Feasibility of radioimmunoguided surgery of colorectal carcinoma using indium 111 CEA specific antibody and simulation with a phantom using 2 steps targetting with bispecific antibody.

The study was undertaken to define the potential use of radiolabelled (Indium 111 or Technetium 99 m) carcinoembryonic antigen specific antibody (CEA f(ab')2) for the radioimmunodetection of colorectal cancer using an intraoperative hand-held gamma probe. A clinical study performed with ten patients showed that tumor with good uptake of CEA specific antibody could be detected with sufficient contrast only in two patients. Results of a biodistribution study performed with tumor fragment and normal tissue countings in a gamma counter showed high tumor uptake in five patients. There was no correlation between tumor uptake and the count rates measured intraoperatively. To increase the signal/background of the gamma probe, a simulation study with a peritoneal cavity phantom was performed. We determined the efficiency of a two steps targetting method compared to the direct method. We simulated different tumor sizes with plexiglas balls (0.5, 1, 2, 5 ml) and tested two scintillators (NaI, BgO). Experiments were performed with 111 In and 99 m Tc. The two steps targetting method was better than direct method. The results of simulation with direct method radiolabelled with 111 in confirmed our clinical study: no efficiency of a gamma probe for the surgeon to detect a tumor. However the two steps targetting method (indirect labelling method) was very encouraging to detect tumors (size 1 and 2 ml) and definitively convincing with 99 m Tc.