Thymic Well Differentiated Neuroendocrine Carcinoma (Carcinoid and Atypical Carcinoid)
Definition
- Carcinoma of the thymus exhibiting uniform cytologic features and neuroendocrine differentiation
Alternate/Historical Names
- Prior to their description in 1972, most were considered variants of thymoma or types of adenoma
- Atypical carcinoid is included in this category
Diagnostic Criteria
- Most cells show evidence of neuroendocrine differentiation
- Synaptophysin and/or chromogranin stains positive in most cases
- Argyrophil stain positive in nearly all cases
- Argentaffin negative
- Neuron specific enolase, PGP9.5 and CD56 are sensitive but not specific
- Electronmicroscopy reveals neurosecretory granules
- Most well differentiated neoplasms exhibit characteristic cytologic and architectural features
- Round regular nuclei
- Stippled (salt and pepper) chromatin
- Inconspicuous nucleoli
- Usually moderate granular eosinophilic cytoplasm
- Various growth patterns
- Insular growth pattern
- Round nests of cells
- Rosettes and gland-like structures
- No clear mucinous gland differentiation
- Trabecular
- Rows and strands of cells
- Other occasional to rare patterns
- Spindle cell
- Oncocytic
- Pigmented (melanin or lipofuscin)
- Angiectatic
- With amyloid stroma (calcitonin positive carcinoid)
- With mucinous stroma (acidic, stromal mucin)
- Sarcomatoid dedifferentiation has been reported in rare cases
- Sustentacular cells may be seen in occasional cases
- Punctate necrosis
- Sharply defined foci of coagulative tumor necrosis
- Mitotic figures frequently are numerous
- 2-20/hpf in most cases
- Nearly all cases would qualify as atypical carcinoid in the lung
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342
