Dosimetric Feasibility of HA-WBRT With an MRI-Guided Linear Accelerator

Abstract

Whole brain radiotherapy with hippocampal avoidance (HA-WBRT) is a technique utilized to treat extensive 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.Twelve patients treated with HA-WBRT plans on a non-MRI-guided C-Arm Linac were selected for retrospective analysis. Treatment plans were developed for comparison using a 0.35T MRI-guided Linac. Hippocampi were contoured by the physician on a T1 MRI image fused to a planning CT image with a 5mm expansion to the hippocampi for the avoidance structure. The PTV was expanded from the CTV excluding the hippocampal avoidance structure. Treatment planning goals were defined as provided in the Phase II Trial NRG CC001. MRI-guided treatment (MRgRT) plans were developed by a dosimetrist and compared with clinical plans. QA plans were developed and a subset were delivered on the MRI-guided Linac and measured with a cylindrical diode detector array. PTV coverage was normalized to ∼95% to provide a control point for comparison of dose to the organs at risk.MRgRT plans were deliverable and met all clinical goals. Mean values demonstrated that the clinical plans were cooler than MRgRT plans with mean V37.5 of 0.00% and 0.66%, respectively. Average hippocampi maximum doses were 14.20 ± 2.04 Gy and 14.33 ± 1.19 Gy for clinical vs MRgRT plans, respectively. Average hippocampi D100% were 8.63 ± 0.52 Gy and 7.87 ± 0.23 Gy, respectively. Average maximum doses to 0.03 cc of optic structures were 30.93 ± 1.10 Gy and 31.78 ± 0.66 Gy, respectively. The gamma analysis comparing planned and measured doses resulted in a mean of 99.9% ± 0.14% of passing points (3%/2mm criteria). MRgRT plans had an average of 38.33 beams with total delivery time ranging from 10.7-15.7 minutes and 4.03 minutes for total beam on time. Clinical plans had average delivery times of 3-7 minutes depending on the number of coplanar arcs.This study demonstrates that HA-WBRT can be treated using an MRI-guided linear accelerator with comparable treatment plan quality and delivery accuracy. Given the equitable dosimetric outcomes to traditional CT based plans, the use of MRgRT for hippocampal sparing whole brain radiotherapy opens the possibility of radiomic analysis and potential adaptive treatments (i.e., boost) to lesions based on disease response.