Preliminary investigations of two types of silica-based dosimeter for small-field radiotherapy

• SiO2–GeO2 doped fibres and glass beads (GB) investigated as TLDs for their small field dosimetry.
• The maximum inter-difference between FWHM measurements was 1.8 mm.
• The maximum difference for penumbra measurements (80% to 20% beam fall off points) was<1 mm.

Two thermoluminescent dosimeters (SiO2–GeO2 doped fibres and glass beads (GB)) were used to measure small photon field doses and compared against GAFCHROMIC film, a small ionisation chamber (RK-018) and a p-type silicon diode (SCANDITRONIX, F1356), as well as Monte Carlo simulations with FLUKA and BEAMnrc/DOSXYZnrc. Ge-doped SiO2 fibres have been shown by this group to offer a viable system for use as dosimeters. The fibres and GB offer good spatial resolution (~120 μm and 2 mm respectively), large dynamic dose range (with linearity from tens of mGy up to well in excess of many tens of Gy), are non-hygroscopic and are of low cost. Measurements of beam profiles for field sizes of 10 mm×10 mm, 20 mm×20 mm, 30 mm×30 mm, 40 mm×40 mm, and 100 mm×100 mm were carried out. Through the use of a customised solid water phantom, doped optical fibres and GBs were placed at defined positions along the x-and y-axes to allow accurate beam profile measurement. The maximum difference between FWHM measurements was 1.8 mm. For penumbra measurements (measured between 80% and 20% isodoses), the maximum difference was<1 mm. These measurements indicate good agreement, within measurement uncertainty, with Gafchromic film, data obtained from the use of two commonly used detectors and FLUKA and BEAMnrc/ DOSXYZnrc simulations.