e15056 Background: Classical Circulating Tumor Cells (CTCs) are defined as EPCAM+, PanCK+, CD45-, whereas, Circulating Tumor Associated Cells (CTACs) are defined as EPCAM+, PanCK+, CD45+/-. CTACs comprise a heterogenous mix of tumor-associated cells including classical CTCs, cancer stem cells, circulating micro-emboli, tumor cell fused macrophages and other tumor-educated cells including junction endothelial and stromal cells. These cells play either a causative or synergistic role in the metastatic spread of cancer. Although it has been speculated that CTCs and CTACs would be rapidly cleared once tumor burden diminishes, the effect of radical surgery in cases of solid organ cancers and the change in the quantum of these cells after surgery has not been fully explored. Methods: We obtained 15 ml peripheral blood from 20 patients diagnosed with solid organ cancers comprising head and neck (11/20, 55%), breast (4/20, 20%), and colorectal (5/20, 25%) before and immediately after (within 24 hrs.) radical surgery. Peripheral Blood Mononucleated Cells (PBMCs) were isolated after RBC lysis and CTCs and CTACs were identified and enumerated from the same sample using differential apoptosis based negative enrichment followed by fluorescent imaging. Results: Pre-surgery, CTCs ranged from 12 to 0 cells per ml and 0 to 2 per ml post-surgery. CTACs ranged from 17 to 11057 cells per ml and, post-surgery, from 4 to 2041 cells per ml of blood. After surgery, CTCs were eliminated in 19/20 (95%) specimens, and in 1/20 (5%) specimens, there was a slight increase in the number of CTCs post-surgery. In 16/20 (80%) patients, the CTACs reduced by > 80%; in 2/20 (10%) patients, the CTACs reduced by > 50%-<80% and, in 2/20 (10%) patients, CTACs increased by > 5%. Conclusions: The enumeration of CTCs appears to correlate with the tumor burden pre- and post-surgery. On the other hand, although there appears to be a significant reduction in CTACs, residual cells of this phenotype appear to persist. The clinical correlation of this divergent trajectory of CTCs and CTACs pre- and post-surgery warrants further long-term studies.