A phantom-based experimental and Monte Carlo study of the suitability of in-vivo diodes and TLD for entrance in-vivo dosimetry in small-to-medium sized 6 MV photon fields

Abstract

In-vivo dosimetry is recommended for radiotherapy treatment verification. The main purpose of the study is to investigate the suitability of various dosimeters for performing in-vivo measurements in small 6 MV photon fields by measurement as well as Monte Carlo simulation. Two types of diode dosimeters designed for placement on skin for the purpose of in-vivo dosimetry (EDD-5 and EDP-10), 3.1 × 3.1 × 0.89 mm3 LiF TLDs, and an electron field diode (EFD) were used for dosimetric measurements in a custom-made PMMA phantom. An Elekta accelerator’s treatment head was Monte Carlo modeled in detail, which was used together with validated Monte Carlo models of the diodes. Dose perturbation effects of the dosimeters and their suitability for in-vivo measurements were investigated. In fields ≥2 × 2 cm2, differences in the measured response of EDD-5, EDP-10 and TLD from the Monte Carlo gold-standard were <2%. For the 1 × 1 cm2 field, however, the differences from the Monte Carlo gold-standard were all greater than 2% due to the influence of dosimeter size as well as any misalignment and set-up uncertainties. In fields ≥1 × 1 cm2, EDD-5, EDP-10, and TLD produced 2.2%–3.2%, 5.3%–6.6% and 0.8–1.1% dose perturbation (≤10 cm depths), respectively. A better than 2% dosimetric accuracy was not achieved by these diodes for field sizes smaller than 2 × 2 cm2. Routine patient in-vivo dosimetry is feasible with the EDD-5 diode and TLD for field sizes down to 2 × 2 cm2 without exceeding the 5% tolerance level for overall uncertainty. But the EDP-10 diode produces a higher perturbation making it much less suitable for this purpose.