Gorovets D, et al. Pract Radiat Oncol 2020.
BACKGROUND: This study reports clinical experience using a linear accelerator-based MV-kV imaging system for intra-fraction motion management during prostate SBRT.
METHODS: From June 2016 to August 2018, 193 prostate SBRT patients were treated using MV-kV motion management (median dose 40 Gy in 5 fractions). Patients had three fiducials implanted then simulated and treated with a full bladder and empty rectum. Pre-treatment orthogonal kVs and cone beam computed tomography were used to position patients and evaluate internal anatomy. Motion was tracked during volumetric modulated arc therapy delivery using simultaneously acquired kV and MV images from standard on-board systems. Treatment was interrupted to reposition patients when motion >1.5-2mm was detected. Motion traces were analyzed and compared to CalypsoTM traces from a previously treated similar patient cohort. To evaluate "natural motion" (i.e., if we had not interrupted treatment and repositioned), intra-fraction couch corrections were removed from all traces. Clinical effectiveness of the MV-kV system was explored by evaluating toxicity (CTCAE v3.0) and biochemical recurrence rates (nadir + 2ng/mL).
RESULTS: Median number of interruptions for patient repositioning was 1 per fraction (range: 0-9). Median overall treatment time was 8.2 minutes (range: 4.2-44.8 minutes). Predominant motion was inferior and posterior, and probability of motion increased with time. Natural motion >3mm and >5mm in any direction was observed in 32.3% and 10.2% of fractions, respectively. Calypso monitoring (n=50) demonstrated similar motion results. In the 151 MV-kV patients with ≥3-month follow-up (median: 9.5 months; range: 3-26.5 months), grade ≥2 acute GU/GI and late GU/GI toxicity was observed in 9.9%/2.0% and 11.9%/2.7%, respectively. Biochemical control was 99.3% with a single failure in a high-risk patient.
CONCLUSIONS: The MV-kV system is an effective method to manage intra-fraction prostate motion during SBRT, offering the opportunity to correct for prostate CTV displacements that would have otherwise extended beyond typical PTV margins.