A Rare Case of Invasive Cribriform Carcinoma in Male Breast.

BACKGROUND Male breast cancer is a very rare disease that represents 0.6% of all breast carcinomas. Among breast carcinomas, invasive cribriform carcinoma (ICC) is a rare type of breast carcinoma and is more common in older women, with only a few cases in men reported. We present a case of male breast cancer with ICC. CASE REPORT A 58-year-old man presented with a painful and palpable mass in the left breast, which was diagnosed as breast cancer of the ICC type. The patient underwent total mastectomy plus sentinel lymph node biopsy. On the microscope, the tumor was composed of more than 90% cribriform glands with comedo necrosis and dystrophic calcification. On immunohistochemical (IHC) staining, it appeared to be a luminal breast cancer. The IHC staining for c-erb B2 was equivocal (2 positive); hence fluorescence in situ hybridization was performed, and showed no amplification of the HER2/neu oncogene. The Ki-67 labeling index was 30%. The patient received radiotherapy and adjuvant systemic chemotherapy (4 cycles of docetaxel and cyclophosphamide), and has been on antiestrogen therapy (daily tamoxifen, 20 mg) for 30 months with no evidence of disease. CONCLUSIONS ICC is a rare type of invasive carcinoma of the breast, and ICC from the male breast is extremely rare. We report, in this case, the final pathologic results of a male patient diagnosed with ICC breast cancer and treated with surgery, chemotherapy, and radiotherapy.

Usefulness of BRAF VE1 immunohistochemistry in non-small cell lung cancers: a multi-institutional study by 15 pathologists in Korea.

BACKGROUND: Next-generation sequencing (NGS) is an approved test to select patients for BRAF V600E targeted therapy in Korea. However, the high cost, long turnaround times, and the need for sophisticated equipment and skilled personnel limit the use of NGS in daily practice. Immunohistochemistry (IHC) is a rapid and relatively inexpensive assay available in most laboratories. Therefore, in this study, we evaluate the usefulness of BRAF VE1 IHC in terms of predictive value and interobserver agreement in non-small cell lung cancers (NSCLCs). METHODS: A total of 30 cases with known BRAF mutation status were selected, including 20 cases of lung adenocarcinomas, six cases of colorectal adenocarcinomas, and four cases of papillary thyroid carcinomas. IHC for BRAF V600E was carried out using the VE1 antibody. Fifteen pathologists independently scored both the staining intensity and the percentage of tumor cell staining on whole slide images. RESULTS: In the lung adenocarcinoma subset, interobserver agreement for the percentage of tumor cell staining and staining intensity was good (percentage of tumor cell staining, intraclass correlation coefficient = 0.869; staining intensity, kappa = 0.849). The interobserver agreement for the interpretation using the cutoff of 40% was almost perfect in the entire study group and the lung adenocarcinoma subset (kappa = 0.815). Sensitivity, specificity, positive predictive value, and negative predictive value of BRAF VE1 IHC were 80.0%, 90.0%, 88.9%, and 81.8%, respectively. CONCLUSIONS: BRAF VE1 IHC could be a screening test for the detection of BRAF V600E mutation in NSCLC. However, further studies are needed to optimize the protocol and to establish and validate interpretation criteria for BRAF VE1 IHC.

ARL6IP5 reduces cisplatin-resistance by suppressing DNA repair and promoting apoptosis pathways in ovarian carcinoma.

