Original article Experimental study of radioimmunotherapy versus chemotherapy for colorectal cancer G. M. de Jong 1 , R. P. Bleichrodt 1 , A. Eek 2 , W. J. G. Oyen 2 , O. C. Boerman 2 and T. Hendriks 1 1 Department of Surgery, Division of Oncology and Abdominal Surgery, and 2 Department of Nuclear Medicine, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands Correspondence to: Ms G. M. de Jong, 490 Department of Surgery, Division of Oncology and Abdominal Surgery, Radboud University Nijmegen Medical Centre, PO Box 9101, 6525 GA Nijmegen, The Netherlands (e-mail: g.dejong@chir.umcn.nl) Background: Radioimmunotherapy (RIT) has been shown to reduce the incidence of local recurrence of colorectal cancer in an experimental model. The aim of the present study was to investigate the survival benefit of RIT compared with chemotherapy. Methods: An anastomosis was constructed in male Wag/Rij rats after intraluminal injection of CC531 tumour cells. The therapeutic efficacy of 177 Lu-labelled MG1 (single intravenous dose of 300 MBq/kg, n = 20) was compared with that of 5-fluorouracil-based chemotherapy (6 weekly cycles administered intraperitoneally, n = 20) and no treatment (n = 20). The primary endpoint was survival. Toxicity was monitored by bodyweight measurement. Results: Both chemotherapy and RIT affected bodyweight, but the weight of animals in the RIT group remained significantly higher than in the chemotherapy group (median slope of bodyweight plot 0·48 versus 0·30 g/day; P < 0·001). Kaplan–Meier analysis showed that overall survival in the RIT and chemotherapy groups was significantly better than that in the control group (50 and 46 per cent versus 25 per cent respectively after 170 days; P = 0·024 and P = 0·029). Survival after treatment with RIT did not differ from that after chemotherapy (P = 0·911). Conclusion: RIT is as effective as chemotherapy in experimental colorectal cancer. Presented to a meeting of the Society of Nuclear Medicine, Salt Lake City, Utah, USA, June 2010, and a meeting of the European Society of Surgical Oncology, Bordeaux, France, September 2010; and published in abstract form as J Nucl Med 2010; 51(Suppl 2): 59 and Eur J Surg Oncol 2010; (in press) Paper accepted 12 October 2010 Published online 15 December 2010 in Wiley Online Library (www.bjs.co.uk). DOI: 10.1002/bjs.7361 Introduction Local recurrence after potentially curative colorectal can- cer surgery has a major impact on the prognosis 1 . These local recurrences are defined as arising in the initial tumour bed, operative field, anastomosis, or in peri- toneal structures contiguous or adherent to the primary tumour 2 . The reported 5-year incidence varies between 4 and 28 per cent, depending mainly on tumour site and tumour node metastasis stage 3–6 . Peritoneal carcinomato- sis is an example of regional recurrence, with diffuse metastatic nodules on the visceral and parietal surfaces. Current treatment involves surgery with hyperthermic intraperitoneal chemotherapy 7 . Radiotherapy has a limited role and toxicity to the small bowel is an important limiting factor. Radioimmunotherapy (RIT), using radiolabelled mono- clonal antibodies directed against tumour-associated antigen, offers the opportunity to irradiate tumour cells selectively while sparing normal tissues. Several preclinical and clinical studies have indicated that RIT is suitable for the treatment of minimal or microscopic residual disease 8–13 . In a previous study in rats, adjuvant RIT prevented perianastomotic tumour growth after colonic resection 13 . The present study tested the hypothesis that both RIT and chemotherapy could improve colorectal cancer in an experimental model.  2010 British Journal of Surgery Society Ltd British Journal of Surgery 2011; 98: 436–441 Published by John Wiley & Sons Ltd