Purpose: In radioembolization using Y90 microspheres for liver cancer, Lung is considered the main dose-limiting organ to avoid radiation-induced pneumonitis. Thus, lung shunt fraction (LSF) is used as a criterion to exclude patients considered for this treatment modality or limit the administered activity. We performed voxel-based dosimetry with Monte Carlo (MC) methods to assess the relation between LSF and accumulated dose in lungs.
Methods: Using the MC code for radiation transport TOPAS, we computed the dose per injected activity to lung for 45 patients treated for hepatocellular carcinoma with both glass and resin Y90 microspheres. Calculations were based on pre-treatment Tc99m-MAA-based hybrid SPECT/CT images. Prior to treatment, all patients underwent angiography studies to discard extrahepatic shunts. LSF was determined from 5-min planar acquisitions with gamma-cameras using a low-energy high-resolution parallel collimator. We compared calculations with MC and an analytical algorithm using pre-calculated voxel-based S-values for soft tissue with no density correction.
Results: Resin microspheres showed higher LSF than glass microspheres (3.8% vs 3.2% median). MC-calculated mean dose to the right lung increased with respect to the non-density corrected analytical algorithm (1.69 vs 0.90 Gy/mCi median). No clear relation between the determined LSF and the MC-calculated mean dose to the right lung was observed in our study. Dose to the left lung showed closer agreement with LSF. However, doses to the right lung can be affected by respiratory motion artifacts showing artificially high activity in the lung contour.
Conclusion: LSF in the right lung did not correlate mean absorbed dose per unit activity. Nonetheless, the apparent lack of correlation between LSF and mean dose to the right lung might indicate that motion artifact is too significant to make accurate assessments of LSF based on right lung counts, and shunts might need to be determined through counts on left lung.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the National Institutes of Health/National Cancer Institute (NIH/NCI) grant no. K99 CA267560. Eric Wehrenberg-Klee receives preclinical research funding, clinical research funding, and consulting fees from Boston Scientific, and consulting fees and honoraria from Sirtex.
Nuclear Medicine, Monte Carlo, MIRD
IM/TH- Radiopharmaceutical Therapy: Dose estimation: Monte Carlo