Dermatology: Practical and Conceptual Research Letter | Dermatol Pract Concept. 2023;13(1):e2023030 1 Identifying Pitfalls for Diagnosing Pigmented Bowen Disease on Reflectance Confocal Microscopy: Misleading Dendritic Cells Banu Farabi1,2,3, Babar K. Rao4, Manu Jain4,5 1 Department of Dermatology, New York Medical College, Valhalla, New York, NY, USA 2 Department of Dermatology, NYC Health + Hospitals/Metropolitan, New York, NY, USA 3 Department of Dermatology, NYC Health + Hospitals/Coney Island, Brooklyn, NY, USA 4 Department of Dermatology, Weil Cornell Medical School, NY, USA 5 Dermatology Department, Memorial Sloan Kettering Cancer Center, NY, USA Key words: Bowen’s disease, squamous cell carcinoma in situ, reflectance confocal microscopy, dermatoscopy, dermoscopy, dendritic cells Citation: Farabi B, Rao BK, Jain M. Identifying pitfalls for diagnosing Pigmented Bowen’s Disease on Reflectance Confocal Microscopy: Misleading dendritic cells. Dermatol Pract Concept. 2023;13(1):e2023030. DOI: https://doi.org/10.5826/dpc.1301a30 Accepted: June 13, 2022; Published: January 2023 Copyright: ©2023 Farabi et al. This is an open-access article distributed under the terms of the Creative Commons Attribution- NonCommercial License (BY-NC-4.0), https://creativecommons.org/licenses/by-nc/4.0/, which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original authors and source are credited. Funding: None. Competing Interests: None. Authorship: All authors have contributed significantly to this publication. Corresponding Author: Banu Farabi, 1901 First Avenue, New York, NY, 10029. Telephone: +1 (212) 423 7467, Fax: +1 (212) 423 8464, Email: banufarabi91@gmail.com Introduction Pigmented Bowen disease (pBD) is a rare variant of squa- mous cell carcinoma in situ of the skin. Precise diagnosis of pBD can be difficult based only on clinical and derma- toscopic findings. Reflectance confocal microscopy (RCM) plays an important role by showing atypical keratinocytes and full thickness atypia in vivo. Previous studies showed confounding presence of hyper-refractile elongated den- dritic cells in pigmented actinic keratosis (AK)/pBD on RCM [1]. We present a case of pBD located on the facial skin mis- diagnosed as lentigo maligna with RCM due to the presence of abundant hyper-refractile, atypical dendritic cells in the interfollicular spaces. Case Presentation A 73-year-old female with skin type II was seen for a 5 mm pigmented lesion on the right cheek (Figure 1A). Derma- toscopic examination showed an asymmetrical pigmented lesion with multiple colors, pigmented circles, and dotted and fine linear vessels on an erythematous background (Figure 1B). RCM images at the spinous and supra-papillary/ basal layer showed an atypical honeycomb (Figure 1, C and D) pattern and numerous bright edged papillae (Figure 1E) and dispersed bright fusiform and stellate shaped cells with thin dendrites (Figure 1F). A complete surgical excision of the lesion was performed with 2 mm of tumor-free mar- gins. Histopathology revealed a pBD including full thickness keratinocytic atypia, prominent basal layer pigmentation, 2 Research Letter | Dermatol Pract Concept. 2023;13(1):e2023030 and dermal melanophages (Figure 2). The patient continues routine care via skin cancer surveillance. Conclusions The diagnosis of pBD can be challenging clinically due to relative rarity and various clinical presentations. Dermatos- copy can be a helpful tool in diagnosing these lesions [2]. However, in equivocal cases, RCM plays an important role in differentiating pBD from lentigo maligna (LM). Typical RCM features of BD are full thickness keratinocytic atypia and architectural disorganization of the epidermis which presents as atypical or disarranged honeycomb pattern. Re- cent studies highlighted that intraepidermal dendritic cells can be found in pigmented AK/pBD which creates a poten- tial diagnostic pitfall. Moscarella et al reported dendritic cells in 12/17 cases of AK/BD [3]. Persechino et al found bright interfollicular dendritic cells in 53% of AK cases [4]. RCM features of LM includes atypical melanocytes and nests surrounding adnexal openings, sheets of cells composed of mainly dendritic cells giving a ‘medusa head’ appearance at dermo-epidermal junction (DEJ). Folliculotropism is a typi- cal feature of LM and is visualized as dendritic, atypical cells infiltrate the follicles [4]. Thus, it is important to visualize Figure 1. (A) Clinical examination of the lesion on the right preauricular area shows a 5 mm diameter asymmetrical pigmented. (B) Derma- toscopy reveals an asymmetrical pigmented lesion with multiple colors, pigmented circles, dotted and fine linear vessels on an erythematous background. (C) RCM images at the spinous and suprapapillary/basal layer shows an atypical honeycomb pattern and numerous bright edged papillae. (D) Higher magnification of atypical honeycomb pattern adjacent to the lesion which is highlighted with red bracket in (C). (E) Higher magnification of bright ringed-edged papillae at the level of dermo-epidermal junction shown with yellow arrow in (C). (F) Higher magnification of dendritic cells between ringed edged papillae (this image is obtained with VivaStack mode). Figure 2. Histologic examination shows full thickness keratinocyte atypia, disorganization of the keratinocyte and prominent basal layer pigmentation with dermal melanophages. Research Letter | Dermatol Pract Concept. 2023;13(1):e2023030 3 DEJ in detail and follicular structures for signs of dendritic cell infiltration to rule out LM. Other clues for pBD are nu- merous marked small bright rings at DEJ [5,6]. Since DEJ is infiltrated by malignant melanocytes in LM, presence of regularly shaped bright rims can signify pigmented AK/pBD. Intraepithelial hyper-reflective dendritic cells are found quite high in pBD. The exact nature of these cells is unknown, and density of dendritic cells can correlate with clinical pig- mentation of the lesion. Thus, it is imperative to be aware of the presence of dendritic cells. In the presence of dendritic cells in a pigmented lesion should not prompt extensive sur- gical excision without the evidence of melanocytic neoplasm. In doubt, incisional biopsy of the lesion should be considered to avoid extensive surgical treatment. 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