Case Report: A rare adult multisystem Langerhans cell histiocytosis involving the skin, thyroid, jaw-periodontal region, and bone: diagnostic challenges and complementary PET/CT-SPECT/CT findings.

Langerhans cell histiocytosis (LCH) is a rare clonal myeloid neoplasm with prominent inflammatory features and heterogeneous manifestations in adults, often leading to diagnostic delay. We report a 32-year-old man who presented with a two-year history of recurrent ulcerative intertriginous skin lesions refractory to empirical treatment for presumed inflammatory or infectious dermatoses. Biopsy from the ulcer edge showed atypical histiocytoid cells with characteristic nuclear grooves and numerous eosinophils; immunohistochemistry was positive for CD1a, S-100, and Langerin with a Ki-67 index of ~40%, confirming cutaneous LCH. Baseline 18F-FDG PET/CT (August 2022) demonstrated multisystem disease with extensive cutaneous involvement and imaging-suggestive involvement of multiple extracutaneous sites, including multifocal osseous lesions, the thyroid, gastrointestinal tract, lymph nodes, lung (cystic lesions), and an intramuscular nodule. After six cycles of cytarabine-based induction chemotherapy, follow-up PET/CT (April 2023) showed near-complete metabolic remission with only residual low-grade uptake in the perineal skin and maxillomandibular/gingival region. In contrast, 99mTc-MDP whole-body bone scintigraphy with SPECT/CT performed one year later (July 2024) revealed persistent multifocal tracer uptake, most prominent in the craniofacial skeleton and long bones, with corresponding CT changes suggestive of ongoing remodeling. This case highlights the importance of early biopsy of persistent unexplained ulcerative intertriginous lesions in adults to expedite systemic staging, and demonstrates the complementary roles of serial 18F-FDG PET/CT and bone scintigraphy with SPECT/CT in longitudinal assessment of osseous disease in adult multisystem LCH.