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

• Published in: Medical physics (Lancaster) 2021-07, Vol.48 (7), p.3438-3452
• Main Authors: Hooshangnejad, Hamed, Youssefian, Sina, Guest, James K., Ding, Kai
• Format: Article
• Language: English
• Subjects: FEMOSSA; finite element model; hydrogel spacer placement; hydrogel spacer simulation; patient‐specific model; prostate cancer; radiation therapy
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1002/mp.14990

Purpose 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