[Progress in methodological research on bridging the efficacy-effectiveness gap of clinical interventions(2): to improve the extrapolation of efficacy].

Objective: Randomized controlled trials (RCT) usually have strict implementation criteria. The included subjects' characteristics of the conditions for the intervention implementation are quite different from the actual clinical environment, resulting in discrepancies between the risk-benefit of interventions in actual clinical use and the risk-benefit shown in RCT. Therefore, some methods are needed to enhance the extrapolation of RCT results to evaluate the real effects of drugs in real people and clinical practice settings. Methods: Six databases (PubMed, Embase, Web of Science, CNKI, Wanfang Data, and VIP) were searched up to 31st December 2022 with detailed search strategies. A scoping review method was used to integrate and qualitatively describe the included literature inductively. Results: A total of 12 articles were included. Three methods in the included literature focused on: ①improving the design of traditional RCT to increase population representation; ②combining RCT Data with real-world data (RWD) for analysis;③calibrating RCT results according to real-world patient characteristics. Conclusions: Improving the design of RCT to enhance the population representation can improve the extrapolation of the results of RCT. Combining RCT data with RWD can give full play to the advantages of data from different sources; the results of the RCT were calibrated against real-world population characteristics so that the effects of interventions in real-world patient populations can be predicted.

[Analysis methods and case analysis of effect modification (3): effect modification in individual patient data Meta-analysis].

This paper briefly introduces the unique advantages, overall analysis ideas and existing analysis methods of individual patient data Meta-analysis in terms of effect modification. In addition to Meta-regression and subgroup analysis, this paper also introduces the analysis methods based on part of individual patient data integrated with aggregated data and summarizes the current reporting of the above mentioned methods. In addition, the application and results interpretation of the above mentioned methods in individual patient data Meta-analysis are presented in this paper by taking "Effects of sodium-glucose cotransporter 2 inhibitors on SBP in patients with type 2 diabetes" as an example and by introducing their advantages and limitations.

[Analysis methods and case analysis of effect modification (2): effect modification in network Meta-analysis].

This paper briefly introduces the characteristics, research significance, and global reporting status of effect modification in network Meta-analysis, demonstrates the heterogeneity caused by effect modification in network Meta-analysis, and emphasizes the importance of exploring effect modification in network Meta-analysis. This paper also summarizes the normalized description and analysis strategies of effect modification in network Meta-analysis. Finally, by the case of "comparison of efficacy of three new hypoglycemic drugs in reducing body weight in type 2 diabetes patients", this paper demonstrates the realization of subgroup analysis and network Meta-regression in exploring effect modification, summarizes the advantages and disadvantages of the two methods, to provide references for future researchers.

[Progress in methodological research on bridging the efficacy-effectiveness gap of clinical interventions (1): to improve the validity of real-world evidence].

Objective: Differences between randomized controlled trial (RCT) results and real world study (RWS) results may not represent a true efficacy-effectiveness gap because efficacy-effectiveness gap estimates may be biased when RWS and RCT differ significantly in study design or when there is bias in RWS result estimation. Secondly, when there is an efficacy- effectiveness gap, it should not treat every patient the same way but assess the real-world factors influencing the intervention's effectiveness and identify the subgroup likely to achieve the desired effect. Methods: Six databases (PubMed, Embase, Web of Science, CNKI, Wanfang Data, and VIP) were searched up to 31st December 2022 with detailed search strategies. A scoping review method was used to integrate and qualitatively describe the included literature inductively. Results: Ten articles were included to discuss how to use the RCT research protocol as a template to develop the corresponding RWS research protocol. Moreover, based on correctly estimating the efficacy-effectiveness gap, evaluate the intervention effect in the patient subgroup to confirm the subgroup that can achieve the expected benefit-risk ratio to bridge the efficacy-effectiveness gap. Conclusion: Using real-world data to simulate key features of randomized controlled clinical trial study design can improve the authenticity and effectiveness of study results and bridge the efficacy-effectiveness gap.

