H. Choi, S. J. Lee, Y. Park, W. Kim, H. Lee, and J. Cho; Department of Radiation Oncology, Yonsei Cancer Center, Heavy Ion Therapy Research Institute, Yonsei University College of Medicine, Seoul, Korea, Republic of (South)
Purpose/Objective(s): Rectal hydrogel spacer is an effective method for reducing rectal irradiation with minimal impact on dosimetric parameters. This study quantifies morphological variation of the inserted spacer to assess spacer effectiveness and stability. Materials/
Methods: Following the LDR-BT procedure, the same radiation oncologist performed the spacer insertions, focusing on accurate saline dissection, needle placement, and injection timing for optimal hydrogel positioning and prostate apex coverage. Dosimetric parameters were assessed from intraoperative ultrasonography, immediate postoperative CT, and 1-month follow-up CT/MRI. Spacer contours were delineated on the 1-month CT scans fused with T2-weighted MRI. Spacer semivolumes divided by axial and sagittal planes at the prostate centroid, and thicknesses perpendicular to the coronal plane at the prostate centroid and at 1cm intervals in four directions, were measured. A 10-point variability score criterion was established based on the measurement distributions. Medical records were reviewed for biopsy results, MRIs, and initial, preoperative, and 1-month follow-up serum prostate-specific antigen levels, as well as androgen deprivation therapy (ADT). Results: From August 2022 to April 2023, 149 patients received spacers. The average patient age was 68 years, with an initial PSA level of 9.06 ng/ml. As patients diagnosed with early-stage localized prostate cancer or organ-confined high-risk prostate cancer, the majority of patients presented a Gleason score of 6 (3+3) (n=53) or 7 (3+4) (n=38), and frequently classified as T2a (n=72). Half of the patients received preoperative ADT (n=80). Prostate volume averaged 30cc, with 76 seeds and 20 needles planned per patient. Prostate V100% averaged 97.6%, 97.3%, and 97.2% pre, post-spacer insertion, and at 1-month follow-up, with urethra V200% consistently at 0.00cc. Rectum D1cc averaged 40.9 Gy, 28.2% of the prescription dose. Spacer median volume was 15cc, with semivolumes 11, 5, 7, 8cc superior, inferior, left, right to the prostate centroid, and median thickness at the prostate centroid level was 12mm, with 12, 11, 10, 10mm thickness at 1cm intervals in the superior, inferior, left, right side. A linear summated 10-point variability score was established, assigning one point for each measurement criterion mets for volume over 5cc or thickness over 5mm. The most vulnerable criteria were the inferior volume (n=66, 44.3%), followed by the thickness (n=113, 75.8%). Initially, scores ranged from 4 to 10 in the first 30 cases, but in the last 30 cases, only 3 scored below 8, indicating a learning curve. Conclusion: The proposed morphological variation scoring criterion, considering volumes and thickness, enhances the evaluation of spacer effectiveness and stability. After achieving proficiency in the procedure, the presence of persistent outliers with lower morphological variation scores highlights an unexpected high variance, which can be attributed to rapid absorption.
.jpg "Hyeok Choi, MD photo")