Comparing two methods for ⁹⁰Y PET based dosimetry as a predictor of metabolic response for patients receiving radioembolisation

Objectives To investigate the relationship between ⁹⁰Y PET absorbed dose estimates with metabolic response measured by FDG PET in patients receiving SIR-Spheres microspheres to treat of unresectable liver disease.

Methods Patients receiving SIR-Spheres microspheres underwent baseline FDG PET/CT to define metabolically active liver lesions; ⁹⁰Y PET/CT within 24hrs of therapy; and follow-up FDG PET/CT within 100 days to measure lesion specific response (as change in standardised uptake value (SUV) and total lesion glycolysis (TLG)). Mean lesion absorbed dose (D_ave) was derived from the ⁹⁰Y PET using both the local deposition method (LDM) and dose kernel convolution method (DKCM), which was also used to generate dose volume histograms and lesion volume receiving at least 50Gy (V50), and dose to 70% of lesion (D70).

Results 33 lesions were assessed (21 mCRC). A correlation was found between D_ave and variation in TLG which followed a log relationship (agreeing with literature [1]), with a superior coefficient of determination (R²) when using the LDM (0.72) versus the DKCM (0.34). A significant response (TLG variation > 50%) was observed for all lesions that received D_ave of at least 30Gy. Five lesions had a complete response at D_ave < 50Gy, each with a low initial TLG (mean = 40cc) when compared to the mean of the overall lesion population (350cc), implying that lesion volume and/or metabolic grade may also affect response. No correlation was found for either method when using variation in SUV to define response. Separating mCRC from other pathologies did not change the dose-response relationship. V50 and D70 led to similar relationships when compared to TLG variation, with R² = 0.50 and 0.36, respectively.

Conclusions The dose-response relationship was strongest when using TLG as a measure of response against D_ave derived through the LDM. A D_ave of at least 30 Gy led to a significant partial response in lesions, regardless of pathology.

Research Support KW is supported through an Australian Research Council Linkage Grant, in association with Sirtex Medical.