Primary Mediastinal (Thymic) Large B-cell Lymphoma (PMBL)

Epidemiology and Etiology:

• 2-4% of NHL
• Young adults (median age ~35 y)
• Female:male 2:1

Common sites:

• Thymus
• Bulky mass often
• Invasion of adjacent structures such as lungs, pleura, or pericardium
• Supraclavicular and cervical lymph node spread maybe
• ABSENCE of BM involvement or other lymph node involvement (otherwise it is considered systemic DLBCL with secondary mediastinal involvement)
• At progression, dissemination to distand extranodal sites including kidney, adrenal, liver, or central nervous system is relatively common

Gross features:

Histologic features:

• Compartmentalizing alveolar fibrosis common
• Medium to large sized cells
• Abundant pale cytoplasm
• Regular round or ovoid nuclei
• Pleomorphic and/or multilobated in some cases, may resemble Hodgkin cells

Immunophenotype:

Marker:	Sensitivity:	Specificity:
CD19,20,22,79a
Ig (neg)	As a rule
CD30	> 80% (weak, heterogeneous)
CD15	Occasionally
IRF4/MUM1	75%
CD23	70%
Bcl-2	55-80%
Bcl-6	45-100%
CD10	8-32%
MAL antigen
CD54
CD95
TRAF and nuclear REL coexpression
HLA class I and/or class II molecules – variable defects in expression	Most
Ki-67 > 60%	?usually

Molecular features:

• Rearrangements of BCL2, BCL6, and MYC are absent or rare

Other features:

• Outcomes equivalent or superior to DLBCL

References:

• Swerdlow et al. (eds.) WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues, 4^th ed. (2008)