Parathyroid carcinoma
Parathyroid carcinomas are very rare, and the overwhelming majority of discrete parathyroid gland lesions seen will be parathyroid adenomas. Carcinomas often present with profound hyperparathyroidism, as most of them are functional. Early metastasis is not uncommon. Imaging is important in their diagnosis, usually a combination of ultrasound and Tc-99m-sestamibi.
Epidemiology
Parathyroid carcinomas are said to be the rarest of all endocrine malignancies . In the USA, estimated incidence is 0.015 per 100,000, with a prevalence of 0.005% . No gender or ethnic bias is known. They are most commonly found in those in their late 40s.
Risk factors
Most cases are sporadic, with only a few developing in the context of a known genetic disease. There is no evidence that parathyroid carcinomas arise from the transformation of pre-existing parathyroid adenomas .
- hyperparathyroidism/jaw tumor syndrome (HPT-JT)
- multiple endocrine neoplasia
- familial isolated hyperparathyroidism (FIHP)
Associations
Several studies have reported associations between parathyroid carcinoma and other neck pathologies :
- secondary and tertiary hyperparathyroidism
- neck irradiation
- thyroid carcinoma
Clinical presentation
Most parathyroid malignancies are hyperfunctional, with only 10% non-secreting of parathyroid hormone (PTH). Many patients present with symptoms and signs related to profound hyperparathyroidism and metastatic disease at initial presentation is not unusual .
- palpable neck lump (40-70% )
- severe hyperparathyroidism
- may present with a hypercalcemic crisis, also known as parathyrotoxicosis
- metastatic adenopathy may be seen in 15-30% at presentation
Pathology
A histopathological diagnosis of parathyroid carcinoma can be difficult, as like other endocrine malignancies, there is a marked overlap of the appearances of benign and cancerous lesions .
The lung and liver are the most common sites for metastases with rarely described sites including bone, pleura, pericardium, and pancreas .
Radiographic features
In general, a combination of ultrasound and radioisotope studies are employed to diagnose parathyroid carcinoma in most cases . The main role of CT and MRI is to assess for metastatic disease and recurrence .
Ultrasound
A study of 69 cases of parathyroid lesions >15 mm in size, of which eight were carcinomas, and the rest adenomas, examined the useful differentiating sonographic features . It was found that the presence of invasiveness or calcification of the lesion had a 100% positive predictive value (PPV) for carcinoma. Conversely, those features with a high negative predictive value (NPV) were:
- absence of suspicious vascularity (NPV 98%)
- radial vascularity without clear feeding vessels
- absent thick capsule (NPV 97%)
- homogeneity of contents (NPV 100%)
The presence/absence of necrotic/cystic change is sufficiently common in benign disease that it only has a limited role as a discriminator .
Features favoring parathyroid carcinoma:
- heterogeneity of contents
- lobulated morphology
- larger: often >3 cm
Features favoring parathyroid adenoma:
- homogeneity of contents
- regular morphology
- smaller
Nuclear medicine
Technetium-99m sestamibi scintigraphy is very helpful in the localization of parathyroid adenomas (sensitivity up to 98%) but struggles to differentiate carcinomas and adenomas .
Treatment and prognosis
Surgical resection is the mainstay of therapy. Chemotherapy has been found to be generally ineffective. Most parathyroid carcinomas are radioresistant.
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