CIO 2021-17 Calculated Activity Comparison Based on Contouring Technique for Resin Y-90 Microsphere Liver Radioembolization

Purpose: Delineation of liver, hepatic lobes, and gross tumor volumes (GTV) is necessary for activity calculation of resin Y-90 microsphere radioembolization. Hepatic lobe delineation can be based on middle hepatic vein (MHV) or perfused volume using cone beam CT (CBCT) during pre-treatment angiography. This work investigates variations in calculated activity based on contouring technique.

Material and Methods: Eighty-five radioembolization cases were retrospectively examined. Clinically, patients were contoured based on CT from SPECT/CT images post 99mTc-MAA study. Contours were fused to CBCT images acquired during angiography to visualize perfused volumes intended for lobar therapy. Liver, left/right hepatic lobes and GTVs were drawn on this image fusion. Retrospectively, the CT was fused to MR or contrast CT scans to identify MHV. Since MHV classically divides the liver, left/right lobes were re-drawn based on MHV, and GTVs were reassigned to respective lobes. Y-90 activity was re-calculated using BSA model with volumes from re-drawn contours and compared to original activity calculations based on CBCT perfused volumes.

Results: Activities differed in 94% (80/85) of cases, based on contouring technique. Median liver volume was 1662 cc. Using a Wilcoxon signed rank test, no significant difference in activity was noted for the right lobe. With the MHV approach, a 31% higher (median 11.6 vs. 15.2 mCi, p<0.01) activity for the left lobe was noted versus perfusion-based calculations. For patients with smaller livers (906-1636 cc), there was a 23% (p < 0.01) increase in median left lobe activity from perfusion to MHV approaches. In larger livers (1662-4504 cc), there was a 53% (p < 0.01) increase in median left lobe activity. There is no correlation between difference in left lobe activity as a function of GTV or liver volumes. Clinically, 27% (23/85) of cases received only right lobar treatment. Utilizing MHV-based contouring, some GTVs were reassigned to the left lobe, resulting in left-sided activity calculations.

Conclusions: Significant differences in left lobe activity were noted based on contouring technique. Practices not using a perfusion CBCT during treatment planning should consider this contouring technique for accurate activity calculation, especially in patients with variant arterial anatomy or after hepatic artery embolization performed for redistribution.