Evaluation of perturbation effects for various size TLDs in small field dosimetry

Small field treatments in radiotherapy have been increasingly developed during the last years due to the po-tential to produce more conformal dose in the target volume and to enhance the therapeutic ratio. On the other hand, dosimetric task for small field techniques is more challenging because of the production of small dimension high dose areas with rapid dose rate fall off margins. Dosimetry of such small fields necessitate a significant decrease in the size of routine radiation dosimeters. In this work we look at the effect of dosimeter size on the perturbation effect caused by the presence of the dosimeter in the radiation field. Monte Carlo simulations were used to characterize the radiation field and obtain distribution of energy deposition in a water phantom. Changes in electron spectra within various size (LiF and SiO2) TLDs in different size radiation fields 10 x 10 cm2, 2 x 2 cm2 and 0.5 x 0.5 cm2 were investigated and perturbation effect caused by the presence of dosimeter was observed. The results showed significant perturbations produced by conventional LiF TLDs that is especially important in small radiation fields. The use of small size silica fibre (SiO2) TL dosimeters for application in small field dosimetry was observed to be reasonably advantageous. Based on the simulation results, cylindrical silica fibres with 0.5 mm diameter do not produce a significant perturbation in the 10 x 10 cm2, 2 x 2 cm2 fields. The perturbation in the field by 0.125 mm diameter silica fibre was observed to be negligible even in 0.5 x 0.5 cm2 radiation field.

Automated summary

Small field treatments in radiotherapy have become increasingly important for better control of target volume dose and improved therapeutic ratio. However, the dosimetry of small field techniques is more challenging due to the production of high dose areas with rapid dose rate fall off margins. In this study, Monte Carlo simulations were used to investigate the perturbation effect caused by dosimeter size in radiation fields. The results showed that small size silica fibre TL dosimeters have advantages in small field dosimetry, while conventional LiF TLDs produced significant perturbations, especially in small radiation fields.