Surgical Pathology Criteria

Thymic Carcinoma

Differential Diagnosis

Metastatic Carcinoma must be ruled out before the diagnosis of primary thymic carcinoma is made

 

Metastatic Small Cell Neuroendocrine Carcinoma of Lung Thymic Basaloid Carcinoma
History and imaging may reveal primary site History and imaging should reveal no other primary site
TTF1and synaptophysin positive in >90% of lung neuroendocrine carcinomas Synaptophysin and TTF1 negative
p63 negative p63 positive

 

Thymic Well Differentiated Neuroendocrine Carcinoma Thymic Basaloid Carcinoma
Most cases have a moderate amount of cytoplasm Scant cytoplasm
Inconspicuous nucleoli Variable, may have large nucleoli
Synaptophysin positive Synaptophysin negative
p63 negative p63 positive
Both have peripheral palisading and necrosis

 

Thymoma (general features) Thymic Carcinoma
Cytologically bland, moderate atypia at most Typically cytologically malignant
Lacks desmoplastic response to invasion Aggressive, destructive invasion with desmoplastic response
Lobular growth pattern Irregular growth pattern
Thick fibrous capsule and septa Lacks prominent capsule and septa
Immature T cells present Mature T and B cells may be present, but lacks immature T cells
Epithelial cells CD5 (except for B3) and CD117 <5% Epithelial cells CD5 and CD117 >70%
Micronodular thymoma with lymphoid stroma may lack a capsule while basaloid carcinoma may form a capsule

Special types of thymoma:

Type A Thymoma Thymic Spindle Cell Carcinoma
Mild cytologic atypia at worst Cytologically atypical
Epithelial cell mitotic figures rare Epithelial cell mitotic figures may be frequent
Some immature T cells usually present Lymphocytes are mature T and B cells
Usually well circumscribed, with only microscopic invasion Usually clearly invasive

 

Type B2 Thymoma Thymic Lymphoepithelial Carcinoma
Mild cytologic atypia at worst Cytologically atypical
Epithelial cell mitotic figures rare Epithelial cell mitotic figures may be frequent
Lymphocytes are immature T cells Lymphocytes are mature T and B cells

 

Type B3 Thymoma Thymic Low Grade Squamous Carcinoma
Cytologic atypia usually mild, moderate at worst Cytologically atypical
Epithelial cell mitotic figures usually <2/10hpf Epithelial cell mitotic figures may be frequent
Lymphocytes few, but with immature T phenotype Lymphocytes are mature T and B cells
Overt squamous differentiation usually scant and focal Overtly squamous, with intercellular bridges usually identifiable
Lobular growth pattern Infiltrative growth pattern
Frequent perivascular spaces surrounded by palisaded epithelial cells Lacks perivascular spaces and palisading
Epithelial cells CD117 negative to rare Epithelial cells CD117 65-80% positive
Epithelial cells CD5 negative to at most focally positive Epithelial cells CD5 70% positive

 

Metaplastic Thymoma Thymic Carcinoma with Sarcomatoid Differentiation
Cytologically bland Usually cytologically atypical
Mitotic figures rare Mitotic figures may be numerous
Metaplastic thymoma may have scattered bizarre nuclei but the general epithelial population is bland

 

Micronodular Thymoma with Lymphoid Stroma Metastatic Carcinoma in a Lymph Node
Epithelial nodules usually scattered throughout Carcinoma usually subcapsular, sinusoidal or focal
Cytologically very bland Usually cytologically atypical
Mitotic figures very rare Mitotic figures may be numerous
Lacks a subcapsular sinus Subcapsular sinus present
Presence of adjacent nonneoplastic thymus supports a thymic neoplasm Typically no adjacent thymus
Immature T cells may be present Immature T cells absent
Rare micronodular thymic carcinomas have been reported; these show all features of micronodular thymoma but exhibit atypia and mitotic figures

 

Mediastinal (thymic) Germ Cell Neoplasms are separable morphologically and immunologically

Mediastinal (Thymic) Large B Cell Lymphoma is easily separated immunologically

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