Speaker
Description
The development of radiation measurement instruments that enable in-vivo assessment and control of radiation treatments in brachytherapy is highly recommended by the European Society for Radiotherapy and Oncology (ESTRO), and it is supported by the Agència Valenciana de la Innovació (AVI). In-vivo dosimetry for high dose rate (HDR) brachytherapy requires a miniature-sized dosimeter, that can be inserted into a catheter, to monitor source dwell positions and optimal dose delivery to the tumors.
Plastic fibre scintillation dosimeters are among the most promising devices for this application, due to their notable dosimetric properties, as water equivalence, response linearity with increasing dose, independence from temperature and small size. Their main drawback is the optical noise caused by Cerenkov radiation generated in the irradiated media, which may seriously compromise the accuracy of the measured dose.
Common Cerenkov removal techniques as the twin-fibre method and the spectral removal technique may suffer large uncertainties depending on the irradiation configuration, or may require complex calibrations that limit their clinical use. A simple alternative method is the air-core light guide method consisting in the suppression of Cerenkov radiation at its source, by replacing the optical fibre light guide by an air-core fibre. This method was proved to provide accurate dose measurement, but with the drawbacks of high signal loss and rigidity of the dosimeter’s air-core light guide made of silica.
In the present study, an air-core scintillation dosimeter prototype with a silvered PTFE light guide was developed and compared to a standard prototype with solid-core light guide. This all-polymer dosimeter has the advantages of flexibility and robustness over the existing air-core dosimeters, based on silica, quoted in the litterature. These features enable its use with most source afterloaders. Using experimental measurements and simulations, we found Cerenkov noise in the silvered PTFE air-core dosimeter to be less than 0.3% when the light guide is fully irradiated, compared to up to 50% with a solid core light guide. On the other hand, its light collection efficiency was optimized, so its light loss is reduced to less than 40% ( less than a factor 2.5) compared to the standard dosimeter. This is much less that the factor 10 loss quoted in the litterature for the silica based air-core dosimeters. GATE (Geant4 Application for Tomographic Emission) simulations of the silvered PTFE ai-core dosimeter were implemented in a prostate HDR brachytherapy scenario using an Iridum radioactive source (model mHDR V2r). The dosimetric performance and the Cerenkov-to-signal ratio of this all-polymer air-core dosimeter are presented and its accuracy and practical suitability for dose measurement in real clinical conditions are discussed.
