2. Anatomy of thymus
• Thymus is a lymphoepithelial organ located in the
superior mediastinum.
• It is fully formed and functional at birth and slowly
undergoes involution after puberty but never
disappears completely.
• Connective tissue surrounds thymus and subdivides
into lobules.
• The thymic parenchyma contains developing T cells
in an extensive meshwork formed by
epithelioreticular cells.
3.
4. Thymic or Hassalls corpuscles are ( derived from type VI
epithelioreticular cells ) a distinguishing features of thymic medulla
5.
6. Pathology
Primary immunodeficiency
– Thymic dysplasia
• Small size < 5g
• Primitive appearing epithelium
• Tubules and rosettes
• Absence of Hassall corpuscles
• Total absence of lymphocytes.
– X linked severe combined immunodeficiency
– Ataxia telagiectasia
– Nezelof’s syndrome
– Incomplete form of DiGeorge syndrome
• D/D – acute thymic involution
7. Cysts
• Unilocular thymic cysts
– Developmental origin
• Multilocular thymic cyst
– Acquired process of reactive nature
– Inflammation and fibrosis
– Acquired cystic dilatation of medullary duct epithelial
derived structures.
– Nodular sclerosis Hodgkin’s lymphoma and seminoma
• Lymphocyte induced cystic ductular dilatation
11. Thymoma
General features
• Neoplasm of thymic epithelial cells exhibiting
organotypic features, independent of presence or
number of lymphocytes
• Organotypic features
– Lobulation
– Medullary differentiation
– Perivascular spaces
– Immature T lymphocytes
12. • Nearly all cases occur in adults but can occur in
young children
• Usual location is anterosuperior mediastinum, can
occur in other mediastinum but rare in posterior
mediastinum
• CT, MRI or simple X ray and Fine needle aspiration
can be used as a preoperative diagnosis
15. Myasthenia gravis
– More likely to have thymoma if male and/or
developing symptoms after 50 years of age
– Almost always have epithelial cells of stellate or
cuboidal rather than spindle type.
– Follicular hyperplasia is more intimately
associated than thymoma
– Thymectomy is always done irrespective of
presence or absence of thymoma
16. • Pure red cell aplasia
– Most of the thymoma associated with this have
epithelial cells of spindle cell type
– Autoantibodies to red cell precursors or excessive CD8
cells which release various inhibitory substances for
erythropoisis.
• Hypogammaglobulinemia
– Often with spindle cell variant
– Despite lymphocytosis and substantial persisting T cell
function there is immunodeficiency
17. Clinical features
• 5th and 6th decades
• Slight female preponderance
• 1/3 – asymptomatic incidental finding
• 1/3 – mediastinal mass
• 1/3 – paraneoplastic syndromes
37. Thymic carcinoma – Type C thymoma
• It is an epithelial neoplasm of thymus showing overt
cytological features of malignancy and lacking organotypic
features
General features
• 15% of epithelial tumors
• Most occur in adults (mean age 50yrs) with slight male
preponderance
• Asymptomatic mediastinal masses in some patients
• Most are Aggressive tumors, frankly invasive and often
unresectable and metastasize to lymph nodes, bone, lung
and liver
45. • TNM staging – is a misleading one
Type of thymoma + following descriptive equalent
• encapsulated
• minimally invasive
• widely invasive
• metastatic
Microscopic subtype and likelihood of invasion
A<AB<B1<B2<B3<C
47. Treatment
• Surgery and post operative radiotherapy for
invasive tumors other than type A and AB
Prognosis
• Stage - regardless of the systems used
• Microscopic type
• Completeness of excision
• Myasthenia gravis – no relation
• Proliferation index and DNA ploidy – Not an
independent factor
48. Cervical tumors of thymic and related
branchial pouch derivation
• Ectopic cervical thymoma
• Ectopic hamartomatous thymoma
• Spindle epithelial tumor with thymus like
element (SETTLE)
• carcinoma with thymus like element
(CASTLE)
49.
