Dermatology: Practical and Conceptual Letter | Dermatol Pract Concept 2021;11(1):e2020086 1 Dermatology Practical & Conceptual Introduction Rosai-Dorfman disease (RDD) is a reactive and histioprolif- erative disorder of unknown cause. It predominantly affects the cervical lymph nodes, with extranodal lesions in 40% of cases (skin lesions comprise 16%). The presence of charac- teristic RDD symptoms (fever, night sweats, and weight loss), termed B symptoms, is considered systemic RDD. Primary cutaneous RDD (CRDD) comprises 3% of all RDDs. It is localized to the skin with no systemic symptoms or abnor- mal blood examination findings. Although it often occurs on the face, there are very few reports of primary CRDD of the scalp. We report a case of primary CRDD of the scalp that needed to be distinguished from angiosarcoma. Case Presentation A 51-year-old man noticed a mass on his head 6 months before his first visit, and 2 months prior, biopsy had been per- formed at a nearby hospital owing to gradual mass enlarge- ment. Histopathological findings indicated angiosarcoma. PET/CT detected a skin lesion with accumulation in the right temporal region and no metastasis (Figure 1A). Therefore, he was referred to our hospital. Clinical examination revealed a smooth, soft, and mobile red mass measuring 2.5 × 2 cm in the right temporal region (Figure 1B), and the pathological diagnosis was reconsidered because he was younger than the typical angiosarcoma patient and had no purpura around the mass. Numerous inflammatory cells (plasma cells) were seen in the dermis (Figure 2A). Many large histiocytes formed a nodular/diffuse pattern (Figure 2B). Normal lymphocytes and neutrophils were detected in the cytoplasm of large histiocytes, termed emperipolesis. Tumor cells were positive for S100 and CD68 and negative for CD1a, CD34, and ERG. (Figure 2C). Thus, CRDD was diagnosed. At follow-up 10 months after the patient’s first visit, with no treatment, the tumor reduced in size. Conclusions Five cases of CRDD of the scalp have been reported (4 primary skull RDD with subcutaneous infiltration and 1 primary CRDD of the scalp) [1]. Accumulation of RDD on PET/CT has been reported [2]. Therefore, it must be differ- entiated from other malignant diseases. Primary Cutaneous Rosai-Dorfman Disease of the Scalp Haruki Mizuta1,2, Eiji Nakano3, Naoya Yamazaki1 1 Department of Dermatologic Oncology, National Cancer Center Hospital, Tokyo, Japan 2 Department of Plastic and Reconstructive Surgery, Graduate School of Medicine, Osaka City University, Osaka, Japan 3 Division of Dermatology, Department of Internal Medicine, Kobe University Graduate School of Medicine, Hyogo, Japan Key words: Rosai-Dorfman disease, cutaneous Rosai-Dorfman disease, scalp, angiosarcoma Citation: Mizuta H, Nakano E, Yamazaki N. Primary cutaneous Rosai-Dorfman disease of the scalp. Dermatol Pract Concept. 2021;11(1):e2020086. DOI: https://doi.org/10.5826/dpc.1101a86 Accepted: May 14, 2020; Published: December 7, 2020 Copyright: ©2020 Mizuta et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License BY-NC-4.0, which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported in part by the National Cancer Center Research and Development Fund (29-A-3). Competing interests: The authors have no conflicts of interest to disclose. Authorship: All authors have contributed significantly to this publication. Corresponding author: Eiji Nakano, MD, PhD, Division of Dermatology, Department of Internal Medicine, Kobe University Graduate School of Medicine, Hyogo, 7-5-1 Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan. Email: einakano@med.kobe-u.ac.jp 2 Letter | Dermatol Pract Concept 2021;11(1):e2020086 regression and has a good prognosis. Minimally invasive modalities, such as excision and topical, local injection of, and internal use of steroids, are options for treatment; how- ever, a benign or malignant diagnosis greatly influences the treatment strategy. Thus, it is crucial to consider CRDD in the diagnosis of atypical skin cancers. References 1. Kaskas NM, Kern M, Johnson A, et al. A case of cutaneous Rosai-Dorfman disease with underlying calvarial involvement and absence of BRAF V600E mutation. JAAD Case Rep. 2015;1(6):408-411. DOI: 10.1016/j.jdcr.2015.10.003. PMID: 26858984. 2. Hadchiti J, Kamar FG, Ghosn JA, et al. 18F-fluoro-2- deoxyglucose positron emission-computed tomography in a rare cutaneous form of Rosai-Dorfman disease: a case report. Mol Clin Oncol. 2018;8(2):236-241. DOI: 10.1016/10.3892/ mco.2017.1507. PMID: 29435284. Our case mimicked an atypical angiosarcoma that pre- sented as classical angiosarcoma found on sun-damaged skin of the head and neck of elderly patients, lymphedema- associated angiosarcoma in postmastectomy or lymphatic malformations, and post-radiation angiosarcoma in radia- tion fields. Histopathological CRDD findings revealed large his- tiocytes, which form nodular/diffuse patterns with skin infiltration and are positive for S100 and CD68 but negative for CD1a. These histological features indicate RDD as a non- Langerhans cell histiocytosis. Further, immunohistochem- ical staining of CD34 and ERG, markers of vascular endothelium, distinguishes CRDD from angiosarcoma. Emperipolesis, in which normal lymphocytes and neutrophils are recognized in the cytoplasm of histiocytes, is a charac- teristic feature of CRDD. These 3 features must be present to suspect angiosarcoma. CRDD undergoes spontaneous Figure 1. Clinical findings: (A) PET/CT taken at the nearby hospital. A skin lesion with an accumulation is seen in the right temporal region away from the skull. (B) Clinical findings at the first visit: a smooth, soft, and mobile red mass measuring 2.5 × 2 cm with no surrounding purpura. A B Figure 2. Histopathological features: (A) There are many large histiocytes forming a nodular/diffuse pattern (H&E, ×200). (B) Normal lymphocytes and neutrophils are detected in the cytoplasm of the large histiocytes, which is called emperipolesis (H&E, ×400). (C) The large histiocytes are positive for S100 (immunohistochemical staining, ×200). A B C