This is a Phase II study to determine the efficacy of SBRT to treat liver metastases in patients with Colorectal Adenocarcinoma, Carcinoma of the Anal Canal and Gastrointestinal Neuroendocrine Tumors that are not amenable to surgery. Patients should have no evidence of extra-hepatic disease or have disease that is planned to be treated with curative intent.

Therefore, SBRT is being considered as a potentially curative procedure.

Inclusion Criteria:

• Karnofsky Performance Scale (KPS) equal or greater than 70
• 1 to 4 liver metastases with an individual maximum diameter of up to 5 cm
• Lesions considered unresectable or patients considered unfit for surgery
• Histology of the primary tumor: colorectal adenocarcinoma, carcinoma of the anal canal or gastrointestinal neuroendocrine tumors.
• Absence of evidence of extra-hepatic disease or extra-hepatic disease to be treated with curative intent.
• Minimum interval of 2 weeks between systemic chemotherapy and SBRT.
• Adequate bone marrow function defined as:
• absolute neutrophils count > 1,800 cells / mm 3
• platelets > 100,000 cells / mm 3
• hemoglobin > 8.0 g / dl ( transfusion or other intervention accepted)

Exclusion Criteria:

• Concomitant chemotherapy
• Prior radiotherapy to the upper abdomen
• Pregnancy
• Underlying Cirrhosis
• Active hepatitis or clinically significant liver failure
• Prior invasive neoplasm except for non-melanoma skin cancer, or unless more than five years without evidence of disease
• Severe Comorbidity
• Current anticoagulant treatment