ANALYSIS OF POSITIONING UNCERTAINTY AND PTV MARGIN OBTAINED WITH THE USE OF A PELVIC IMMOBILIZING DEVICE IN THE TREATMENT OF PROSTATE CANCER

Objectives: To evaluate positioning uncertainty and PTV margins in the treatment of prostate cancer with and without the use of a pelvic immobilizing device. Methods and Materials: The clinical trial evaluated 127 patients with prostate cancer treated with Image Guided (EPID) Intensity Modulated Radiation Therapy. Patients were split in two arms: First arm with 67 patients who did not use the pelvic immobilizing device and second arm with 60 patients who used the device. In average, eight pairs of portal images (Anterior and Lateral) were acquired per patient during the course of Radiotherapy. Planning Target Volume (PTV) margins were calculated using Van Herks formula so that 99% of patient would receive 95% coverage. Results: The group of patients using the pelvic device showed slightly less uncertainty in positioning (lateral, vertical and longitudinal) than the group that did not use it. Furthermore, the staff had the impression that the positioning process became more robust and standardized with the device. Population PTV margin dimensions for the 67 patients who did not used the pelvic device were 0.73, 0.75 and 0.47-cm in the lateral, vertical and longitudinal directions, respectively. Population margins for the 60 patients evaluated with the use of the pelvic device were 0.64, 0.69 and 0.46-cm in the same directions. A slight margin reduction was observed for the arm using the device, especially in the lateral direction (12,3%). Conclusion: Immobilization using the pelvic device had only a small contribution in treatment precision and accuracy. Even though the treatment table has digital millimeter resolution, the study did not show statistically significance in margin reduction and positioning reproducibility with the use of pelvic devices for prostate patients, for which PTV margins are as big as 1.0-cm. The numerical results did not proportionally reflect the staff impressions.