Background: To evaluate outcomes and dosimetric parameters of ablative stereotactic body radiotherapy (SBRT) with functional treatment planning for localized hepatocellular carcinoma (HCC) in patients with Child-Pugh B (CP-B) hepatic cirrhosis. Methods: Liver SPECT with ^99mTc-sulfur colloid was co-registered to 3D-CT for identification and avoidance of functional hepatic parenchyma during SBRT in patients with advanced cirrhosis. Functional liver volumes (FLV-SPECT) were compared to anatomical liver volumes, as were dosimetric parameters when radiation dose constraints were adapted exclusively to FLV-SPECT. Hepatic function, toxicity, and radiographic response were documented every 4–6 months following SBRT. Results: With a median follow-up of 25 months 37 patients (48 lesions treated) with CP-B cirrhosis received SBRT to a median dose 48 Gy (4–5 fractions). FLV-SPECT volume loss (509 cc or 41.3%, p < 0.001) was observed in all patients, while the functional and anatomical liver volumes matched well in a control group of non-cirrhotic/non-HCC patients. While tumors received ablative irradiation, mean dose to FLV-SPECT was maintained at 1.3 – 16 Gy (median 9.17 Gy), well below the liver threshold tolerance to radiation. Seventeen patients successfully completed liver transplant at a median time to transplant of 6.5 months. The dropout rate from the transplant list was 9 % with intrahepatic progression outside treated tumors. Eight of 10 patients with intrahepatic progression received additional SBRT during follow-up. Overall 2-year survival rate was 65% with no incidence of RILD or CP class migration from B to C was observed at 6+ months post SBRT. Two patients completed liver SBRT on a hybrid linear accelerator combined with MRI scanner (Unity MR-Linac, Elekta) utilizing Super-Paramagnetic Iron Oxide (SPIO) nanoparticle agent as an alternative contrast media for functionally active liver parenchyma. Prolonged SPIO-contrast retention also allowed per fraction treatment plan adaptation and enhanced tumor imaging. Conclusions: SBRT planning with ^99mTc sulfur colloid SPECT allows identification and conformal avoidance of residual functionally active hepatic parenchyma in patients with CP-B cirrhosis. We report high local control and low toxicity leading to satisfactory pre- and post-liver transplant outcomes. Prospective clinical trial investigating MRI-SPIO functional treatment planning for liver SBRT is ongoing at our institution.