A COMPARISON OF 4 PATIENT IMMOBILIZATION DEVICES IN THE TREATMENT OF PROSTATE-CANCER PATIENTS WITH 3-DIMENSIONAL CONFORMAL RADIOTHERAPY

Purpose: To determine the variability of patient positioning during th ree-dimensional conformal radiotherapy (3D-CRT) for prostate canter tr eated with no immobilization or one of four immobilization devices, an d to determine the effects of patient body habitus and pelvic circumfe rence on patient movement with each individual immobilization techniqu e. Methods and Materials: To see whether our immobilization techniques have improved day-to-day patient movement, a retrospective analysis w as carried out. A total of 62 patients treated at one facility on a si ngle machine with 3D-CRT via a four-field box technique (anterior-post erior and opposed laterals) in the supine position with either no immo bilization or one of four immobilization devices. Five groups of patie nts were compared: (a) group 1-no immobilization; (b) group 2-alpha cr adle from the waist to upper thigh; (c) group 3-alpha cradle from wais t to below the knees; (d) group 4-styrofoam leg immobilizer (below kne es); and (e) group 5-aquaplast cast encompassing the entire abdomen an d pelvis to midthigh with alpha cradle immobilization to their lower l egs and feet, Prior to starting radiotherapy, portal films of all four treatment fields were obtained 1 day before treatment, Subsequently, portal films were then obtained at least once a week. Portal films wer e compared with the simulation films and appropriate changes were made and verified on the next day prior to treatment. A deviation of great er than 0.5 cm or greater was considered to be clinically significant in our analysis. We studied the difference among the types of immobili zation and no immobilization by looking at the frequency of movements (overall, and on each of the three axes) that a patient had during the course of his treatment Using a logistic regression model, the probab ility of overall and individual directional movement for each group wa s obtained. In addition, the effects of patient body habitus and pelvi c circumference on movement were analyzed. Results: The maximum deviat ion was 2 cm and the median deviation was 1.2 cm. For each patient, th e probability of movement ranged from 0 to 76%, with a mean of 39%. Th ere was no significant difference seen in overall movement with any of the immobilization devices compared to no immobilization, but there w as less vertical (9 vs. 18%; p = 0.03) and AP (6 vs. 15%; p = 0.14) mo vement with the aquaplast than any other group. However, when examinin g the lateral direction, the aquaplast had significantly more movement (32 vs. 9%; p < 0.001). When accounting for body habitus and pelvic c ircumference, no immobilization device was effective in reducing movem ent in obese patients or in patients with pelvic circumference greater than 105 cm. The aquapIast group had a significantly increased amount of lateral movement with obesity (42 vs. 23%; p < 0.05), and with pel vic circumference >105 cm (33 vs. 29%; p < 0.05). Conclusions: There w as no significant reduction in overall patient movement noted with any of the immobilization devices compared to no immobilization. The aqua plast group had reduced vertical and AP movement of greater than 0.5 c m. There was significantly more lateral movement with aquaplast apprec iated in obese patients or patients with pelvic circumferences greater than 105 cm. The aquaplast immobilization appears to be useful in red ucing movement in two very clinically important dimensions (AP and ver tical). Despite our findings, other immobilization may still be useful especially in the treatment of nonobese patients. It is clear that th e optimal immobilization technique and patient positioning are yet to be determined.