[Analysis methods and case analysis of effect modification (1): effect modification in epidemiology and traditional Meta-analysis].

This paper briefly introduces the definition, classification and significance of effect modification in epidemiological studies, summarizes the difference between effect modifier and confounders, and analyze the influence as well as the role of effect modification in epidemiological studies and Meta-analysis. In this paper, the possible scenarios of effect modification and related analysis strategy in Meta-analysis are indicated by graphics, aiming to arouse researchers' attention to effect modification. This paper also demonstrates how to identify and deal with effect modification in Meta-analysis through a study case of "Efficacy of sodium-glucose cotransporter 2 inhibitors in patients with type 2 diabetes", and shows the analysis process and interpretation of results of subgroup analysis and Meta-regression methods respectively. The advantages and disadvantages of these two methods are summarized to provide reference for the method selection of future research.

[Introduction and application of European Academy of Paediatric Dentistry judgment criteria and scoring system for molar-incisor hypomineralization].

Molar-incisor hypomineralization (MIH) is defined as an enamel mineralization defect caused by systemic factors, which is characterized by demarcated opacities. These opacities are liable to result in brittle hypomineralized enamel breakdown, which expediting the eventual development of cavities, even tooth loss. Early diagnosis and prompt intervention are essential. The MIH scoring system based on the diagnostic criteria of the European Academy of Paediatric Dentistry (EAPD) is internationally recognized. This system is particularly helpful to diagnose and evaluate the MIH, as well as conductive to the performance of epidemiological investigations. This paper gives a presentation on the EAPD judgment criteria and scoring system as well as their applications, based on the current situation of MIH studies and our findings of MIH epidemiological investigation.

[Clinical features and influencing factors of long-term prognosis in patients with tuberculous meningitis].

Objective: To investigate the clinical features and influencing factors of long-term prognosis of tuberculous meningitis(TBM), and to provide a recommendation for treatment and early intervention of TBM. Methods: Clinical data of TBM patients were retrospectively collected at Peking Union Medical College Hospital from January 2014 to December 2021. Patients who were followed-up more than one year were divided into two groups according to modified Rankin Scale (mRS). Risk factors associated with long-term prognosis were analyze by conditional logistic stepwise regression. Results: A total of 60 subjects were enrolled including 33 (55%) males and 27 (45%) females with age 15-79 (44.5±19.8) years. There were 30 cases (50%) complicated with encephalitis, 21 cases (35%) with miliary tuberculosis. The diagnosis was microbiologically confirmed in 22 patients (36.7%), including 5 cases (22.7%, 5/22) by acid-fast staining, 8 cases (36.4%, 8/22) by Mycobacterium tuberculosis (MTB) culture, and 20 cases (90.9%, 20/22) by molecular biology. The median follow-up period was 52(43, 66 ) months in 55 cases surviving more than one year. Among them, 40 cases (72.7%) were in favorable group (mRS 0-2) and 15 cases (27.3%) were in unfavorable group (mRS 3-6) with poor prognosis. The mortality rate was 20% (11/55). Elderly (OR=1.06, P=0.048 ) , hyponatremia(OR=0.81,P=0.020), high protein level in cerebrospinal fluid (CSF) (OR=3.32,P=0.033), cerebral infarction(OR=10.50,P=0.040) and hydrocephalus(OR=8.51,P=0.049) were associated with poor prognosis in TBM patients. Conclusions: The mortality rate is high in patients with TBM. Molecular biology tests improves the sensitivity and shorten the diagnosis time of TBM. Elderly, hyponatremia, high protein level in CSF, cerebral infarction and hydrocephalus are independent risk factors of long-term survival in TBM patients.

[Introduction of a tool to assess Risk of Bias in Non-randomized Studies-of Environmental Exposure (ROBINS-E)].

