BACKGROUND:This invited review for the special issue is a celebration of the continued growth of knowledge in the field of serous fluid cytopathology. SUMMARY:The review begins by addressing the value of cytological examination of the fluid samples in conjunction with clinical, radiological, and biochemical data interpreted by the treating physician. It then covers the advances in cytopreparatory techniques with special reference to existing and emerging ancillary techniques available to obtain maximal diagnostic, prognostic, and therapeutic information for the practice of precision medicine. The concept of mesothelioma as a cytological diagnosis has been made possible in recent years through clinical research and advocacy by experts and this has added value to serous fluid samples especially when this is the only sampling method available for a patient. Advances in the recommended repertoire of immunostains and a standardized approach through the use of diagnostic categories has been made possible by The International System for Reporting Serous Fluid Cytopathology. The reporting system has found application to other types of fluid samples, most recently, cerebrospinal fluid cytology, which will be included in the forthcoming edition of the terminology system and is introduced in this special issue. KEY MESSAGES:The review charts a journey commemorating important landmarks in our current understanding of serous fluids in diagnostic cytopathology.

Background: Mesothelial proliferations range from reactive lesions to benign and malignant tumors, bearing witness to the plasticity of these cells. Their diagnosis often requires combination of morphological assessment, immunohistochemistry, and molecular testing. Summary: Considerable progress has been made in recent years in all the above aspects. The separation of reactive from neoplastic mesothelial proliferations has become more robust. Mesothelioma in situ is now recognized as precursor of invasive mesothelioma and better tools exist for its differentiation from benign mimics. Differentiating mesothelial tumors from other malignancies, including metastatic carcinomas, sarcomas, and other tumors, is now more easily achieved. Genetic conditions which predispose to development of mesothelioma at a young age have been identified and characterized. These issues are discussed in this review. Key messages: Mesothelial pathology is an evolving field and more knowledge of these processes is likely to be gained in coming years. The diagnosis of these tumors requires experts with subspecialty in cytology, thoracic, soft tissue/bone, and gynecologic pathology, cooperating with surgeons, radiologists, and oncologists at specialized centers.

There are numerous benign cellular changes that occur in the lung due to diverse underlying diseases that can be encountered in exfoliative samples and transbronchial and CT-guided samples. Entities such as bronchial cell hyperplasia, pneumocyte type II proliferation, repair, etc. should be recognised and diagnosed. Yet, many pathologists encounter difficulties in their interpretation and frequently either diagnose them as 'Atypical' resulting in more invasive procedures or misdiagnose them as 'Malignant' with drastic consequences to the patient. Other entities may be dismissed as benign lung elements, resulting in misclassification as non-diagnostic/non-representative, resulting in repeated or more invasive procedures. In this review, we will present an overview of many of these diagnostic pitfalls and mimickers and describe clues to their diagnosis and how to differentiate them from malignant conditions. As in all cytologic diagnoses, both false positive and false negative errors can occur and both types of errors can have grave consequences to patient management, resulting in increased unnecessary morbidity such as pneumonectomy and, at times, litigation.

Background: Mesothelioma is an aggressive malignancy of the serosal surfaces with very poor prognosis. It traditionally manifests in older patients and at an advanced stage which results in minimal improvement in prognosis despite the recent advances in management. Early detection would therefore significantly impact management and potentially improve survival. Mesothelioma frequently presents with recurrent effusions, posing cytology as the initial procedure in the workup. A definitive diagnosis would not only spare the patients additional diagnostic procedures but also potentially afford them an opportunity for early surgical intervention and therapy. Summary: In this article we review the role of immunocytochemistry (ICC) in the work up of mesothelioma. The various ICC markers to confirm or rule out mesothelial lineage are reviewed. In addition, newly introduced molecular surrogates that confirm the malignant nature of the mesothelial cells and support a definitive diagnosis of mesothelioma are discussed. We also briefly discuss the theranostic implications of such markers and potential impact of such recent advances on the cytological diagnosis and reporting of mesothelioma. Key messages: The cytological diagnosis of mesothelioma no longer requires the extensive expertise in morphological analysis and can be offered based on supporting ICC that confirms the mesothelial lineage and malignant nature of the cells.