Ovarian carcinoma (OC) is the most lethal gynecological malignancy due to frequent recurrence resulting from cisplatin-resistance. ARL6IP5 is a novel gene implicated to suppress cisplatin-resistance by activating apoptosis and inhibiting DNA repair through XRCC1 and PARP1. We investigated the clinicopathological and prognostic significance of the immunohistochemical ARL6IP5 expression on 79 post-chemotherapy OC patient tissue samples; in vitro, the effect of ARL6IP5 overexpression (OE) and knockdown (KD) on cancer hallmark functions and the effect of ARL6IP5 on the expression of DNA repair and apoptosis-related proteins were observed in OC cells and their cisplatin-resistant (CisR) counterparts. ARL6IP5 expression was significantly associated with chemotherapeutic response and was an independent prognosticator of progression-free and overall survival of high-grade serous OC patients. ARL6IP5-OE decreased cellular proliferation, invasion, migration, adhesion, and increased apoptosis (p < 0.05); the opposite was observed for ARL6IP5-KD. Notably, ARL6IP5-OE reduced cisplatin-resistance of both OC and CisR OC cells, while ARL6IP5-KD increased cisplatin-resistance (p < 0.05). ARL6IP5-OE suppressed the expressions of DNA repair proteins and increased those of pro-apoptotic proteins; the opposite was observed for ARL6IP5-KD. The recombinant ARL6IP5 protein (rARL6IP5) had the greatest apoptotic effect among cisplatin and olaparib, in both OC and CisR OC cells; moreover, rARL6IP5 was the only single agent in CisR OC cells to retain higher apoptotic efficacy compared with control (p < 0.05), indicating that the apoptotic pathway influenced by rARL6IP5 remained effective in CisR OC cells compared to cisplatin and olaparib. In conclusion, we demonstrated that ARL6IP5 is an independent prognosticator of OC patients with cellular functions of a tumor-suppressor, possibly influencing the development of cisplatin-resistance and progression of OC cells through regulation of DNA repair and apoptosis. rARL6IP5 had significantly greater apoptotic efficacy compared to conventional chemotherapeutic agents in both OC and CisR OC cells, suggesting that ARL6IP5 may be a valuable novel chemotherapeutic against CisR OC.

Core needle biopsy and ultrasonography are superior to fine needle aspiration in the management of follicular variant papillary thyroid carcinomas.

PURPOSE: Follicular variant papillary thyroid carcinoma (FVPTC) is a problematic entity. FVPTCs are often misdiagnosed by the standard fine needle aspiration (FNA); in addition, FVPTCs represent a mixed group of tumors with two biologically distinct subtypes: The indolent encapsulated FVPTC and the aggressive infiltrative FVPTC. Recent changes in guidelines suggests that FVPTC management may be improved if subtypes can be determined preoperatively. Preoperative assays, FNA, core needle biopsy (CNB), and ultrasonography (US) were compared for their ability to identify and subtype FVPTCs to determine the most appropriate test to manage FVPTCs. METHODS: The preoperative assays and clinicopathologic variables of 255 resected FVPTCs cases at Samsung Medical Center between 2012 and 2016 were retrospectively evaluated. RESULTS: CNB had the overall best ability to manage FVPTCs with the highest rate of diagnosis indicating surgery, lowest rate of inconclusive results, high sensitivity (88.9%), specificity (87.7%), negative predictive value (97.0%), diagnostic odds ratio (DOR; 56.9), and excellent predictive ability (AUC 0.906) for differentiating FVPTC subtypes. US had a moderate DOR (12.8), good predictive ability (AUC 0.802), high sensitivity (75.0%) and specificity (81.0%). CNB and US both had significantly higher accuracy for discriminating FVPTC subtypes than FNA (AUC 0.908 and 0.877 > 0.671; p < 0.05). The excellent performance of CNB could be attributed to distinct histologic differences between FVPTC subtypes. CONCLUSION: CNB and US had superior performance to FNA in the identification and subtyping of FVPTC. In institutions with skilled and experienced operators, CNB is the preferred method for evaluating possible FVPTC lesions.

Molecular biomarker testing for non-small cell lung cancer: consensus statement of the Korean Cardiopulmonary Pathology Study Group.

