C a n c e r S c ree n i n g Test s f o r Small A nimals Stephanie E. Schleis,
KEYWORDS Cancer Companion animals Screening tests Diagnosis KEY POINTS Biomarkers are best used in combination with other clinical findings. At risk breeds should be screened for the ABCB1-1D mutation prior to chemotherapy administration. No screening test should be used in isolation for diagnosis and treatment.
Cancer is increasingly more common in our aging patient population. Increased owner expectations and advances in veterinary diagnostic capabilities have led to the development and commercial availability of several tests for the diagnosis and treatment of cancer in companion animals. Early detection of disease may lead to more curable tumors and a less diseasedebilitated patient population. However, the role of early detection in human oncology is controversial with regard to the risk-to-benefit ratio for patients. Considerations must be given to the invasiveness of procedures undergone when a screening test is positive compared with the number of patients who actually have the disease and benefit from further scrutiny. Another consideration is that, although for some cancers early detection is of survival benefit, there are others for which no evidence suggests early implementation of treatment improves the quality or quantity of life. These same discussions hold true in veterinary oncology.1 With the aid of objective, scientific data where available, some of the more recent cancer detection tests, albeit not an exhaustive list, are discussed herein. This article provides the reader with a basic understanding of the tests and their appropriate use for cancer diagnosis, monitoring, or predictive value for detecting cancer in a healthy patient (Table 1).
Disclosure: The authors have nothing to disclose. Department of Clinical Sciences, Bailey Small Animal Teaching Hospital, Auburn University, 1010 Wire Road, Auburn, AL 36839, USA E-mail address: [email protected]
Vet Clin Small Anim 44 (2014) 871–881 http://dx.doi.org/10.1016/j.cvsm.2014.05.007 vetsmall.theclinics.com 0195-5616/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
Table 1 Definitions Sensitivity
Proportion of patients with the disease who test positive.
Proportion of patients without the disease who test negative.
Probability of disease given a particular test result. Influenced by how commonly the disease occurs in a specific population.
Characteristics, including substances or compounds, that are objectively measured and evaluated as indicators of normal biology, pathology, or the response to treatment.
Data from Modiano JF, Sharkey LC. A practical guide to diagnostic testing for veterinary cancer patients. 2012. Available at: www.akcchf.org/news-events/library/articles/cancerdiagnostics-1.pdf. Accessed November 1, 2013.
TESTS FOR LYMPHOMA Veterinary Diagnostics Institute TKcanine1
This test is a recently marketed canine blood test by Veterinary Diagnostics Institute (VDI; Simi Valley, CA, USA) for the diagnostic, prognostic, and therapeutic monitoring of dogs with lymphoma and hemangiosarcoma. It employs an indirect, modified 2-step, competitive chemiluminesence immunoassay to quantify the serum level of thymidine kinase 1 (TK1), and an enzyme-linked immunosorbent assay for the quantification of serum levels of canine C-reactive protein (cCRP). Thymidine kinase is an enzyme involved in the 1-step salvage pathway of pyrimidine synthesis. It exists in 2 forms, with TK1 located in the cytosol and TK2 located in the mitochondria.2 TK1 is associated with cellular proliferation; its activity is greatly increased in the S phase of the cell cycle. TK1 can be measured in the serum because the enzyme can leak through the cell membrane with high levels of expression (Fig. 1).2–4 TK1 expression is limited to proliferating cells, and hematopoietic malignancies have very high cell proliferation rates.2–5 In human oncology, serum TK1 levels provide information on prognosis and treatment efficacy in leukemia, multiple myeloma, and Hodgkin and non-Hodgkin lymphoma.2,4,5 In veterinary species, high levels of serum TK1 compared with normal range are detected in canine lymphoma, splenic hemangiosarcoma, and feline lymphoma.2–8 Solid tumors in dogs and humans do not show consistently increased levels precluding serum TK1 levels as a general biomarker for all cancer histologies.3 However, it is used in humans for breast, lung, and colorectal cancer.4 A recent study also showed increased levels of inactive TK1 in the serum of dogs with solid tumors using an immunoaffinity/Western blot assay for detection of the protein, not solely activity. The types of solid tumors included mammary tumors (n 5 4), hepatic tumors (n 5 2), osteosarcoma (n 5 2), synovial cell sarcoma (n 5 1), mastocytomas (n 5 5), plasmacytoma (n 5 1), fibrosarcoma (n 5 1), melanoma (n 5 2), histiocytic sarcoma (n 5 1), lung carcinoma (n 5 1), and infiltrative fibrolipoma (n 5 1).9 Other conditions can cause an increase in the activity of serum TK1, including premalignant conditions, viral infections, and inflammatory conditions.3 For example, TK1 was found to be significantly elevated in dogs with pyometra.10 Based on unpublished data, minor increases in TK1 were noted concurrently with positive tests for rickettsial disease in dogs.3 Although the specificity of TK1 as a biomarker for malignancy is high, its low-end sensitivity, especially for solid tumors, is poor owing to these other instances where TKI levels can be above the normal range.3