This paper summaries the Risk of Bias in Non-randomized Studies-of Environmental Exposure (ROBINS-E), a tool for evaluating risk of bias about non-randomized studies of exposures (NRSE), and introduces the application of ROBINS-E in a published NRSE. According to the characteristics of NRSE, evaluation fields and signaling questions were designed in ROBINS-E to provide essential information about risk of bias for NRSE included in systematic reviews and GRADE. ROBINS-E is the tool in assessment of risk of bias in observational studies and quasi-randomized studies. Although the tool has been used in practice to some extent, but it still needs further improvement. Attention should be paid to its update and progress.

[Introduction to COSMOS-E: Guidance on conducting systematic reviews and Meta-analyses on etiology of observational studies].

This paper introduces the conducting systematic reviews and Meta-analyses of observational studies of etiology (COSMOS-E) and illustrates the critical issues of COSMOS-E with a published systematic review. This document provides researchers with guidance on all steps in systematic reviews of observational studies of etiology, from shaping the research question, defining exposure and outcomes, to assessing the risk of bias and statistical analysis.

[Atypical renal cysts: a clinicopathological and molecular analysis of six cases].

Objective: To investigate the clinicopathological characteristics and molecular genetics of atypical renal cysts. Methods: Six cases of atypical renal cysts were collected from Zhejiang Provincial People's Hospital, Hangzhou, China, between February 2014 and February 2019. The clinicopathological characteristics and disease progression were analyzed. The 3p deletion and trisomy of chromosomes 7 and 17 were detected using fluorescence in situ hybridization (FISH). Results: All of the 6 patients were male, aged 43-63 years (median: 52 years). Preoperative Bosniak classification showed 4 cases of grade Ⅱ, 1 case of grade Ⅰ and 1 of grade Ⅲ. Histologically, atypical renal cysts appeared as unilocular or multilocular cysts, lined by multilayered flattened or cuboidal-shaped clear or eosinophilic cells. They often showed short papillary projections, and lacked solid or nodular growth of the lesional cells within the wall or septa of the cysts. Histologically, these cysts could be classified into three categories: acquired cystic disease-associated renal cell carcinoma (ACKD-RCC)-like (3 cases), clear cell type (2 cases), and eosinophilic papillary type (1 case). Two cases of ACKD-RCC-like atypical renal cysts were accompanied by clear cell renal cell carcinomas. On immunohistochemical staining, ACKD-RCC-like atypical renal cysts were focally CK7+/AMACR+/CD57+, the clear-cell type atypical renal cysts were CK7+/CAⅨ+, and eosinophilic papillary type atypical renal cysts were CK7+/AMACR+. FISH analyses showed that one case of ACKD-RCC-like atypical renal cysts had trisomy 17 and one case of clear cell type had 3p deletion, while no signal abnormality was detected in the other cases. The six patients were followed up for 13 to 70 months (median: 27 months), and no evidence of renal cell carcinoma was noted. Conclusion: Atypical renal cysts are a group of lesions that are heterogeneous in clinical, histological and immunophenotypical and molecular genetic features. FISH analyses suggest that a subset of the cases may be precursors of currently known renal cell carcinomas. Extensively sampling and careful observation of the histological characteristics of the cyst wall are important for distinguishing atypical renal cysts from extensively cystic renal cell carcinomas.

[The perioperative safety of cytoreductive surgery plus hyperthermic intraperitoneal chemotherapy for pseudomyxoma peritonei].