A case of a 45-year-old female presented with an anterior neck mass measuring 5 cm in diameter. She had noticed a soft tissue swelling 2 months ago, and it had been increasing in size. CT, ultrasound, and fine needle aspiration were concerning for malignancy.

INTRODUCTION:The diagnosis of mesothelioma in situ (MIS) is now accepted by the WHO as a pre-invasive neoplastic mesothelial proliferation and considered a diagnosis based on histologic evaluation only. Although the definition of MIS includes recurrent effusions, little is known about the cytologic features of such effusions. Since mesothelioma is usually diagnosed at an advanced stage and has a poor prognosis, early detection of a neoplastic mesothelial population in such effusions can potentially have a positive impact on the management of such a dire disease.MATERIALS AND METHODS:We reviewed a total of 18 pleural effusions from nine patients with recurrent effusions. Of these, five patients had follow-up biopsies diagnosed as MIS and the remaining four cases had negative radiology and malignant cytology proven by molecular markers (BAP1, MTAP or CDKN2A deletion) and at least 1 year follow-up with no overt mass identified by radiology.RESULTS:Initial effusions may mimic reactive mesothelial hyperplasia or exhibit atypia. As effusions recur, the cellularity and atypia increase and the mesothelial proliferation becomes morphologically indistinguishable from mesothelioma. Molecular alterations diagnostic of mesothelioma can be detected in these effusions, even in the initial-benign/reactive appearing ones. The cellularity and atypia detected in such effusions surpassed those noted on the biopsies, raising questions regarding the cause of such discrepancy.CONCLUSION:The diagnosis of MIS can be suspected based on malignant effusion cytology supported by molecular alterations. We propose that the proliferation of neoplastic mesothelial clones represent a clinically silent "liquid phase MIS stage" corresponding to in situ stage in other organs.

Quality control in the cytology laboratory comprises many aspects of laboratory practice that are common to all medical laboratories but some that are unique. In the application to cytology, there are further, special considerations. This chapter describes quality control and quality assurance activities specific to cytology, which require continuous monitoring and are mandatory by the Clinical Laboratory Improvement Amendments of 1988 (CLIA '88) regulations. This chapter also describes how to document review activities and monitor their effectiveness in improving performance in gynecological, non-gynecological, and fine-needle aspiration cytology techniques.

The World Health Organization (WHO), the International Academy of Cytology, and the International Agency for Research on Cancer have developed an approach to standardized reporting of pancreaticobiliary cytopathology. The WHO Reporting System for Pancreaticobiliary Cytopathology (WHO System) revises the Papanicolaou Society of Cytopathology (PSC) System for Reporting Pancreaticobiliary Cytology published in 2015 and replaces the 6 PSC categories with 7 categories: “Insufficient/Inadequate/Nondiagnostic”; “Benign/Negative for malignancy”; “Atypical”; “Pancreaticobiliary neoplasm, low risk/grade (PaN-low)”; “Pancreatic neoplasm, high risk/grade (PaN-High)”; “Suspicious for malignancy”; and “Malignant”. In the PSC system, there is a single category for “Neoplastic” lesions that includes 2 groups, 1 for benign neoplasms and 1 named “Neoplastic-other”, dominated by premalignant intraductal neoplasms primarily intraductal papillary mucinous neoplasms and low-grade malignant neoplasms (pancreatic neuroendocrine tumors (PanNET) and solid pseudopapillary neoplasms (SPN). In the WHO System, benign neoplasms with virtually no risk of malignancy are included in the “Benign” category and low-grade malignancies (PanNET and SPN) are included in the “Malignant” category, as per the 5th edition of the WHO Classification of Digestive System Tumors, while the non-invasive pre-malignant lesions of the ducts are divided by the cytomorphological grade of the epithelium into PaN-low and PaN-high with distinctly different risks of malignancy. Within each category, key diagnostic cytopathologic features and the ancillary studies for diagnostic and prognostic evaluation, as well as the implications of diagnosis for patient care and management, are outlined. Reporting and diagnostic management options recognize the variations in the availability of diagnostic and prognostic ancillary testing modalities in low- and middle-income countries.

