Cutaneous mastocytosis with atypical mast cells and giant cytoplasmic granules

A 12-year-old male presented with an 8-year history of five firm cream colored papules on the right vertex of the scalp. A biopsy showed a dense infiltrate of monomorphous mast cells involving the dermis and extending into the subcutis. A relatively well-circumscribed cluster of larger cells showed pleomorphic nuclei with bilobed and multilobed morphology. Both mast cell populations had an eosinophilic cytoplasm filled with granules ranging in size from small to giant forms. By immunohistochemistry, the cells expressed CD117, tryptase and CD68, and were negative for AE1/AE3, CD1a, CD2 and CD25. S-100 staining revealed only faint cytoplasmic positivity and myeloperoxidase had an inhomogeneous patchy pattern, with an overall staining of less than 5% of the cells. A diagnosis of cutaneous mastocytosis was made and after 6 months follow-up, no progression observed. Clinical correlation and awareness of these unusual morphologic features as being part of the spectrum of cutaneous mastocytosis are important to avoid an erroneous diagnosis of malignancy. Although pleomorphic, multilobed nuclear morphology and giant cytoplasmic granules have not been associated with an aggressive behavior or systemic mastocytosis, close clinical observation is warranted in this context.