Automatic applicator digitization for MRI-based cervical cancer brachytherapy planning using two surface models

摘要

Modern image-guided cervical cancer brachytherapy involves the insertion of hollow applicators in the uterus andsurrounding the cervix to deliver a radioactive source. These applicators are imaged and manually digitized followinginsertion for treatment planning. We present an algorithm to automatically digitize these applicators using MRI forcervical cancer brachytherapy planning. Applicators were digitized in vivo using T2-weighted MR images (1.5 T) from21 brachytherapy fractions including 9 patients. The model-to-image registration algorithm was implemented in C++involving a 2D matched filter to identify the applicator center, and a 3D surface model to identify local position bymaximizing the intensity gradient normal to the surface. Surface models were produced using training MR images.Errors in the algorithm results were calculated as the 3D distances of the applicator tip and center from those identifiedmanually. A model based on manufacturer data was also used for applicator digitization to assess algorithm sensitivity tosurface model variation. The algorithm correctly identified the applicator in 20 out of 21 images with mean executiontime of 2.5 s. Mean±SD error following digitization using the MRI and manufacturer-based surface models was 1.2±0.6mm and 1.3±0.7 mm for the tandem tip (p = 0.52), and 1.4±0.9 mm and 1.3±0.7 mm for the ring center (p = 0.61). Thealgorithm requires no manual initialization with consistent results across surface models, showing promise for clinicalimplementation.