Commercial photodiodes and phototransistors as dosimeters of photon beams for radiotherapy

Abstract

Purpose The response to radiation typically used in radiotherapy treatments has been experimentally evaluated for three samples of two phototransistors (BPW85B and OP505A) and two PIN photodiodes types (VTB8440BH and BPW34S). Methods To that end, a staggered irradiation cycle has been applied which included dose rate values from 0.81 to 4.87 cGy/s, achieving a total absorbed dose of 21.4 Gy. The samples have been irradiated with a linear accelerator and the relations between the induced photocurrent and the average and instantaneous dose rates, and between the accumulated charge and the absorbed dose, have been determined. The radiation‐induced output currents were measured by means of an external interface of the devices to a previously designed readout unit. Results Experimental results of Si PIN photodiode BPW34S have shown a sensitivity of (13.9 ± 0.5) nC/cGy, slight sensitivity dependence on dose rate, and a high linearity of the current with the average and instantaneous dose rate, requiring only 10 V of reverse bias voltage. This device thermal drift has characterized and modeled for temperature effect compensation. Conclusions Silicon PIN photodiode BPW34S, previously tested for X‐rays and Co‐60 gamma ray source, can also be a reliable candidate for dose rate and absorbed skin dose determination in typical radiotherapy treatments irradiations. A low sensitivity loss below 2% up to 21.4 Gy has been measured, allowing its use as an affordable reusable skin dosimeter. Moreover, no significant difference has been observed between its response to dose‐per‐pulse and changing pulse repetition frequency in terms of sensitivity and dependence with dose‐rate value.

Caracterización física de fotorresistencias como dosímetros en radioterapia

“…El coste de estos sistemas se reduciría sensiblemente si fuera posible utilizar dispositivos comerciales que no hayan sido fabricados ex profeso para dosimetría, lo cual ya se ha realizado con transistores MOSFET 3,4 o con fotodiodos. 5 En este trabajo se estudia el empleo de fotorresistencias comerciales, en adelante LDR por sus siglas en inglés (light-dependent resistor), como detectores asociados a los electrómetros clínicos disponibles en los hospitales, de manera que tengamos dosímetros de bajo coste para los que no haya que diseñar un sistema de lectura específico. Una LDR es un componente electrónico que es sensible a luz y cambia su resistencia cuando se expone a ella, disminuyendo a medida que la intensidad de la luz aumenta.…”