Studies have shown that the early detection of breast cancer leads to better patient prognosis and a greater five-year survival rate. Diagnostic and prognostic markers play a key role in classifying tumors and determining the best treatment plan for a patient. The most widely used and established prognostic markers for breast cancer recurrence are tumor size, tumor grade, lymph node involvement, and tumor hormone receptor status. These indicators, although well established, are all related to tumor aggressiveness. Recent evidence has shown that proliferation markers, such as Ki-67 and proliferating cell nuclear antigen (PCNA), may have independent prognostic value [1–3]. Although these proliferation markers have potential, recent studies indicate that thymidine kinase 1 (TK1), another marker associated with proliferation, may be a better prognostic marker than either Ki-67 or PCNA.
Thymidine kinase 1 (TK1) has been studied extensively, primarily as a diagnostic biomarker for a variety of cancer types. TK1 is a nucleotide salvage pathway repair enzyme that is primarily responsible for the phosphorylation of thymidine to thymidine monophosphate. TK1 is associated with proliferating cells and is primarily elevated during S phase. As a biomarker, higher serum TK1 activity levels correlate with a more advanced cancer stage and grade. Serum TK1 levels also show prognostic potential as their levels help predict future relapse at the time of primary diagnosis in breast and colorectal cancer patients.