Molecular biomarker testing is the standard of care for non-small cell lung cancer (NSCLC) patients. In 2017, the Korean Cardiopulmonary Pathology Study Group and the Korean Molecular Pathology Study Group co-published a molecular testing guideline which contained almost all known genetic changes that aid in treatment decisions or predict prognosis in patients with NSCLC. Since then there have been significant changes in targeted therapies as well as molecular testing including newly approved targeted drugs and liquid biopsy. In order to reflect these changes, the Korean Cardiopulmonary Pathology Study Group developed a consensus statement on molecular biomarker testing. This consensus statement was crafted to provide guidance on what genes should be tested, as well as methodology, samples, patient selection, reporting and quality control.

Current status and future perspectives of liquid biopsy in non-small cell lung cancer.

With advances in target therapy, molecular analysis of tumors is routinely required for treatment decisions in patients with advanced non-small cell lung cancer (NSCLC). Liquid biopsy refers to the sampling and analysis of circulating cell-free tumor DNA (ctDNA) in various body fluids, primarily blood. Because the technique is minimally invasive, liquid biopsies are the future in cancer management. Epidermal growth factor receptor (EGFR) ctDNA tests have been performed in routine clinical practice in advanced NSCLC patients to guide tyrosine kinase inhibitor treatment. In the near future, liquid biopsy will be a crucial prognostic, predictive, and diagnostic method in NSCLC. Here we present the current status and future perspectives of liquid biopsy in NSCLC.

Interobserver Reproducibility of PD-L1 Biomarker in Non-small Cell Lung Cancer: A Multi-Institutional Study by 27 Pathologists.

BACKGROUND: Assessment of programmed cell death-ligand 1 (PD-L1) immunohistochemical staining is used for treatment decisions in non-small cell lung cancer (NSCLC) regarding use of PD-L1/programmed cell death protein 1 (PD-1) immunotherapy. The reliability of the PD-L1 22C3 pharmDx assay is critical in guiding clinical practice. The Cardiopulmonary Pathology Study Group of the Korean Society of Pathologists investigated the interobserver reproducibility of PD-L1 staining with 22C3 pharmDx in NSCLC samples. METHODS: Twenty-seven pathologists individually assessed the tumor proportion score (TPS) for 107 NSCLC samples. Each case was divided into three levels based on TPS: <1%, 1%-49%, and ≥50%. RESULTS: The intraclass correlation coefficient for TPS was 0.902±0.058. Weighted κ coefficient for 3-step assessment was 0.748±0.093. The κ coefficients for 1% and 50% cut-offs were 0.633 and 0.834, respectively. There was a significant association between interobserver reproducibility and experience (formal PD-L1 training, more experience for PD-L1 assessment, and longer practice duration on surgical pathology), histologic subtype, and specimen type. CONCLUSIONS: Our results indicate that PD-L1 immunohistochemical staining provides a reproducible basis for decisions on anti-PD-1 therapy in NSCLC.

Molecular Diagnostic Assays and Clinicopathologic Implications of MET Exon 14 Skipping Mutation in Non-small-cell Lung Cancer.

BACKGROUND: Recent studies revealed MET exon 14 skipping (METex14) as a biomarker that predicts the response to MET inhibitors in non-small-cell lung cancer (NSCLC). However, METex14 genomic alterations exhibit a highly diverse sequence composition, posing a challenge for clinical diagnostic testing. This study aimed to find a reasonable diagnostic assay for METex14 and identify its clinicopathologic implications. MATERIALS AND METHODS: We performed a comprehensive analysis of METex14 in 414 EGFR/KRAS/ALK/ROS1-negative (quadruple negative) surgically resected NSCLCs. We used real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) and Sanger sequencing for the first assay, followed by next-generation sequencing (NGS; hybrid-capture targeted DNA/RNA sequencing). Clinicopathologic implications of the METex14 group were analyzed in a total of 880 NSCLCs. RESULTS: METex14 was confirmed in 13 (3.1%) patients by DNA- and RNA-NGS. After comparison of assay results, qRT-PCR and NGS demonstrated the highest concordance rate. The mean variant allele frequency was 10.5% and 49% in DNA- and RNA-NGS, respectively. DNA-NGS revealed various lengths of indel and substitutions around and in exon 14. Moreover, METex14 was associated with adenocarcinoma (4.8%; 11/230) or sarcomatoid carcinoma (9.5%; 2/21), old age, never-smokers, and early stage of disease. CONCLUSIONS: METex14 occurs in about 3% of NSCLCs and has characteristic clinicopathologic features. NGS should be the first assay of choice as a multiplex testing. Sanger sequencing can detect METex14, but sensitivity can be hampered by large deletions or low allele frequency. qRT-PCR, an mRNA-based method, is sensitive and specific and can be appropriate for screening METex14 as a single gene testing.

