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BACKGROUND AND OBJECTIVES—Skin lesions in Langerhans cell histiocytosis (LCH) are often painful and difficult to treat. Topical application of nitrogen mustard (0.02% mechlorethamine hydrochloride, mustine), an alkylating cytostatic agent, has been shown to be effective. There is, however, concern about potentially harmful long term side effects. STUDY DESIGN—In a retrospective study 20 children with LCH (average extent of initial skin involvement: 16.4% body surface) were followed up for an average of 8.3 years after completion of topical mustine therapy. They had received a total of 34 courses (mean duration 14.2 weeks) of topical mustine. Disease status on follow up was assessed according to the Histiocyte Society classification. RESULTS—After mustine was introduced, 16 patients were able to discontinue systemic steroids and/or chemotherapy. Topical mustine was well tolerated in 18 patients, but two developed irritant dermatitis. On follow up, the disease was inactive in 10 patients. Among the children with active disease, six had mild skin disease and four had progressive disease, two of them with skin lesions unresponsive to mustine treatment. Scars confined to areas of formerly active skin disease were found in six patients. There was no evidence of premalignant or malignant skin disease in the treated areas. CONCLUSION—Topical mustine is an effective and safe treatment for skin disease in most children with LCH. Residual scarring was probably a result of the disease itself rather than to mustine. Although no evidence of skin cancer was found in this study, continued long term follow up is advisable.

Most human tumours are monoclonal, which suggests that they originate from a single cell. “Clonality” can be investigated by several techniques. In plasma a dominant monoclonal class of immunoglobulin suggests a neoplasm derived from a single altered plasma cell, which gives it a survival and growth advantage over non-neoplastic cells. In lymphoid malignancies, studies on rearrangements of genes coding for antigen receptors provide an excellent system of clonal markers, with consistent rearrangement of clonal immunoglobulin heavy chains (in B cell leukaemias and lymphomas)1 and T cell receptor s chains (in T cell tumours). In other tumours careful cytogenetic and molecular analysis has shown consistent reciprocal chromosomal translocations and partial and complete chromosomal deletions or duplications which reflect tumour clonality as well as indicating stretches of altered sequences of DNA that may have a role in the initial neoplastic change. Cytogenetic and DNA analysis has not yet, however, provided clonal markers for many solid tumours.2 An alternative approach, applicable only to females, is to take advantage of the phenomenon of “Lyonisation” (X inactivation mosaicism).3 4 Early in …