Precursor hematodermic neoplasm (also known as CD4+/CD56+ hematodermic neoplasm and previously as blastic NK-cell lymphoma) is a rare, usually fatal neoplasm, the understanding of which is in evolution. Precursor hematodermic neoplasm (PHDN) was previously believed to be a natural killer lymphoma due to CD56-positivity, but an increasing body of data implicates a plasmacytoid dentritic cell origin rather than an NK-cell lineage. Given the clinical and histologic presentation, the high propensity to develop a leukemic phase, as well as a treatment approach similar to acute myeloid leukemia, we place PHDN under the rubric of leukemia cutis.

Comprising less than 1% of all primary cutaneous lymphomas in the French Study Group on Cutaneous Lymphomas, PHDN is rare. Males are affected with PHDN twice as frequently as females. The median survival of PHDN is 14 months overall, 38 months for patients less than 40 years of age and 10 months for patients over 40 years of age.

Cutaneous involvement in PHDN is nearly universal at initial presentation and at relapse. Extracutaneous involvement is present in approximately 40% of patients at first presentation, but most patients ultimately develop a leukemic phase during the disease course. Skin lesions are typically characterized by deep red to purple papules or nodules that may be few and scattered or innumerable and confluent (Fig. 12-13).

Histopathologic examination of skin biopsies in PHDN may resembles cutaneous involvement by acute myeloid leukemia, with a diffuse dermal and frequently subcutaneous infiltrate composed of medium-sized, monomorphous blast-like cells with scant cytoplasm. These cells do not stain for B-cell,

A FIGURE 12-13 Deep red and purple nodules on the face in precursor hematodermic neoplasm.

T-cell, or myeloid cell markers, but do stain for CD4 and CD56. No clone is identified with T-cell receptor and immunoglobulin heavy chain gene rearrangement studies.

Treatment of PHDH is usually similar to that of acute myeloid leukemia, but early treatment with bone marrow transplantation may improve survival.40

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