This study tests the feasibility and reproducibility of patient-specific motion models. These will be used for quantification of safe margin reduction. Patient-specific motion models will be built by post-processing 4D MRI data with non-rigid registration. By comparing these models between visits, model reproducibility will be assessed, and the methodology refined.

The study will use a real time position management (RPM) system for respiratory gating. The RPM system senses the respiration motion of the patient by tracking a pair of reflective markers on a lightweight "marker block" constructed of a synthetic material. This block is placed on the patient's abdomen.

During Radiotherapy, the beam is only switched on at pre-determined phases of respiration. The appropriate phase of respiration is determined by the RPM system. This implies a reliable correlation of external block motion to internal hepatic movement. This correlation is not a simple one, and requires "modeling".

Inclusion Criteria:

• Histologically confirmed adenocarcinoma compatible with primary disease in the colon or rectum
• Aged 18 years or older
• Unequivocal contrast-enhanced CT and/or MRI evidence of malignant single or multiple metastases in the liver, metastases at other sites will not be an exclusion criterion
• WHO performance status 0-2 and life expectancy of at least 12 weeks
• Informed written consent

Exclusion Criteria:

• Pregnancy or breast feeding
• Evidence of ascites
• Patients unable for whatever reason to lie flat
• Patients unable to give fully informed consent
• Age< 18 years

10 RMH patients aged over 18 years with single or multiple hepatic metastases secondary to colorectal cancer. Metastases at other sites will not be an exclusion criterion. WHO performance status 0, 1 or 2.