Radiotherapy or stereotactic body radiosurgery (SBRT) requires a sufficient functional liver volume to tolerate the treatment. The current study extended the work of de Graaf et al. (2010)  on the use of 99mTc-mebrofenin imaging for presurgery planning to radiotherapy planning for liver cancer or metastases. Patient was immobilized and imaged in an identical position on a single-photon emission computed tomography/computed tomography (SPECT-CT) system and a radiotherapy simulation CT system. 99mTc-mebrofenin SPECT was registered to the planning CT through image registration of noncontrast CT from SPECT-CT system to the radiotherapy planning CT. The voxels with higher uptake of 99mTc-mebrofenin were transferred to the planning CT as an avoidance structure in optimizing a 2-arc RapidArc plan for SBRT delivery. Excellent dose coverage to the target and sparing of the healthy remnant liver volume was achieved. This report illustrated a procedure for the use of 99mTc-mebrofenin SPECT for optimizing radiotherapy for liver cancers and metastases. © 2014 American Association of Medical Dosimetrists.