Inclusion criteria:  Age 18 or older Willing to provide informed consent Biopsy proven colon or rectal cancer No distant metastases  Exclusion criteria:  Serious medical comorbidities or other contraindications to surgery +/- adjuvant therapy as determined by the treatment team Previous pelvic radiotherapy or radiotherapy planned prior to surgery Allergy or intolerance to iron oxide compounds Iron overload disorder Allergy or intolerance to iodides Pregnant or lactating women*