Topographic Genotyping and Cancer
By Joan Whetzel
Topographic genotyping is a diagnostic tool used to identify cancers of the GI tract, pancreas, head and neck, breast tissue, as well as genitourinary and gynecologic tumors. It is also used to determine the current stage of the cancer and its prognosis. Solid tumors and fluid specimens are tested with Topographic Genotyping (TG) by staining cancer cells, then performing microscopic analysis.
Tissue Testing
All methods of tissue testing involve examining sold tumors and suspect cystic lesions. Microscopic inspection of cells and tissue structures (with TG) are performed to identify DNA sequencing associated with specific cancers and any new mutations than may have occurred. Completed test results are sent to the physician, including the types of DNA found, the quantities of each DNA type, any mutations, and a summary of the molecular profile.
Purpose of TG
TG is used to diagnose and identify specific cancers through molecular genetics involving micro-dissection of tissue samples and gene profiling. Tissue samples are examined microscopically for genetic changes caused by the growth of tumors. To perform the test, the tissue sample is placed in a container along with a lysis buffer (an antibody that breaks down tissues), which is spun out with a centrifuge to separate the clear fluid from the red blood cells (the suprernatent). The supernatant is extracted for the cancer DNA sequencing.
Accuracy
In one study performed by multiple cancer centers, PathfinderTG was compared to other forms of cancer tests on 113 pancreatic cancer patients. The PathfinderTG was found to have a 96% accuracy rate as compared to a 75% accuracy rate for cytology results. The study had its limitations, though. The study did not provide adequate follow ups on the patients, there was evidence of some bias in the selection of patients, and the testing was not performed as a blind study. Other studies have been performed using TG tests other than the PathFinder TG test, but they were small studies and only included results obtained in the past, not testing on current patients.
Drawbacks of the TG Studies
According to Cigna's Medical Coverage Policy, the studies that have been performed were published online rather than in peer-reviewed journals. Peer-reviewed journals have a long history of thoroughly examining testing prcedures, test results, and study results for accurate reporting, to verfy that the testing and results have a solid basis in science, to determine if the testing was performed with scientific soundness, and to determine the impact of the testing on clinical results. In studying the success rates of TG testing compared to other forms of cancer testing (i.e. cytology), Cigna felt that the apparent failures of the other testing methods may have been the result of insufficient tissue sample sizes. According to Anthem Medical Policy, the study results were considered incomplete because they only compared the DNA tests to standard testing techniques and described how the results changed the management of the disease, but they did not follow up with patients to see if the earlier diagnosis and change in treatments improved the outcome for these patients.
An Additional Note
As of 2010 Cigna does not cover topographic genotyping because it was considered too experimental and the results could not be confirmed. Athem still deems this test to be "investigational and not medically necessary for all indications." Their medical policy online states that it may or may not cover the test, and that patients should check with their carrier before having the test performed or to get pre-approval.
Reference
Anthem Medical Policy. Topographic Genotyping (PathFinderTG® Test). http://www.anthem.com/medicalpolicies/policies/mp_pw_b088866.htm
Finklestein, Sydney David and Patricia Anne. Patent Store. Topographic Genotyping. http://www.devileye.net/patents/high_voltage_protection_resistor/topographic_genotyping.html
CignaMedical Coverage Policy. Topographic Genotyping (PathFinderTG® Test). http://www.cigna.com/customer_care/healthcare_professional/coverage_positions/medical/mm_0487_coveragepositioncriteria_topographic_genotyping.pdf
UPMC Health Plan Policy And Procedure Manual. Topographic Genotyping.
http://www.upmchealthplan.com/pdf/PandP/PAY%20027%20Topographic%20Genotyping%20Ver%20Sept%2009.pdf
Stedman's Medical Dictionary for the Health Professions and Nursing. New York: Lippincott, Williams & Watkins. 2011.