Cancer Screening Tests for Small Animals
Fig. 1. Role of TK in the salvage pathway of pyrimidine synthesis. TK2 is expressed in the mitochondrion and is present during the entire cell cycle. Leakage of TK (ie, TK1) through the cell membrane reflects either the overall degree of DNA synthesis or the number of cells dying in the replicative stage. In the presence of ATP, TK1 catalyzes the conversion of deoxythymidine (dT) to deoxythymidine monophosphate (dTMP). dTMP is subsequently phosphorylated to its triphosphate analogue (dTTP) before being a substrate for DNA synthesis. (From von Euler H, Einarsson R, Olsson U, et al. Serum thymidine kinase activity in dogs with malignant lymphoma: a potent marker for prognosis and monitoring the disease. J Vet Intern Med 2004;18(5):696–702.)
In efforts to better the low-end sensitivity of TK1, VDI also combined with this test the detection of cCRP in serum samples.3 cCRP is an accepted biomarker for inflammation. It is produced in the liver in response to proinflammatory cytokines as part of the acute phase response. It is an effective measure of systemic inflammation and correlates with the duration and severity of the inflammatory stimuli. However, as a biomarker it is not disease specific.3,11–13 Coupling biomarkers for rapid cell proliferation and inflammation has a strong relevance to the neoplastic process and malignant transformation. Chronic inflammation has been shown to predispose a patient to the development of neoplasia. The continued inflammatory state is essential in all aspects of cancer development and maintenance.3 In a 2013 study funded by the VDI, the resulting serum levels for each biomarker is used to create a Neoplasia Index (NI) ranging from 0 to 9. Three hundred sixty apparently healthy German shepherd dogs and golden retrievers had serum collected and analyzed for both biomarkers. The patients were followed out for a minimum of 6 months and up to 1 year after sample collection with the goal of assessing the utility of these combined biomarkers and NI to detect occult disease.3 During the study, 11 dogs developed malignant neoplasia and 10 developed benign tumors with an overall 3% incidence of cancer in the population. Of the 11 malignant tumors, 3 were hematopoietic and 8 were nonhematopoietic (hemangiosarcoma, abdominal sarcoma, intestinal sarcoma, anal sac adenocarcinoma, and parathyroid tumor). Results of the study showed a sensitivity of 82% and a specificity of 91% for the NI to detect occult malignant disease 6 months before the onset of clinical signs.3 VDI-TKcanine1 is advertised as a dual biomarker panel for dogs with suspected or confirmed cancer. Data provided by the company on their website, but not found in any peer-reviewed journals at the time of this article writing, list a patient cohort of
390 dogs with 52 cancers of various types and 22 benign neoplasms. In this population, for the diagnosis of cancer, this test had a sensitivity of 96% and specificity of 85%. A high positive sensitivity of 99% was noted when the NI was 8.0 or greater. The different varieties of cancers are not listed.14 Based on several other studies in dogs and humans, the ability of TK1 to function as a good biomarker for highly proliferative hematopoietic diseases such as lymphoma is known. However, caution should be used in interpreting a positive result in relation to other tumors where the biomarker has not been more thoroughly investigated. A study by Kumar and colleagues9 demonstrated a large fraction of inactive TK1 protein in the serum of dogs with solid tumors, and an abstract by Selting and colleagues15 in the 2013 Proceedings of the Veterinary Cancer Society may show some future significance for this biomarker combination in other tumors, but more studies need to be conducted. The VDI also gives data on the use of TK1 as a prognostic indicator for canine lymphoma and hemangiosarcoma. It lists favorable median survival times when values are less than 30 U/L with a 50% survival probability of around 150 days. When the TK1 values are greater than 100 U/L, the median survival time is poor, at approximately 50% survival probability of 50 days. Regarding therapeutic monitoring, the company states that after successful treatment TK1/cCRP/NI should return to normal (