ESTRO 2024 - Abstract Book

S3302

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

phantom were chosen as the reference cause of the very small sensitive volume which minimizes the volume average problem for small fields. No correction factor were available for RD and RC measurements.

OFs were normalized at the reference 10x5 cm2 field size at 85 cm SSD and 10 cm of depth. EBT3 film (GAF) were used in all centers to evaluate the equivalent effective fields.

Equivalent square fields were calculated by sqrt(X*Y) where X is the geometric value of the horizontal upper entry times the scale factor 95/85 and Y is the measured longitudinal FWHM at 10 cm depth of the three field sizes on the vertical left entry. Measurement results were calculated as the average response from 3 repeated irradiations, with an associated errors calculated as one standard deviation of these repetitions. The uncertainty related to detector position was estimated to be between 0.1% and 1% for equivalent effective field from 15mm to 10mm and negligible for field sizes larger than 15 mm.

Results:

The analysis of the equivalent effective field size evidenced a good agreement between all dataset with standard deviations from the nominal effective fields less than 0.4 mm (see table 1) in all cases. In fig.1a and 1b are reported the percentage differences of OFs in solid water phantom respect the reference ones for the 12 configured fields in Tomotherapy linac for RC and RD detectors respectively. . RC and RD showed a good inter-department agreement with a max deviation from reference less than 2% for 5 cm and 2.5cm jaws. For 1 cm jaw’s size, a max deviation of 5% were observed for RC in two centers, for the others the agreement was lower than 2%. Considering only RD measurements, the deviations were reduced to 4%.