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Erythema nodosum is the most frequent clinicopathologic variant of panniculitis. The process is a cutaneous reaction that may be associated with a wide variety of disorders, including infections, sarcoidosis, rheumatologic diseases, inflammatory bowel diseases, medications, autoimmune disorders, pregnancy, and malignancies. Histopathologically, erythema nodosum is the stereotypical example of a mostly septal panniculitis with no vasculitis. The composition of the inflammatory infiltrate in the septa varies with age of the lesion. Treatment of erythema nodosum should be directed to the underlying associated condition, if identified.

Nevus sebaceus of Jadassohn is a hamartoma that combines epidermal, follicular, sebaceous, and apocrine gland abnormalities. Classically, several types of cutaneous neoplasms have been associated with this hamartoma, with basal cell carcinoma being the most frequently described malignancy. We studied a series of 155 examples of nevus sebaceus of Jadassohn with clinicopathologic correlation. Several histopathologic changes related to the age of the patients were found. In our series, we could not identify any cases of authentic basal cell carcinoma. In contrast, several examples of primitive follicular induction and of trichoblastomas were seen. Other cutaneous hamartomas, hyperplasias, and neoplasms found in our series of nevus sebaceus of Jadassohn included syringocystadenoma papilliferum, sebomatricoma, apocrine gland cyst, poroma, different histopathologic variants of warts (classic warts, tricholemmoma, and desmoplastic tricholemmoma), primitive follicular induction, and ductal induction. In our series, no examples of malignant neoplasms were identified. On the basis of these findings, the classically recommended treatment for this hamartoma, which consists of early excision to prevent the development of malignancy, seems to be inappropriate.

Spitz nevus is an infrequent, usually acquired melanocytic nevus composed of epithelioid and/or spindle melanocytes that can occasionally be confused with melanoma. Currently, there are no immunohistochemical markers or molecular biology techniques that can be used to make an entirely safe diagnosis of Spitz nevus or melanoma in problematic cases. A retrospective study has been carried out that included all the cases diagnosed as Spitz nevus from our files. Follow-up information of the patients was unavailable. Three hundred forty-nine cases of unequivocal Spitz nevi were included, and their clinical and histopathological parameters were reviewed. One hundred and forty patients (40%) were 15 years old or younger, with a male to female ratio of 1:1. In patients older than 15 years, there was an evident predominance of women, with a male to female ratio of around 1:3. Spitz nevus was most commonly located on the lower extremities, followed by the trunk in both children and adults. Despite the fact that the head and neck were the third most common location in children, it was a much more frequent location in children than in adults. The constitution by epithelioid and/or spindled cells was the only histopathological finding present in 100% of cases. The other pathological findings studied were, from more to less frequent: maturation (72%), inflammatory infiltrate (70%), epidermal hyperplasia (66%), melanin (50%), telangiectasias (40%), Kamino bodies (34%), desmoplastic stroma (26%), mitosis (23%), pagetoid extension (13%), and hyalinization of the stroma (8%). Hyalinization was the only histopathological parameter that was statistically more frequent in adults than in children.

Reactive angioendotheliomatosis is a rare benign process that has been mainly described in patients with systemic infections, such as subacute bacterial endocarditis or tuberculosis, and in association with intravascular deposition of cryoproteins. Histopathologically, it is characterized by a proliferation of endothelial cells within vascular lumina resulting in the obliteration of the involved vessels. Another rare variant of reactive angioendotheliomatosis has been described in the lower extremities of patients with severe peripheral vascular atherosclerotic disease. It consists of violaceous and purpuric plaques histopathologically characterized by diffuse proliferation of endothelial cells interstitially arranged between collagen bundles of the reticular dermis. This second variant has been named diffuse dermal reactive angioendotheliomatosis. We report two patients with reactive cutaneous angioendotheliomatosis appearing distally to arteriovenous fistulas used for hemodialysis because of chronic renal failure. The first patient showed intravascular reactive angioendotheliomatosis, while the second one had purpuric plaques that were characterized histopathologically by diffuse dermal angioendotheliomatosis. Both patients showed an arteriovenous "steal" syndrome with distal ischemia, and it is possible that a local increase of vascular endothelial growth factor, as is the case in hypoxia situations, induces the endothelial proliferation. To the best of our knowledge, cutaneous reactive angioendotheliomatosis has not been previously described in association with arteriovenous shunts.

Atypical fibroxanthoma is a superficial variant of pleomorphic malignant fibrous histiocytoma. Histopathologically, it is characterized by a dermal nodule composed of bizarre cells arranged in a haphazard-to-fascicular pattern. These cells are spindle or rounded, pleomorphic and with numerous atypical mitotic figures. Some cells appear polygonal with ample and foamy cytoplasm. We recently encountered two elderly patients with atypical fibroxanthoma on their face. Histopathologically, one of the lesions was composed, almost entirely, of clear cells, whereas in the other one aggregations of clear cells constituted a half of the neoplasm. Atypical multinucleated cells with a Touton-like appearance were present. In addition to clear cells, areas of more conventional atypical spindle cells arranged in fascicles were seen, supporting the diagnosis of atypical fibroxanthoma. PAS staining failed to demonstrate glycogen in neoplastic cells. Immunohistochemistry revealed that neoplastic cells expressed positivity for vimentin, muscle-specific actin, and alpha smooth muscle actin, whereas cytokeratin, S-100 protein, EMA, CEA, and desmin were negative. Ultrastructural studies showed that neoplastic cells contained abundant rough endoplasmic reticulum, mitochondria, and numerous lipid vacuoles within the cytoplasm. Clear-cell atypical fibroxanthoma is a rare variant of atypical fibroxanthoma that should be differentiated from other clear-cell neoplasms of the skin.

Objective: To describe a series of 41 patients with fresh lesions of Sweet syndrome in which the histopathologic study demonstrated an inflammatory infiltrate mostly composed of histiocytoid mononuclear cells. Design: Histopathologic, immunohistochemical, and cytogenetic studies of the inflammatory infiltrate in a case series of histiocytoid Sweet syndrome. Setting: University departments of dermatology and a private laboratory of dermatopathology. Methods: Conventional histopathologic study as well as immunohistochemical investigations were performed using the alkaline phosphatase antialkaline phosphatase technique with a large panel of antibodies. In some cases, fluorescent in situ hybridization studies were performed to investigate the presence of the bcr/abl gene fusion. Results: Immunohistochemical studies demonstrated that most cells of the infiltrate showed immunoreactivity for CD15, CD43, CD45, CD68, MAC-386, HAM56, and ly-sozyme, which is consistent with a monocytichistiocytic immunoprofile. However, intense myeloperoxidase reactivity was detected in most of the cells with histiocytic appearance, which raised the possibility of specific cutaneous involvement by myelogenous leukemia. Nevertheless, cytologic peripheral blood examinations, fluorescent in situ hybridization studies to investigate the bcr/abl gene fusion, and follow-up of the patients, taken all together, ruled out this possibility. Conclusions: This case series demonstrates that some fresh cutaneous lesions of Sweet syndrome are histopathologically characterized by an infiltrate mostly composed of cells that may be misinterpreted as histiocytes, when in fact they are immature myeloid cells. We named this histopathologic variant histiocytoid Sweet syndrome, which should not be mistaken with leukemia cutis or other inflammatory dermatoses that are histopathologically characterized by histiocytes interstitially arranged between collagen bundles of the dermis.