Cutaneous solitary fibrous tumor: Report of three cases with review of histopathological mimics.

Solitary fibrous tumor (SFT) is a relatively uncommon spindle cell mesenchymal neoplasm that is most often based on the pleura but may rarely arise in extrapleural locations, including the skin. Herein, we describe three cases of cutaneous SFTs. SFT is characterized by epithelioid and spindle cells arranged in random patterns with focal prominent stromal collagen and pericytomatous vessels. Immunohistochemical evaluation is required for definitive distinction of SFT from other benign and malignant cutaneous spindle cell neoplasms. Although aggressive biologic behavior is uncommon, accurate diagnosis of it is required for prognostication and counseling. CD34, bcl-2, and CD99 stains are positive in SFT, but not specific. STAT6 is the most sensitive and specific immunohistochemical marker to confirm diagnosis of SFT.

Melanocytic marker Melan-A detects molluscum contagiosum bodies.

Melan-A is one of the most commonly used immunohistochemical assays (IHC) in dermatopathology laboratories to detect the presence and outline the distribution of melanocytes. It is a cytoplasmic stain that detects a melanocyte-specific cytoplasmic protein involved in the formation of stage II melanosomes. Clinically, Melan-A is primarily used to detect and confirm melanocytic tumors although it is also positively expressed in adrenal cortical tumors and sex cord stromal tumors. We found that Melan-A also detected and highlighted Henderson-Patterson bodies of molluscum poxvirus. To determine if other melanocytic markers detect molluscum contagiosum bodies, S-100, HMB-45, MITF, and SOX-10 were also tested. In 15 tested molluscum cases, Melan-A stains were positive in all cases, whereas the other tested melanocytic markers were negative. Our results confirm that Melan-A is very sensitive in detecting molluscum contagiosum bodies and could be clinically useful to supplement the hematoxylin and eosin (H&E) in cases that are very inflamed or only have limited biopsy material.

Knuckle pads: an ancient disease frequently misdiagnosed because of minimal modern attention.

Knuckle pads are benign painless papules and nodules that most commonly appear on the extensor surfaces of the proximal interphalangeal joints. Knuckle pads are frequently misdiagnosed due to their location overlying joints, which can lead to costly interventions and patient discomfort for a relatively harmless condition. We describe a 44-year-old woman who presented with mildly painful nodules on multiple bilateral proximal interphalangeal joints. The patient did not have a family history of fibromatosis and the rheumatoid factor was negative. Histology showed mild epidermal hyperkeratosis, papillomatosis, and acanthosis with a deep dermal, poorly circumscribed, proliferative nodule made of spindled myofibroblasts without cytological atypia. The diagnosis of knuckle pads was established based on the clinical and morphological presentation of the nodules. Treatment with intralesional triamcinolone acetonide injection produced significant clinical improvement. Our findings highlight the challenging diagnosis of knuckle pads and the importance of increasing the familiarity of knuckle pads in modern medical practice.