A 28-year-old patient presented with painless thyroid swelling that showed features of a hydatid cyst on resection. The preoperative cytology had been reported as unsatisfactory. A review of the smears revealed the presence of acellular laminated membranes overlooked initially. Infection as a cause of thyroiditis is rare and can be bacterial, fungal, and less commonly parasitic or viral. Most patients have a predisposing factor like a congenital surgical anomaly or an immunocompromised status. Infection may also be iatrogenic; post-fine needle cytology being an important cause. Subacute thyroiditis having an overlapping clinical presentation is an important differential diagnosis. The cytological examination provides an integral modality for diagnosis, allowing microscopic evaluation and procurement of material for culture and sensitivity studies. However, a high index of suspicion is essential to avoid missed or misdiagnosis. Here, we review the various causative agents of infectious thyroiditis, discussing the cytological features and differential diagnosis.

The International Academy of Cytology has joined with the International Agency for Research on Cancer (IARC) to bring together a group of experts in lung cytopathology to develop a WHO Reporting System for Lung Cytopathology (WHO System). This WHO System defines five categories for reporting lung cytopathology, that is, "Insufficient"/"Inadequate"/"Non-diagnostic," "Benign," "Atypical," "Suspicious for malignancy," and "Malignant," each with a clear descriptive term for the category, a definition, a risk of malignancy and a suggested management algorithm. The key diagnostic cytopathology features of each of the lesions within each category have been established by consensus and will be presented more fully in a subsequent IARC e-book and published hard cover book.The WHO System provides the best practice application of ancillary testing, including immunocytochemistry and molecular pathology, and provides a review to guide sampling and processing techniques to optimize the handling and preparation of the cytopathology sample emphasizing the cytomorphological differential diagnosis to aid low-resourced settings. The authors recognize that local medical and pathology resources will vary, particularly in low- and middle-income countries, and have developed the WHO System to make it applicable worldwide based on cytomorphology with options for further diagnostic management of the patient.The online WHO System provides a direct link to the WHO Tumour Classification for Thoracic Tumours 5th Edition. It will raise the profile and use of cytopathology by increasing awareness of its current role and its potential role in the era of personalized medicine based on molecular pathology utilizing "small biopsies." Ultimately, the System will improve patient care and outcomes.This System aims to improve and standardize the reporting of cytopathology, facilitate communication between cytopathologists and clinicians and improve patient care. The System is based on the current role of lung cytopathology and synthesizes the existing evidence while highlighting areas requiring further research and the future potential role of lung cytopathology.

Abstract Background Bile duct brush specimens are difficult to interpret as they often present inflammatory and reactive backgrounds due to the local effects of stricture, atypical reactive changes, or previously installed stents, and often have low to intermediate cellularity. As a result, diagnosis of biliary adenocarcinomas is challenging and often results in large interobserver variability and low sensitivity Objective In this work, we used computational image analysis to evaluate the role of nuclear morphological and texture features of epithelial cell clusters to predict the presence of pancreatic and biliary tract adenocarcinoma on digitized brush cytology specimens. Methods Whole slide images from 124 patients, either diagnosed as benign or malignant based on clinicopathological correlation, were collected and randomly split into training (ST, N = 58) and testing (Sv, N = 66) sets, with the exception of cases diagnosed as atypical on cytology were included in Sv. Nuclear boundaries on cell clusters extracted from each image were segmented via a watershed algorithm. A total of 536 quantitative morphometric features pertaining to nuclear shape, size, and aggregate cluster texture were extracted from within the cell clusters. The most predictive features from patients in ST were selected via rank‐sum, t‐test, and minimum redundancy maximum relevance (mRMR) schemes. The selected features were then used to train three machine‐learning classifiers. Results Malignant clusters tended to exhibit lower textural homogeneity within the nucleus, greater textural entropy around the nuclear membrane, and longer minor axis lengths. The sensitivity of cytology alone was 74% (without atypicals) and 46% (with atypicals). With machine diagnosis, the sensitivity improved to 68% from 46% when atypicals were included and treated as nonmalignant false negatives. The specificity of our model was 100% within the atypical category. Conclusion We achieved an area under the receiver operating characteristic curve (AUC) of 0.79 on Sv, which included atypical cytological diagnosis.