CML-Blast phase in the Form of Myeloid Sarcoma with Megakaryoblastic Differentiation

Lessons From the Friday Unknowns

The patient has a history of chronic myeloid leukemia (CML) status post multiple systemic therapies and stem cell transplant. PET-CT showed hypermetabolic adenopathies in the right neck

Histologic sections show fragments of tissue extensively involved by a malignant neoplasm in a diffuse pattern.

The neoplasm is composed of sheets of immature cells with open chromatin, small distinct nucleoli, and amphophilic cytoplasm.

Scattered eosinophils and megakaryocytes are identified.

Scattered mitotic figures and apoptotic bodies are present. No areas of necrosis are noted.

Immunohistochemical stains show that the neoplastic cells are positive for CD5 (diffuse and strong), CD33, CD34, CD43, CD61 (partial), CD117 (partial), and MPO (dim/partial) and are negative for CD3, CD20, lysozyme, and TDT. The proliferation index measured by Ki-67 is estimated approximately 50–60%.

The concurrent flow cytometry analysis detects an aberrant blast population (23% of total events), positive for CD5, CD33 partial, CD34, CD38 decreased, CD45, CD56 dim/partial, CD71 partial, CD117 partial, HLA-DR subset, TdT small subset, and negative for CD2, sCD3, cytoCD3, CD4, CD7, CD13, CD14, CD15, CD19, CD22, CD25, CD41, CD64, CD123, and MPO.

Overall, the morphologic and immunophenotypic findings support a diagnosis of myeloid sarcoma with megakaryocytic differentiation. In the context of patient’s history of CML, the myeloid sarcoma represents blastic phase of CML.

Links to digital slides: https://bit.ly/3s9RDDj | Slides labeled case 3