Quantifying Post- Laser Ablation Prostate Therapy Changes on MRI via a Domain-Specific Biomechanical Model: Preliminary Findings

Abstract

Focal laser ablation destroys cancerous cells via thermal destruction of tissue by a laser. Heat is absorbed, causing thermal necrosis of the target region. It combines the aggressive benefits of radiation treatment (destroying cancer cells) without the harmful side effects (due to its precise localization). MRI is typically used pre-treatment to determine the targeted area, and post-treatment to determine efficacy by detecting necrotic tissue, or tumor recurrence. However, no system exists to quantitatively evaluate the post-treatment effects on the morphology and structure via MRI. To quantify these changes, the pre- and post-treatment MR images must first be spatially aligned. The goal is to quantify (a) laser-induced shape-based changes, and (b) changes in MRI parameters post-treatment. The shape-based changes may be correlated with treatment efficacy, and the quantitative effects of laser treatment over time is currently poorly understood. This work attempts to model changes in gland morphology following laser treatment due to (1) patient alignment, (2) changes due to surrounding organs such as the bladder and rectum, and (3) changes due to the treatment itself. To isolate the treatment-induced shape-based changes, the changes from (1) and (2) are first modeled and removed using a finite element model (FEM). A FEM models the physical properties of tissue. The use of a physical biomechanical model is important since a stated goal of this work is to determine the physical shape-based changes to the prostate from the treatment, and therefore only physical real deformations are to be allowed. A second FEM is then used to isolate the physical, shape-based, treatment-induced changes. We applied and evaluated our model in capturing the laser induced changes to the prostate morphology on eight patients with 3.0 Tesla, T2-weighted MRI, acquired approximately six months following treatment. Our results suggest the laser treatment causes a decrease in prostate volume, which appears to manifest predominantly at the site of ablation. After spatially aligning the images, changes to MRI intensity values are clearly visible at the site of ablation. Our results suggest that our new methodology is able to capture and quantify the degree of laser-induced changes to the prostate. The quantitative measurements reflecting of the deformation changes can be used to track treatment response over time.

3D Registration of mpMRI for Assessment of Prostate Cancer Focal Therapy

“…This important finding shows the ability to successfully model tissue changes at the location of cancer that can be visualized on baseline mpMRI. Intensity changes at the location of the ablation were also reported by Toth et al 20 .…”

“…This is significantly lower than the volume of the AZ, although the two measures were significantly correlated. Toth et al 20 reported a similar decrease in gland volume at the same follow-up time delay in response to laser ablation. Volume shrinkage is likely due to the process of cicatrization with fibrosis 49 .…”

“…There is very limited literature on longitudinal registration describing the deformation of the gland after local treatment 16,46 . A recent report 20 aims to quantify changes of the gland after focal laser ablation using the finite elements method (FEM) align pre- and post-operative T2W images. The study notes the importance of knowing biomechanical properties of the tissue, including surrounding bladder and rectum.…”

“…Toth and associates 20 provide preliminary validation of a competing framework based on FEM and requiring modeling the elastic effects of the bladder and the rectum. A direct comparison between FEM and purely image-based framework would be of interest.…”

“…Using a finite elements method (FEM), Marami et al 19 validated a registration approach between MRI acquired with an endorectal coil and the intraoperative MRI. Toth et al 20 also used FEM to model the changes in prostate shape after laser ablation.…”