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Dosimetric Evaluation of Irradiation Geometry and Potential Air-Gaps in An Acrylic Miniphantom Used for External Audit of Absolute Dose Calibration for a Hybrid 1.5T MR-Linac (MRL) System

N Tyagi*, E Subashi, D Lovelock, S Lim, Memorial Sloan-Kettering Cancer Center, New York, NY

Presentations

PO-GePV-M-192 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To investigate the impact of partial lateral scatter (LS), back scatter (BS) and presence of airgaps on OSLD measurements in a standard IROC acrylic miniphantom on the Unity MRL.

Methods: The following irradiation geometries were investigated using OSLDs, A26MR/A12MR ion-chamber (IC) and Monaco Monte Carlo system: (a) IC/OSLD in IROC miniphantom (partial LS, partial BS) (b) IC/OSLD in a miniphantom placed on a solid water (SW) stack at a depth of 1.5cm (partial LS, full BS) (c) IC/OSLD placed at a depth of 1.5cm inside a 3cm slab of SW/buildup (full LS, partial BS) (d) IC/OSLD centered inside a 3 cm slab of SW/buildup at a depth of 1.5cm placed on top of a SW stack (full LS, full BS). Average of two irradiated OSLDs with and without water was used at each setup. An airgap of 1mm and 2mm, mimicking presence of potential airgap around the OSLDs in the IROC geometry was also simulated. The calibration condition of the machine was 1cGy/MU at SAD=143.5cm, d=5cm, G90, and 10x10cm².

Results: The expected dose at d=1.5cm was 110cGy. The Monaco calculation (0.5% uncertainty and 1.0mm voxel-size) for the 4 setups were 108.2, 108.1, 109.4 and 110.0 cGy. The corresponding IC measurements were 109.0±0.03, 109.5±0.06, 110.2±0.02 and 109.8±0.03 cGy. Without water, OSLDs measurements were ~10% higher than the expected. With added water to minimize airgaps, the measurements were significantly improved to within 2.2%. The dosimetric impacts of 1mm and 2mm airgaps were also verified with Monaco to be 13.3% and 27.9% higher respectively due to the electron-return-effect.

Conclusion: IROC MP geometry with partial LS and BS did not have any impact at the measurement depth. A minimal amount of air around or within the OSLDs can cause measurement discrepancies of 10% or higher when placed in a high b-field Unity MRL.

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