Ancillary Tests Research Articles

Fine Needle Aspiration and Core Biopsy of Liver Mass as First Time Diagnosis of Sarcomatoid Anaplastic Thyroid Carcinoma, A Rare Malignancy Presenting at an Unexpected Body Site

Abstract Introduction/Objective Anaplastic thyroid carcinoma (ATC) is a fatal malignancy. We report an unusual case of a patient with ATC whose chief complaint and clinical history were listed as “jaundice and liver mass” without mention of disease in the neck. Methods/Case Report A 66-year-old female presented to the emergency department with worsening “yellow-green skin discoloration” for a few weeks. She also reported worsening dyspnea and recent dysphagia with solid foods, fatigue, abdominal pain and recent hemoptysis. Physical examination revealed conjunctival icterus, jaundice, a tender neck mass, right upper chest wall tenderness and palpable lymphadenopathy in the lower neck. Assessment notes indicated inability to carry out self-care and being bedridden, even though she reported having been active, swimming and cycling, up to one month before her presentation. Computed tomography studies confirmed findings compatible with widespread malignancy with mass lesions in her liver, lungs, adrenals, kidneys, neck, pancreas, gallbladder, and brain. Mild anemia, hemoglobin 10.5g/dL, mild leukocytosis, leukocytes 17,000/mm3, were confirmed. Bilirubin was 15µmol/L, and hepatic enzymes were elevated. An ultrasound-guided liver biopsy was performed and evaluated for cytologic features, histologic features and pattern of immunoreactivity. The combined cytomorphologic and histologic features were those of a pleomorphic sarcomatoid high-grade malignancy with variably spindled to epithelioid morphology and associated necrosis. Immunohistochemical studies performed on sections from the accompanying core biopsy confirmed the malignant cells to be positive with CK-AE1/AE3, CK 7, TTF-1, GATA3, PAX8, SATB2, and BRAF V600E. The malignant cells were nonreactive with CK 20, thyroglobulin, calcitonin, napsin, CDX2, PCEA, and ER. The combined clinical setting, imaging findings, cytomorphology, histomorphology and ancillary testing results together allowed for a specific diagnosis of metastatic anaplastic thyroid carcinoma. The patient was referred for palliative care services and died within a few days of diagnosis. Results (if a Case Study enter NA) NA Conclusion Metastatic disease from a sarcomatoid ATC, as diagnosed in this case, is an important differential diagnosis to keep in mind when an elderly patient presents with multisite metastases from an undefined primary source. Clinical, radiological and laboratory correlations are of paramount importance in correlating with cytomorphology/histomorphology and in helping to determine the best panel of IHC studies to consider.

Recurrent de novo SPTLC2 variant causes childhood-onset amyotrophic lateral sclerosis (ALS) by excess sphingolipid synthesis

BackgroundAmyotrophic lateral sclerosis (ALS) is a neurodegenerative disease of the upper and lower motor neurons with varying ages of onset, progression and pathomechanisms. Monogenic childhood-onset ALS, although rare, forms an...

The spectrum of malignant diagnoses in cerebrospinal fluid cytology from an adult population: a multi-institutional retrospective review

IntroductionLimited updated literature exists about the prevalence and spectrum of malignancies involving cerebrospinal fluid (CSF). In this multi-institutional study, we review our experience with focus on first time malignancy diagnosis in CSF samples of adults. Materials and MethodsInstitutional databases at 4 academic centers were queried retrospectively for CSFs over a 10-year period. The following data elements were collected: total # of CSFs, total # of CSFs with a malignant diagnosis; for each patient with a first time CSF diagnosis of malignancy: age, gender, diagnosis, prior history of malignancy and ancillary studies. Results24142 CSFs were collected with a positive for malignancy rate of 2.3% (n=551). Out of 347 (1.4%) adults with a first-time diagnosis of CSF malignancy 182 (52%) were female (age range: 19-89/mean: 57) and 165 (48%) were male (age range: 20-95/mean: 60). Hematolymphoid malignancies (48%, n=168) were overall the most common neoplasm. In women, metastatic carcinomas (63%, n=114) were the leading malignancy, of which the majority were breast primaries. In men, lymphomas/leukemias (64%, n=106) were the leading malignancy, of which the majority were B-cell lymphomas. Ancillary studies aided the final diagnosis in 110 (32%) cases. For 286 (82%) cases, a prior history of malignancy was available to correlate CSF findings. ConclusionsA malignancy diagnosis in the CSF of adults is rare. The most common malignancies in females and males are metastatic breast carcinoma and hematolymphoid malignancies, respectively. Metastatic neoplasms account for the majority, with primary CNS neoplasms being quite uncommon. History of malignancy and ancillary tests can be helpful.

