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* Context.--Mesothelioma is an uncommon tumor that can be difficult to diagnose. Objective.--To provide updated, practical guidelines for the pathologic diagnosis of mesothelioma. Data Sources.--Pathologists involved in the International Mesothelioma Interest Group and others with expertise in mesothelioma contributed to this update. Reference material includes peer-reviewed publications and textbooks. Conclusions.--There was consensus opinion regarding guidelines for (1) histomorphologic diagnosis of mesothelial tumors, including distinction of epithelioid, biphasic, and sarcomatoid mesothelioma; recognition of morphologic variants and patterns; and recognition of common morphologic pitfalls; (2) molecular pathogenesis of mesothelioma; (3) application of immunohistochemical markers to establish mesothelial lineage and distinguish mesothelioma from common morphologic differentials; (4) application of ancillary studies to distinguish benign from malignant mesothelial proliferations, including BAP1 and MTAP immunostains; novel immunomarkers such as Merlin and p53; fluorescence in situ hybridization (FISH) for homozygous deletion of CDKN2A; and novel molecular assays; (5) practical recommendations for routine reporting of mesothelioma, including grading epithelioid mesothelioma and other prognostic parameters; (6) diagnosis of mesothelioma in situ; (7) cytologic diagnosis of mesothelioma, including use of immunostains and molecular assays; and (8) features of nonmalignant peritoneal mesothelial lesions. (Arch Pathol Lab Med. 2024;148:1251-1271; doi: 10.5858/arpa.2023-0304-RA), Pathologic diagnosis of mesothelioma continues to evolve. These updated guidelines (1) reflect the authors' expert opinion, informed by a thorough review of relevant literature. The contents were discussed at the [...]

* Context.--Respiratory infections complicate lung transplantation and increase the risk of allograft dysfunction. Allograft lungs may have different susceptibilities to infection than native lungs, potentially leading to different disease severity in lungs of single lung transplant recipients (SLTRs). Objective.--To study whether infections affect allograft and native lungs differently in SLTRs but similarly in double LTRs (DLTRs). Design.--Using an institutional database of LTRs, medical records were searched, chest computed tomography studies were systematically reviewed, and histopathologic features were recorded per lung lobe and graded semiquantitatively. A multilobar-histopathology score (MLHS) including histopathologic data from each lung and a bilateral ratio ([MLHS.sub.ratio]) comparing histopathologies between both lungs were calculated in SLTRs and compared to DLTRs. Results.--Six SLTRs died of infection involving the lungs. All allografts showed multifocal histopathologic evidence of infection, but at least 1 lobe of the native lung was uninvolved. In 4 of 5 DLTRs, histopathologic evidence of infection was seen in all lung lobes. On computed tomography, multifocal ground-glass and/or nodular opacities were found in a bilateral distribution in all DLTRs but in only 2 of 6 SLTRs. In SLTRs, the [MLHS.sub.Allograft] was higher than [MLHS.sub.Native] (P = .02). The [MLHS.sub.ratio] values of SLTR and DLTR were significantly different (P < .001). Conclusions.--Allograft and native lungs appear to harbor different susceptibilities to infections. The results are important for the management of LTRs. doi: 10.5858/arpa.2023-0227-OA), The median overall survival of lung transplant recipients (LTRs) is only 6.7 years, which is shorter than for other solid organ transplantations. (1) Noncytomegalovirus (nonCMV) infections play a major role [...]

