[Dosimetric studies using glass fibers].

PURPOSE

Ionizing radiation may cause discolouring of glasses by creation of colour centers. So radiation induced optical loss is a measure of absorbed dose. With a doped glass fiber a small volume optical dosimeter is developed for clinical purposes providing real time dosimetry with high spatial resolution.

MATERIALS AND METHODS

Discolouring of glass by ionizing radiation is dose dependent and can be measured as light attenuation at a fixed wavelength. Light power of usual light emitting diodes (LED) is sufficient for this purpose. The readout light is conducted through a transmission fiber of arbitrary length. Concurrent measurements were performed at several wavelengths (660 to 850 nm) using a time multiplexing technique. We investigated the radiation induced light attenuation of a lead doped silica fiber with 60 Wt-% PbO2, diameter < 0.5 mm, and length < 0.1 m. The fiber was exposed to high energy photons of cesium-137, cobalt-60, 12-MV-photons, and 18-MV-photons generated by a linear accelerator, respectively. The influence of various temperatures, doses, and dose rates was tested. When sensor fiber is termed by a mirror reflected light can be detected with one transmission fiber and optical pathway is doubled.

RESULTS

In a wide dose range (0 to 112 Gy) radiation induced loss represents absorbed dose in a linear manner without saturation effects. Optical loss is diminished by partial recovery of radiation damage depending on time and temperature. In order to compensate fading a phenomenological model was fitted to experimental data. Temperature dependence may be corrected by measurements with several readout wavelengths. Above 1 MeV there is merely a slight dependence on photon energy. At a size of the glass fiber reflection sensor of L = 2 cm doses of 0.04 Gy may be detected. The reproducibility at 1 Gy is about 4%.

CONCLUSION

Lead doped silica fiber is suitable for radiation dosimetry in a dose range interesting for clinical practice. Fading may be compensated during irradiation using a phenomenological model. The size of a reflection sensor is comparable to thermoluminescence dosimeters. In contrast to TLD glass fiber provides real time dose measurements. By this means optical glass fiber dosimeter may be appropriate for in-vivo dosimetry in radiation therapy.