Registering prostate external beam radiotherapy with a boost from high-dose-rate brachytherapy: a comparative evaluation of deformable registration algorithms

Abstract

BackgroundRegistering CTs for patients receiving external beam radiotherapy (EBRT) with a boost dose from high-dose-rate brachytherapy (HDR) can be challenging due to considerable image discrepancies (e.g. rectal fillings, HDR needles, HDR artefacts and HDR rectal packing materials). This study is the first to comparatively evaluate image processing and registration methods used to register the rectums in EBRT and HDR CTs of prostate cancer patients. The focus is on the rectum due to planned future analysis of rectal dose-volume response.MethodsFor 64 patients, the EBRT CT was retrospectively registered to the HDR CT with rigid registration and non-rigid registration methods in VelocityAI. Image processing was undertaken on the HDR CT and the rigidly-registered EBRT CT to reduce the impact of discriminating features on alternative non-rigid registration methods applied in the software suite for Deformable Image Registration and Adaptive Radiotherapy Research (DIRART) using the Horn-Schunck optical flow and Demons algorithms. The propagated EBRT-rectum structures were compared with the HDR structure using the Dice similarity coefficient (DSC), Hausdorff distance (HD) and average surface distance (ASD). The image similarity was compared using mutual information (MI) and root mean squared error (MSE). The displacement vector field was assessed via the Jacobian determinant (JAC). The post-registration alignments of rectums for 21 patients were visually assessed.ResultsThe greatest improvement in the median DSC relative to the rigid registration result was 35 % for the Horn-Schunck algorithm with image processing. This algorithm also provided the best ASD results. The VelocityAI algorithms provided superior HD, MI, MSE and JAC results. The visual assessment indicated that the rigid plus deformable multi-pass method within VelocityAI resulted in the best rectum alignment.ConclusionsThe DSC, ASD and HD improved significantly relative to the rigid registration result if image processing was applied prior to DIRART non-rigid registrations, whereas VelocityAI without image processing provided significant improvements. Reliance on a single rectum structure-correspondence metric would have been misleading as the metrics were inconsistent with one another and visual assessments. It was important to calculate metrics for a restricted region covering the organ of interest. Overall, VelocityAI generated the best registrations for the rectum according to the visual assessment, HD, MI, MSE and JAC results.Electronic supplementary materialThe online version of this article (doi:10.1186/s13014-015-0563-9) contains supplementary material, which is available to authorized users.

Accumulation of rectum dose‐volume metrics for prostate external beam radiotherapy combined with brachytherapy: Evaluating deformably registered dose distribution addition using parameter‐based addition

“…The algorithm performs the registration in multiple passes from low resolution to high resolution using a B‐splines algorithm with the Mattes mutual information metric and the limited‐memory Broyden–Fletcher–Goldfarb–Shanno optimiser. The DIR is described in additional detail elsewhere . A detailed evaluation of the registration, including qualitative visual inspection by experts, has previously been reported .…”

“…The variation in accumulated rectum doses obtained via the DVH‐based addition and registration‐based distribution addition methods is a concern for combined EBRT/HDR‐BT as the planning CTs contain substantial anatomical changes (e.g. HDR‐BT rectal‐packing material and variable rectal filling) . The difference between the methods has been reported for cervical cancer treatments and endometrial cancer treatments .…”

“…HDR-BT rectal-packing material and variable rectal filling). 10 The difference between the methods has been reported for cervical cancer treatments 2,7,[11][12][13][14] and endometrial cancer treatments. 15 However, such studies are lacking in the context of combined EBRT/HDR-BT prostate cancer treatments where rectum dose accumulation 4,9 has not yet progressed to include deformable image registration.…”

Prostate external beam radiotherapy combined with high-dose-rate brachytherapy: dose-volume parameters from deformably-registered plans correlate with late gastrointestinal complications

“…The registration process has been described in detail previously [20,21]. Visual inspections for the 118 patients were undertaken by authors (initials CRM, VL and CIT) and did not identify any major registration misalignments (e.g.…”

“…A general structure overlap of 70 % is considered to be the starting point for satisfactory structure-correspondence in the radiotherapy context [22,23]. The registrations have also previously been extensively evaluated using structure-correspondence metrics, image similarity metrics and qualitative visual inspection by authors (initials CRM, VL and CIT) [21].…”