Dosimetric feasibility of hippocampal avoidance whole brain radiotherapy with an MRI-guided linear accelerator.

PURPOSE/OBJECTIVE(S): Whole brain radiotherapy with hippocampal avoidance (HA-WBRT) is a technique utilized to treat metastatic brain disease while preserving memory and neurocognitive function. We hypothesized that the treatment planning and delivery of HA-WBRT plans is feasible with an MRI-guided linear accelerator (linac) and compared plan results with clinical non-MRI-guided C-Arm linac plans. MATERIALS/METHODS: Twelve HA-WBRT patients treated on a non-MRI-guided C-Arm linac were selected for retrospective analysis. Treatment plans were developed using a 0.35T MRI-guided linac system for comparison to clinical plans. Treatment planning goals were defined as provided in the Phase II Trial NRG CC001. MRI-guided radiotherapy (MRgRT) treatment plans were developed by a dosimetrist and compared with clinical plans. quality assurance (QA) plans were generated and delivered on the MRI-guided linac to a cylindrical diode detector array. Planning target volume (PTV) coverage was normalized to ∼95% to provide a control point for comparison of dose to the organs at risk. RESULTS: MRgRT plans were deliverable and met all clinical goals. Mean values demonstrated that the clinical plans were less heterogeneous than MRgRT plans with mean PTV V37.5 Gy of 0.00% and 0.03% (p = 0.013), respectively. Average hippocampi maximum doses were 14.19 ± 1.29 Gy and 15.00 ± 1.51 Gy, respectively. The gamma analysis comparing planned and measured doses resulted in a mean of 99.9% ± 0.12% of passing points (3%/2mm criteria). MRgRT plans had an average of 38.33 beams with average total delivery time and beam-on time of 13.7 (11.2-17.5) min and 4.1 (3.2-5.4) min, respectively. Clinical plan delivery times ranged from 3 to 7 min depending on the number of noncoplanar arcs. Planning time between the clinical and MRgRT plans was comparable. CONCLUSION: This study demonstrates that HA-WBRT can be treated using an MRI-guided linear accelerator with comparable treatment plan quality and delivery accuracy.

MR-guided stereotactic body radiation therapy for intracardiac and pericardial metastases.

AIMS: To assess the safety and efficacy of MR-guided stereotactic body radiation therapy (MRgSBRT) for cardiac metastases. MATERIALS/METHODS: This single institution retrospective analysis evaluated our experience with MRgSBRT for cardiac metastases. Response rate was compared between pre-RT and post-RT imaging. Symptomatic changes were also tracked and documented. RESULTS: Between 4/2019 and 3/2020, five patients with cardiac metastases (4 intracardiac and 1 pericardial) were treated with MRgSBRT. Median age at treatment was 73 years (range 64-80) and two patients had pre-existing cardiac disease. Histologies included melanoma and breast adenocarcinoma. Median lesion diameter was 2 cm (range 1.96-5.8 cm). Three patients were symptomatic, one of whom had pulmonary hypertension and RV enlargement. Another patient had an asymptomatic arrythmia. Median PTV prescribed dose was 40 Gy (range 40-50 Gy) and delivered in five fractions on nonconsecutive days. Median PTV volume was 53.4 cc (range 8.7-116.6 cc) and median coverage was 95% (range 84.1-100%). A uniform 3 mm margin was used for real-time gating, allowing a median 7% (range 5-10%) pixel excursion tolerance. Median follow-up was 4.7 months (range 0.9-12.3). Two patients exhibited stable disease, two had a partial response and one exhibited a complete response. All symptomatic patients experienced some relief. There were no acute adverse events, however, one patient without prior cardiac disease developed atrial fibrillation 6 months after treatment. Two patients died of causes unrelated to cardiac MRgSBRT. CONCLUSION: In this largest known series of cardiac metastasis MRgSBRT, real-time image guidance enables safe treatment resulting in good response with improving presenting symptoms without acute adverse events.