2312 - Optimal Planning Organs-at-Risk Volume Margins Considering Displacement of Implanted Fiducial Markers and Organ Motions in High-Risk Prostate Radiation Therapy

S. N. Huh^1,2, and J. Y. Park^1,2; ¹Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, FL, ²University of Florida Health Proton Therapy Institute, Jacksonville, FL

Purpose/Objective(s): To determine optimal planning target and organs-at-risk volume (PRV) margins in high-risk prostate external beam treatment, we evaluated triangular area changes between prostate fiducial markers (FM) and 3D internal organ motions in daily cone-beam computed tomography (CBCT) images and cine-MRI, respectively. Materials/

Methods: We selected 25 high-risk prostate cases treated in photon radiation therapy with daily CBCT and scanned with cine-MRI to monitor intra-fractional organ motions. All cases had three FM and a minimum of one calcification within the prostate gland and used a rectal spacer. The centers of each FM were defined via automatic marker detection to deduce the prostate volume deformation and changes during the treatment course. Under the assumption that the variations of the triangle areas (V_FM) created by the three fiducial markers and calcifications can present prostate volume deformation, the V_FM was analyzed in linear regression along treatment fractions. In addition, motions of the bladder, rectum, and rectal spacer referring to pubic and pelvic bones were evaluated in cine MRI acquired using a balanced fast field echo (b-FFE) sequence. The displacement of organ motions was distinguished in a motion map generated by subtracting maximum and minimum intensity projections.
Results: The MR-motion map was clinically valuable and straightforward in defining PRV co-registering with planning CT images. The triangular extent changes tracking with prostate FM showed correlations with prostate volume deformation. Excessive rectal gas showed rectal movements up to 4 mm. However, the rectal spacer showed reliable positions less than 2 mm with insignificant motions. Even if the daily bladder filling was variable, the inferior bladder wall movement adjacent to the base of prostate volume was less than 3 mm.
Conclusion: The retrospective evaluation of extent changes between prostate FM and internal organ motions from MR-motion maps was useful in determining intrafraction motions and appropriate margins for target volume and PRV. The motion evaluation can be applied to hypo-fractionated prostate treatment and other treatment delivery techniques to reduce toxicities and suggest proper treatment robustness considering internal organ motions.