Simulation of focal boosting in online adaptive MRI-guided SBRT for patients with locally advanced prostate cancer with seminal vesicle involvement

Abstract

To evaluate the feasibility and accuracy of focal boosting in online adaptive MRI-guided stereotactic body radiation therapy (SBRT) for patients with prostate cancer (PCa) with seminal vesicle invasion (T3b) by analyzing the impact of intra-fraction motion on the dose planned to the gross tumor volume (GTV) and clinical target volume (CTV). Data of 23 patients with T1-T3a PCa who received focal boosting SBRT on a 1.5T MR-Linac was used. A radiation oncologist replaced clinical GTVs with artificial GTVs representative for T3b tumor(s). For each MRI used for daily adaptation (MRIadapt) an automated treatment plan was generated (Df1-5) using the adapted contours. Patients were planned to receive 35 Gy to the CTV, with an isotoxic focal boost to the GTV up to 50 Gy. During each fraction an additional MRI was acquired to assess intra-fraction motion (MRIduring). Dose accumulation of all fractions was performed by deformable registration of MRIadapt, f2-5 to MRIadapt, f1 (DACC, planned). The Df1-5 were projected to their corresponding MRIduring, which were used to reconstruct DACC, delivered, likewise. Our results were compared to patients with tumor(s) without seminal vesicle invasion (T1-T3a). The median (10th-90th percentile) D98%ACC, planned to the GTV, which correlates most strongly with outcome, was 41.1 Gy (40.1 - 43.0 Gy) in the plans for patients with artificial T3b tumors, compared to 43.0 Gy (40.4 - 47.2 Gy) in the plans for patients with T1-T3a tumors. The D98%ACC, delivered to the GTV, taking into account intra-fraction motion, was 41.0 Gy (39.3 - 42.6 Gy) and 42.5 Gy (40.0 - 46.6 Gy) in the plans for the artificial and clinical GTVs, respectively. MRI-guidance can ensure high accuracy of focal boosting in patients with T3b disease. Due to the unfavorable location of the GTV, a lower boost dose was feasible compared to patients with T1-T3a PCa.