Thyroid Fine-Needle Aspiration Cytology Practice in Korea.

We reviewed the current status of thyroid fine-needle aspiration cytology (FNAC) in Korea. Thyroid aspiration biopsy was first introduced in Korea in 1977. Currently, radiologists aspirate the thyroid nodule under the guidance of ultrasonography, and cytologic interpretation is only legally approved when a cytopathologist makes the diagnosis. In 2008, eight thyroid-related societies came together to form the Korean Thyroid Association. The Korean Society for Cytopathology and the endocrine pathology study group of the Korean Society for Pathologists have been updating the cytologic diagnostic guidelines. The Bethesda System for Reporting Thyroid Cytopathology was first introduced in 2009, and has been used by up to 94% of institutions by 2016. The average diagnosis rates are as follows for each category: I (12.4%), II (57.9%), III (10.4%), IV (2.9%), V (3.7%), and VI (12.7%). The malignancy rates in surgical cases are as follows for each category: I (28.7%), II (27.8%), III (50.6%), IV (52.3%), V (90.7%), and VI (100.0%). Liquid-based cytology has been used since 2010, and it was utilized by 68% of institutions in 2016. The categorization of thyroid lesions into "atypia of undetermined significance" or "follicular lesion of undetermined significance" is necessary to draw consensus in our society. Immunocytochemistry for galectin-3 and BRAF is used. Additionally, a molecular test for BRAF in thyroid FNACs is actively used. Core biopsies were performed in only 44% of institutions. Even the institutions that perform core biopsies only perform them for less than 3% of all FNACs. However, only 5% of institutions performed core biopsies up to three times more than FNAC.

Molecular Testing of Lung Cancers.

Targeted therapies guided by molecular diagnostics have become a standard treatment of lung cancer. Epidermal growth factor receptor (EGFR) mutations and anaplastic lymphoma kinase (ALK) rearrangements are currently used as the best predictive biomarkers for EGFR tyrosine kinase inhibitors and ALK inhibitors, respectively. Besides EGFR and ALK, the list of druggable genetic alterations has been growing, including ROS1 rearrangements, RET rearrangements, and MET alterations. In this situation, pathologists should carefully manage clinical samples for molecular testing and should do their best to quickly and accurately identify patients who will benefit from precision therapeutics. Here, we grouped molecular biomarkers of lung cancers into three categories-mutations, gene rearrangements, and amplifications-and propose expanded guidelines on molecular testing of lung cancers.

Mesothelin Expression in Gastric Adenocarcinoma and Its Relation to Clinical Outcomes.

BACKGROUND: Although surgical resection with chemotherapy is considered effective for patients with advanced gastric cancer, it remains the third leading cause of cancer-related death in South Korea. Several studies have reported that mesothelial markers including mesothelin, calretinin, and Wilms tumor protein 1 (WT1) were positive in variable carcinomas, associated with prognosis, and were evaluated as potential markers for targeted therapy. The aim of this study was to assess the immunohistochemical expression of mesothelial markers (mesothelin, calretinin, and WT1) in gastric adenocarcinoma and their relations to clinocopathological features and prognosis. METHODS: We evaluated calretinin, WT1, and mesothelin expression by immunohistochemical staining in 117 gastric adenocarcinomas. RESULTS: Mesothelin was positively stained in 30 cases (25.6%). Mesothelin expression was related to increased depth of invasion (p = .002), lymph node metastasis (p = .013), and presence of lymphovascular (p = .015) and perineural invasion (p = .004). Patients with mesothelin expression had significantly worse disease-free survival rate compared with that of nonmesothelin expression group (p = .024). Univariate analysis showed that mesothelin expression is related to short-term survival. None of the 117 gastric adenocarcinomas stained for calretinin or WT1. CONCLUSIONS: Mesothelin expression was associated with poor prognosis. Our results suggest that mesothelin-targeted therapy should be considered as an important therapeutic alternative for gastric adenocarcinoma patients with mesothelin expression.

