MICROSCOPIC DESCRIPTION: Paraffin sections show a densely hypercellular lesion. This is composed predominantly of large cells with irregular round, oval and indented nuclei some with conspicuous nucleoli and variable amount of pale cytoplasm. These are arranged in diffuse sheets and are admixed with large numbers of eosinophils. Many multinucleated giant cells are noted. An occasional mitotic figure is identified. The tumour cells are S-100 and CD1a positive. They are AE1/3 negative. The features are of Langerhans cell histiocytosis (eosinophilic granuloma). DIAGNOSIS: Frontal scalp subgaleal lesion: Langerhans cell histiocytosis (eosinophilic granuloma).