ESTRO 2024 - Abstract Book

S3396

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Material/Methods:

A self-shielded Liac HWL (6, 8, 10, 12 MeV, dose rate 11 – 12.9 Gy/min at Dmax) was positioned in the center of the OR with tilt and rotation of 0°. Measurements were performed with a reference PMMA applicator (100 mm, bevel 0°) positioned on a 15 cm thick PMMA phantom on a treatment table, at a height of 115 cm above the floor. A beam stopper with a diameter of 90 cm was aligned with the beam and positioned under the treatment table. The OR was cleared, and no additional shielding was used. A digital calibrated STEP OD-02 set in dose mode was used to determine Hp(10), in a horizontal plane 115 cm above the floor, at 2 m around the Liac, every 45° with the 0° position in front of the Liac. The detector is used with and without a lead enclosure foreseen with an opening in the direction of the source. Alternatively, drywall or RW3 slabs were placed in front of the detectors to minimize electron contamination. Additional measurements were performed outside the OR to validate the setup with previous measurements. For 3D mapping assessment, five direct ion storage Mirion Instadose2 detectors were placed on a vertical tripod, 50 cm spaced, from 15 to 215 cm above the floor, and at a distance of 2 and 3 meters from the source. A comparison was performed between the STEP OD-02, the solid-state detector MGP DMC2000 S, and the Instadose2 to determine the response under different conditions. The MGP detector shows a flat response with an increasing dose (rate) for 6 and 12 MeV. For 12 MeV, values are a factor 42 and 130 lower compared to STEP and Instadose2, respectively. This detector strongly underestimates the SR and is, therefore, not suitable for determining photon SR from an MV linear accelerator. The response of Instadose2 and STEP follow the inverse square law, as expected. Therefore, STEP and Instadose2 measurements only are reported. Instadose2 reports 4 times higher values compared to STEP OD-02, probably due to lateral anisotropy of SR on top of the volume averaging effect of the STEP. The highest measured scattered radiation value is 8.61 mSv/h (10.37 µSv/Gy) at 90 degrees and 10 MeV at 2 m (not the highest electron energy). The use of a lead enclosure was relevant only for lateral positions. When electron and low penetration SR is eliminated with 62.5 mm drywall (TVL = 42 cm for 10 MeV) or 20 mm RW3 in front of the detector, values are a factor 6-38 lower (0.19 - 1.14 mSv/h or 0.26 - 1.38 µSv/Gy) compared to the setup without slabs (6.26 - 8.61 mSv/h or 9.27 - 10.37 µSv/Gy). Therefore, the main contribution of SR can be attributed to electron and scattered low penetration SR. We suggest using only standard slabs as the drywall composition can vary which is not suitable for data comparison. A cleared OR produced the same SR in five reference points outside the OR. Results: