Tumor Dose Response in Yttrium-90 Resin Microsphere Embolization for Neuroendocrine Liver Metastases: A Tumor-Specific Analysis with Dose Estimation Using SPECT-CT

Purpose: To evaluate dose-response relationship in yttrium-90 (⁹⁰Y) resin microsphere radioembolization for neuroendocrine tumor (NET) liver metastases using a tumor-specific dose estimation based on technetium-99m-labeled macroaggregated albumin (^99mTc MAA) single photon emission computed tomography (SPECT)-CT. Materials and Methods: Fifty-five tumors (mean size 3.9 cm) in 15 patients (10 women; mean age 57 y) were evaluated. Tumor-specific absorbed dose was estimated using a partition model. Initial (median 2.3 months) follow-up data were available for all tumors; last (median 7.6 months) follow-up data were available for 45 tumors. Tumor response was evaluated using Modified Response Evaluation Criteria in Solid Tumors (mRECIST) on follow-up CT. Tumors with complete or partial response were considered responders. Mean tumor absorbed dose was 231.4 Gy ± 184.3, and mean nontumor liver absorbed dose was 39.0 Gy ± 18.0. Results: Thirty-six (65.5%) and 30 (66.7%) tumors showed response at initial and last follow-up, respectively. Mean absorbed doses in responders and nonresponders at initial and last follow-up were 285.8 Gy ± 191.1 and 128.1 Gy ± 117.1 (P = .0004) and 314.3 Gy ± 195.8 and 115.7 Gy ± 117.4 (P = .0001). Cutoff value of ≥ 191.3 Gy for tumor-specific absorbed dose predicted tumor response with 93% specificity, whereas < 72.8 Gy predicted nonresponse with 100% specificity at last follow-up. Estimated mean absorbed tumor dose per patient was significantly higher in responders versus nonresponders over the follow-up period (224.5 Gy ± 90.3 vs 70.0 Gy ± 28.0; P = .007). Conclusions: Tumor-specific absorbed dose, estimated with a partition model, was significantly associated with tumor response in NET liver metastases. An estimated dose ≥ 191.3 Gy predicted treatment response with high sensitivity and specificity.