Investigation of a 3D dosimetry system utilizing radio-chromic gel and telecentric optical-CT readout.

Background: Sophisticated modern radiation therapy treatments require comprehensive validation in 3D.

Purpose: Investigation and characterization of a novel 3D dosimetry system consisting of ClearView radiochromic gel dosimeters (commercially available from Modus Inc) and an in-house telecentric optical CT scanner DLOS (the Duke Large Field of View Optical-CT Scanner).

Methods: Spectrophotometry measurements were made on small volumes of ClearView gel irradiated with 6X photon doses up to 40 Gy to determine linearity and temporal stability of dose response. Clinical evaluation of Clearview/DLOS system was conducted in two phases. Phase one involved simple photon and electron benchmark irradiations, delivered to 15 and 10 cm diameter dosimeters, at various energies and doses. Phase 2 investigated application to the verification of two single isocenter multi-target (SIMT) stereotactic radiosurgery (SRS) deliveries. These were patient treatments for two and five brain lesions, respectively, and delivered to 15 cm diameter dosimeters. SIMT treatments were delivered by Varian TrueBeam 6X with doses of 40 Gy. For dose read-out, dosimeters were optically scanned in the DLOS both pre- and post- irradiation (within 24 h). 3D reconstructions (1 mm resolution) of the change in linear-optical- attenuation (proportional to dose) was obtained using in-house software and 3D Slicer. Measured and predicted (Eclipse TPS) doses were compared through percent depth-dose (PDD), cross plane and in-plane profiles, and relative 3D gamma analysis (performed at a range of 7%/4 mm down to 2%/2 mm). Regions of known artifacts were excluded from analysis (jar base, neck, and wall). The SIMT SRS deliveries were additionally compared to SciMoca, an independent Monte Carlo second check software.

Results: Linearity of dose response was confirmed with R2 ≥ 0.9986 at both 520 and 630 nm wavelengths and at three post-irradiation time points: 21 h, 6 and 10 days. Dose profiles of all benchmark irradiations, in both 15 and 10 cm dosimeters, show good agreement in useable areas of the gel compared to Eclipse dose calculations, with root mean square errors (RMSE) ≤ 0.0054, and R2 ≥ 0.9808. Gamma pass rates for the 15 cm dosimeter benchmark irradiations were ≥ 94% at 2%/2 mm (central axis), ≥ 90% at 3%/3 mm (left lateral), ≥ 90% at 2%/2 mm (electron), and ≥ 94% at 3%/2 mm (stacking field). Similar high passing rates were observed for benchmark irradiations to the smaller 10 cm diameter dosimeters. Very high Gamma pass rates were found for SIMT SRS deliveries, with 99.82% and 97.80% at 3%/2 mm, for the two and five target plans, respectively.

Conclusion: This work presents the first investigation of ClearView dosimeters in combination with a telecentric optical-CT scanner (DLOS). Simple benchmark irradiations demonstrate ClearView/DLOS can accurately recreate and measure relative 3D dose within non-artifact regions (i.e., > 1 cm away from walls). Application to SIMT SRS deliveries demonstrated the viability of the system as a means for comprehensive 3D verification of complex treatment deliveries as well as confirming the treatment planning system dose distribution. The results indicate that DLOS/ClearView system is a highly effective 3D verification tool for SIMT irradiations and can be applied with 3%/2 mm gamma criteria where passing rates of > 95% are to be expected.