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The oncocyte is a cell characterized by capacious, eosinophilic, finely granular cytoplasm, and lesions composed primarily of oncocytes are termed oncocytomas. Whereas oncocytic metaplasia has been reported in various cutaneous neoplasms, oncocytomas typically occur in the kidneys, thyroid and salivary glands and are uncommon in the dermatopathology literature. We present three cases of cutaneous oncocytoma so that dermatopathologists are cognizant of this uncommon entity. Although some believe that oncocytomas are locally aggressive lesions, our cases indicate that their clinical course is perhaps as banal as their histology.

Skeletal muscle (SM) regeneration is a pleomorphic yet benign reaction pattern that follows injury to SM and has been reported to simulate histologically various malignancies such as squamous cell carcinoma and rhabdomyosarcoma. A 61-year-old man presented with a 4-month history of an enlarging nodule on the upper lip following blunt trauma. An initial punch biopsy was non-diagnostic. A deeper biopsy revealed a multilobular proliferation of atypical and pleomorphic cells with vesicular nuclei, prominent nucleoli, and large amphophilic cytoplasm. Immunoperoxidase studies showed these cells to be positive for muscle-specific actin and desmin and negative for S-100 protein and smooth muscle actin. Based on these findings, a diagnosis of skeletal muscle regeneration (SMR) was made. To our knowledge, this is the first report of SMR presenting clinically as a rapidly growing cutaneous nodule on the lip following blunt trauma.

Determining the underlying etiology of recurrent erythema multiforme (EM) can be a difficult endeavor. Although infection with herpes simplex virus (HSV) has been implicated in some cases, the precise trigger of a given patient's recurrent EM often remains elusive. We discuss the case of a woman with a recurrent blistering eruption that was clinically and histopathologically consistent with EM. An investigation into the etiology of the patient's EM suggested that HSV was not the causative factor but instead pointed toward a hormonal influence that we interpret as autoimmune progesterone dermatitis (APD). This case is presented to highlight the importance of considering hormonal triggers in women with recurrent EM that consistently flares during the luteal phase of the menstrual cycle, the point at which serum progesterone levels peak. A brief review of the literature regarding the diagnosis, histopathology, etiology and treatment of APD is further provided.

Patients who have had malignant melanoma are at an increased risk of developing a second primary melanoma compared with the general population risk of developing a first melanoma. Many of these second primary melanomas occur at a similar anatomic site as the first lesion. Determining whether a second lesion is indeed a separate primary vs. a metastasis or locoregional recurrence can be very difficult histologically. We report the case of a patient who developed a second melanoma, 2 years after the initial diagnosis, within 3 cm of the site of the original lesion. Because of distinct histomorphologic features, the second lesion was favored to be a separate primary. However, because of the nearly identical anatomic location, molecular testing for loss of heterozygosity and BRAF mutation was performed to help further make this distinction. The first lesion was found to have loss of heterozygosity and a BRAF mutation that were not present in the second lesion. While these tests cannot elucidate the true molecular origin of these lesions, they provide a useful clinical tool to assess whether a second lesion should be treated as a recurrence or as a separate lesion with unique biologic potential.

Given that nevi may grow in size, mitotic figures may be expected in melanocytic nevi. We reviewed the literature for studies addressing this issue. We sought to determine the number of mitotic figures we might discover upon review of a group of randomly collected, conventional nevi. We reviewed 157 nevi from patients and found seven nevi exhibiting mitotic figures, comprising 4% of our sample. We noted the location of the mitoses within the nevi, the presence of any congenital features, as well as other features such as signs of inflammation or irritation. Through this study we have shown that occasional mitoses occur within unremarkable, conventional nevi.

Clear cell hidradenoma is a benign eccrine tumor with nodular-cystic architecture, ductal differentiation and poroid cytology. Squamous islands occurring in eccrine neoplasms are classified into three types, according to their location, histologic characteristics and degrees of cellular atypia. Type I squamous islands resemble normal epidermis and are present in the superficial to middle portion of eccrine poromas. Type II squamous islands are localised in the deeper portion of eccrine poromas and show lumen formation suggestive of acrosyringeal differentiation. Type III squamous islands show atypical features and are limited to malignant eccrine tumors. A 55-year-old man was biopsied for a dermal nodule on his left arm. The initial shave biopsy revealed the surface of an intradermal cystic nodule with numerous whorls and horn pearls of squamatization, reminiscentÂof a well-differentiated squamous cell carcinoma; however, no cytologic atypia, necrosis or mitotic activity were identified; focal clear cell and ductal differentiation were also present. Complete excision revealed clear cell hidradenoma with extensive mixed type I and type II squamous islands. The presence of whorls and pearls of squamoid cells is a rare finding in clear cell hidradenoma that, moreover, in the setting of a superficial biopsy, may be a diagnostic pitfall.

The histologic diagnosis of desmoplastic melanoma is challenging, particularly when the overlying junctional component is absent and when the spindle cells lack melanin pigment. In these instances, the importance of immunohistochemistry cannot be overemphasized. S-100 protein, with a sensitivity approaching 90%, is the primary immunohistochemical stain used for this purpose. HMB-45, although a more specific marker for melanocytes, is often negative in these cases. A marker of schwannian differentiation, p-75 NGF-R has been shown to be a useful confirmatory marker for desmoplastic and neurotropic melanoma, with staining intensity comparable to or better than S-100 protein. We report two cases of desmoplastic melanoma which stained focally and weakly with S-100 protein in comparison with the stronger staining of p-75 NGF-R. In both cases, S-100 staining could have been missed or interpreted as non-confirmatory, thus misguiding the diagnosis. We therefore suggest that p-75 NGF-R be a complementary marker to S-100 protein in differentiating desmoplastic melanoma from other non-neural crest-derived spindle cell tumors.