ESTRO 2024 - Abstract Book

S3330

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

All measurements were performed at a Varian Clinac 2300C/D facility operating at 6 and 20MV photon beams. A solid phantom was set-up with water equivalent plate (CNMC Plastic Water® – The Original) and a plexiglass plate manufactured to position RPLGD at different distances from the beam axis. Out-of-field dose measurements were performed for different field sizes (5 cm x 5 cm ; 10 cm x 10 cm ; 15 cm x 15 cm) and at different distances from the beam central axis ranging from 10 cm to 40 cm at different depths (1 cm, 10 cm, 15 cm). The calibration factor of reference, N W,Q0 , was determined as the ratio of the dose measured with the ionization chamber (PTW 30010) and the reading of the RPLGD under the reference conditions as defined by the TRS 398 (IAEA) for the 6 MV photon beam (TPR20,10 = 0.675). Calibration factors, N W,Q , were defined as the ratio of the doses measured with the ionization chamber and the reading of the RPLGD. Energy correction factors, k en , were calculated as the ratio of the calibration factors for a certain field size, depth and distance from the beam axis and the calibration factor of reference, N W,Q0 .

The RPLGD response was modelled as a function of out-of-field distance and depth, for different field sizes.

Results:

The uniformity was ±1.2% at one standard deviation level. The N W,Q0 was equal to 1.144 ± 0.018 Gy per Reading Unit.

Table 1 summarizes the values obtained for the depth 10 cm for both GD-302M and GD-352M. The calibration factors were closer to 1 with the 20 MV photon beam. Energy correction factors k en ranged from 0.745 to 1.294 and 0.817 to 1.367 for 6 and 20MV photon beam, respectively, with 4 % uncertainties (k = 2). Values of k en were greater than 1 for 1 cm depth.

Table I. Energy correction factors outside the treatment field for 10 cm depth for three field sizes, and for 6 and 20 MV photon beam.