Development of a 3D remote dosimetry protocol compatible with MRgIMRT.

Abstract

To develop a novel remote 3D dosimetry protocol to verify Magnetic Resonance-guided Radiation Therapy (MRgRT) treatments. The protocol was applied to investigate the accuracy of TG-119 IMRT irradiations delivered by the MRIdian® system (ViewRay® , Oakwood Village, OH, USA) allowing for a 48-hour delay between irradiation at a field institution and subsequent readout at a base institution. The 3D dosimetry protocol utilizes a novel formulation of PRESAGE® radiochromic dosimeters developed for high postirradiation stability and compatibility with optical-CT readout. Optical-CT readout was performed with an in-house system utilizing telecentric lenses affording high-resolution scanning. The protocol was developed from preparatory experiments to characterize PRESAGE® response in relevant conditions. First, linearity and sensitivity of PRESAGE® dose-response in the presence of a magnetic field was evaluated in a small volume study (4ml cuvettes) conducted under MRgRT conditions and irradiated with doses 0-15Gy. Temporal and spatial stability of the dose-response were investigated in large volume studies utilizing large field-of-view (FOV) 2kg cylindrical PRESAGE® dosimeters. Dosimeters were imaged at t=1hr and t=48hrs enabling the development of correction terms to model any observed spatial and temporal changes postirradiation. Polynomial correction factors for temporal and spatial changes in PRESAGE® dosimeters (CT and CR respectively) were obtained by numerical fitting to time-point data acquired in six irradiated dosimeters. A remote dosimetry protocol was developed where PRESAGE® change in optical-density (ΔOD) readings at time t=X (the irradiation to return shipment time interval) were corrected back to a convenient standard time t=1hr using the CT and CR corrections. This refined protocol was then applied to TG-119 (American Association of Physicists in Medicine, Task Group 119) plan deliveries on the MRIdian® system to evaluate the accuracy of MRgRT in these conditions. In the small volume study, in the presence of a 0.35T magnetic field, PRESAGE® was observed to respond linearly (R2 =0.9996) to Co-60 irradiation at t=48hrs postirradiation, within the dose ranges of 0 to 15Gy, with a sensitivity of 0.0305(±0.003) ΔODcm-1 Gy-1 . In the large volume studies, at t=1hr postirradiation, consistent linear response was observed, with average sensitivity of 0.0930±0.002 ΔODcm-1 Gy-1 . However, dosimeters gradually darkened with time (OD<5% per day). A small radial dependence to the dosimeter sensitivity was measured (<3% of maximum dose), which is attributed to a spherically symmetric dosimeter artifact arising from exothermic heating legacy in the PRESAGE® polyurethane substrate during curing. When applied to the TG-119 IMRT irradiations, the remote dosimetry protocol (including correction terms) yielded excellent line-profile and 3D gamma agreement for 3%/3mm, 10% threshold (mean passing rate=96.6%±4.0%). A novel 3D remote dosimetry protocol is introduced for validating off-site dosimetrically complex radiotherapy systems, including MRgRT. The protocol involves correcting for temporal and spatially dependent changes in PRESAGE® radiochromic dosimeters readout by optical-CT. Application of the protocol to TG-119 irradiations enabled verification of MRgRT dose distributions with high resolution.