Rapid MR-Only On-Table Planning and Immediate Treatment for Palliation of Metastatic Spine Disease

Abstract

Patients suffering from spine mets would benefit greatly from a radiation therapy workflow that speeds up the start of treatment. Two significant sources of delay are CT-SIM and off-line planning. Each of these could be eliminated by using MRgRT systems for MR-only on-table planning and immediate treatment. Here we present the development and characterization of a planning protocol that uses default parameters and bulk electron densities for MR-only planning. We hypothesize this process could be used to deliver appropriate treatment for spine mets in less than 30 min from the time a patient lies on the table. We developed an MR-only planning protocol to rapidly generate plans (3 Gy × 10 & 8 Gy × 1) for the palliation of spine mets. The critical elements of the protocol were (1) a library of 4 PTV structures, (2) default beam templates designed to cover the PTVs and avoid OARs, (3) bulk density assignments for dose calculations, and (4) a single set of dose optimization parameters. The 4 default PTV structures covered approximately 3 vertebrae over the following ranges: C spine (C3-C7), superior T spine (T1-T5), mid-T spine (T6-T11), and inferior T spine/ L spine (T11-L4). Editing of the structures can be done quickly within the TPS. We generated 10 plans for the default C spine PTV (five 3 Gy × 10 & five 8 Gy × 1) using 5 patient MR-SIM scans. For every other default PTV, 20 plans were generated (ten 3 Gy × 10 & ten 8 Gy × 1) on 10 patient MR-SIM scans. We evaluated the accuracy of the MR-only plans by transferring each plan to the patient’s CT-SIM images and comparing dosimetric parameters. After analysis of the planning protocol, 5 volunteers participated in a workflow time study that consisted of (1) set-up, (2) MR-SIM, (3) on-table planning, (4) patient reimaging and positioning, (5) dose reoptimization and online QA, and (6) TPS prediction of delivery time. All plans generated using bulk densities for MR-only planning resulted in acceptable dose distributions when transferred to the patient’s CT-SIM images. The maximum point dose was within 3% (range -3% to 3%), and the minimum dose to 90% of the target volume was greater than 90% of the prescription dose. The dose distributions calculated on the CT-SIM images were reviewed by two physicians and 100% were determined to be appropriate for treatment. The time study found that the entire workflow could be completed in under 30 min (mean 23.9 min, max 29.3 min). The mean times of each step were (1) 4.6 min for set-up, (2) 1.5 min for MR-SIM, (3) 7.9 min for planning, (4) 2.8 min for reimaging and positioning, (5) 2.8 min for dose reoptimization and online QA, and (6) 4.2 min for predicted delivery. The results of this study demonstrate that it is possible to provide safe and fast palliation of spine mets in under 30 min using MR-SIM, bulk density on-table planning, and immediate treatment delivery. This procedure would eliminate the need for CT-SIM and allow patients to be treated on their first visit to the clinic.