Objective: This study was to investigate the perioperative safety of cytoreductive surgery (CRS) plus hyperthermic intraperitoneal chemotherapy (HIPEC) for pseudomyxoma peritonei (PMP), and analyze the risk factors of serious adverse events (SAEs). Methods: The occurrences of perioperative SAEs were retrospectively analyzed in 254 PMP patients treated with CRS plus HIPEC. Univariate and multivariate analysis were performed to identify independent risk factors. Results: Among the 272 CRS plus HIPEC procedures for 254 PMP patients, a total of 93 (34.2%) perioperative SAEs occurred, including 26 in infection, 22 in digestive system, 17 in respiratory system, 15 in cardiovascular system, 8 in hematological system, and 4 in urinary system. In terms of severity, the vast majority was grade Ⅲ with 76 cases, followed by grade Ⅳ with 13 cases and grade Ⅴ with 4 cases. Univariate analysis revealed 3 risk factors of perioperative SAEs: HIPEC regimen (P=0.020), intraoperative red blood cell transfusion volume (P=0.004), and intraoperative blood loss volume (P=0.002). Multivariate analysis by logistic regression model analysis revealed that intraoperative red blood cell transfusion volume was an independent risk factor for perioperative SAEs (OR=1.160, P=0.001). Conclusion: In conclusion, the perioperative safety of CRS plus HIPEC was acceptable. Moreover, intraoperative blood loss volume and red blood cell transfusion volume are expected to be reduced in order to prevent SAEs for PMP patients.

[Clinicopathological features of composite pheochromocytoma].

Objective: To detect the clinicopathological features, immunophenotype, diagnosis, and differential diagnosis of composite pheochromocytoma(CP). Methods: Five cases of CP were collected at Zhejiang Provincial People's Hospital from January 2011 to January 2019. The clinical, radiological, histologic, immunohistochemical and outcome data were analyzed; the diagnosis and differential diagnosis were discussed. Results: The patients' age range was 52-68 years (mean 59 years, median 54 years), There were 4 males and 1 female, and the male to female ratio was 4∶1. Tumor size was 3-4 cm (mean 3.6 cm, median 3.5 cm). The most common clinical manifestation was adrenal mass. Histologically, the classical feature was two distinct morphologic components, one with tumor cells arranged in irregular nests, and with fine granular and basophilic oramphophilic cytoplasm; the other was composed of scattered ganglion cells in the background of Schwann cells organized in interwoven bundles. The components of pheochromocytoma expressed PHOX2B(5/5), synaptophysin (5/5), CgA (5/5), the sustentacular cells expressed S-100 protein; the components of ganglioneuroma expressed S-100 protein (5/5), NF (5/5), the ganglion cells were weakly positive for PHOX2B, synaptophysin and CgA. All the cases were surgically resected and all patients were free of recurrence at follow-up. Conclusions: CP is rare adrenal tumor, and it has typical histologic features but no specific clinical manifestations. Attention should be paid to its characteristic histomorphology with the use of PHOX2B, CgA, synaptophysin and S-100 protein immunohistochemistry that is helpful for its diagnosis.

[Establishment and characterization of patient derived xenograft model of malignant peritoneal mesothelioma in nude mice].

Objective: To establish patient derived xenograft (PDX) model of malignant peritoneal mesothelioma (MPM), and to identify the key characteristics of tumor biology of the model, so as to provide an experiment platform for studying the pathologic mechanisms and new therapeutic strategies for MPM. Methods: Surgically excised MPM tumor tissues were inoculated subcutaneously in BALB/c-nu/nu mice for 3 stable passages. In the 4th passage, the subcutaneous tumors were harvested under aseptic conditions, cleaned and made into MPM tumor cell homogenate. Four nude mice (two males and two females) were selected and one male and one female nude mouse were inoculated in the abdominal cavity at the dose of 100 µL, others were inoculated at a dose of 200 µL. The PDX model of MPM was established. The changes of body mass in nude mice were measured regularly, the extent of abdominal and pelvic tumors was judged by experimental peritoneal cancer index (ePCI) score, and the pathologic characteristics of tumors were analyzed. Results: The subcutaneous and abdominal animal models of MPM were successfully established. The subcutaneous tumor model grew into tumor on the 20th day, followed by a slow growth stage between the 20th and 29th day, then a rapid growth stage between the 30th and 57th day. According to the dose of tumor cells (100, 200 µL) and timing (14th and 69th days after grafting), the abdominal tumor model successfully simulated the early and late clinical stages of MPM. The HE staining results of the MPM nude mice model showed that the tumor was epithelial mesothelioma and invaded most of the organs, including liver, spleen, pancreas, mesentery. Immunohistochemical staining for calretinin, cytokeratin 5/6, WT1 and Ki-67 were positive. Whole-genome exon sequencing identified 26 and 36 high frequency gene mutations in tumors derived from the PDX model and clinical sample from patients, including 21 common gene mutations. Conclusions: The PDX model of MPM is established. The model is characterized by highly malignant tumor with rapid growth and high invasiveness.