* Context.--Diffuse parenchymal lung disease (DPLD) is a well-recognized complication of systemic connective tissue disease (CTD) but rarely arises in patients with psoriasis or psoriatic arthritis, a poorly understood phenomenon. Objective.--To characterize DPLD associated with psoriasis or psoriatic arthritis, with or without prior immunomodulation. Design.--Pathology consultation files were searched for patients having psoriasis or psoriatic arthritis and DPLD. After excluding cases with active infection or smoking-related DPLD only, 44 patients (22 women; median age, 60 years; range, 23-81 years) were enrolled. Clinical history and pathology slides were reviewed. Results.--Twenty-seven of 44 patients (61 %) had psoriatic arthritis; the remainder had psoriasis alone. Most presented many years later with nonspecific respiratory symptoms. Nearly one-third had no prior immunosuppression, and most had no concomitant CTD. Radiographically, ground-glass opacities, consolidation, and/or reticulation were typical. Histologically, nonspecific interstitial pneumonia and unclassifiable fibrosis were seen in 24 patients (55%) and 8 patients (18%), respectively; usual interstitial pneumonia and airway-centered fibrosis were rare. Super-imposed acute lung injury was common, usually manifesting as organizing pneumonia. Lymphoplasmacytic infiltrates, lymphoid aggregates, and chronic pleuritis were frequent. Interstitial granulomas were seen in 17 patients (39%) but were usually rare, poorly formed, and non-necrotizing. No histologic differences were apparent among patients with or without concomitant CTDs or prior therapy. Conclusions.--Some patients with psoriasis or psoriatic arthritis develop clinically significant DPLD, even without prior therapy. Histopathologic findings mirror changes seen with other CTDs. Additional studies are warranted to clarify the association between psoriasis or psoriatic arthritis and DPLD. doi: 10.5858/arpa.2021-0616-OA, Psoriasis is a common chronic autoimmune inflammatory disorder of the skin that may exhibit a variety of clinical manifestations. Some cases of psoriasis are complicated by the development of psoriatic [...]

* Context.--Organ weights are an essential part of autopsy analysis. Deviations from normal organ weights provide important clues to disease processes. The assessment of normal organ weights depends on reliable reference tables, but most widely available reference tables are based on data that are either decades old or derived from relatively small sample sizes. Objective.--To provide an updated reference table of organ weights based on contemporary sources and a large sample size. Design.--Organ weights from 4197 carefully screened autopsies performed on adults at the Palm Beach County Medical Examiner's Office in West Palm Beach, Florida, and the Mayo Clinic Hospital in Rochester, Minnesota. Results.--Height and body weight data in this study reflect the well-recognized increases in both variables, but most particularly in body weight, seen during the last decades. The study data show a strong positive association between organ weight and body weight for the heart, liver, and spleen. There is a similar but weaker association between body weight and the weight of the lungs and kidneys. Brain weight is independent of body weight but shows a strong negative association with age. Even when controlling for body weight, men's organs are heavier, except for the weight of the liver, which is comparable in men and women. These associations are in agreement with the findings of previous studies. The current study suggests that, for some of the commonly weighed organs, there has been an increase in median organ weight when compared with existing references. Conclusions.--The tables presented here provide an updated reference that should prove useful to autopsy pathologists in the forensic and hospital settings., Organ weights are a cornerstone of autopsy gross pathology. By weighing the major organs with a simple hanging balance and comparing the results with standard tables, pathologists can draw inferences [...]

