Published response rates to preoperative or definitive radiation therapy (RT) for cholangiocarcinoma appear to be relatively high. For instance, a complete response proportion of 48% was recently reported for perihilar cholangiocarcinoma patients who received preoperative chemoradiation followed by liver transplant. Moreover, small series have demonstrated superior progression free and overall survival with the combination of external beam RT and chemotherapy compared to that derived from chemotherapy alone for many unresectable hepatic malignancies, including IHC, colorectal cancer liver metastases, and hepatocellular carcinoma. For example, addition of external beam RT to cisplatin chemotherapy was associated with prolonged progression free (median 4.3 vs. 1.9 months, p=0.001) and overall (median 9.3 vs. 6.2 months, p=0.048) survival compared to cisplatin alone among 92 total patients with unresectable IHC. Traditional thoughts in radiation biology of tumors suggested that doses of at least 1.2 Gy were required to overcome the initial shoulder of the cell survival curve. In practice, the standard dose per fraction is considered to be 0.015-0.022 Gy per fraction although the vast majority of patients are treated with either 1.8 Gy or 2 Gy fractions.