Characterization of an Iodinated Rectal Spacer for Prostate Photon and Proton Radiotherapy.

ABSTRACT Purpose Conventional rectal spacers (nonI-SPs) are low-contrast on CT, often necessitating an MRI for accurate delineation. A new formulation of spacers (I-SPs) incorporates iodine to improve radiopacity and CT visualization. We characterized placement, stability, and plan quality of I-SPs compared to nonI-SPs. Methods N=50 intact prostate cancer patients treated with I-SPs and photons were compared to N=50 randomly selected patients with nonI-SPs (photon or proton therapy). The I-SP was contoured on the planning CT and CBCTs at three timepoints: first, middle, and final treatment (n=200 scans). I-SPs Hounsfield Units (HU), volume, surface area (SA), centroid position relative to prostate centroid, and distance between prostate/rectum centroids were compared on the planning CTs between each cohort. I-SP changes were evaluated on CBCTs over courses of treatment. Dosimetric evaluations of plan quality and robustness were performed. I-SP was tested in a phantom to characterize its relative linear stopping power (RLSP) for protons. Results I-SPs yielded a distinct visible contrast on planning CTs compared to nonI-SPs ( 138 vs. 12, p 8.9 vs. 10.6 ml, p 28 vs. 35 cm2, p 0.1). Dosimetric analysis concluded there were no significant changes in plan quality or robustness for I-SPs compared to nonI-SPs. The I-SP RSLP was 1.018, necessitating HU override for proton planning. Conclusion I-SPs provide a manifest CT contrast, allowing for delineation on planning CT alone with no MRI necessary. I-SPs radiopacity, size, and relative position remained stable over courses of treatment from 28-44 fractions. No changes in plan quality or robustness were seen comparing I-SPs and nonI-SPs.