Hematologic Neoplasia of Unusual Cell Type

Question 1

Immunophenotype of blastic plasmacytoid dendritic cell leukemia

Answer 1

Blastic plasmacytoid dendritic cell leukemia essentially has the same immunophenotype as normal plasmacytoid dendritic cell precursors (CD4 positive, CD123 positive), but with the additional aberrant expression of CD56, CD43, and variable TdT; EBER is negative in these leukemia cells.

Question 1

Answer 1

Blastic plasmacytoid DC leukemia

Question 2

Answer 2

Mast Cell Sarcoma

Rare, aggressive neoplasm composed of cytologically high-grade mast cells presenting clinically as a discrete mass. Medium-sized to large epithelioid cells, including bizarre multinucleated cells, and does not closely resemble either normal mast cells or the spindle cells of systemic mastocytosis.

As with classical systemic mastocytosis, accompanying eosinophils are often present, tryptase expression is retained, and aberrant expression of CD2, CD25, and CD30 is often seen. KIT expression is usually present and may be strong or variable, but do not rely on this alone for diagnosis - tryptase is really essential.

Some of these neoplasms display the classical KIT exon 11 and exon 9 mutations, which are imatinib sensitive. The imatinib-resistant D816V mutation has not yet been reported in mast cell sarcoma, but can probably happen. Some of these neoplasms display no KIT variants, unlike classical mastocytosis.