Dosimetry and Response Modeling In 90y-SIRT: Impact of Monte-Carlo SPECT and Segmentation Method
Abstract
Purpose
Treatment planning and dosimetry in 90Y-SIRT rely on accurate determination of perfused volumes (PV). Bremsstrahlung 90Y-SPECT imaging remains limited, with poor resolution and erroneous counts throughout the imaging volume. “Ground truth” identification of treated regions and PVs for dosimetry calculations therefore remains elusive. In this work, different 90Y-SPECT reconstructions and PV segmentation methods were assessed in 43 HCC tumors treated with ultra-selective 90Y-SIRT.
Methods
PV segmentation was performed using HU-threshold guidance from planning-CBCT and three post-therapy 90Y-SPECT/CT thresholds at 10, 20, and 30 pct of maximum counts. Three 90Y-SPECT recons were employed: clinical standard, window compensation, and Monte-Carlo (MC) corrected. Post-therapy, 90Y-SPECT/CT voxelized dosimetry was computed using the local deposition method and PV-based activity normalization. PV volumes, PV/tumor dose metrics, and dose-response models were compared.
Results
Median (IQR) CBCT PV was 64.5 (44.9–94.1) mL. PVs from 90Y-SPECT were 94.9 (58.2–48.1), 49.5 (31.4–85.8), and 27.7 (17.8–63.9) mL for 10, 20, and 30 pct thresholds (standard recon), and 87.2 (53.6 – 135.1), 47.5 (30.0 – 77.8), and 30.9 (14.2 – 53.6) mL using MC-SPECT. All PVs were significantly different (p≤0.03) from CBCT except for standard 20 pct (p=0.152). The variations in PVs produced inverse changes in PV and tumor doses, at times substantial (>1000 Gy) in specific cases. The performance of dose-response models was not strongly impacted by the dose metric variations, with tumor D95 connecting most strongly with response. The percentage of tumor volumes within 90Y-SPECT threshold PVs also aided in identifying cases with a high risk for incomplete response.
Conclusion
Tumor response and patient outcomes may be impacted if targeted regions are under-dosed from inaccurate estimation methods for PV volumes. The results in this study highlight that post-therapy 90Y-SPECT can help identify discrepancies between planned and delivered volumes, while also informing adequate tumor alignment within treated regions.