MassARRAY, pyrosequencing, and PNA clamping for EGFR mutation detection in lung cancer tissue and cytological samples: a multicenter study.

BACKGROUND: Testing for epidermal growth factor receptor (EGFR) mutation is an important process in the therapeutic plan of patients with lung cancer. Recently, MassARRAY, based on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, has been shown to be a useful method for somatic mutation analysis with pyrosequencing and peptide nucleic acid clamping (PNAc). METHODS: A total of 107 tissues and 67 cytological samples, which were confirmed to have lung adenocarcinoma at nine hospitals in Korea, were collected. Among the MassARRAY, pyrosequencing, and PNAc, the concordance rates and sensitivity of EGFR mutation detection were analyzed and validated in comparative tissue and cytological specimens. RESULTS: The concordance rate between pyrosequencing and PNAc was higher than that between MassARRAY and either of the pyrosequencing and PNAc in both tissue and cytological samples. In a comparison of diagnostic performance, MassARRAY (sensitivity: 85.7 %) was higher than pyrosequencing (74.3 %) and PNAc (70 %) in tissue, although pyrosequencing (80.5 %) was more highly sensitive, compared to MassARRAY (70.7 %) and PNAc (70.7 %) in terms of cytology. Unexpectedly, use of MassARRAY resulted in a significantly different EGFR mutation detection rate between tissue and cytological samples. CONCLUSIONS: When used for the detection of EGFR mutations, MassARRAY was more sensitive than pyrosequencing or PNA clamping in tissue, but not in cytological samples. In EGFR mutation detection between tissues and cytology, PNAc showed relatively higher concordance than MassARRAY or pyrosequencing.

Comparison of EGFR mutation detection between the tissue and cytology using direct sequencing, pyrosequencing and peptide nucleic acid clamping in lung adenocarcinoma: Korean multicentre study.

BACKGROUND: The importance of sensitive methods for the detection of epidermal growth factor receptor (EGFR) mutation is emphasized. The aim of this study is to perform comparative and concordance analyses of direct sequencing, pyrosequencing and peptide nucleic acid (PNA) clamping for detecting EGFR gene mutations using archived tissue and cytology specimens. METHODS: Samples from a total of 112 cases, which were diagnosed with adenocarcinoma of the lung at nine hospitals in Korea were collected. Using the above three methods, the concordance rates of EGFR mutations in exons 18, 19, 20 and 21 were analysed and validated in comparative tissue and cytology specimens. RESULTS: Comparison of EGFR mutation detection between the tissue and cytology had a high concordance rate. The diagnostic performance of pyrosequencing and PNA clamping in tissue was higher than that of direct sequencing as well as cytology. Additionally, among some of the patients who had EGFR wild type by single method, EGFR mutations were detected by other methods. Cytology specimens had a diagnostic performance for the detection of EGFR mutations. CONCLUSIONS: Cytology specimens had a diagnostic performance for the detection of EGFR mutations that was comparable to that of tissues. For detecting EGFR mutations, pyrosequencing or PNA clamping was more sensitive than direct sequencing. In EGFR mutation negative patients who are difficult to obtain tissue, repeating test using pyrosequencing or PNA clamping is recommended to improve the detection rate of EGFR mutation than only one, especially in cytology.

