10 mm compared with 10.6% in the immobilized arm ( p = 0.001). For the control group, average deviations in the anteroposterior, right-left, and superior-inferior directions were 5.2 mm, 3.2 mm, and 4.3 mm, respectively, compared with 2.9 mm, 2.1 mm, and 3.9 mm for the immobilized group ( p = <0.001, p < 0.001, p = 0.55). The RTOG skin reaction was greater with in patients with a cast (28% having Grade 2 toxicity vs. 10% in the control arm), although this was not statistically significant ( p = 0.68). Patients in both groups found the treatment position comfortable: 90% in the immobilized group and 87% in the control group scored the treatment position either reasonably or very comfortable. Treatment times were very similar between the two groups: the average treatment time was 15.5 min in the control group vs. 16.1 min in the immobilized group ( p = 0.82). Conclusions The use of rigid immobilization improves the accuracy of treatment delivery for the prone position, especially in the anteroposterior direction. Of clinical importance, the number of major deviations >10 mm (that is, that would result in a geographic miss) was reduced from 31% to 11%.