De Novo Mast Cell Leukemia Without CD25 Expression and KIT Mutations: a Rare Case Report in a 13-year-old Child

Background: Mast cell leukemia (MCL) is a very rare form of systemic mastocytosis (SM) and accounts for less than 0.5% of all mastocytosis. The diagnosis of MCL requires the presence of SM criteria, accompanied by leukemic infiltrating of atypical mast cells (MCs) in bone marrow (BM), peripheral blood as well as extracutaneous organs. MCL is a fatal disease that almost always behaves aggressively, and the median survival time is only about six months. Herein, we present a rare case of de novo MCL without CD25 expression and KIT mutations.

Case Presentation: A previously healthy 13-year-old boy was referred to our hospital due to incidental discovery of an enlarged right tonsil. Diffuse infiltration of medium-sized hematopoietic blasts was found in his right tonsil, BM and multiple lymph nodes. The neoplastic cell population was subsequently revealed to exhibit differentiation towards the mast cell lineage by expressing CD117 and tryptase, but the cell population lacked expression of CD25/CD2 and the activating mutation of the KIT gene. An abnormal karyotype was identified, but no leukemia-associated fusion genes were found. Involvement of peripheral blood, bone and lung was subsequently demonstrated. The most important differential diagnosis included tryptase-positive (T+) acute myeloid leukemia, myelomastocytic leukemia and basophilic leukemia. The morphological characteristics and infiltrating patterns of the abnormal MCs supported the final diagnosis of MCL. Although intensive chemotherapy and allogeneic stem cell transplants were performed on the patient, he died 18 months after initial presentation.

Conclusion: Due to its rarity, the diagnosis of MCL without typical immunophenotype and genetic aberrations is particularly challenging. Comprehensive investigation of clinical and pathological features to exclude other T+ myeloid neoplasms is necessary.