Development of a 3D remote dosimetry protocol compatible with MR g IMRT

Purpose To develop a novel remote 3D dosimetry protocol to verify Magnetic Resonance‐guided Radiation Therapy ( MR g RT ) treatments. The protocol was applied to investigate the accuracy of TG ‐119 IMRT irradiations delivered by the MRI dian ® 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. Methods 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 (4 ml cuvettes) conducted under MR g RT conditions and irradiated with doses 0–15 Gy. Temporal and spatial stability of the dose‐response were investigated in large volume studies utilizing large field‐of‐view ( FOV ) 2 kg cylindrical PRESAGE ® dosimeters. Dosimeters were imaged at t = 1 hr and t = 48 hrs 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 ( C T and C R 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 (Δ OD ) readings at time t = X (the irradiation to return shipment time interval) were corrected back to a convenient standard time t = 1 hr using the C T and C R corrections. This refined protocol was then applied to TG ‐119 (American Association of Physicists in Medicine, Task Group 119) plan deliveries on the MRI dian ® system to evaluate the accuracy of MR g RT in these conditions. Results In the small volume study, in the presence of a 0.35 T magnetic field, PRESAGE ® was observed to respond linearly (R 2  = 0.9996) to Co‐60 irradiation at t = 48 hrs postirradiation, within the dose ranges of 0 to 15 Gy, with a sensitivity of 0.0305(±0.003) Δ OD  cm −1  Gy −1 . In the large volume studies, at t = 1 hr postirradiation, consistent linear response was observed, with average sensitivity of 0.0930 ± 0.002 Δ OD  cm −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%/3 mm, 10% threshold (mean passing rate = 96.6% ± 4.0%). Conclusion A novel 3D remote dosimetry protocol is introduced for validating off‐site dosimetrically complex radiotherapy systems, including MR g RT . 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 MR g RT dose distributions with high resolution.