Clinical factors and ancillary tests used in modern clinical practice for diagnosing brain death

Clinical factors and ancillary tests used in modern clinical practice for diagnosing brain death

Validation of a Gene Expression Approach for the Cytological Diagnosis of Epithelioid and Biphasic Pleural Mesothelioma on a Consecutive Series

Cytological diagnosis of pleural mesothelioma (PM) is controversial, even using ancillary markers (BAP1, MTAP and CDKN2A). Here, we aimed to prospectively validate a previously developed 117-gene expression panel for the differential cytological diagnosis of epithelioid, biphasic PM and mesothelial hyperplasia. Seventy-seven pleural effusions were classified using the 117-gene expression levels (NanoString system). Sixty-eight cases were also screened for ancillary markers. The performance of both gene panel and ancillary markers was evaluated using ROC metrics. A score using the top consistently deregulated genes between epithelioid and biphasic PM was built to subtype malignant effusions. The panel alone reached a diagnostic accuracy (0.89) comparable to the best marker combination (BAP1 plus MTAP: 0.88). Ancillary tests missed 8 PMs, 7 of which were correctly classified by the panel. The score built by averaging the expression levels of MSLN, CLDN15 and CFB showed an accuracy of 0.80 in subtyping epithelioid and biphasic effusions. The 117-gene panel is effective for PM cytological diagnosis of epithelioid and biphasic PM. This tool can be complementary to ancillary markers, reducing invasive procedures and allowing an earlier diagnosis. Finally, the possibility to subtype PM on effusions strengthens the panel’s role in PM diagnosis and management.

Immunocytochemical markers, molecular testing and digital cytopathology for aspiration cytology of metastatic breast carcinoma.

Fine-needle aspiration cytology (FNAC) is a versatile diagnostic procedure uniquely suited for tissue biopsy of breast carcinomas and axillary metastases and/or recurrences. With the expanding treatment options and accompanying theragnostic tests, it is crucial to recognize the developments on ancillary testing and digital cytopathology techniques related to aspiration cytology of metastatic breast carcinoma. In this review, we aim to summarize and update the evidence of immunocytochemistry, for the detection of carcinoma cells (epithelial markers), confirmation of breast primary (breast-specific markers), assessment of surrogate immunostains (hormone receptors, ki-67 proliferative index and HER2) and theragnostic biomarkers, with discussion on potential diagnostic pitfalls, followed by the application of molecular tests, and digital cytopathologic techniques for assessing metastatic breast carcinoma in cytology.

Second edition of the Milan System for Reporting Salivary Gland Cytopathology: Refining the role of salivary gland FNA.

The use of standardized reporting systems for non-gynecologic cytopathology has made enormous gains in popularity during the past decade, including for thyroid fine-needle aspiration, urine cytology, serous effusions, pancreas, lymph nodes, lung, and more. In February 2018, the first edition Atlas of the Milan System for Reporting Salivary Gland Cytopathology (MSRSGC) was published. The MSRSGC defines six diagnostic fine-needle aspiration categories encompassing the spectrum of non-neoplastic, benign, and malignant lesions of the salivary glands. The goal of the MSRSGC is to combine each diagnostic category with a defined risk of malignancy and a specific clinical and/or surgical management algorithm. Since its initial publication in 2018, more than 200 studies and commentaries have been published confirming the role of the MSRSGC. The second edition of the MSRSGC, published in July 2023, includes refined risks of malignancy based on systematic reviews and meta-analyses, a new chapter summarizing the use of salivary gland imaging, new advances in ancillary testing, and updates in nomenclature.