Context.--Studies of lungs in patients with COVID-19 have focused on early findings. Objective.--To systematically study histopathologic and imaging features and presence of SARS-CoV-2 RNA in lung tissue from patients in later stages of COVID-19. Design.--Autopsies, explants, surgical lung biopsies, transbronchial biopsies, cryobiopsies, and needle biopsies from patients with COVID-19 whose onset of symptoms/ confirmed diagnosis was more than 28 days before the procedure were studied. Available images were reviewed. Reverse transcription droplet digital polymerase chain reaction for SARS-CoV-2 RNA was performed on lung tissue. Results.--Of 44 specimens (43 patients; median age, 59.3 years; 26 [60.5%] male) features of acute lung injury (ALI) were seen in 39 (88.6%), predominantly organizing pneumonia and diffuse alveolar damage, up to 298 days after onset of COVID-19. Fibrotic changes were found in 33 specimens (75%), most commonly fibrotic diffuse alveolar damage (n = 22) and cicatricial organizing pneumonia (n = 12). Time between acquiring COVID-19 and specimen was shorter in patients with diffuse ALI (median, 61.5 days) compared with patients with focal (140 days) or no AlI (130 days) (P =.009). Sixteen (of 20; 80%) SARS-CoV-2 reverse transcription droplet digital polymerase chain reaction tests were positive, up to 174 days after COVID-19 onset. Time between COVID-19 onset and most recent computed tomography in patients with consolidation on imaging was shorter (median, 43.0 days) versus in patients without consolidation (87.5 days; P =.02). Reticulations were associated with longer time to computed tomography after COVID-19 onset (median, 82 versus 23.5 days; P =.006). Conclusions.--ALI and SARS-CoV-2 RNA can be detected in patients with COVID-19 for many months. ALI may evolve into fibrotic interstitial lung disease., Coronavirus disease 2019 (COVID-19) is a viral infection caused by SARS-CoV-2. More than 235 million people have been infected globally, with a death burden in excess of 4.8 million since [...]

* Context.--Small case series have evaluated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) detection in formalin-fixed, paraffin-embedded tissue using reverse transcription-polymerase chain reaction, immunohistochemistry (IHC), and/or RNA in situ hybridization (RNAish). Objective.--To compare droplet digital polymerase chain reaction, IHC, and RNAish to detect SARS-CoV-2 in formalin-fixed, paraffin-embedded tissue in a large series of lung specimens from coronavirus disease 2019 (COVID-19) patients. Design.--Droplet digital polymerase chain reaction and RNAish used commercially available probes; IHC used clone 1A9. Twenty-six autopsies of COVID-19 patients with formalin-fixed, paraffin-embedded tissue blocks of 62 lung specimens, 22 heart specimens, 2 brain specimens, and 1 liver, and 1 umbilical cord were included. Control cases included 9 autopsy lungs from patients with other infections/inflammation and virus-infected tissue or cell lines. Results.--Droplet digital polymerase chain reaction had the highest sensitivity for SARS-CoV-2 (96%) when compared with IHC (31%) and RNAish (36%). All 3 tests had a specificity of 100%. Agreement between droplet digital polymerase chain reaction and IHC or RNAish was fair ([kappa] = 0.23 and [kappa] = 0.35, respectively). Agreement between IHC and in situ hybridization was substantial (k = 0.75). Interobserver reliability was almost perfect for IHC ([kappa] = 0.91) and fair to moderate for RNAish ([kappa] = 0.380.59). Lung tissues from patients who died earlier after onset of symptoms revealed higher copy numbers by droplet digital polymerase chain reaction (P =.03, Pearson correlation = -0.65) and were more likely to be positive by RNAish (P =.02) than lungs from patients who died later. We identified SARS-CoV-2 in hyaline membranes, in pneumocytes, and rarely in respiratory epithelium. Droplet digital polymerase chain reaction showed low copy numbers in 7 autopsy hearts from ProteoGenex Inc. All other extrapulmonary tissues were negative. Conclusions.--Droplet digital polymerase chain reaction was the most sensitive and highly specific test to identify SARS-CoV-2 in lung specimens from COVID-19 patients. (Arch Pathol Lab Med. 2021;145:785-796; doi: 10.5858/arpa.2021-0008-SA), Coronavirus disease 2019 (COVID-19), the disease that is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, can be life-threatening, largely because of development of acute respiratory distress syndrome. [...]

