Thymic carcinoid tumor refers to a carcinoid tumor arising in the thymus. It is the most common histologic type for a neuroendocrine tumor of the thymus.
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Epidemiology
Affected patients are typically in the fourth or fifth decades of life. There is a recognized male predominance with M:F ratios around 3:1 2,7.
Associations
- non-functioning thymic carcinoids may be seen in association with multiple endocrine neoplasia (MEN) syndrome type 1 2,6
Clinical presentation
The majority of patients present with symptoms related to mass effect or invasion of mediastinal and other thoracic structures 7. Carcinoid syndrome is uncommon 1,4.
Up to 50% of affected patients have hormonal abnormalities 2. Approximately 40% of patients have Cushing syndrome as a result of adrenocorticotropic hormone secretion by the tumor; these patients tend to have smaller lesions at the time of diagnosis since they present early with signs of corticosteroid excess. Up to 30% of patients with have advanced-stage disease at presentation 6.
Pathology
Neuroendocrine tumors of the thymus are rare and believed to arise from thymic cells of neural crest origin (Kulchitsky cells).
Thymic carcinoid tumors are similar to lesions arising within the bronchi, ranges in differentiation and behavior from typical carcinoid to atypical carcinoid to small cell carcinoma 1,3.
Approximately 50% of thymic carcinoids are functionally active 6,7.
They generally tend to be large masses at the presentation that range in size from 6-20 cm.
Radiographic features
Radiologically, these tumors typically manifest as large masses with a propensity for local invasion. Focal areas of necrosis and punctate calcification may be present 2,5.
CT/MRI
On CT or MRI the masses are usually of heterogeneous attenuation or signal intensity, respectively.
Nuclear medicine
Ga-68 Dotatate
Well differentiated thymic carcinoid tumors will be intensely avid on Dotatate PET, indicating the presence of somatostatin receptors14. Poorly differentiated neuroendocrine tumors may have variable/minimal intensity of Dotatate tracer uptake (these tumors are more likely to be FDG avid).
F-18 FDG-PET
The FDG uptake of carcinoid tumors on CT-PET is variable, with a tendency for the uptake to be low.
On cross-sectional imaging, thymic carcinoids may be indistinguishable from a thymomas 6. However, increased uptake has been reported to correlate with aggressive behaviors such as local invasion and distant metastases 2.
In-111–diethylenetriaminepentaacetic acid (DTPA)-D-Phe1]-octreotide
Accumulates in thymic carcinoids but is not specific as it also accumulated in thymomas and metastatic thymic tumors 7. In-111 imaging has been superseded by Ga-68 Dotatate PET-CT in many centers.
Treatment and prognosis
They are generally thought to carry a poor prognosis due to high rates of recurrence and metastases. Follow up after resection (if resectable) if often recommended 10.
History and etymology
They are thought to have been initially described by Rosai and Higa in 1972 as a distinct entity from thymomas 11.
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