[Establishment of patient derived xenograft model of high-grade mucinous carcinoma peritonei accompanied with signet ring cells and identification of biological characteristics].

Objective: To establish the patient derived xenograft (PDX) model of pseudomyxoma peritonei (PMP), and identify the key characteristics of tumor biology of this model, in order to provide a reliable model for studying the pathological mechanisms and new therapeutic strategies of PMP. Methods: PMP tumor tissue was obtained from surgery and cut into pieces after washing. Then tumor pieces were implanted subcutaneously in BAL B/c-nu mice for 6 stable passages. In the 7th passage, tumor tissue was implanted orthotopically into abdomen. Subcutaneous tumor and orthotopic tumor were then homogenized to make tumor cell suspension, implanted into abdomen of 10 BAL B/c-nu mice through midline laparotomy, 100 µl for each. The key experimental parameters including body weight changes in the observation period, experimental peritoneal cancer index (ePCI) score at the autopsy, histopathological and immunohistochemical characteristics, and gene expression profiles by high-throughput whole-genome exon sequencing were detected and recorded. Results: The successful rate of established orthotopic PDX model of human PMP was 100% (10/10). The animals showed smooth body weight increases after tumor inoculation until day 27, then the body weight began to decrease steadily. Widespread tumor dissemination of PMP tumor through the whole abdomen was found by autopsy, including the diaphragm, liver, spleen, stomach, kidney, parietal peritoneum, bowel and mesenterium. Gelatinous ascites was also observed in abdominopelvic cavity. The ePCI score ranged from 5 to 9, with a 8 of median ePCI. Histopathological studies showed peritoneal mucinous carcinomatosis accompanied with signet ring cells (PMCA-S), obvious tumor cell atypia and parenchymal invasion.Immunohistochemistry showed the expressions of MUC1, MUC2, MUC5AC, CEA, CA199, CK20, CDX-2 and Ki-67 were positive, MUC6, CK7 and p53 were negative. Whole-exome sequencing identified that the most significant genetic alteration is the exon10 missense mutation c. 1621A>C of KIT gene, the mutation abundance was 89.7%. Conclusion: PDX model of PMCA-S is successfully established, which displays the characters of high-degree malignancy, high proliferation and strong aggressiveness.

[Analysis on the transmission characteristics of newly reported human immunodeficiency virus/acquired immunodeficiency syndrome cases based on the molecular transmission network in Huzhou, Zhejiang, 2017].

