Lav K. Goyal

Large deformation three-dimensional image registration in image-guided radiation therapy

In this paper, we present and validate a framework, based on deformable image registration, for automatic processing of serial three-dimensional CT images used in image-guided radiation therapy. A major assumption in deformable image registration has been that, if two images are being registered, every point of one image corresponds appropriately to some point in the other. For intra-treatment images of the prostate, however, this assumption is violated by the variable presence of bowel gas. The framework presented here explicitly extends previous deformable image registration algorithms to accommodate such regions in the image for which no correspondence exists. We show how to use our registration technique as a tool for organ segmentation, and present a statistical analysis of this segmentation method, validating it by comparison with multiple human raters. We also show how the deformable registration technique can be used to determine the dosimetric effect of a given plan in the presence of non-rigid tissue motion. In addition to dose accumulation, we describe a method for estimating the biological effects of tissue motion using a linear-quadratic model. This work is described in the context of a prostate treatment protocol, but it is of general applicability.

Automatic segmentation of intra-treatment CT images for adaptive radiation therapy of the prostate

We have been developing an approach for automatically quantifying organ motion for adaptive radiation therapy of the prostate. Our approach is based on deformable image registration, which makes it possible to establish a correspondence between points in images taken on different days. This correspondence can be used to study organ motion and to accumulate inter-fraction dose. In prostate images, however, the presence of bowel gas can cause significant correspondence errors. To account for this problem, we have developed a novel method that combines large deformation image registration with a bowel gas segmentation and deflation algorithm. In this paper, we describe our approach and present a study of its accuracy for adaptive radiation therapy of the prostate. All experiments are carried out on 3-dimensional CT images.

SU-FF-J-08: Calculating Biological Effective Dose in the Presence of Organ Deformation

Purpose: Evaluate the total biologically effective dose (BED) delivered to the prostate over the course of treatment in ART, accommodating organ motion using deformable image registration. Method and Materials: Using an in-treatment-room CT-on-rails, scans are acquired on the first five days of treatment and twice weekly thereafter. For each daily image, large-deformation diffeomorphic registration is performed to determine, for each voxel in the planning image, the location of the corresponding volume of tissue on the patient at the time of a given fractionated treatment. Knowing its position at the time of treatment, it is possible to evaluate the dose acquired by that volume of tissue on the treatment day. These varying doses can be summed for each voxel, yielding a map of the total delivered dose. However, this total dose does not correctly account for the cumulative radiobiological effect. To incorporate the fractionation biological effect, we use the Linear-Quadratic model. That is, for each voxel in the planning image, we accumulate the dose from each day, taking into account the quadratic dose response incorporating the tissue sensitivity (α/β) for the tissue type of the given voxel. This process yields a distribution of delivered BED. Results: For a series of patients we evaluate dose distributions and DVHs for planned and delivered BED using published values of α/β. These will be used to compare ART and standard IMRT treatment. Total dose and BED to the prostate are compared to the planned dose and BED, showing appreciable differences. Conclusion: Voxel-based radiobiological effects instead of voxel-based radiation dose should be accumulated in the fractionated treatment course for patients with organ motion and setup variation.