Update on juvenile xanthogranuloma: unusual cutaneous and systemic variants.

Juvenile xanthogranuloma (JXG) is a well-recognized benign disorder of infancy and early childhood characterized by yellowish cutaneous nodules that spontaneously regress over months to years. In the vast majority of children, JXG is limited to the skin and requires no treatment. Over the past two decades, unusual cutaneous and systemic forms of JXG have been increasingly reported. JXGs have been discovered, usually unexpectedly, in every organ system of the body. Correct diagnosis is crucial to prevent unnecessary invasive diagnostic and therapeutic procedures. Unusual clinical and histological variants of JXG often require immunohistochemical studies and/or electron microscopy to establish the diagnosis. Nonlipidized, giant, intramuscular, subcutaneous, and clustered JXG are but some of the variants that are discussed in this article. The immunohistochemistry of JXG, current nosology, and hypotheses regarding the origins of JXG are also reviewed.