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Background Accurate risk stratification of Langerhans cell histiocytosis (LCH) is essential as management can range from conservative in single system, low risk for central nervous system (CNS) involvement lesions to intensive chemotherapy for multisystem or high-risk disease. Additionally, being able to differentiate metabolically active from inactive lesions is essential for both prognostic reasons and to avoid potentially unnecessary treatment. Methods A retrospective review was performed on all patients with histopathology-confirmed LCH at Cincinnati Children's Hospital Medical Center (CCHMC) between 2009 and 2019. Results One hundred seven positron emission tomography (PET)/computerized tomography (CT) images were included in the review. A discrepancy between PET/CT and conventional imaging occurred on 53 occasions. On 13 occasions, increased uptake was observed on PET in an area with no identifiable lesion on conventional imaging. On 40 occasions, lesions were found on conventional imaging where no increased uptake was observed on PET. On eight skeletal surveys, three other radiographs, four diagnostic CTs, five localization CTs, and one bone scan, no lesion was identified in an area with increased fluorodeoxyglucose (FDG) uptake. This occurred exclusively in bone. On nine skeletal surveys, one other radiograph, four diagnostic CTs, six localization CTs, 19 magnetic resonance imaging (MRI) scans, and one bone scan, a lesion was identified in a location without increased FDG uptake. This occurred in bone, CNS, and lungs. Conclusion F-18-FDG PET/CT is vital in the evaluation of LCH lesions given its ability to detect LCH lesions not detectable on conventional imaging modalities, as well as its ability to distinguish metabolically active from inactive disease. MRI and diagnostic CT are still useful adjunctive tests for identification of CNS and lung lesions.

BACKGROUND Post-transplant lymphoproliferative disorders (PTLDs) are well characterized in tissue sections, but their evaluation in exfoliative cytology specimens is limited. This study reports a 25-year experience with PTLDs in exfoliative cytology specimens. METHODS All solid organ or allogeneic stem cell transplant recipients with PTLDs and exfoliative cytology specimens from 1987 to 2011 were identified. The cytomorphology, Epstein-Barr virus (EBV) status, flow cytometry, immunohistochemistry, and molecular studies were reviewed from all exfoliative cytology specimens previously diagnosed as atypical lymphoid proliferations or PTLDs. RESULTS A total of 55 patients (age range, 1-72 years) with PTLDs had 434 exfoliative cytology specimens. Thirty-six of the 55 patients (65%) had 54 specimens with abnormal lymphoid proliferations (12% of the specimens), and 26 of these patients had 37 specimens available for review (15 cerebrospinal fluid specimens, 12 peritoneal fluid specimens, 9 pleural fluid specimens, and 1 bronchoalveolar lavage fluid specimen). Thirty percent of the reviewed cytology specimens were diagnostic of PTLDs, including 8 cases of monomorphic post-transplant lymphoproliferative disorder (M-PTLD) with abnormal B/T-cell populations identified with flow cytometry/immunohistochemistry and 3 EBV-positive specimens with a differential diagnosis of polymorphic PTLD versus M-PTLD. All cases diagnostic of a PTLD had 1 to 3 ancillary studies performed. Forty percent of the cytology specimens (15 of 37) were suspicious for a PTLD, but ancillary studies were performed for only a third of them, and they did not support a definitive diagnosis of a PTLD. Thirty percent of the cytology specimens (11 of 37) appeared reactive, but they lacked sufficient ancillary studies to exclude a PTLD. CONCLUSIONS Atypical lymphoid proliferations are common in exfoliative cytology specimens from patients with PTLDs, and they require ancillary studies at least including immunophenotyping and EBV evaluations for a definitive diagnosis. Cancer (Cancer Cytopathol) 2016. © 2016 American Cancer Society.

Langerhans cell histocytosis (LCH) and Erdheim-Chester disease are two rare histiocytic disorders. Their occurrence in the same patient is more infrequent, but has been described. We report a case of a 38-year-old woman who presented with a diagnosis of single system cutaneous LCH. Subsequently, she developed multiple papules on her extremities consistent with a non-LCH xanthogranuloma type lesion. BRAF(V600E) mutation immunostain, VE1 was positive in the skin lesion, which was confirmed by molecular polymerase chain reaction (PCR) studies, initiating a complete systemic workup for Erdheim-Chester disease. Systemic involvement was confirmed with bilateral sclerotic bone lesions and retroperitoneal and pelvic fibrosing disease. She was also found to have a BRAF(V600E) mutation positive papillary thyroid carcinoma. New suspicious cutaneous lesions presenting in patients with a history of LCH need to be biopsied. A BRAF(V600E) mutation in a non-LCH histiocytic lesion with a xanthogranuloma phenotype (CD163/CD68/CD14/fascin/Factor 13a) should prompt an Erdheim-Chester disease workup. This is a unique case of a woman with BRAF(V600E) mutation positive Erdheim-Chester disease and cutaneous LCH, while also being, to our knowledge, the first reported case in the English literature of it occurring in a patient with a BRAF(V600E) mutation positive papillary thyroid carcinoma.