Second edition of the Milan System for Reporting Salivary Gland Cytopathology: Refining the role of salivary gland FNA.

The use of standardised reporting systems for non-gynaecologic cytopathology has made enormous gains in popularity during the past decade, including for thyroid fine-needle aspiration, urine cytology, serous effusions, pancreas, lymph nodes, lung and more. In February 2018, the first edition of the Atlas of the Milan System for Reporting Salivary Gland Cytopathology (MSRSGC) was published. The MSRSGC defines six diagnostic fine-needle aspiration categories encompassing the spectrum of non-neoplastic, benign and malignant lesions of the salivary glands. The goal of the MSRSGC is to combine each diagnostic category with a defined risk of malignancy and a specific clinical and/or surgical management algorithm. Since its initial publication in 2018, more than 200 studies and commentaries have been published, confirming the role of the MSRSGC. The second edition of the MSRSGC, published in July 2023, includes refined risks of malignancy based on systematic reviews and meta-analyses, a new chapter summarising the use of salivary gland imaging, new advances in ancillary testing and updates in nomenclature.

Performance of the 23-gene Expression Profile (23-GEP) Test by Histopathological Evaluation in an Independent, Multi-Center Performance Cohort of Cutaneous Melanocytic Neoplasms

Histopathologic evaluation can effectively diagnose most melanocytic neoplasms; however, lesions considered to be difficult-to-diagnose pose challenges for accurate classification of malignant potential, which can lead to over- or under-treatment. Ancillary testing to provide additional information is available for such cases. The validated diagnostic 23-GEP test provides an objective result of benign, malignant, or intermediate. In this large, independent cohort, the performance of the 23-GEP in its current laboratory is presented. Lesions from patients ≥18 years old were enrolled from eight centers or from melanoma cases clinically submitted for prognostic 31-GEP testing. Lesions were independently reviewed by 3–5 dermatopathologists and included in the study if they were fully concordant or non-concordant without conflicting diagnoses (i.e., both benign and malignant designations) or majority of unknown malignant potential designations, resulting in a cohort (n=2512) of benign nevi (n=1140) and malignant melanomas (n=1372). Accuracy metrics and two-tailed 95% confidence intervals (CIs) were calculated using resampling x10,000 iterations to establish a balanced number of benign versus malignant samples. The 23-GEP performance within this cohort was 91.3% (95% CI, 89.2–93.2%) sensitivity, 91.9% (89.8–93.8%) specificity, 92.2% (9.30–94.0%) positive predictive value, and 91.0% (89.0–92.9%) negative predictive value; 7.8% of lesions received an intermediate result. These metrics do not deviate significantly from previously published studies. These data demonstrate that the 23-GEP has an overall accuracy of 91.6% (90.1–93.0%), further supporting its use as an ancillary test that can be integrated with clinical and histopathologic information to guide final diagnosis.

Granulomatous dermatitis caused by Exserohilum mcginnisii infection in a cow.

