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BACKGROUND: Body fluid cytology (BFC) is an important tool in the diagnosis and staging of malignancy and is aided by the judicious use of immunohistochemistry (IHC). The aim of this study was to determine the usage rates of IHC stains in BFC, their type and indications, and their diagnostic impact. We also attempted to estimate the optimal rate of IHC use in BFC by comparing the entire laboratory's and each individual cytopathologist's IHC use rates with their respective indeterminate and malignant diagnosis rates. METHODS: We conducted a retrospective study of IHC stain use in BFC during a 5.5-year interval (2013-2018) and determined the laboratory's and each individual cytopathologist's IHC usage patterns according to the final diagnosis, site, and indications for their use. RESULTS: A total of 477 out of 4144 (11.5%) BFC cases had 2128 individual immunostains performed, with an average of 4.5 immunostains per case. Individual cytopathologists usedIHC stains on 6.7% to 22% of their BFC cases. Pathologists with higher rates of IHC stain use than the laboratory's mean were less experienced and had higher rates of indeterminate but not of malignant diagnoses. The most common indication for the use of IHC stains was differentiating mesothelial from malignant cells. MOC31, calretinin, Ber-EP4, CD68, and D2-40 were the most commonly used of the 67 different IHC stains used in BFC. CONCLUSIONS: The laboratory's mean may represent the optimal IHC use rate, as higher IHC use rates did not lead to more diagnostic certainty or higher pickup rates of malignant cells. Cancer Cytopathol 2020;128:384-391.

She underwent a Whipple procedure. Her pathology of both the pancreatic mass and RML lung wedge resection showed sinus histiocytosis with massive lymphadenopathy, along with extensive fibrosis intertwined with nodular mixed inflammatory infiltrate. The histiocytes characteristically showed "emperipolesis," in which lymphocytes had penetrated the cytoplasm and remained viable within the histiocytes (lymphocytes continued to have free movement in the histiocyte). In addition, the histiocytic cells were positive with S-100 protein and CD68, hallmarks of RDD. Although rare, Rosai-Dorfman disease should be considered in the differential diagnosis of patients presenting with pancreatic and/or lung nodules, especially when biopsy or cytology results report atypical inflammatory findings.

Malignant gastrointestinal neuroectodermal tumor (GNET) is a rare disease with a handful of cases described in literature. GNET has only become a well-known/widely accepted entity in the recent years, but it is still not listed in the database of rare diseases. Due to the rarity of disease, there are no guidelines on standard therapeutic approaches in the adjuvant or metastatic setting. Here, we describe a unique case of GNET with a 7-year disease-free survival following adjuvant cisplatin and etoposide chemotherapy. This is the longest disease-free survival that has ever been described in literature and may support using this combination in a larger cohort of patients in the context of a global clinical trial. We will also review the histopathologic features of GNET and potential therapeutic options in the metastatic setting.

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