Abstract No. 439 Histological outcomes in resected tumor specimens after Yttrium-90 transarterial radioembolization using resin microspheres

To evaluate histological outcomes in resected tumor specimens of patients treated with Yttrium-90 transarterial radioembolization (Y90-TARE) using resin microspheres. IRB-approved, retrospective review of resected tumor specimens of 18 consecutive liver cancer patients (median age = 68 years [IQR: 64–71 years], 61% male) treated with Y90-TARE using resin microspheres from November 2014 to March 2020. Type of tumor was primary liver cancer in 17 (94%) patients and metastatic colorectal cancer in 1 (6%) patient. Prescription dosimetry model for Y90-TARE was body surface area (BSA) model (n = 5) and medical internal radiation dosimetry (MIRD) model with target dose of 120 Gy (n = 4), 150–180 Gy (n = 5) and 200 Gy (n = 4). A blinded, independent pathologist assessed the histological specimen for tumor to normal liver ratio (TNR) of microsphere count and pattern of microsphere distribution (peripheral dominant vs. diffuse). Nonviable tumor ≥ 90% was defined as extensive histological response (EHR) and 100% as complete pathological response (CPR). Fischer’s exact test and t-test were used for data analysis. Median largest tumor size was 7 cm (IQR: 5–8 cm). Segmental TARE was performed in 9/18 (50%) patients. Median target area volume was 593 cm3 (IQR:469–920 cm3). EHR was achieved in 10/18 (56%) patients. EHR occurred in 1/5 (20%) of patients treated with BSA, 2/4 (50%) for MIRD 120Gy, 4/5 (80%) for MIRD 150 to 180Gy, and 3/4 (75%) for MIRD 200Gy. EHR occurred in 4/9 (44%) with lobar and 6/9 (67%) with segmental treatment (P = 0.64). Median TNR microsphere count was 10 (IQR: 2–25) with EHR versus 5 (IQR: 1–6) in specimens without EHR (P = 0.10). EHR rate was similar regardless of peripheral dominant or diffuse microsphere distribution (7/13 [54%] vs. 3/5 [60%], P = 1.00). CPR was achieved in 4/13 (31%) specimens for patients treated with MIRD dosimetry (median target absorbed dose: 152Gy [IQR:131–184Gy]) but no CPR was seen in patients treated with BSA model. ≥ 90% pathological necrosis can occur with microsphere distribution limited to the tumor periphery and occurs more frequently in patients treated with MIRD dosimetry using target absorbed dose greater than 150Gy.