A 5-y-old, Piedmontese cow had a 4-mo history of ongoing development of skin masses. This was the only cow affected in a herd of 20 cows. Up to 12, hairless, red-to-black, raised nodules-to-plaques were distributed along the dorsum and tail head. Biopsies were taken for histopathology and ancillary testing. An ulcerated skin section contained dermal infiltrates of eosinophils, plasma cells, neutrophils, macrophages, lymphocytes, and multinucleate giant cells, and pyogranulomas. Fungal hyphae were seen within the dermis, multinucleate giant cells, and pyogranulomas. In pyogranulomas, fungi were surrounded by a Splendore-Hoeppli reaction. Dematiaceous (pigmented) hyphae were rarely observed with H&E-stained and unstained (cleared and mounted) sections, but stained well with a Fontana-Masson stain. Exserohilum mcginnisii was identified by fungal culture, followed by PCR assay and sequencing. Exserohilum is a dematiaceous fungus that causes disease in humans and rarely in animals. The use of unstained sections and Fontana-Masson stain are important to demonstrate pigment because dematiaceous fungi have little melanin and appear as hyaline hyphae histologically. PCR assay and sequencing aid in the differentiation and classification of fungal species. To our knowledge, E. mcginnisii dermal granulomas have not been reported previously in cattle.

Do young children with known cannabis intoxication benefit from further neurological-based testing or imaging?

BackgroundRecent work has demonstrated that children with unintentional cannabis ingestions often undergo extensive ancillary testing such as head imaging or lumbar puncture. To better understand the yield of these tests, our objective was to describe the frequency of additional significant diagnoses in children with cannabis ingestion. MethodsWe performed a retrospective cross-sectional study of the Pediatric Health Information System (PHIS) database, including ED encounters from January 2016 to April 2023 with a diagnosis indicating cannabis exposure in children <6 years of age. We assessed the frequency of co-diagnoses that would be found on head imaging, lumbar puncture, or toxicology testing. ResultsWe included 4132 ED encounters for cannabis ingestion from 47 hospitals. Of these, 1243 (30%) received head imaging and 130 (3.1%) underwent lumbar puncture. There were 23 children (0.6%) with diagnosis of skull fracture or intracranial hemorrhage, 4 (<0.1%) with intracranial neoplasm, and 0 (0%) with a diagnosis for meningitis or intracranial abscess. Presence of discharge diagnosis for other drugs was also uncommon. The most frequent drug ingestion co-diagnoses were cocaine in 43 (1.0%) and opioids in 22 (0.5%) encounters. ConclusionIn children with cannabis intoxication, high rates of head imaging and lumbar puncture are likely driven by the signs of altered mental status at presentation. These data suggest that if cannabis ingestion is considered early and identified quickly with testing, neuroimaging, particularly that with ionizing radiation, may be low yield.

Immunohistochemistry for PRAME in Dermatopathology.

Preferentially expressed antigen in melanoma (PRAME) is a tumor-associated antigen first identified in a melanoma patient and found to be expressed in most melanomas as well as in variable levels in other malignant neoplasms of epithelial, mesenchymal, or hematolymphoid lineage. Detection of PRAME expression in formalin-fixed paraffin-embedded tissue is possible by immunohistochemistry (IHC) with commercially available monoclonal antibodies. In situ and invasive melanoma frequently show a diffuse pattern of nuclear PRAME immunoreactivity which contrasts with the infrequent and typically nondiffuse staining seen in nevi. In many challenging melanocytic tumors, results of PRAME IHC and other ancillary tests correlate well, but not always: The tests are not interchangeable. Most metastatic melanomas are positive for PRAME, whereas nodal nevi are not. Numerous studies on PRAME IHC have become available in the past few years with results supporting the value of PRAME IHC as an ancillary tool in the evaluation of melanocytic lesions and providing insights into limitations in sensitivity and specificity as well as possible pitfalls that need to be kept in mind by practicing pathologists.

Transcranial Doppler ultrasound to evaluate the risk of hyperperfusion after endovascular stroke thrombectomy.

