What diagnostic testing is recommended for a patient with secondary skin involvement of lymphoma?

Diagnostic Testing for Secondary Skin Involvement of Lymphoma

When lymphoma presents with secondary skin involvement, perform an adequate excisional skin biopsy (preferably 4mm or larger) with comprehensive histology, immunohistochemistry (including CD3, CD20, CD79a, surface/cytoplasmic immunoglobulins, Bcl-2, Bcl-6, CD10, CD5, cyclin D1, and Ki-67), and molecular studies (T-cell receptor or immunoglobulin gene rearrangements) to distinguish secondary cutaneous involvement from primary cutaneous lymphoma and guide systemic staging. 1

Tissue Diagnosis Requirements

Biopsy Technique and Size

• Obtain excisional biopsy whenever possible rather than punch biopsy; if punch biopsy is necessary, use at least 4mm diameter to ensure adequate tissue architecture 1
• Multiple biopsies from different sites may be required if initial sampling is non-diagnostic 1
• Include both dermis and subcutaneous fat tissue in the specimen, as lymphoma cells may preferentially involve deeper layers 2, 3

Essential Immunohistochemical Panel

The following markers are critical to distinguish secondary systemic lymphoma from primary cutaneous lymphoma:

For B-cell lineage confirmation and characterization:

• CD20 and/or CD79a to confirm B-cell origin 1
• Surface and cytoplasmic immunoglobulins (sIg/cIg) to detect monotypic expression 1
• Bcl-2, Bcl-6, and CD10: Strong expression of all three in follicular structures strongly suggests systemic follicular lymphoma with secondary skin involvement rather than primary cutaneous disease 1
• CD5 and cyclin D1 to differentiate mantle cell lymphoma (CD5+, cyclin D1+) from primary cutaneous marginal zone lymphoma (CD5-, cyclin D1-) 1

For T-cell lineage:

• CD3 for T-cell confirmation 1
• CD4 and CD8 for subset identification 1

Additional markers:

• Ki-67 proliferative index to distinguish neoplastic from reactive follicles 1
• CD21 or CD35 to visualize dendritic networks 1

Molecular Studies

• T-cell receptor (TCR) gene rearrangement analysis for T-cell lymphomas using fresh tissue when possible 1
• Immunoglobulin heavy chain (IgH) gene rearrangement using BIOMED-2 primers for B-cell lymphomas 1
• Test for t(14;18) translocation if Bcl-2, Bcl-6, and CD10 are strongly positive, as this confirms systemic follicular lymphoma 1

Systemic Staging Evaluation

Clinical Assessment

• Complete physical examination documenting all sites of lymphadenopathy and organomegaly 1
• Assessment for B symptoms (fever, night sweats, weight loss >10% body weight in 6 months) 1, 4
• Excisional biopsy of any bulky palpable peripheral lymph nodes (preferred over core or fine needle biopsy) 1

Laboratory Studies

Mandatory blood work:

• Complete blood count with differential to assess for cytopenias or lymphocytosis 1
• Comprehensive metabolic panel including liver and renal function 1
• Lactate dehydrogenase (LDH) as prognostic marker 1
• Flow cytometry of peripheral blood if lymphocytosis is present or Sézary syndrome is suspected 1
• Lymphocyte subsets and CD4/CD8 ratios for T-cell lymphomas 1
• HTLV-I serology for T-cell lymphomas 1
• Serum protein electrophoresis to exclude monoclonal gammopathy in selected B-cell cases 1

Imaging Studies

CT scans of chest, abdomen, and pelvis are mandatory for all patients with secondary skin involvement to document extent of systemic disease 1

Consider PET/CT for more accurate staging, particularly in aggressive lymphoma subtypes 1

Bone Marrow Evaluation

• Bone marrow aspirate and trephine biopsy are indicated for cutaneous lymphomas with intermediate or aggressive clinical behavior 1
• Required for all T-cell lymphoma variants (except lymphomatoid papulosis) and stage IIB or higher mycosis fungoides 1
• Consider for B-cell lymphomas based on other staging findings 1

Critical Diagnostic Pitfalls

Distinguishing Primary vs. Secondary Cutaneous Lymphoma

The immunophenotype pattern is decisive:

• Strong Bcl-2, Bcl-6, and CD10 expression in follicular structures = systemic follicular lymphoma until proven otherwise 1
• CD5+ cyclin D1+ = mantle cell lymphoma (systemic disease) 1
• CD5+ CD23+ with dim CD20 = chronic lymphocytic leukemia/small lymphocytic lymphoma (systemic disease) 1

Avoiding Misdiagnosis

• Do not rely on morphology alone—immunohistochemistry is mandatory to distinguish reactive from neoplastic processes 1
• BCL2 positivity in T cells does not indicate malignancy; it is commonly expressed in reactive T cells 5
• Flow cytometry alone cannot substitute for tissue immunohistochemistry due to loss of architectural information 1
• Negative initial biopsy does not exclude lymphoma—repeat biopsies from multiple sites if clinical suspicion remains high 1

Special Consideration: Intravascular Lymphoma

If systemic symptoms (fever, weight loss, neurological deterioration) are present without obvious lymphadenopathy:

• Perform random skin biopsies from at least 3 sites (both upper thighs and abdomen) even from normal-appearing skin 2, 3, 6
• Include subcutaneous fat tissue where intravascular lymphoma cells preferentially accumulate 2, 3
• This technique has 13-21% diagnostic yield in fever of unknown origin cases ultimately diagnosed as lymphoma 6