A nutritional supplement containing fish oil and selenium has been identified that potentiates radiotherapy of tumors in a synergistic fashion. Cotherapy with such a supplement and radiotherapy results in substantially improved reductions in tumor volume of animal models relative to radiotherapy alone, and reduces common side effects such as weight loss, inflammation, and damage to the gastrointestinal tract. Such cotherapy was also found to reduce the incidence of metastasis from the primary implantation site, and to modulate expression of genes associated with apoptosis in tumor cells. A formulation for such a supplement that is both well tolerated and palatable is also provided.