Hemorrhagic transformation (HT) has been reported in up to 50% of acute ischemic stroke (AIS) patients with a large vessel occlusion (LVO) treated with endovascular thrombectomy (EVT). HT may be driven by postrecanalization hyperperfusion injury and is independently associated with worse functional outcomes. Strategies to identify patients at risk for HT may assist in developing preventive therapies. We prospectively included adult AIS patients with an anterior circulation LVO achieving successful recanalization after EVT. Consenting participants received transcranial Doppler ultrasound (TCD) within 18 hours of procedure completion. We compared flow velocities according to the presence of HT on the computed tomography scan performed within the first 24±12 hours from the end of EVT. We also evaluated the association of flow velocities with systemic blood pressure (BP) readings at the time of insonation. A total of 48 patients consented to participate in the study. Six (12%) were excluded due to the absence of temporal windows. HT was detected in 20 participants (48%). Those with HT had higher peak systolic velocities on the middle cerebral arteries compared to those without HT for both the symptomatic (107±42vs. 82±25cm/second, p = .024) and asymptomatic (97±21vs. 81±25cm/second, p = .040) sides. No correlation of flow velocities on either the symptomatic or asymptomatic side and BP measurements at the time of insonation was detected. TCD can identify patients at risk of HT following successful EVT. TCD could serve as an inexpensive ancillary test to guide participant selection for clinical trials targeting postprocedural reperfusion injury.

Targeted RNA Sequencing Highlights a Diverse Genomic and Morphologic Landscape in Low-grade Endometrial Stromal Sarcoma, Including Novel Fusion Genes.

Low-grade endometrial stromal sarcoma (LGESS) represents a morphologically and genetically heterogenous mesenchymal neoplasm. Previous work has shown that approximately half of LGESS are characterized by JAZF1::SUZ12 gene fusions, while a smaller proportion involves rearrangement of other genes. However, a subset of cases has no known genetic abnormalities. To better characterize the genomic landscape of LGESS, we interrogated a cohort with targeted RNA sequencing (RNA-Seq). Cases previously diagnosed as low-grade endometrial stromal neoplasia (n=51) were identified and re-reviewed for morphology and subjected to RNA-Seq, of which 47 were successfully sequenced. The median patient age was 49 years (range: 19 to 85). The most commonly detected fusions were JAZF1::SUZ12 (n=26, 55%) and BRD8::PHF1 (n=3, 6%). In addition to the usual/typical LGESS morphology, some JAZF1::SUZ12 fusion tumors showed other morphologies, including fibrous, smooth muscle, sex-cord differentiation, and myxoid change. Novel translocations were identified in 2 cases: MEAF6::PTGR2 and HCFC1::PHF1 . Ten tumors (21%) had no identifiable fusion, despite a similar morphology and immunophenotype to fusion-positive cases. This suggests that a subset of cases may be attributable to fusion products among genes that are not covered by the assay, or perhaps altogether different molecular mechanisms. In all, these findings confirm that RNA-Seq is a potentially useful ancillary test in the diagnosis of endometrial stromal neoplasms and highlight their diverse morphology.

Liposarcoma Involving Serous Fluid Cavities-A Case Series Illustrating Clinical Implications and the Diagnostic Role of Exfoliative Cytology.

Introduction: Cytological diagnosis of sarcomas requires detailed cytomorphological assessment and integration of immunocytochemistry and/or molecular testing. The role of exfoliative cytology, as compared to aspiration cytology, is less understood. This case series describes well-differentiated/dedifferentiated liposarcomas in effusions, with cytomorphological features, ancillary test results and clinical outcomes detailed. Methods: A computerized search of the department pathology archives was performed for sarcomatous effusions with histological diagnosis or clinical history of well-differentiated/dedifferentiated liposarcoma. Clinical progress, cytology slides, immunocytochemistry and molecular test results were reviewed. Results: Six patients were identified. In 5 patients with clinical follow up, 4 (80%) were deceased within 5 months of malignant effusion. One patient was alive with 12 years disease-free survival after radical resection with adjuvant radiotherapy. Three patients showed dedifferentiation on histology, and high-grade (dedifferentiated) tumor cells were present in effusion cytology of 2 patients. Two showed well-differentiated components only on biopsy, but high-grade (dedifferentiated) tumor cells were identified in cytology. The high-grade tumor cells displayed marked nuclear irregularity, enlargement, size variation, with macronucleoli and multinucleation. Well-differentiated lipomatous components were demonstrated in 4 patients (66.7%), comprising of multivacuolated lipoblasts and atypical lipocytes. CDK4 and MDM2 immunoreactivity in all 3 cases with cell blocks, and CDK4 and MDM2 amplification in one were successfully demonstrated. Conclusion: Lipomatous and dedifferentiated components can be sampled and cytomorphologically identified on effusion fluids of liposarcomas, with sufficient cellularity for immunocytochemistry and molecular testing. Although generally associated with poor prognosis, long disease-free survival with sarcomatous effusion is possible with radical surgery and adjuvant treatment.

