Reconstruction of a Fletcher type applicator with a CT/MR compatible catheter using MR imaging for cervical cancer brachytherapy

Abstract

The purpose of this study was to investigate the clinical feasibility of Fletcher-Williamson (FW) applicator reconstruction and to report on the use of such reconstructions in treatment planning for magnetic resonance imaging (MRI)-based brachytherapy of locally advanced cervical cancer. We constructed a phantom containing a fixed standard CT/MRI applicator set, and six fixed spheres filled with Ultravist, representing reference dose points (P1 ∼ P6). CT and two MRI image sets of the phantom, including a standard CT/MR applicator with inserted National Cancer Center (NCC) CT/MR catheter sets developed in-house, were scanned for brachytherapy treatment planning. The results were compared with traditional two-dimensional film-based treatment planning. Catheter reconstruction for each plan was performed with a 2.5 mm source step and repeated five times using a routine protocol for four different imaging sets to verify reproducibility in radiation treatment planning. From all calculated dose distributions, doses at the P1 P6 reference points were acquired for all reconstruction methods, and treatment planning was repeated. Based on MR imaging, 3D applicator reconstruction was performed for FW type applicators, and the Plan and the PlanMR−N demonstrated excellent agreement, with a maximum difference of 1.5% in the average dose distribution at all six reference points. However, Plan and PlanMR−G showed significant variations (6.5%) at some points and for relative standard deviations (SD). Our preliminary phantom study indicates that, for cervical cancer brachytherapy, reconstruction of a Fletcher-type applicator with a CT/MR-compatible catheter using MRI is feasible and shows excellent reproducibility and accuracy for clinical implementation.