OC-0046 SHORT TERM HORMONE THERAPY AND DOSE ESCALATED RADIATION FOR LOCALIZED PROSTATE CANCER: A RANDOMIZED PHASE III STUDY

Abstract

consequences of rotational errors, which remain uncorrected if six degree of freedom couches are missing, and intra-fractional patient motion, which appears larger compared to the invasive frame-based approach, are unknown. It was the purpose of this study to investigate geometric and dosimetric accuracy of frame-less IG-RS for brain metastases. Materials and Methods: Single fraction IG-RS was practiced in 72 patients with 98 brain metastases. Patient positioning and immobilization used either double(n=72) or single-layer (n=27) thermoplastic masks. Pre-treatment set-up errors (n=98) were evaluated with cone-beam CT (CBCT) based IG and were corrected in six degrees of freedom without an action level. CBCT imaging after treatment measured intra-fractional errors (n=64). Preand posttreatment errors were simulated in the Pinnacle planning system based on the patient-specific RS treatment plans and target coverage and Paddick conformity index were evaluated. Three scenarios of 0mm, 1mm and 2mm GTV-to-PTV (gross / planning tumor volume) safety margins (SM) were simulated. Results: Errors prior to IG were 3.9mm±1.7mm (3D vector) and the maximum rotational error was 1.7°±0.8° on average. The posttreatment 3D error was 0.9mm±0.6mm. No significant differences between the more rigid double-layer (0.8mm±0.6mm) and single-layer (1.0mm±0.6mm) masks were observed. Simulation of RS without IG reduced target coverage and conformity by 25% and 40% for 0mm SM, respectively. Each 3D set-up error of 1mm decreased target coverage and dose conformity by 6% and 10% on average, respectively. After simulation of 1mm and 2mm GTV-to-PTV SM, target coverage was still reduced by 18% and 10% on average, respectively, if no IG was practiced. Pre-treatment correction of translations only but not rotations did not affect target coverage and conformity, irrespective of the SM. Post-treatment errors reduced target coverage by >5% in 14% and 0% of the patients for a 0mm and 1mm GTV-to-PTV SM, respectively. Conclusions: IG-RS with online correction of translational errors achieved high accuracy with residual errors of about 1mm on average after treatment. Application of 1mm GTV-to-PTV safety margins ensured target coverage within a 5% range compared to the planned dose distributions in all patients.