Impact of detector selection on commissioning of Leipzig surface applicators with improving immobilization in high-dose-rate brachytherapy.

PURPOSE: Commission and treatment setup of Leipzig surface applicators, because of the steep dose gradient and lack of robust immobilization, is challenging. We aim to improve commissioning reliability by investigating the impact of detector choice on percentage depth dose (PDD) verifications, and to enhance accuracy and reproducibility in calibration/treatment setup through a simple and novel immobilization device. METHODS AND MATERIALS: PDD distributions were measured with radiochromic films, optically stimulated luminescent dosimeters (OSLDs), a diode detector, and both cylindrical and parallel plate ionization chambers. The films were aligned to the applicators in parallel and transverse orientations. PDD data from a benchmarking Monte Carlo (MC) study were compared with the measured results, where surface doses were acquired from extrapolation. To improve setup accuracy and reproducibility, a custom-designed immobilization prototype device was made with cost-effective materials using a 3D printer. RESULTS: The measured PDD data with different detectors had an overall good agreement (<±10%). The parallel plate ionization chamber reported unreliable doses for the smallest applicator. There was no remarkable dose difference between the two film setups. The two-in-one prototype device provided a rigid immobilization and a flexible positioning of the applicator. It enhanced accuracy and reproducibility in calibration and treatment setup. CONCLUSION: We recommend using radiochromic films in the transverse orientation for a reliable and efficient PDD verification. The applicator's clinical applicability has been limited by a lack of robust immobilization. We expect this economical, easy-to-use prototype device can promote the use of Leipzig applicators in surface brachytherapy.

A Monte Carlo model for independent dose verification in serial tomotherapy.

Due to the very high complexity of IMRT treatment plans, it is imperative to perform dose verification, preferably before patient delivery. The aim of this project is to develop a Monte-Carlo-based model to verify the final dose distributions of plans developed using the Peacock system (CORVUS Treatment Planning System and MIMiC collimator). The system delivers radiation through arc therapy and uses sinogram files to determine the state of each of the multileaf collimator leaves. In-house software was developed using Matlab to decode the sinograms and create blocklets that are used as input in an MCSIM model of the MIMiC collimator attached to a Varian Clinac 600C. After validating the model, a prostate and head and neck case were simulated. The CORVUS-predicted dose distributions were compared with the Monte Carlo dose distributions. As expected, the results agreed very closely for the homogeneous case of the prostate but there were large discrepancies observed for the more heterogeneous head and neck case.