Mammary Tumors in Dogs, Recent Perspectives and Antiangiogenesis as a Therapeutic Strategy: Literature Study

A particular type of tumor that is frequently detected in female dogs who are sexually active is a mammary tumor. Neoplasia results from DNA-based alterations in cell cycle regulating genes. The mammary gland is prone to the formation of tumors due to its dynamic structure. The development of this tumor is supported by numerous variables. It has been recently discovered that there is substantial evidence linking the BRCA2 gene to the process of cancer. Standard examination techniques, such as fine needle aspiration, histopathology, and immunohistochemistry, are used along with ancillary tests to determine the tumor type and degree of malignancy. The primary treatment option for malignant tumors is surgical resection followed by adjuvant chemotherapy; benign tumors necessitate surgical resection as well. Adjuvant therapy options include hormone therapy and non-steroidal anti-inflammatory medications. Tumor tissue undergoes angiogenesis as it grows and develops to accommodate the abundant supply of nutrients. Therefore, angiogenesis-inhibiting therapies can be utilized to halt the growth of tumor cells. A number of antiangiogenic medications are now being studied in clinical settings on humans, and several more are undergoing trials on animals. In addition to pharmaceuticals, viruses may be used as a therapeutic to block tumor angiogenesis.

Diagnostic Approaches, Classification and Management in Retinoblastoma

Retinoblastoma is the most common intraocular tumor of childhood. Although retinoblastoma classically presents with leukocoria and strabismus, the definitive diagnosis is made by evaluating fundoscopic findings together with the findings of ancillary tests such as ultrasonography, fundus fluorescein angiography, optical coherence tomography and magnetic resonance imaging. The management of the disease is closely related to the presence of sporadic or hereditary disease, the presence of unilateral or bilateral disease and the presence of local or distant metastases. The aim of this article is to summarize the findings of the traditional methods together with the innovations in the diagnostic tools of retinoblastoma, and to explain the classification systems and the treatment methods adopted accordingly.

Pituitary Adenoma Hidden by Comorbid Cataract

Background: Background: A pituitary adenoma is an abnormal growth in the pituitary gland. Pituitary adenomas are the most common cause of sellar masses accounting for approximately 10% of all intracranial neoplasms. Early visual symptom of pituitary adenoma can be hidden by comorbid cataract in elderly patients. Case Case Report: Report: A 74 y.o. male presented to the eye clinic with blurry vision at all distances and has trouble with glare from lights. His best corrected visual acuity (BCVA) was 20/20-1 OD, 20/25-2 OS, but subjectively of poor quality. Slit lamp exam confirmed mild nuclear sclerotic cataract OU and normal fundus exam. Patient underwent uncomplicated cataract extraction with intraocular implant, OS first then OD. Post-op course was unremarkable until one month later when patient stated that his vision OS was blurry to the left. BCVA was 20/25 OD, 20/25-2 OS. Post-op exam was still normal including FCF. Humphrey visual field (HVF) however showed a bitemporal hemianopia. Patient was referred for a brain imaging. Brain MRI confirmed a 24 x 22 x 28 mm T2 isointense enhancing mass of the sella with suprasellar extension consistent with a pituitary macroadenoma. Patient underwent endonasal transphenoidal craniectomy for debulking of the tumor subsequently by an ENT specialist. Conclusion Conclusion: Clinicians need to be sure the cataract matches the visual complaint. When in doubt further ancillary testing such as comprehensive HVF is crucial to detect visual pathway disorder and brain tumors. Co-management with corresponding specialist is essential to preserve vision and save life.

