Cutaneous Mastocytosis: Histopathologic Spectrum and Comparison of Mast Cell Counts by Manual versus Digital Methods

Abstract Background: Cutaneous mastocytosis (CM) is classified into maculopapular cutaneous mastocytosis (MPCM), solitary mastocytoma (SM), and diffuse mastocytosis (DM). Our aim is to highlight the histopathologic spectrum of CM and throw light upon the utility of manual versus digital mast cell (MC) counts. Materials and Methods: All histologically proven cases of CM over the last 13 years were reviewed. CD117 immunohistochemistry was performed for all MPCM. The number of MC/mm 2 was counted manually on toluidine blue (TB) and CD117 slides and digitally on only the CD117 slides. Results: Out of 34 cases, 24 (70.6%) were MPCM, 7 (20.6%) were SM, and 3 (8.8%) were DM. Histopathologically, epidermal changes, such as atrophy and acanthosis were seen. Subepidermal blisters were seen in 3 cases. Hematoxylin and Eosin and TB highlighted a deep, diffuse pattern of infiltration by MCs in the majority of SM and DM cases. Perivascular infiltration was prominent in MPCM (41.7%). Over 71% of the cases showed normal morphology, spindled MCs were also noted. TB showed MC count range from 8 to 2980 with a mean of 730. Manual counts on CD117 ranged from 23 to 6899, with a mean of 2190, and digital counts ranged from 81 to 8563, with a mean of 2575. Conclusion: Most of the histopathologic features are consistent with literature, but there is a wide variation in MC counts, with a third of MPCM having lower counts than recommended. As there is no standardized cut-off, one should be aware of these variations, particularly in unsuspected cases. Digital imaging and counting further enhance the MC counts.