Thyroid Cancer — The Role Of Molecular Testing
Thyroid cancer occurs in 5% to 15% of thyroid nodules detected.1 Accurate diagnosis is therefore essential to avoid unnecessary thyroid surgery for benign disease and to select the most appropriate treatment option for cancerous nodules.
Drs. Richard Reitz and Frederic Waldman, Medical Directors, Quest Diagnostics Nichols Institute, discuss recent developments in the field of thyroid cancer, in particular the significance of genetic mutations in thyroid cancer and how their identification through molecular testing can guide clinical decision-making.
Thyroid cancer is the fastest-increasing cancer in the United States, with an estimated 56,000 new cases identified in 2012 and an annual age adjusted incidence rateof 11.6 per 100,000 men and women per year2.
Dr. Reitz attributes this increase partially to the availability of highly sensitive, portable ultrasound machines as well as a real increase in thyroid cancer. The new ultrasound machines are being used in the endocrinologist’s office. “Traditionally one would examine a patient, and if one felt a nodule larger than 1cm the patient would be referred for an imaging study and a biopsy,” says Dr. Reitz. “Now smaller nodules are identified in the physician’s office with the ultrasound, frequently under 1cm,with a resultant FNA if the physician feels appropriate. A small proportion of these are malignant, so the benefit is we’re catching them at an earlier stage, before they’ve invaded the surrounding tissue.”
Fine Needle Aspiration
The biopsy is performed by fine needle aspiration (FNA), using a very small gauge needle, inserted into the lesion under ultrasound guidance. Several samples are collected, made into a slide and sent for cytopathologic analysis. “80% of specimens are benign,” says Dr. Waldman. “The challenge comes in making a diagnosis for those which are malignant or indeterminate. If the pathologist can determine the size and pattern of the cells, he or she can make a diagnosis of malignancy. The two main categories of thyroid tumors, known as differentiated tumors, are papillary thyroid cancer (PTC) and follicular thyroid cancer (FTC) and account for 95% of thyroid cancer. These have distinct patterns, enabling a diagnosis to be made, provided there are sufficient cells.”
“But often there’s not an adequate number of cells, or they don’t form a clear pattern,” continues Dr. Waldman. “In these cases, the lesions are categorized as suspicious for malignancy or as atypical cells of uncertain significance.” The patient will then typically undergo surgery even though a large proportion of the lesions are benign. However, the introduction of molecular tests, which detect the presence or absence of certain genetic mutations, can help determine the likelihood a lesion is malignant or benign, and may help avoid unnecessary surgery. “Physicians are beginning to feel that for patients in the indeterminate category molecular markers can help decide whether or not the patient needs to go to the operating room,” notes Dr. Reitz.
Molecular tests are relatively new tools for further assessing nodules with atypical, suspicious, or indeterminate FNA cytology results. These tests can detect molecular alterations involved in the pathogenesis of thyroid cancer. Tests are available to identify alterations in BRAF, RAS, RET/PTC and PAX8/PPARγ.
Dr Waldman explains the significance of these alterations. “In BRAF, alterations can occur in the DNA of the neoplastic cells. There’s one ‘hot spot’ at a specific nucleotide, that changes from one amino acid to another, activating that gene and others. This changes activity in the nucleus, whereby new proteins are produced causing the behavior we associate with cancer. The presence of a BRAF mutation is a worse prognosis for classic papillary thyroid cancer.”
“The RAS protein may also contain hot spots for mutation, which have a downstream effect, activating mutations that cause more proliferation. This results in a tumor that will grow faster and more invasively.”
“The other two mutations are actually translocations, where two pieces of DNA break and are fused back together in the wrong way, forming two different chromosomes. This results in a new, activated, protein that produces new proteins, thereby causing proliferation. If you find a PAX8/PPARγ it’s very likely to be a follicular neoplasm, while RET/PTC helps diagnose a papillary carcinoma.”
More than 70% of papillary thyroid carcinomas harbor a point mutation in BRAF or RAS or a RET/PTC rearrangement, and over 70% of follicular thyroid carcinomas harbor either RAS mutations or PAX8/PPARγ rearrangements.3 Nodules with indeterminate cytology lacking these alterations are less likely to be malignant (6% to 28% risk).4
Molecular testing can also help the clinician determine the appropriate intervention, explains Dr. Reitz. “If, for example, it’s positive for the BRAF mutation, there will be more aggressive surgery - a complete thyroidectomy - to remove everything except a tiny portion. There can also be an anterior compartment dissection, to take out any lymph nodes within the boundaries of the sternocleidomastoid muscles. If the test result is negative some doctors will still opt to perform a total thyroidectomy, while others may take out only one side of the thyroid, depending on the size of the nodule. The molecular test certainly helps doctors make more informed decisions for surgical intervention, as well as the subsequent treatment with radioactive iodine and the approach to follow-up.”
Following surgery radioactive iodine may be administered to remove any residual thyroid. Patients are then monitored for thyroglobulin, which would indicate the presence of thyroid cells, and therefore the recurrence or persistence of disease.
“This immunoassay is an accurate test for about 80% of patients, who do not have autoantibodies.” notes Dr. Waldman. “The problem comes when testing people, who do have an autoantibody. This test platform is then of very limited value, because you get a result that is antibody positive, yet their thyroglobulin is unmeasurable.” He is hopeful that developments underway will soon lead to the availability of tests that will provide more accurate information for patients with autoantibodies.
Toward More Targeted Therapy
Other advances in the field include the potential for more targeted therapies based on gene alterations. “There are drugs that target the specific mutation in BRAF, for example, that have been shown to work in melanoma,” says Dr. Waldman. “And there are now drugs being tested in thyroid cancer that will be specific for mutations in these genes, as well as in others yet to be identified. So molecular analysis could provide a basis for treating patients with therapies, which target individual gene alterations.”
- Nikiforov YE, Steward DL, Robinson-Smith TM,. Molecular Testing for Mutations in Improving the Fine-Needle Aspiration Diagnosis of Thyroid Nodules. J Clin Endocrinol Metab. June 2009, 94(6):2092–2098.
- Surveillance Epidemiology and End Results. National Cancer Institute. National Institutes of Health. http://seer.cancer.gov/statfacts/html/thyro.html
- Nikiforov YE. Molecular diagnostics of thyroid tumors. Arch Pathol Lab Med. 2011;135:569-577.
- Nikiforov YE, Ohori NP, Hodak SP, et al. Impact of mutational testing on the diagnosis and management of patients with cytologically indeterminate thyroid nodules: A prospective analysis of 1056 FNA samples. J Clin Endocrinol Metab. 2011;96:3390-3397.
Released on Monday, June 04, 2012