Medullary thyroid carcinoma accounts for 1.6% of thyroid cancers . Given the infrequent occurrence and highly variable cytomorphology of medullary thyroid carcinoma, diagnosis can be challenging. Maintaining a high level of clinical suspicion and appropriately triaging material at rapid on-site evaluation (ROSE) can simplify the diagnosis and optimize patient care.
Medullary thyroid carcinoma can be suspected based on clinical history. Twenty to thirty percent of medullary cancers are inheritable germline mutations , so patients often have a strong family history of endocrine cancers and several bilateral suspicious thyroid nodules. The most common familial form, Multiple Endocrine Neoplasia (MEN) type 2A (Sipple’s syndrome), results from point mutations in the RET gene (chromosome 10q11, exons 10 or 11), causing medullary thyroid carcinoma and pheochromocytomas between ages 20-39. Familial medullary thyroid carcinoma (FMTC) is limited to thyroid disease and tends to occur between ages 40-59, similar to sporadic medullary carcinoma. FMTC is also strongly associated with RET mutations in exons 10 or 11. MEN2B is less common but more aggressive, presenting in childhood with a tall, thin Marfanoid habitus, mucosal neuromas, medullary thyroid carcinoma, and pheochromocytoma. MEN2B typically involves a RET exon 16 M918T point mutation, which is also common in sporadic medullary thyroid carcinoma . So, a personal or familial history of multiple endocrine tumors should raise concern for medullary carcinoma.
During ROSE, cytomorphologic characteristics that might prompt consideration of medullary thyroid carcinoma include moderate to abundant cellularity, dispersed plasmacytoid to spindled cells, and occasional syncytial clusters. Cytoplasm tends to be moderate to abundant, granular, and vacuolated in liquid-based preparations. Romanowsky stain highlights occasional fine red-purple granules in medullary thyroid carcinoma, which contrasts with the blue-grey granules of Hurthle cells. Rarely, melanin pigment can be observed and raise concern for melanoma. Nuclei tend to be eccentric and contain granular chromatin; prominent nucleoli and intranuclear inclusions are infrequent but have been described. Nuclear grooves are uncommon. Like all endocrine glands, occasional cells show bizarre atypia and multinucleation, but the overall pleomorphism tends to be mild to moderate, in contrast to anaplastic thyroid carcinoma’s striking pleomorphism. Numerous variants (giant-cell, clear cell, oncocytic, small cell, etc.) have been described, but the clinical significance of these different morphologic patterns is unclear.
Because of the widely varied appearance of medullary thyroid carcinoma, immunocytochemical stains on cell blocks are useful for confirmation. Unlike most primary thyroid cancers that derive from thyroglobulin-producing follicular cells, medullary carcinoma originates from the calcitonin-secreting parafollicular C-cells. Thus, it is uniquely positive for calcitonin and negative for thyroglobulin. Medullary carcinoma also expresses CEA and neuroendocrine markers, as well as TTF1. PAX8 expression is variable. If amyloid is present, Congo red staining may show apple-green birefringence. Useful stains to exclude other differential diagnoses include S100, HMB45, Melan-A (negative in medullary, positive in melanoma) and CD138, kappa, lambda (negative in medullary, positive in plasmacytoma).
In addition to cell block material, needle rinses for calcitonin measurements should also be collected if a medullary thyroid carcinoma is suspected at ROSE . Dedicated needle rinses of thyroid nodules and lymph nodes are cheap, quick, and highly accurate. This is especially useful if the cell block is insufficient for confirmatory stains.
Material should also be reserved for molecular testing if possible. Studies for germline RET mutations may assist in classifying a patient’s risk for other endocrine tumors, as well as identifying disease in family members. Up to 25% of presumed sporadic cases can harbor germline mutations, so testing is recommended for all patients. 
Since staging, prognosis, and therapy depend on the presence of lymph node metastases, it is important to assess nodes at the time of thyroid FNA. Lymph node metastases are present at diagnosis in 20-30% of micro-medullary thyroid cancers (diameter less than 1 cm), 50% with a diameter of 1-4 cm, and 90% with a diameter greater than 4 cm. [5,6] Dedicated needle rinses can be helpful in assessing for nodal metastases. Medullary cancer confined to the thyroid has a >99% 5-year survival rate, which decreases to 90% for regional metastases and 39% for distant metastases. 
Following a diagnosis of medullary thyroid carcinoma and prior to surgical resection, serum CEA and calcitonin levels should be obtained, along with assessment for distant metastases. Patients with serum calcitonin values <20pg/mL are unlikely to have lymph node metastases, but values >500pg/mL raise the likelihood of distant metastases . It is important to note that serum calcitonin can also be increased in severe renal failure, lactation, and rare upper gastrointestinal tract neuroendocrine tumors.  Patients should also be examined for related endocrine tumors with potential significant surgical complications. An unidentified pheochromocytoma can release excess catecholamines, causing a hypertensive crisis and possible death. The elevated serum calcium from parathyroid adenomas can cause cardiac arrhythmias, along with bone remodeling and kidney stones.
In conclusion, a cytologist’s actions and advice during ROSE can greatly improve patient care, by encouraging assessment of lymph nodes, collection of familial history, consideration of other tumors such as pheochromocytoma, procurement of material for immunocytochemical and molecular studies and needle rinses for calcitonin measurement.