Mast cell sarcoma (MCS) is an extremely rare variant of systemic mastocytosis characterized by uncontrolled and progressive proliferation of mast cells into a solid tumor of a malignant nature.

Case Report:
 A 22 year-old female presented with back pain and bilateral lower extremity weakness. Physical examination revealed loss of motor function in lower extremities and bladder dysfunction. Laboratory examination showed WBC 16.2, Hemoglobin 12.7gm/dL, PLT 410,00. Magnetic resonance imaging (MRI) of the spine revealed an extradural mass at T2 and T3 levels. Bone scan showed multiple lytic lesions. Patient underwent resection of the tumor and laminectomy with decompression of the spinal cord. Initial evaluation of the biopsy of the spinal cord mass revealed diffuse proliferation of large, dyscohesive cells with pleomorphic nuclei and giant multinucleated tumor cells with frequent abnormal mitoses. The immunohistochemical profile of the tumor cells was inconclusive; positive stains included c-kit (CD117), vimentin, and HHF-35, and negative stains included LCA (CD45), CD3, L26 (CD20), BerH2 (CD30), CD34, factor VIII, KP1 (CD68), cytokeratin, S100, HMB45, myogenin, myoD1, desmin, HCG, and PLAP. Giemsa stain was negative. Electron microscopy was done. The preliminary diagnosis was “high grade, undifferentiated, malignant neoplasm”of soft tissue versus hematopoeitic origin. Additional staining at an outside facility found the tumor cells to express tryptase, LCA, CD31, CD34, and KP1. A final diagnosis of “mast cell sarcoma” was made. Bone marrow sampling revealed numerous immature and dyspoietic mast cells. The granulated forms were positive in specific esterase stain and metachromatic in toluidine blue stain. The mast cells expressed LCA, KP1, and c-kit in immunohistochemical stains done on the core biopsy and CD15, CD33 and c-kit in flow cytometric immunoanalysis done on the marrow aspirate. Overall findings were consistent with bone marrow involvement by mast cell sarcoma, “mast cell sarcoma, leukemic phase”. Further analysis of the type of c-kit mutation, in particular D816V, has been requested.

Patient was started on imatinib mesylate 400 mg orally once daily, but showed progression of the tumor at T3 and new multiple metastatic lesions throughout the cervical and thoracic spine within ten days. Neurosurgical decompression was done a second time followed by radiosurgery at the time of abstract submission.

 This case was not only unusual and dramatic in clinical presentation, but proved to be extremely challenging for both diagnosis and management. Positive staining for LCA, tryptase, CD68 in combination with c-kit was crucial for diagnosis. There was progression of disease despite initiating treatment with tyrosine kinase inhibitor imatinib impliying either primary resistance or higher dose requirement. Analysis of the type of c-kit mutation, in particular, D816V (pending) may give us some insight in this regard. Historically, mast cell sarcoma has been reported as chemotherapy and radiation therapy resistant, and therefore it has an extremely poor prognosis. Treatment options include newer tyrosine kinase inhibitors such as dasatinib or nilotinib. Aggressive chemotherapy with AML-like regimens has been reported as successful in patients with mastocytosis and an associated hematological disorder, but not in patients with mast cell leukemia.

Disclosure: No relevant conflicts of interest to declare.

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