Microscopic lymph node tumor burden quantified by macroscopic dual-tracer molecular imaging.

Lymph node biopsy is employed in many cancer surgeries to identify metastatic disease and to determine cancer stage, yet morbidity and diagnostic delays associated with lymph node biopsy could be avoided if noninvasive imaging of nodal involvement were reliable. Molecular imaging has potential in this regard; however, variable delivery and nonspecific uptake of imaging tracers have made conventional approaches ineffective clinically. Here we present a method of correcting for nonspecific uptake with injection of a second untargeted tracer that allows for quantification of tumor burden in lymph nodes. We confirmed the approach in an athymic mouse model of metastatic human breast cancer by targeting epidermal growth factor receptor, a cell surface receptor overexpressed by many cancers. We observed a significant correlation between in vivo (dual-tracer) and ex vivo measures of tumor burden (r = 0.97, P < 0.01), with an ultimate sensitivity of approximately 200 cells (potentially more sensitive than conventional lymph node biopsy).