50. Neuroendocrine Tumors
• Carcinoid Tumors
– Malignant neoplasm, aggressive than bronchial carcinoid
– Lacks endocrine manifestation but associated with
Cushing’s in 1/3 of cases
– Associated with carcinoid tumors of other sites, MEN I
and IIa and Neurofibromatosis I
• Small cell neuroendocrine carcinoma
• Large cell neuroendocrine carcinoma
51.
52.
53. Germ cell tumors
• 20% of mediastinal tumors and cysts
• Significantly related to thymus gland and when small they are totally
encased within this organ
• No association with testicular intrtubular germ cell neoplasia but
associated with Klinefelter’s disease
• More incidence in males except in mature cystic teratoma – affects
equally
• Presence of i(12p)
• Nonseminomatous germ cell tumors of mediastinum do not respond
well to chemotherapy
• Occasionally they may develop as somatic type malignant tumor
• A Rare but well documented event is hematological neoplasia
54. Seminoma
• Almost always in thymus
gland
• Anaplastic seminoma
• Presence of fibrous septa
infiltrated by plasma cells,
lymphocytes, granulomas,
germinal centers, large
amount of cytoplasmic
glycogen and irregular skein
like nucleolus
• Immunoreactive for
placental alkaline phosphtase,
CD117 but negative for CD45
and focally positive for keratin
55.
56. Mature cystic teratoma
• Most common type of
mediastinal germ cell
neoplasia
• Grow in large size in a
well delineated wall and
predominantly cystic
• Sebaceous material can
be expressed and also
xanthogranumatous
inflammation can be seen
• Cysts are lined by
stratified squamous
epithelium and contain
sebaceous glands and hair
follicles
• Prognosis is excellent
57. • Immature teratoma
• Embryonal carcinoma
– Invasive highly necrotic neoplasm, by definition poorly
differentiated
• Yolk sac tumors
– Yolk sac elements are more common in mediastinum
– May have a prominent spindle cell features, hepatoid component
or cystic changes
– Prognosis is poor
• Teratocarcinoma
• Choriocarcinoma
– In 3rd decade, often with gynecomastia and elevated hCG
– Prognosis is extremely poor
58.
59. Malignant Lymphoma
• Hodgkin’s lymphoma
– It can primarily involve thymus
– Most are young adults, often females, and nearly always
nodular sclerosis type
– May present with local pressure symptoms
– Immunohistochemical stains for CD15, CD30 and keratin
sharply delineate lymphoid neoplastic tissue and
reactive epithelial component.
– Syncytial type may simulate NHL, germ cell tumor and
carcinoma, peripheral fibrosis may confuse with
sclerosing mediastinitis and germinal centers in
lymphoid follicle may simulate castleman’s disease.
60.
61. Lymphoblastic lymphoma
– It has a predilection for thymic origin and usually
of immature T cell type
– Typically presents with acute respiratory distress
in an adolescents, often in males
– Usually restricted to supradiaphragmatic region
62.
63. Large cell lymphoma
– Most are young adult females and usually
presents with superior vena caval syndrome
– Represents a distinct type of lymphoma, possibly
arising from a subset of intrathymic lymphoid cells
(asteroid B cells)
– CD45, CD30 (70%), bcl-6 and CD10 are expressed
– Excellent response to radiation and
chemotherapy
64.
65. Marginal zone B cell lymphoma
– Increasing incidences in thymus in Asians
– Marked predilection for females and associated
with Sjogren’s syndrome and rheumatoid arthritis
– Presence of typical lymphoid hyperplasia of sheets
of centrocyte like B cells disrupting the cytokeratin
positive medullary epithelial cells should suggest
the possibility of lymphoma
– Express IgA but lack API2-MALT1 gene fusion
66.
67. Stromal and other tumors
• Thymolipoma
– Encapsulated huge benign thymic lesion
– Associated with autoimmune disease in some
cases
– It is an admixture of variable mature adipose
tissue and unremarkable thymic tissue
• Thymic stromal sarcoma