* Context.--Distinguishing pulmonary sarcomatoid carcinoma from pleural sarcomatoid mesothelioma is challenging because of overlapping histology, immunophenotype, and clinical features. Reliable immunohistochemical markers to aid in this distinction would be very valuable. Recent studies have proposed that MUC4 expression is common in sarcomatoid carcinoma but not in sarcomatoid mesothelioma, with the converse pattern reported for GATA3. Objective.--To further explore the utility of MUC4 and GATA3 in distinguishing pulmonary sarcomatoid carcinoma from sarcomatoid mesothelioma. Design.--Well-characterized cases of sarcomatoid carcinoma (n = 32) and sarcomatoid mesothelioma (n = 64) were included. Diagnoses were confirmed by thoracic pathologists with incorporation of immunophenotype, clinical, and radiographic features. Whole-tissue sections were stained for GATA3 and MUC4. Results.--Patients with sarcomatoid carcinoma and sarcomatoid mesothelioma had similar mean age and male predominance. GATA3 was positive in 63 of 64 sarcomatoid mesotheliomas (98%; 42 diffuse, 16 patchy, 5 focal), and 15 of 32 sarcomatoid carcinomas (47%; 3 diffuse, 8 patchy, 4 focal). MUC4 was positive in 2 of 64 sarcomatoid mesotheliomas (3%; 1 patchy, 1 focal), and in 12 of 32 sarcomatoid carcinomas (38%; 5 diffuse, 6 patchy, 1 focal). Conclusions.--Diffuse GATA3 expression favors sarcomatoid mesothelioma over sarcomatoid carcinoma, which rarely shows diffuse expression (sensitivity and specificity of diffuse staining 66% and 94%, respectively). Focal and patchy GATA3 expression is observed in both tumor types, and therefore is not helpful in this distinction. Sensitivity of MUC4 for sarcomatoid carcinoma was low in our cohort, positive in only 38% with frequent patchy staining, but it was quite specific. doi: 10.5858/arpa.2019-0647-OA, Pathologic diagnosis of sarcomatoid neoplasms in the lung and pleura is quite difficult, because the differential diagnosis includes a long list of primary and metastatic tumors. While many entities, such [...]

* Context.--Respiratory failure appears to be the ultimate mechanism of death in most patients with severe coronavirus disease 2019 (COVID-19) infection. Studies of postmortem COVID-19 lungs largely report diffuse alveolar damage and capillary fibrin thrombi, but we have also observed other patterns. Objective.--To report demographic and radiographic features along with macroscopic, microscopic, and microbiologic postmortem lung findings in patients with COVID19 infections. Design.--Patients with confirmed COVID-19 infection and postmortem examination (March 2020-May 2020) were included. Clinical findings were abstracted from medical records. Lungs were microscopically reviewed independently by 4 thoracic pathologists. Imaging studies were reviewed by a thoracic radiologist. Results.--Eight patients (7 men, 87.5%; median age, 79 years; range, 69-96 years) died within a median of 17 days (range, 6-100 days) from onset of symptoms. The median lung weight was 1220 g (range, 960-1760 g); consolidations were found in 5 patients (62.5%) and gross thromboemboli were noted in 1 patient (12.5%). Histologically, all patients had acute bronchopneumonia; 6 patients (75%) also had diffuse alveolar damage. Two patients (25%) had aspiration pneumonia in addition. Thromboemboli, usually scattered and rare, were identified in 5 patients (62.5%) in small vessels and in 2 of these patients also in pulmonary arteries. Four patients (50%) had perivascular chronic inflammation. Postmortem bacterial lung cultures were positive in 4 patients (50%). Imaging studies (available in 4 patients) were typical (n = 2, 50%), indeterminate (n = 1, 25%), or negative (n = 1, 25%) for COVID-19 infection. Conclusions.--Our study shows that patients infected with COVID-19 not only have diffuse alveolar damage but also commonly have acute bronchopneumonia and aspiration pneumonia. These findings are important for management of these patients. (Arch Pathol Lab Med. 2021;145:11-21; doi: 10.5858/arpa.2020-0491-SA), Acute lung injury appears to be the most serious complication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and the subsequent clinical disease, coronavirus disease 2019 (COVID-19), resulting in [...]