Radioimmunoguided-intraoperative radiation therapy in colorectal carcinoma: a new technique to precisely define the clinical target volume.

PURPOSE The clinical target volume (CTV) to be irradiated by intraoperative radiation therapy (IORT) after resection is generally based on the surgeon's estimation of close margins. We have developed a new technique, radioimmunoguided-intraoperative radiation therapy (RIG-IORT), that uses an intraoperative hand-held gamma-detecting probe to define areas of residual microscopic disease containing radiolabeled monoclonal antibodies to tumor associated antigen, to more precisely delineate the CTV for IORT. METHODS AND MATERIALS Patients were injected i.v. with 2 mCi 125I- radiolabeled CC49 antibody approximately 3 weeks before surgery. They then underwent radioimmunoguided surgery (RIGS) with maximal resection of tumor. A hand-held gamma-detecting probe (Neoprobe 1000) was used intraoperatively to detect and resect areas of high radioactivity, representing tumor. Areas with persistently high probe counts after resection were the areas of occult residual disease, and represented the CTV to be irradiated. The IORT was given with either 6-9 MeV electron beam from a dedicated linear accelerator, or with high-dose-rate brachytherapy from a remote afterloader. If all RIGS-positive tissue had been resected, or if widely disseminated disease remained, the patient was not considered for IORT. RESULT This technique was used in 31 patients with colorectal adenocarcinoma recurrent into the pelvis (n = 23) or paraortic nodes (n = 8). The CTV for IORT was delineated by increased RIGS count in 13 of 19 patients (68%) with microscopic residual, and in 11 of 12 patients (92%) with gross residual. In the other 7 patients, the tumor area did not accumulate the radiolabeled antibody; therefore, these tumor beds were irradiated based on the surgeon's estimation of close margins. Hence, overall, the RIG-IORT technique was used to define the tumor bed for IORT in 24 of 31 patients (77%). This technical report focuses on the development of the RIG-IORT technique and does not address the outcome results of the treated patients. CONCLUSION A new technique, RIG-IORT, which uses radiolabeled monoclonal antibodies to precisely determine the CTV for IORT, is described. Whether the use of this technique will lead to improved tumor control will only be known upon the outcome analysis of RIG-IORT-treated patients compared with those obtained using traditional IORT techniques.