Multi-point calorimeter using distributed fiber Bragg gratings for small field dosimetry in radiotherapy

Abstract

Background: The interest of using fiber Bragg gratings (FBGs) dosimeters in radiotherapy (RT) lies in their (i) microliter detection volume, (ii) customizable spatial resolution, (iii) multi-point dose measurement, (iv) real-time data acquisition and (v) insensitivity to Cherenkov light. These characteristics could prove very useful for characterizing dose distributions of small and nonstandard fields with high spatial resolution.

Purpose: We developed a multi-point FBGs dosimeter customized for small field RT dosimetry with a spatial resolution of $\sim$ 1 mm.

Methods: The 3 cm-long multi-point dosimeter is made by embedding a 80 $\mu m$ silica fiber containing an array of thirty (30) co-located $\sim$ 1 mm-long fs-written FBGs inside a plastic cylinder with an UV curing optical adhesive. With its higher thermal expansion coefficient, the plastic cylinder increases the sensitivity of the dosimeter by stretching the fiber containing the FBGs when the temperature rises slightly due to radiation energy deposition. Irradiations (2000 MU at 600 MU/min) were performed with a Varian TrueBeam linear accelerator.

Results: The dose profile of a 2 $\times$ 2 cm ${}^2$ 6 MV beam was measured with a mean relative difference of 1.8% (excluding the penumbra region). The measured output factors for a 6 MV beam are in general agreement with the expected values within the experimental uncertainty (except for the 2 $\times$ 2 cm ${}^2$ field). The detector response to different energy of photon and electron beams is within 5% of the mean response ( $0.068\pm 0.002$ pm/Gy). The calorimeter's post-irradiation thermal decay is in agreement with the theory.

Conclusions: An energy-independent small field calorimeter that allows dose profile and output factor measurements for RT using FBGs was developed, which, to our knowledge, has never been done before. This type of detector could prove really useful for small field dosimetry, but also potentially for MRI-LINAC since FBGs are insensitive to magnetic fields and for FLASH since FBGs have been used to measure doses up to 100 kGy.

Keywords: FBG captor; FBG dosimeter; optical fiber dosimeter.