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

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 (<HU> 138 vs. 12, p<0.001), allowing delineation on CT alone. The delineated volume and SA of I-SPs was smaller than nonI-SPs (<volume> 8.9 vs. 10.6 ml, p<0.001; <SA> 28 vs. 35 cm2, p<0.001), yet relative spacer position and prostate-rectal separation were similar (p=0.79). No significant change in HU, volume, SA, or relative position of the I-SPs hydrogel occurred over courses of treatment (all 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.

Sophia C. Kamran, MD,David M. McClatchy III, PhD,Jennifer Pursley, PhD,Alexei V. Trofimov, PhD,Kyla Remillard, CMD,Anurag Saraf, MD,1,2  Anushka Ghosh, BS,Ashraf Thabet, MD,Patrick Sutphin, MD,David T. Miyamoto, MD, PhD,Jason A. Efstathiou, MD, DPhil1

  1. Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA
  2. Harvard Radiation Oncology Program, Boston, MA, USA
  3. Department of Interventional Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA

Source: Kamran S., McClatchy D., Pursley J. et al. "Characterization of an Iodinated Rectal Spacer for Prostate Photon and Proton Radiotherapy." Practical Radiation Oncology. 2021. https://doi.org/10.1016/j.prro.2021.09.009.