FEMOSSA: Patient‐specific finite element simulation of the prostate–rectum spacer placement, a predictive model for prostate cancer radiotherapy

Abstract

Major advances in delivery systems in recent years have turned radiotherapy (RT) into a more effective way to manage prostate cancer. Still, adjacency of organs at risk (OARs) can severely limit RT benefits. Rectal spacer implant in recto-prostatic space provides sufficient separation between prostate and rectum, and therefore, the opportunity for potential dose escalation to the target and reduction of OAR dose. Pretreatment simulation of spacer placement can potentially provide decision support to reduce the risks and increase the efficacy of the spacer placement procedure. Methods: A novel finite element method-oriented spacer simulation algorithm, FEMOSSA, was developed in this study. We used the finite element (FE) method to model and predict the deformation of rectum and prostate wall, stemming from hydrogel injection. Ten cases of prostate cancer, which undergone hydrogel placement before the RT treatment, were included in this study. We used the pre-injection organ contours to create the FE model and post-injection spacer location to estimate the distribution of the virtual spacer. Material properties and boundary conditions specific to each patient's anatomy were assigned. The FE analysis was then performed to determine the displacement vectors of regions of interest (ROIs), and the results were validated by comparing the virtually simulated contours with the real post-injection contours. To evaluate the different aspects of our method's performance, we used three different figures of merit: dice similarity coefficient (DSC), nearest neighbor distance (NND), and overlapped volume histogram (OVH). Finally, to demonstrate a potential dosimetric application of FEMOSSA, the predicted rectal dose after virtual spacer placement was compared against the predicted post-injection rectal dose. Results: Our simulation showed a realistic deformation of ROIs. The post-simulation (virtual spacer) created the same separation between prostate and rectal wall, as post-injection spacer. The average DSCs for prostate and rectum were 0.87 and 0.74, respectively. Moreover, there was a statistically significant increase in rectal contour similarity coefficient (P < 0.01). Histogram of NNDs showed the same overall shape and a noticeable shift from lower to higher values for both post-simulation and post-injection, indicative of the increase in distance between prostate and rectum. There was less than 2.2-and 2.1-mm averaged difference between the mean and fifth percentile NNDs. The difference between the OVH distances and the corresponding predicted rectal dose was, on average, less than 1 mm and 1.5 Gy, respectively. Conclusions: FEMOSSA provides a realistic simulation of the hydrogel injection process that can facilitate spacer placement planning and reduce the associated uncertainties. Consequently, it increases the robustness and success rate of spacer placement procedure that in turn improves prostate cancer RT quality.

The Feasibility of Haar Feature-Based Endoscopic Ultrasound Probe Tracking for Implanting Hydrogel Spacer in Radiation Therapy for Pancreatic Cancer

“…Similarly, 2D EUS images are not capable to represent all these deformations and changes in three-dimensional view for guiding the injection process. Our lab’s previous research ( 41 ) proposed a FEMOSSA simulation model to predict and simulate the realistic prostate-rectum spacer placement procedure. This method made it possible to design a pre-treatment injected plan based on CT scans for increasing the robustness and success rate of hydrogel placement, thereby potentially improving the clinical outcome of prostate cancer radiation therapy.…”

“…All the simulated injected points were selected based on axial CT scan which duodenum and HOP were adjacent to each other. The detail of this simulation of injection was published in the previous paper from our lab ( 41 ). We generated 7 interpolated MRI slices on every simulated injected point.…”

Finite Element-Based Personalized Simulation of Duodenal Hydrogel Spacer: Spacer Location Dependent Duodenal Sparing and a Decision Support System for Spacer-Enabled Pancreatic Cancer Radiation Therapy

“…Thus, as the first step, in the validation phase, we fine-tuned the FEMOSSA parameters to spacer insertion in the duodenum–pancreas interface problem and validated the simulation performance with the pair pre–post-injection data. FEMOSSA has already been validated for rectal spacer simulation and has shown strong performance in providing a patient-specific simulation of rectal hydrogel insertion compared to other studies ( 30 , 33 ). Previously, our group also simulated the virtual spacer by shifting the structures ( 34 ), and more recently, another group used contour overriding ( 19 ).…”

“…FEMOSSA parameters were, first, fine-tuned, and then the simulation result was validated on data from two cadavers and two patients that have been injected with duodenal hydrogel. Since, in this study, we used the same principles and only fine-tuned FEMOSSA for the duodenal spacer, for more detail on the different components of FEMOSSA we encourage the readers to refer to our previous study (Hamed 30 ). Similar figures of merit as our previous study were used to evaluate the duodenal spacer from different aspects: (1) dice similarity coefficient (DSC) between the target and duodenum in postinjection (ground truth) and post-simulation scans, (2) radial definition of nearest neighbor distance (RNND), adapted to the C-loop-like anatomy of the duodenal loop, and (3) overlapped volume histogram (OVH) L 1 CC , L 3 CC , L 5 CC , L 10 CC , and L 20 CC defined as the amount of uniform expansion of the target to have 1-, 3-, 5-, 10-, and 20-cc-volume overlaps with the duodenum ( 30 ).…”

“…The purpose of this study is to provide a preoperative decision support system by performing a systematic study of the different scenarios of spacer location in the duodenal loop. To do so, we took advantage of our in-house, anatomical-based and patient-specific hydrogel spacer simulation algorithm, FEMOSSA, to simulate the different scenarios of hydrogel spacer placement ( 30 , 31 ). Next, we designed SBRT plans and used the result of dosimetric analysis of the RT plans to create a predictive decision support system (DSS) for the duodenal hydrogel injection procedure.…”