Author: Mohamad Ahmad Alissa, Carson Brame, Damian Czarnecki, Olivier Evrard, Charbel A. Habib, Jeff Hildreth, Andy Murray, Andreas Schönfeld, Ann-Britt Schönfeld, David Towle, Gerd Wessing 👨🔬
Affiliation: CDT-West - Centrum für Diagnostik und Therapie, Radiologie Vechta, Technische Hochschule Mittelhessen, MyMichigan Health, Sun Nuclear Corp., Mirion Technologies 🌍
Purpose: To characterize field output correction factors of novel shielded and unshielded silicon diode detectors.
Methods: Novel shielded and unshielded Silicon Diode Detectors (SDD) were characterized by means of Monte Carlo using the EGSnrc code system. To determine the Field Output Correction Factors (FOCF), the dose deposition was calculated in the SDD model and a “point-like” water voxel. A full linear accelerator head model was used to generate realistic square photon fields from 0.4 to 40 cm and electron fields from 6 to 25 cm.
The calculated data was validated in water phantom measurements using detectors featuring consensus FOCFs published in TRS 483 as reference for small fields, and an ionization chamber as reference for larger fields. Measurements were performed using various linear accelerators with and without flattening filter, with nominal energies ranging from 4 to 25 MV. The silicon diode detectors’ FOCFs were determined by evaluation against corrected reference measurements. For each small field measurement, beam centering was performed and dosimetric field sizes were determined by lateral beam scans.
Analog simulations and measurements were performed with the unshielded SDD for common clinical electron field sizes and nominal electron beam energies ranging from 6 to 22 MeV.
Results: The photon FOCFs determined by Monte Carlo simulations showed excellent agreement with measured data. FOCFs remain within 5 % for all investigated field sizes within the intended range of use stated by the manufacturer. Electron field output factors measured with the unshielded silicon diode detector matched those measured with the reference detectors.
Conclusion: This study demonstrated, both through Monte Carlo simulation and experimentally, that the novel shielded and unshielded silicon diode detectors are suitable for the measurement of field output factors in photon and electron beams and only requires minor corrections.