THERMOLUMINESCENCE DOSIMETRY OF THERAPEUTIC X-RAYS WITH LIF RIBBONS AND RODS

Extruded LiF ribbons (3.1 x 3.1 x 0.9 mm3) and rods (6 x 1 x 1 mm3) are commonly used thermoluminescence dosimeters (TL dosimeters) for clinical dosimetry in radiotherapy. The dose distribution in these crystals was investigated in a 6 MV x-ray beam using smaller LiF TL dosimeter types (thin ribbons with a thickness of 0. 14 mm and small cubes with 1 mm side length). Using the thin ribbons, the effective depth of measurement in the normal ribbons was found to be shifted towards the surface. Measuring on the surface of a solid water phantom in a 10 x 10 cm2 field it was found to be at a depth of 0.4 mm as compared to the physical centre of the ribbons of 0.44 mm. In the investigations with small cubes assembled in form of ribbons and rods it was found that a higher dose was deposited in the centre of the ribbons and rods. Accordingly it was found that TL dosimeters in close contact with each other increase their respective reading. Using two ribbons in contact with each other on the surface of a phantom leads to an overestimation of dose of about 1 %. The specific dose response of LiF increases with dosimeter size, this is most likely due to increased electron scatter from the additional LiF material with density higher than unity. This emphasizes the role of TL dosimeters as relative dosimeters where standards of equal size and geometry should be used to evaluate the readings.