Pseudomalignancies in Children: Histological Clues, and Pitfalls to Be Avoided

The term “pseudomalignancy” covers a large, heterogenous group of diseases characterized by a benign cellular proliferation, hyperplasia, or infiltrate that resembles a true malignancy clinically or histologically. Here, we (i) provide a non-exhaustive review of several inflammatory skin diseases and benign skin proliferations that can mimic a malignant neoplasm in children, (ii) give pathologists some helpful clues to guide their diagnosis, and (iii) highlight pitfalls to be avoided. The observation of clinical–pathological correlations is often important in this situation and can sometimes be the only means (along with careful monitoring of the disease’s clinical course) of reaching a firm diagnosis.

Langerhans cell histiocytosis: A great imitator

Langerhans cell histiocytosis (LCH) is an uncommon but serious inflammatory neoplasia that affects many organs, including the skin. Though uncommon, it should remain high on a clinician's differential diagnosis in treatment-resistant cases of conditions, such as seborrheic dermatitis, diaper dermatitis, arthropod bites, and many more. A thorough history nd physical examination for each patient can aid in the diagnosis; however, if clinically suspicious for LCH, a punch biopsy should be performed. Histologic evaluation of LCH is often enough to differentiate it from the many clinical mimickers. Characteristic findings include a histiocytic infiltrate with "coffee bean"-cleaved nuclei, rounded shape, and eosinophilic cytoplasm. Immunohistochemical stains, including CD1a, S100, and CD207 (langerin) are often needed for a definitive diagnosis. Electron microscopy also demonstrates the ultrastructural presence of Birbeck granules, but this is no longer needed due to immunohistochemical staining. Treatment is often necessary for LCH, if systemic involvement exists.

Langerhans cell histiocytosis mimicking malignant melanoma: a diagnostic pitfall

Langerhans cell histiocytosis (LCH), especially with an involvement limited to the skin, is a rare entity in adults. In formulating a differential diagnosis of a solitary skin lesion, LCH is rarely considered. Morphologically, cells seen in LCH can mimic those seen in a melanocytic tumor; moreover, they both show S-100 protein reactivity with immunoperoxidase staining. A 63-year-old male presented to a dermatology clinic with a solitary hyperpigmented macule on his right calf. A biopsy specimen showed epithelioid cells within the dermis, singly and in small groups, surrounded and infiltrated by collections of histiocytes and lymphocytes. These cells were diffusely positive for S-100 and negative for Melan-A. A diagnosis of malignant melanoma, spitzoid variant, was rendered, and the patient was sent to our melanoma center for surgical treatment. On histologic examination, some of the lesional cells had reniform, vesicular nuclei with central grooves. Additional immunoperoxidase staining showed strong, diffuse positivity for CD1a, supporting the diagnosis of LCH. LCH is morphologically similar to and can be misdiagnosed as malignant melanoma. It is important to be aware of this pitfall and utilize immunohistochemical and ultrastructural analysis to achieve correct diagnosis.