Study of Treatment Variation in the Radiotherapy of Pelvic Tumors Using a New Custom-Made Immobilization Cast and On-Line Electronic Portal Imaging

Purpose: To evaluate and quantify the accuracy using a new custom-made pelvis immobilization device during the course of radiotherapy. Materials and Methods: A new custom-made pelvic immobilization cast had been developed to provide a stable and comfort position of the patients. It consists of a flat and rigid acrylic plate and thermoplastic cast. Eight patients with pelvic cast were evaluated by on line electronic portal imaging during daily radiotherapy treatment for 5 to 8 consecutive days. While another 10 patients without pelvic casts were used as a control. A total of 221 electronic portal imaging (EPI) were reviewed. Under the assistance of image registration software, the geometric errors from the EPI was identified by interactive techniques. The systematic and random setup errors for each patient were determined. Results: Daily set-up variation was markedly improved using the cast. The total directed setup errors for all patients in anterior-posterior (AP), left-right (LR) and inferior-superior (IS) directions were -0.1±2.2 mm, -0.4±1.2 mm and -0.3 ±2.0 mm respectively with cast as compared to -0.4±2.4 mm, 1.2±1.9 mm and -1.1±3.8 mm respectively without cast. The suggested margin, which was calculated according to the total undirected setup error for each patient, in AP/PA and lateral fields for more than 5 mm were found in 14% and 33% of patients with casts as compared to 60% and 89% of patients without immobilization cast. (p value = 0.065 for AP/PA fields and p value = 0.024 for lateral fields) Conclusion: This custom-made pelvic immobilization cast appears to improve the reproducibility of patient setup during radiotherapy for pelvic tumors. In application to 3-D conformal radiotherapy, this type of immobilization is now routinely used in our department and is recommended for all patients receiving pelvic radiotherapy.