Thymidine kinase 1 (TK1), an enzyme involved in DNA synthesis, is associated with highly proliferating cells, particularly malignancies. Circulating TK1 levels have been measured and used to characterize several human hematopoietic neoplasms. Hemangiosarcoma (HSA) is the most common splenic malignancy in dogs. Because of hemoabdomen’s emergent nature and prognostic differences between splenic masses, a screening test that could detect the presence of splenic neoplasia earlier and further differentiate between benign and malignant disease would be beneficial.

Serum TK1 levels were evaluated comparing 15 normal dogs with 62 dogs presented to emergency facilities with hemoabdomen and a splenic mass. Of the 62 dogs, 31 were diagnosed with HSA, 9 with other splenic malignancies, and 22 with benign lesions. TK1 levels were significantly higher in dogs with HSA compared with those in normal dogs, but not significantly higher compared with dogs with benign splenic disease. Several dogs with benign disease had elevated TK1 levels: 7 were euthanized at diagnosis, and 6 died within 1 year of diagnosis, suggesting the potential for occult neoplasia elsewhere. For the highest level of diagnostic accuracy, a 2-tier, cut-off system for TK1 activity has been proposed, dividing results into low, intermediate, and high ranges.

Use of biomarkers in cancer diagnosis and as barometers during treatment is a valuable addition to our diagnostic array. The TK1 biomarker is now commercially available, and although the study supported value in the biomarker’s ability to support or disprove a diagnosis of cancer, additional diagnostics should always be employed before making a final decision on management. Additional studies with larger populations can help support the use of this test for improving our prowess in early cancer detection.—J.A. Impellizeri, DVM, DACVIM (Oncology)

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Elevated serum thymidine kinase activity in canine splenic hemangiosarcoma. Thamm DH, Kamstock DA, Sharp CR, et al. VET COMP ONCOL 10:292-302, 2012.