Objective: Using field epidemiological investigation and molecular analysis to construct the molecular transmission network of human immunodeficiency virus/acquired immunodeficiency syndrome cases (HIV/AIDS) newly diagnosed in Huzhou in 2017, Zhejiang Province. Methods: A total of 160 participants were obtained through a web-based system from Chinese Center for Disease Control and Prevention (CCDC) with the features of diagnosed in Huzhou in 2017 who also had been collected samples for the first follow-up. The basic information of demographic characteristics and risk factors was extracted from the website. RNA was extracted from plasma samples of untreated cases, followed by RT-PCR and nest-PCR for pol gene amplification, sequencing. Phylogenetic tree was constructed by MEGA software for HIV gene subtyping. TN93 model was used for calculating the distance between two sequences. Cytoscape software was used for drawing molecular transmission network. And then an epidemiological survey was conducted to cases in the primary cluster. Results: A total of 138 sequenced individuals (86.3%) were acquired from 160 individuals. Among which, 123 (89.1%) were male. The highest proportion of subtype was CRF07_BC (60, 43.5%), followed by CRF01_AE (46, 33.3%), and with four cases of Unique Recombinant Form (URF, CRF01_AE and CRF07_BC) and one case of URF (subtype B and C). A total of 18 molecular clusters included 56 individuals (40.6%) were found in the transmission network under the optimal genetic distance threshold (1.0%). The clustering proportion of CRF07_BC (66.1%, 37 cases) was higher than that of CRF01_AE. There were 9 clusters formed among CRF07_BC, including 37 cases (accounting for 61.7%, 37/60). The primary transmission cluster contained 11 cases, among which 9 cases were transmitted by homosexual sex. The first time of the cases to have homosexual behavior is range from 2010 to 2016, whose media number (P(25), P(75)) of partners was 6 (3.5, 8.5). Most of the cases come from Anhui Province and engaged in garment industry (5 cases), between which there were 8 cases used Blued software to seek for casual partners, 1 case seeking for casual partners in garden. Conclusion: With CRF07_BC and CRF01_AE predominantly circulating, HIV genetic diversity had been noticed in this area. The primary cluster was consisted of high proportion of locally new infections, and a specific population aggregation in limited place existed.

[Genotype and evolution of hantavirus in Tiantai of Zhejiang province, 2011-2018].

Objective: By investigating the genotype and evolutionary variation of hantavirus (HV) in Tiantai county, a national surveillance site for hemorrhagic fever with renal syndrome (HFRS) was set in Zhejiang province, from 2011 to 2018, to reveal the molecular epidemiological characteristics of hantavirus (HV) in Tiantai. Methods: Total RNA was extracted from ultrasound treated HV antigen- positive rat lung samples in Tiantai from 2011 to 2018. After cDNA was prepared, nested PCR was used to amplify partial sequence of M fragments by using specific primers of HV. The sequences of HV in Tiantai from 2011 to 2018 were compared with other known HV sequences in order to identify the genotype and analyze the evolution and variation of the virus. Results: In 67 HV antigen-positive lung specimens, 31 were positive in nested PCR amplification with type-specific primers, including 30 Hantaan virus (HTNV) positive samples, 1 Seoul virus (SEOV) positive sample, and all the 31 samples were from Apodemus agrarius. The phylogenetic tree based on partial M segment was divided into monophyletic group, 30 strains were distributed in HTNV group and 1 was in SEOV group. The HTNV strain Tiantai T2018-130 was independently in one branch, sharing 84.8%-87.9% homology with other strains both at home and abroad, including 29 strains in HTNV group in Tiantai. The other 29 HTNV strains in Tiantai showed closer relationship. The SEOV strain T2016-31 from Apodemus agrarius showed closer relationship with previous strains of SEOV, Tiantai ZT71, ZT10 and Z37 strains of Wenzhou, Zhejiang province. Conclusions: HTNV, the main genotype of HV in Tiantai of Zhejiang province, showed obvious geographic clustering, but the strain T2018-130 was distinct from the others in Tiantai. Meanwhile, by sequence analysis, we confirmed that The SEOV strain T2016-31 existed in in Apodemus agrarius, indicating there was a phenomenon of "spillover" between virus and host in SEOV evolution.

[Introduction to PRISMA-CI extension statement and checklist systematic reviews on complex interventions].

Comprehensive interventions have been widely used in health system, public health, education and communities and have become increasingly focus of systematic reviews. There have been many reporting guidelines about systematic reviews, but they do not take the features of comprehensive interventions in medical area into consideration. As a result, PRISMA-CI has been developed as an extension of PRISMA, which adds or modifies the essential items of PRISMA. This paper introduces the items of PRISMA-CI and explains the items with an example to help authors, publishers, and readers understand PRISMA-CI and use it in systematic reviews on comprehensive interventions. As it become more and more popular with comprehensive interventions, PRISMA-CI will provide important structure and guidance for its systematic review and Meta-analysis.

