Medullary thyroid carcinoma

Medullary thyroid carcinoma (MTC) is a subtype of thyroid cancer which accounts for 5-10% of all thyroid malignancies. It occurs both sporadically (80%) and as a familial form (see associations).

Epidemiology

In non-familial case, it typically peaks in the 3^rd to 4^th decades.

Pathology

Thought to arise from parafollicular C cells of the thyroid ⁴. Amyloid components may be seen on histology. It is characterised by consistent production of a hormonal marker (calcitonin), calcification of both primary and metastatic sites, and association with other endocrine neoplasms. Metastatic involvement may be seen in up to 50% at the time of presentation ⁴.

Radiographic features

Ultrasound

Punctate high echogenic foci resembling calcification may be seen both within the primary thyroid lesion as well as metastatic regional lymph nodes ³ and distant metastatic sites. Involved lymph nodes typically calcify.

CT

Both primary and metastatic lesions usually have irregular dense calcific foci within ¹.

In the chest, bullae formation and pulmonary fibrosis might happen as a result of a desmoplastic reaction.

Nuclear imaging

• radioactive iodine: lesions do not concentrate radioactive iodine since the tumour does not arise from thyroid follicular cells
• FDG-PET
  • ~75% (range 60-95%) sensitive for metastatic disease ⁶ 
• Tl-201: has been shown to concentrate Thalliumthallium-201 ⁵
• I-123 MIBG: 30% of MTCs show uptake if the thyroid is blocked with Lugol solution prior to the scan

Management and prognosis

As with any malignancy, staging of disease is an important part of management to inform treatment planning and prognosis. Biochemical evaluation in the form of preoperative CEA and calcitonin levels is helpful to assess neoplastic cells' functional capacity. Higher levels of these markers roughly correspond to size of tumor and extent of nodal metastasis.⁷.

The extent of radiologic evaluation is variable, depending on suspicion for metastatic disease. Evaluation should uniformly begin with a neck ultrasound, to be supplemented by additional imaging if there are findings of extensive neck disease. Additional imaging may consist of CT of neck and thorax, multiphase CT/MRI of the liver, or bone scan. The efficacy of PET imaging for assessment of metastatic disease is variable, and neither FDG nor fluoro-DOPA imaging is recommended for assessment.⁸.

The standard treatment approach involves surgical resection of all known disease, when feasible. This typically consists of total thyroidectomy and neck dissection.⁸.

The prognosis of MTC is generally worse than papillary and follicular thyroid cancer subtypes ¹.