Pediatric and Adult Brain Death/Death by Neurologic Criteria Consensus Guideline.

The purpose of this guideline is to update the 2010 American Academy of Neurology (AAN) brain death/death by neurologic criteria (BD/DNC) guideline for adults and the 2011 American Academy of Pediatrics, Child Neurology Society, and Society of Critical Care Medicine guideline for infants and children and to clarify the BD/DNC determination process by integrating guidance for adults and children into a single guideline. Updates in this guideline include guidance related to conducting the BD/DNC evaluation in the context of extracorporeal membrane oxygenation, targeted temperature management, and primary infratentorial injury. A panel of experts from multiple medical societies developed BD/DNC recommendations. Because of the lack of high-quality evidence on the subject, a novel, evidence-informed formal consensus process was used. This process relied on the panel experts' review and detailed knowledge of the literature surrounding BD/DNC to guide the development of preliminary recommendations. Recommendations were formulated and voted on, using a modified Delphi process, according to the 2017 AAN Clinical Practice Guideline Process Manual. Eighty-five recommendations were developed on the following: (1) general principles for the BD/DNC evaluation, (2) qualifications to perform BD/DNC evaluations, (3) prerequisites for BD/DNC determination, (4) components of the BD/DNC neurologic examination, (5) apnea testing as part of the BD/DNC evaluation, (6) ancillary testing as part of the BD/DNC evaluation, and (7) special considerations for BD/DNC determination.

Contribution of BAP1 loss and p16 (CDKN2A) deletion analysis to the definitive diagnosis of mesothelioma in effusion cytology.

The cytological diagnosis of mesothelioma is a controversial issue, and definitive diagnosis often requires ancillary tests. The aim of this study was to investigate the contribution of BRCA1-associated protein (1) (BAP1) loss and p16 (CDKN2A) homozygous deletion (HD) on the early diagnosis of mesothelioma in effusion fluids. Between 2019-2022, 21 pleural and peritoneal fluid samples diagnosed with atypical mesothelial proliferation in our institution were included in the study. The slides of the cases that underwent BAP1 immunohistochemistry (IHC) were retrieved from the archive and re-examined. Homozygous deletion (HD) of p16 (CDKN2A) was investigated by the fluorescence in situ hybridization (FISH) method in cell blocks of cytology samples. At least 100 atypical mesothelial cells were counted in each case, and the HD threshold value was >10%. The mean age of the cases was 63.47 years (34-90 years), female/male ratio was 3/1. Of the pleural mesothelioma cases, 16 were epithelioid, 2 were biphasic, and 1 were sarcomatoid. Two cases were diagnosed with peritoneal well-differentiated papillary mesothelioma (WDPM). BAP1 loss was observed in 11 (69%) of 16 cases. HD deletion of p16 (CDKN2A) was seen in 11 (58%) patients with FISH. The HD threshold value was 10-20% in 6 of the cases, 30-50% in 3 cases, and above 90% in 2 cases. While HD deletion was observed in p16 (CDKN2A) in all biphasic and sarcomatoid cases (n=3), no deletion was observed in peritoneal WDPM (n=2). Positivity was observed with at least one method in 12 (86%) of 14 pleural mesotheliomas who underwent both BAP1 IHC and p16 (CDKN2A) FISH. Due to technical reasons, the FISH signal could not be obtained in two cell blocks, so no results could be obtained. Asbestos exposure in areas where mesothelioma is endemic and/or the presence of proliferating mesothelial cells in cytological examination are important clues for diagnosis. In controversial cases, BAP1 IHC should be the first step in an ancillary test. Although the FISH method applied to cell blocks has cytology-specific limitations and difficulties, investigating the p16 (CDKN2A) deletion with FISH in selected cases will contribute to the diagnosis.