Feasibility of lateral dose profile measurements in a small field using TLDs

The purpose of this work was to study the feasibility of lateral dose profile measurements in a small field using thermoluminescent dosimeters (TLDs) and to evaluate the impact of the field size on the absorbed dose ratio factor f_md of LiF and Al₂O₃ TLDs. The Monte Carlo package BEAM/EGSNRC was used to simulate the lateral dose profile in solid water phantoms (RW3 slab phantom) with various field sizes beyond the build-up region for 6 MV x-rays, and a LiF : Mg, Cu, P (GR-200) dosimeter with dimensions of 0.1  ×  0.1  ×  0.1 cm³ was used to measure the lateral dose profile under the same conditions as the Monte Carlo simulations. To enable comparisons between dosimeters, Gafchromic^® EBT3 films were used. The results indicate that (1) the measured results are in agreement with the simulated results within the uncertainty of the simulation; (2) the values of f_md for Al₂O₃ and LiF in a 1  ×  1 cm² field are 2.8% and 1.6% less, respectively, than those in a 10  ×  10 cm² field; and (3) within the 80% profile region, the dose differences between TLDs and solid water are less than 1%. In the 80–10% profile region, the TLD results are in agreement with the absorbed doses in the solid water within 1 mm. It is generally acceptable to ignore the impact of field size on the absorbed dose ratio factor f_md when the field sizes are larger than 1  ×  1 cm² for LiF and 2  ×  2 cm² for Al₂O₃. For 6 MV x-rays, the small GR-200 chip can be used to measure the relative lateral dose profiles of small fields. (note)