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General Round Cell Tumors of Skin and Soft Tissue

The round cell tumors of skin are 4 types in all species except the dog which has a unique tumor that is the Canine Transmissable Venereal Tumor (CTVT).

The round cell tumors are

  1. Histiocytic tumors
  2. Lymphomas
  3. Mast cell tumors
  4. Plasma cell tumors

Histiocytic tumors

Skin tumors with a histiocytic phenotype are very rare, except in dogs. There are many different types, but across all species, histiocytic sarcoma is the most consistent diagnosis. I deal with the various histiocytic tumors in the sections on Round cell tumors of each species.

Histiocytic tumors are phenotypic histiocytic dendritic cells. Immunohistochemistry is required for confirmation and interpretation is required - and some experience helps to make sure the staining cells are not resident or intratumoral histiocytic/dendritic cells. CD18 is a common stain but each is species specific for cells of bone marrow origin. CD204 and Iba-1 are used too. They are less species specific.

Lymphomas

Lymphoma classification

If you want a career, you could begin by trying to understand lymphoma classification! Lymphomas are officially divided into a zazillion different types. Here is a brief synopsis - begin with 1 and move through.

  1. Hodgkins vs Nonhodgkins-like lymphoma. NonHodgkins lymphoma is extremely rare so this step is quick! Cutaneous lymphomas are nonHodgkins like lymphomas
  2. Anatomical Site. Cutaneous lymphomas are peripheral (outside the bone marrow) and extranodal. Subcutaneous tumors are very rare and the panniculitis like lymphoma is an example.
  3. Cell arrangement
    1. nodular
    2. diffuse - most cutaneous lymphomas are diffuse.
    3. epitheliotropic or nonepitheliotropic
    4. angiotrophic (intravascular) or angiocentric (perivascular).
  4. Immunophenotype.
    1. T cell. Many cutaneous lymphomas are pure T cell lymphomas. There are also B cell rich T cell lymphomas.
    2. B cell. Some are T cell rich B cell lymphomas
    3. nonT non B,
    4. NK cell,
    5. Large Granular Lymphoma LGL.
  5. Size of cell
    1. Small (1-1.5x the size of a red cell)
    2. Intermediate 1.5-2
    3. Large 2+
  6. Nuclear features.
    1. anaplastic
    2. lymphoblastic
    3. plasmablastic
    4. plasmacytoid
    5. Grade (pHPF verified in 3)
      1. Indolent 0
      2. Low 1-5
      3. Intermediate 6-10
      4. High  >10
  7. Syndromes/common manifestations
    1. panniculitis like lymphoma

Many cutaneous lymphomas are peripheral, epitheliotropic, small cell, low grade, T cell lymphomas.

Lymphoma diagnosis

There are five basic steps in the diagnosis of the lymphomas. Not all steps are required for each and every case. The more unusual, rare or complicated a case may be, the more steps are required. In general, differentiating inflammatory or reactive lymphocytic disease from lymphoma requires all five steps that all support lymphoma.

  1. Signalment and clinical history.
  2. Histopathology.
  3. Immunohistochemistry.
  4. PARR (Polymerase chain reaction antigen receptor rearrangement)
  5. Clinical outcome.

Mast cell tumors

There are mast cell tumors reported in virtually every species. They are most common in the skin and subcutis of dogs.

The general classification of mast cell tumors is

  1. Solitary mast cell tumors (these could be one or more tumors in one animal)
    1. dermal - these begin in the dermis and may extend into the subcutis
    2. subcutaneous - these are only in the subcutis
  2. Cutaneous Mastocytosis - extensive involvement of skin
    1. diffuse cutaneous mastocytosis
    2. Maculopapular mastocytosis (also known as urticaria pigmentosa)

The prognosis for mast cell tumors varies with the type and species.

Plasma cell tumors

Plasma cell tumors - those with a plasma cell phenotype are classified as either

Multiple myleoma

is diagnosed in humans if there is

  1. Clonal bone marrow plasma cells >10% or biopsy proven plasmacytoma, and evidence of end organ damage that can be attributed to the underlying plasma cell proliferative disorder, specifically
    1. Hypercalcemia: Serum calcium > 11.5 mg/dL or
    2. Renal insufficiency: Serum creatinine > 173 micromoles/l (or >2mg/dL) or estimated creatinine clearance less than 40ml/minute
    3. Anemia: Normochromic, normocytic with a hemoglobin value of >2 g/dL below the lower limit of normal or a hemoglobin value <10 g/dL
    4. Bone lesions: Lytic lesions, severe osteopenia or pathologic fractures
  2. Clonal bone marrow plasma cells >60% - This is a plasma cell tumor with bone marrow clonal plasma cells

In veterinary medicine, multiple myeloma is the diagnosis when there is a marrow plasma cell proliferation, multiple plasma cell tumors expecially of bone, monoclonal gamopathy and Bence Jones proteinuria.

It is very rare for multiple myeloma to affect the skin.

Plasma cell leukemia

Extramedullary plasma cell tumor

    Plasmacytosis

    This is the diagnosis if there is extensive plasm cell neoplasia without systemic disorder.

    Solitary plasmacytoma

    Neoplasms with plasma cell morphology or diverse type and monoclonal immunoglobulin product that form a localised mass or swelling

    This is the most common form in animals