Commissioning and Initial Clinical Experience of Automated Breath-Hold Delivery on a 1.5 Tesla MR-Linac
Abstract
Purpose
Report commissioning and initial clinical experience using automated breath hold (BH) delivery with Elekta Unity’s Comprehensive Motion Management (CMM).
Methods
Following clinical evaluation of CMM, BH delivery was commissioned using film analysis of two SBRT-like plans delivered on Quasar MRI 4D Motion Phantom and comparing dose delivered with BH gating to static delivery via gamma analysis (criteria: 3%/2mm 10% threshold). Plans were optimized to 50Gy/5fractions and created with: central target (37.48cc) with 1cm motion and lateral target (34.15cc) with 3cm motion (superior–inferior). Subsequently, two patients with renal cell carcinoma were treated BH with 40-50Gy/5 fractions using daily adaptive planning and supplemental oxygen to facilitate BH. A 3-5mm isotropic gating window around the GTV and 95% Volumetric Overlapping Criterion (VOICE) tolerance were set. Patients were coached and educated for BH before simulation/treatment and a commercial visual guidance was provided during delivery.
Results
Analysis of commissioning plans showed excellent agreement between static and gated deliveries, with 99.56% and 98.88% gamma passing rates and average dose differences of 8.96cGy (0.9%) and 2.54cGy (0.3%) respectively. The mean duty cycle was 32.5±0.08% across all fractions delivered clinically. Beam gating inefficiencies due to low quality factor (QF) during BH occurred on average 55% of delivery time and occasionally necessitated manual gating. The most common QF errors were related to breath hold detection – no motion detection and/or drop in registration score. In our experience, QF was improved by lowering MR coil as much as possible to maximize Cine-MR SNR, ensuring normal breathing during initial template acquisition (~20s before BH), and encouraging slow controlled breathing between BHs.
Conclusion
We present one of the first reports of successful commissioning of BH treatment on Unity CMM. Initial clinical experience highlighted limitations of the system and led to development of clinical procedures to mitigate challenges of treating mobile tumors.