ESTRO 2024 - Abstract Book

S3216

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

758

Digital Poster

Evaluation of spectral distribution of new PTW silicon diodes in small fields

Alexia Delbaere 1,2 , Tony Younes 1,2 , Luc Simon 1,2 , Catherine Khamphan 3 , Laure Vieillevigne 1,2

1 Oncopole Claudius Regaud – Institut Universitaire du Cancer de Toulouse, Medical Physics, Toulouse, France. 2 Centre de Recherches en Cancérologie de Toulouse, UMR1037 INSERM - Université Toulouse 3 – ERL5294 CNRS, Toulouse, France. 3 Institut du Cancer - Avignon Provence, Medical Physics, Avignon, France

Purpose/Objective:

Detector manufacturers aim to provide detectors with field output correction factors in small fields as close to unity as possible. The general objective of this study was to evaluate two new PTW unshielded (microSilicon T60023) and shielded (microSilicon X T60022) silicon diodes compared to their predecessors (Diode E T60017, Diode SRS T60018, Diode P T60016).

Material/Methods:

The effects of the various detector components were assessed through the calculation of total electron (+positron) fluence spectra differential in energy with the user code GATE(v9.0)/Geant4 (v10.6.patch01). The latter were scored for the reference (f ref =10x10 cm 2 ) and narrow (f clin =0.5x0.5 cm 2 ) photons beams using the Varian TrueBeam STx phase space files. The fluence spectra were used to quantify the perturbation effects between f clin and f ref ( p Φ factors) and their relation to field output correction factors ( k factors). The k factors were determined numerically for 6 WFF, 6 FFF, 10 WFF and 10 FFF energy beams at 5 cm depth with an SSD of 95 cm. Nominal field sizes (collimated with MLC) ranging from 0.5 x 0.5 cm 2 to 3 x 3 cm 2 were studied.

Results:

The fluence spectra scored in the sensitive volume of the fully modeled diodes were much higher than in water. It was found that the largest contributor to the spectral difference throughout the entire energy range was due to the detector encapsulation i.e. the density of the epoxy material for unshielded diodes and the shielding for shielded diodes. By contrast, the perturbation related to the silicon sensitive volume was closer to the water voxel, except for the SRS diode due to its larger thickness. For the studied field sizes, the k factors ranged from 0.983 to 1.014 for the microSilicon and from 0.950 to 0.994 for the microSilicon X. Although there were slight differences in the spectra shape, the relative differences between p Φ and k factors remained minor (0.2%).

Conclusion:

The calculation of fluence spectra provides a better understanding of the underlying physics of the diode responses. The consistency between p Φ and k factors showed that k factors could be derived from the difference in the total