ESTRO 2024 - Abstract Book

S3268

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

water phantom at the stated set-up conditions. The results, normalized at the 10x10 cm 2 field, are reported for a 6 MV flattened beam from five TrueBeam units.

The ratio of readings between the test field (of size FS ) and the reference 10x10 cm 2 field, OR , as a function of the field size FS for each aS1200 was finally parametrized as:

2 + A

OR(FS) = A 2 (ln(FS+1))

1 (ln(FS+1)) + A 0

The reading-to-dose conversion RtD was then described as the ratio between the output factor OF from ion chamber measurements and the above OR :

RtD=OF(FS) / OR(FS)

The A n parameters were evaluated for the different specific detectors, and a global set of parameters was determined. The difference between the corrected EPID reading and the ion chamber dose estimation was computed by using the detector-specific set of parameters, as well as the global parameters.

Results:

The OR , together with the fits according to the equation above are reported in Figure 1 for the five detectors; the consistency among the different specific detectors is shown.

Figure 1 – Output Ratio of EPID measurements, with fit, for the five detectors

A global fit was obtained as the average of the single A n parameters of the equation, resulting in:

A 0 =0.689±0.001; A 1 =0.168±0.003 ; A 2 =-0.016±0.001.

The dose difference between the ion chamber and aS1200 was evaluated, with the aS1200 reading: corrected according to the detector-specific RtD conversion (Specific in Figure 2), the global RtD conversion (Global), and in the case where no reading-to-dose conversion is applied (NoCorr). The results for the five detectors are shown in Figure 2. The dose difference is on average <0.1%, with maximum deviations <0.5% in both cases of RtD corrections, either specific or global, while variations larger than 2% are found when no conversion is applied.