In vivo dosimetry with MOSFETs: Dosimetric characterization and first clinical results in intraoperative radiotherapy

Purpose: To investigate the use of metal oxide silicon field effect transistors (MOSFETs) as in vivo dosimetry detectors during electron beams at high dose-per-pulse intraoperative radiotherapy. Methods and Materials: The MOSFET system response in terms of reproducibility, energy, dose rate and temperature dependence, dose-linearity from 1 to 25 Gy, angular response, and dose perturbation was analyzed in the 6-9-MeV electron beam energy range produced by an intraoperative radiotherapy-dedicated mobile accelerator. We compared these with the 6- and 9-MeV electron beams produced by a conventional accelerator. MOSFETs were also used in clinical dosimetry. Results: In experimental conditions, the overall uncertainty of the MOSFET response was within 3.5% (±SD). The investigated electron energies and the dose rate did not significantly influence the MOSFET calibration factors. The dose perturbation was negligible. In vivo dosimetry results were in accordance with the predicted values within ±5%. A discordance occurred either for an incorrect position of the dosimeter on the patient or when a great difference existed between the clinical and calibration setup, particularly when performing exit dose measurements. Conclusion: Metal oxide silicon field effect transistors are suitable for in vivo dosimetry during intraoperative radiotherapy because their overall uncertainty is comparable to the accuracy required in target dose delivery.