Characterization of 90Y‐SPECT/CT self‐calibration approaches on the quantification of voxel‐level absorbed doses following 90Y‐microsphere selective internal radiation therapy.

Purpose: 90Y‐microsphere selective internal radiation therapy (90Y‐SIRT or 90Y‐radioembolization) is used in the management of unresectable liver tumors. 90Y‐SIRT presents a unique situation where the total 90Y activity inside the liver can be determined with high accuracy (> 95%). 90Y bremsstrahlung single‐photon emission computed tomography (SPECT)/computed tomography (CT) can be self‐calibrated to provide quantitative images that facilitate voxel‐level absorbed dose calculations. We investigated the effects of different approaches for 90Y‐SPECT self‐calibration on the quantification of absorbed doses following 90Y‐SIRT. Methods: 90Y bremsstrahlung SPECT/CT images of 31 patients with hepatocellular carcinoma, collected following 90Y‐SIRT, were analyzed, yielding 48 tumor and 31 normal liver contours. We validated the accuracy of absorbed doses calculated by a commercial software against those calculated using Monte Carlo‐based radiation transport. The software package was used to analyze the following definitions of SPECT volume of interest used for 90Y‐SPECT self‐calibration: (a) SPECT field‐of‐view (FOV), (b) chest‐abdomen contour, (c) total liver contour, (d) total liver contour expanded by 5 mm, and (e) total liver contour contracted by 5 mm. Linear correlation and Bland–Altman analysis were performed for tumor and normal liver tissue absorbed dose volume histogram metrics between the five different approaches for 90Y‐SPECT self‐calibration. Results: The mean dose calculated using the commercial software was within 3% of Monte Carlo for tumors and normal liver tissues. The tumor mean dose calculated using the chest‐abdomen calibration was within 2% of that calculated using the SPECT FOV, whereas the doses calculated using the total liver contour, expanded total liver contour, and contracted total liver contour were within 68%, 47%, and 107%, respectively, of doses calculated using the SPECT FOV. The normal liver tissue mean dose calculated using the chest‐abdomen contour was within 1.3% of that calculated using the SPECT FOV, whereas the doses calculated using the total liver contour, expanded total liver contour, and contracted total liver contour were within 73%, 50%, and 114%, respectively, of doses calculated using the SPECT FOV. Conclusions: The mean error of < 3% for commercial software can be considered clinically acceptable for 90Y‐SIRT dosimetry. Absorbed dose quantification using 90Y‐SPECT self‐calibration with the chest‐abdomen contour was equivalent to that calculated using the SPECT FOV, but self‐calibration with the total liver contour yielded substantially higher (~70%) dose values. The large biases revealed by our study suggest that consistent absorbed dose calculation approaches are essential when comparing 90Y‐SIRT dosimetry between different clinical studies. [ABSTRACT FROM AUTHOR]