Real-Time Temperature Correction of Medical Range Fiber Bragg Gratings Dosimeters

Abstract

The interest in fiber Bragg gratings dosimeters for radiotherapy dosimetry lies in their (i) submillimeter size, (ii) multi-points dose measurements, and (iii) customizable spatial resolution. However, since the radiation measurement relies on the thermal expansion of the surrounding polymer coating, such sensors are strongly temperature dependent, which needs to be accounted for; otherwise, the errors on measurements can be higher than the measurements themselves. In this paper, we test and compare four techniques for temperature compensation: two types of dual grating techniques using different coatings, a pre-irradiation and post-irradiation temperature drift technique, which is used for calorimetry, and finally, we developed a real-time interpolated temperature gradient for the multi-points dosimetry technique. We show that, over these four tested techniques, the last one outperforms the others and allows for real-time temperature correction when an array of 13 fiber Bragg gratings spatially extending over the irradiation zone is used. For a 20 Gy irradiation, this technique reduces the measurement errors from 200% to about 10%, making it suitable for a radiotherapy dose range. Temperature correction for medical low-dose range dosimetry is a first in our field and is essential for clinical FBG dosimetry applications.

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

“…Consequently, according to this model, our FBG dosimeter is measuring the temperature change induced by absorbed radiation, 21 otherwise known as absorbed dose calorimetry. Centering a 20 cm long detector (having one FBG positioned every cm) on a 10 $$\times$$ 10 cm$$^{2}$$ field, we have also previously shown that FBGs located outside the irradiation field respond solely to ambient temperature variations, whereas FBGs located inside the irradiation field responded to both ambient temperature and radiation‐induced temperature variations 30 . The collective findings, along with the close alignment between the post‐irradiation signal and the thermal simulation presented in this work, confirm that our FBG detector is effectively measuring a radiation‐induced temperature increase and is therefore, a calorimeter, which is not surprising since FBGs are common temperature sensors.…”

“…Figure 2 presents the Bragg wavelength before, during and after a 20 Gy irradiation. To smooth the detector signal, three methods are compared on the figure: a linear fit during irradiation (post‐irradiation technique), a rolling mean with the nine previous values (real‐time technique compatible with real‐time interpolated temperature gradient for multi‐point dosimetry temperature correction technique 30 ) and a linear digital filter scipy.signal.filtfilt (post‐irradiation technique). For each technique, a mean difference between the measured wavelength and the detector response are respectively (7 $$\pm$$ 6)%, (4 $$\pm$$ 3)% and (4 $$\pm$$ 2)%.…”

“…Temperature correction is crucial during irradiation otherwise the dose measurement errors can reach up to 200%. 30 Hence, using the more accurate temperature correction technique directly impacts the repeatability of measurements. In this paper, we could not use the real-time interpolated temperature gradient technique because it requires that at least two FBGs be located outside of the irradiation field (not the case for a 2 × 2 cm 2 field with this prototype).…”

“…The temperature correction also has an impact on measurement variability. In this study, the pre‐irradiation and post‐irradiation temperature drift technique 30 is employed. This technique utilizes the pre‐irradiation and post‐irradiation temperature drift signals to correct the measurements during irradiation.…”

“…The post‐irradiation slope should be approaching zero but is at ‐0.0002 pm/s (while the signal is 0.006 pm/s). To reduce this variability, the real‐time interpolated temperature gradient technique 30 should be used because, as we have previously shown, it allows for a more accurate temperature correction (smaller correlation coefficients, which means that pre‐irradiation and post‐irradiation slopes are closer to 0 using this technique). Temperature correction is crucial during irradiation otherwise the dose measurement errors can reach up to 200% 30 .…”