Abstract

Cutaneous mastocytoma, a benign localized neoplasm of mast cells mostly occurring in infants, is often a straightforward histologic diagnosis. Typically composed of a dense mass of mature mast cells within the dermis, scattered eosinophils are not an uncommon finding and, in fact, may be a helpful clue. A massive infiltration of eosinophils, however, is a very rare finding and an extensive literature search identified just six isolated case reports over several decades. These cases do not appear to be associated with different clinical findings. We report a single case occurring in a 10-month-old child presenting with a hyperpigmented plaque on the lower back, increasing in size since birth, and with intermittent changes including erythema and induration. On histology, the punch biopsy showed a superficial-to-deep sheet-like infiltrate of relatively monotonous round-to-ovoid histiocytoid cells with abundant, subtly granular pale cytoplasm. Additionally, there was a heavy admixture of eosinophils (up to approximately 800/mm²) most prominent superficially with gradual tapering with increasing depth. A tryptase immunostain confirmed mast cell lineage while Giemsa highlighted the abundant intracytoplasmic metachromatic granules. Stains for S100, CD1a, and langerin were negative in the cells of interest. Given the abundant eosinophils and age of the patient, an important consideration was Langerhans cell histiocytosis. Other plausible considerations based on the histopathologic findings, however, could include arthropod bite reaction, eosinophilic cellulitis (Wells syndrome), and drug eruption. While rare, it is important to recognize dense tissue eosinophilia as a possible presentation of mastocytosis.