Intraoperative localization of colorectal cancers using radiolabelled monoclonal antibodies

Abstract

Abstract Radiation detectors may allow the intraoperative localization of small cancer deposits following administration of radiolabelled tumour-associated antibodies. This technique was evaluated in 16 patients with colorectal tumours (14 cancers, one adenoma, one lipoma) with the 111In-labelled monoclonal antibody (MAb) ICR2 which recognizes the tumour-associated epithelial membrane antigen (EMA). At operation counting was carried out (3 × 20 s per site) using a hand-held radiation probe over the primary lesions and any palpable lymph nodes in the mesocolon. The tumour to normal colon (T/NC) ratio of counts recorded at operation was more than 1·5:1 in eight of the 14 patients with cancer (mean(s.d.), 1·54(0·41):1) and 0·91:1 and 1·06:1 respectively in the two patients with benign tumours. Node to normal colon ratios were higher in lymph nodes containing metastases. The uptake of radiolabelled antibody (T/NC ratio) was higher in EMA-expressing cancers than in those not expressing the target antigen (mean(s.d.), 2·45(0·65):1 versus 1·40(0·20):1, P = 0·019). An abdominal tumour model was also developed. Radioactively filled containers of 0·5–10 ml representing tumour deposits were suspended in a tank of 111In solution representing the background activity found in normal tissues. The ratio of radioactivity in the ‘tumour’ to that of background varied from 2:1 to 8:1. The ‘tumour’ was considered to be detectable if the mean counts recorded over the ‘tumour’ exceeded the mean of counts recorded over background by three standard deviations. At a ratio of 2:1 only ‘tumours’ greater than 5 ml could be detected with a sodium iodide probe and those over 10 ml could be detected with a cadmium telluride (CdTe) probe. At a ratio of 8:1, ‘tumours’ of 0·5 ml could be detected with either probe. At all ratios and counting periods the Nal probe was more sensitive than the CdTe.