[Introduction on 'assessing the risk of bias of individual studies' in systematic review of health-care intervention programs revised by the Agency for Healthcare Research and Quality].

This paper summarizes the Risk of Bias of Individual Studies in Systematic Reviews of Health Care Interventions revised by the Agency for Healthcare Research and Quality (AHRQ) and introduces how to use Revman software make risk of bias graph or risk of bias summary. AHRQ tool can be used to evaluate following study designs: RCTs, cohort study, case-control study (including nested case-control), case series study and cross-sectional study. The tool evaluates the risk of bias of individual studies from selection bias, performance bias, attrition bias, detection bias and reporting bias. Each of the bias domains contains different items, and each item is available for the assessment of one or more study designs. It is worth noting that the appropriate items should be selected for evaluation different study designs instead of using all items to directly assess the risk of bias. AHRQ tool can be used to evaluate risk of bias individual studies when systematic reviews of health care interventions is including different study designs. Moreover, the tool items are relatively easy to understand and the assessment process is not complicated. AHRQ recommends the use of high, medium and low risk classification methods to assess the overall risk of bias of individual studies. However, AHRQ gives no recommendations on how to determine the overall bias grade. It is expected that future research will give corresponding recommendations.

[Pigmented microcystic chromophobe renal cell carcinoma: a clinicopathologic analysis of five cases].

Objective: To investigate the clinicopathologic features, diagnostic and differential diagnostic aspects of pigmented microcystic chromophobe renal cell carcinoma (ChRCC). Methods: Five cases of pigmented microcystic ChRCC were collected at Zhejiang Provincial People's Hospital from January 2013 to January 2018. The clinical features, gross and histological appearances, immunohistochemistry and prognosis were analyzed and the relevant literature was reviewed. Results: There were 3 men and 2 women with age range of 45 years to 72 years (mean 57 years). All tumors were incidentally identified by imaging examinations. Grossly, the tumors were well-demarcated and showed diameters ranging from 1.8 cm to 4.0 cm(mean 2.9 cm). On cross section, the tumors were brown to gray tan with solid cut-surface mixed with multiple cysts of variable sizes. Hemorrhage was common, central scar was not seen. Microscopically, the tumors were composed predominantly of irregular and variable-sized microcystic or tubulocystic patterns, with extensive cribriform structures formation and focal adenomatous rearrangements seen in one case each, and focal pseudo-papillary structures (lacking true fibro-vascular cores) seen in two cases. Microscopic calcifications and psammoma bodies were present in all tumors. Four tumors composed mostly of eosinophilic cells whereas 1 predominated in plant-like cells. Brown pigmentations, either intracytoplasmic or extracytoplasmic, were noted in all five cases. The tumor cells had irregular, low-grade nuclei (Paner grade: 1) frequently with binucleation and perinuclar halos. Tumor necrosis or sarcomatous transformation was not seen. By immunohistochemistry, the tumor cells expressed CK, EMA, and E-cadherin diffusely and strongly in five cases; and CK7 and CD117 diffusely in four cases. They were negative for vimentin, CD10, CA9, AMACR/P504s, TFE3, HMB45, Melan A, S-100 protein, synaptophysin and chromogranin. Partial nephrectomies were performed for all five patients; there was no tumor recurrences or metastases at a follow-up of 2 to 55 months (mean, 17 months). Conclusions: Pigmented microcystic ChRCC is a rare histological variant of ChRCC with relatively indolent behavior, and shows morphologic heterogeneity which can elicit a wide range of differential diagnoses. Careful attentions to search for typical features of classic ChRCC with the use of immunohistochemistry can help to distinguish this tumor from its many mimickers.