The pathological distinction between "deep penetrating" dermatofibroma and dermatofibrosarcoma protuberans.

In selected cases, the clinicopathological distinction between deep penetrating dermatofibroma (DPDF), which involves the subcutis, and dermatofibrosarcoma protuberans (DFSPs) may be challenging. In most instances, attention to the cytological constituency of the lesions and the overall architecture is sufficient to make this separation. DPDF is typified by cellular heterogeneity, including giant cells and lipidized histiocytes; when it extends into the hypodermis, it does so either using the interlobular subcuticular fibrous septa as scaffolds or in the form of broad pushing fronts of tumor. In contrast, DFSP is a cytologically monotypical tumor, which entraps subcutaneous adipocytes diffusely or grows in stratified horizontal plates in the hypodermis. In the minority of cases where conventional morphological analysis of optimal biopsy specimens is diagnostically indeterminate, immunostaining for CD34 and factor XIIIa (FXIIIa) is helpful; it is also often necessary when a poorly-representative sample of the lesion has been obtained by the clinician. Characteristically, DF is diffusely FXIIIa-reactive and CD34-negative, whereas DFSP manifests the converse of those findings. Other markers such as Ki-M1p, mutant p53 protein, and metallothionein may also provide adjuvant diagnostic information in this context, as may the observation of abnormalities in chromosomes 17 and 22 by direct karyotypic analysis.