Session Item

To characterise and calibrate 1 mm core PMMA optical fibre sensors as a first step in the integration of optical fibre sensors, developed as part of the EU funded H2020 project “ORIGIN”, within HDR pelvic brachytherapy for use in adaptive brachytherapy.

An optical fibre connected to a scintillation detector of 0.5 mm radius was used to undertake measurements on an Elekta Flexitron HDR system. Initial measurements were performed by placing the fibre within a HDR prostate Perspex needle phantom capable of holding 20 HDR needles in parallel, placed between two solid water blocks. The fibre was placed within a plastic HDR needle inserted in a channel 2 cm from the Iridium-192 source within the phantom, for a dwell time of 35 seconds (Fig. 1(a) and 1(b)). Measurements were performed three times without removing either the fibre or the source from the setup to assess repeatability.

The fibres were then placed in a water tank jig at 1 cm intervals, with positioning supported between the lid of the tank and a support bridge within the water(Fig. 2(a) and 2(b)) The Perspex support bridge was built within the tank to anchor the distal end of the brachytherapy needles containing the fibre and source, ensuring vertical placement within the jig. The distance from the lid of the water tank to the support bridge was approximately 15 cm to simulate full scatter conditions with water. The output of the fibres was assessed using an ¹⁹²Ir radioactive source for a dwell time of 15 seconds and compared with estimated outputs from the treatment planning system (TPS).

The repeatability measurements of the optical fibre within the Perspex needle phantom demonstrated variation of less than 1% (Fig. 1(c)). The output signal, as the source-to-sensor distance is increased in 1 cm increments from 1 cm to 10 cm is shown in Fig. 2(c). As expected, given the inverse square law, a large drop in the optical signal was observed for measurements closest to the source, with a slower taper at greater distances. The relative difference between the output of the optical fibre and that of the TPS increased significantly with increasing distance, due to the fibre sensor’s energy-dependence, and a correction factor must be determined to reduce this deviation (Fig. 2(c)).

The optical fibre sensors demonstrated reproducible measurements with the need for a correction factor to be applied to account for differences in the fibre output and TPS output. Angular and temperature dependence need to be assessed to determine appropriate correction factors. Calibration of the fibres with a well chamber will be performed via intercomparison with a Farmer chamber, following which the fibres will be tested in anthropomorphic phantoms prior to clinical testing.