PTV MARGIN EVALUATION FOR TREATMENT OF PROSTATE CANCER WITH IGRT

Introduction: Conformational radiotherapy planning is performed in pre-defined volumes. According to ICRU (International Commission on Radiation Units and Measurements), volumes are defined as GTV (Gross Tumor Volume) – the visible extent of the tumor; Clinical Target Volume (CTV) – margin comprising GTV, a region where subclinical involvement usually exists; and PTV (Planning Target Volume) – a geometric concept that encompasses CTV, defined from the positioning uncertainties and internal movements of organs. IGRT (Image Guided Radiotherapy) allows a correction of possible positioning variations through image acquisition mechanisms such as EPID (Electronic Portal Imaging Device). These corrections are usually based on bone structures. Another mechanism is the OBI (On- Board Imager) device, where Cone Beam Computed Tomography (CBCT) images are obtained allowing an analysis of the organs internal movements. The use of this system ensures a reduction of random and systematic errors, allowing an evaluation of PTV margin sizes. Objective: Evaluate PTV margins for prostate cancer patients treated with the IMRT technique associated with IGRT. Method: 730 CBCT images were evaluated with the Offline Review tool, available in the Eclipse planning system version 13.6; 54 patients in total; all of them with prostate present; treated with 74 Gy; irradiated with the IMRT technique with IGRT; 6MV photon beam energy of the Trilogy linear accelerator (Varian Medical Systems) with integrated OBI. Results: The displacements were statistically evaluated (mean and standard deviation), then these values were applied in the Van Herk equation, obtaining 0.9 cm vertical, 0.6 cm longitudinal and 0.9 cm lateral as mean values for margins. Furthermore, considering vertical direction individually, values of 0.6 cm anterior and 0.7 cm posterior were obtained. Conclusion: It was found that the PTV margin size adopted by the institution is sufficient to guarantee CTV coverage. From this study, taking into account the current imaging protocol, it is possible to suggest an adaptation of PTV margins adjusting them in different directions. These adjustments make it is possible to improve adjacent tissue dose control maintaining CTV coverage.