A Case of Metastatic Low-Grade Endometrial Stromal Sarcoma Treated with Letrozole after Ovarian Ablation by Radiotherapy.

A 50-year-old woman was admitted to our hospital due to multiple lung nodules detected incidentally on a chest X-ray. A video-assisted thoracoscopic lung biopsy revealed low-grade endometrial stromal sarcoma (LG-ESS). She had undergone a simple hysterectomy 1 year earlier owing to a diagnosis of adenomyosis. A review of her previous hysterectomy specimen showed not endometriosis but LG-ESS. According to the patient's levels of serum follicle stimulating hormone and estradiol, she was in the premenopausal state with retained and normally functioning ovaries. She then underwent ovarian ablation by radiotherapy, after which she was administered 2.5 mg of letrozole once per day. Three months later, the size of the metastatic nodules in both lungs had decreased. The patient was followed up for 24 months while continuing on letrozole, and maintained a partial remission. We report herein on a case of metastatic LG-ESS treated with letrozole after ovarian ablation by radiotherapy.

Practical utility of insulin-like growth factor II mRNA-binding protein 3, glucose transporter 1, and epithelial membrane antigen for distinguishing malignant mesotheliomas from benign mesothelial proliferations.

The differentiation of malignant mesotheliomas and benign mesothelial proliferations is crucial in determining patient care and prognosis. But, this distinction can be extremely difficult, particularly in small biopsies. Recently, insulin-like growth factor II mRNA-binding protein 3 (IMP3) and glucose transporter 1 (GLUT-1) have been reported as specific and sensitive markers in the distinction of mesotheliomas from benign mesothelial proliferations. The purpose of this study is to evaluate the utility of IMP3, GLUT-1, and epithelial membrane antigen (EMA) immunohistochemistry for distinguishing mesotheliomas from benign mesothelial proliferations. Immunoexpression of IMP3, GLUT-1, and EMA was evaluated in 88 malignant mesotheliomas, 35 adenomatoid tumors, and 20 benign lung tissues with reactive mesothelial cells. The sensitivity for IMP3, GLUT-1, and EMA was 37%, 21%, and 41%, respectively. The specificity for IMP3, GLUT-1, and EMA was 100%. When IMP3, GLUT1, and EMA combined, the sensitivity was 66% for IMP3/EMA staining, 53% for GLUT-1/EMA staining, and 45% for IMP3/GLUT-1. Use of IMP3 and EMA together is more helpful to distinguish malignant mesotheliomas from benign mesothelial proliferations than the use of IMP3 or EMA alone.

Guideline Recommendations for EGFR Mutation Testing in Lung Cancer: Proposal of the Korean Cardiopulmonary Pathology Study Group.

Mutations of the epidermal growth factor receptor (EGFR) are the strongest predictive factor for response to EGFR tyrosine kinase inhibitors (TKIs), such as gefitinib and erlotinib. EGFR TKIs are approved in Korea as a first-line treatment for lung cancer patients with mutated EGFR. Rapid and accurate EGFR mutation testing is essential for patient selection and establishing targeted therapies with EGFR TKIs. Thus, a standard set of guideline recommendations for EGFR mutation testing suitable for the Korean medical community is necessary. In this article, we propose a set of guideline recommendations for EGFR mutation testing that was discussed and approved by the Cardiopulmonary Pathology Study Group of the Korean Society of Pathologists.

Cytologic Findings of Thyroid Carcinoma Showing Thymus-like Differentiation: A Case Report.

Carcinoma showing thymus-like differentiation (CASTLE) is a rare carcinoma of the thyroid or adjacent soft tissue of the neck with a histologic resemblance to thymic epithelial tumors. Although the fine-needle aspiration (FNA) plays a central role in the initial evaluation of thyroid nodules, few reports about the cytologic findings of CASTLE have been found according to a review of literatures. We report cytologic findings of a case of CASTLE. A 34-year-old woman presented with a 2-month history of sore throat. The FNA showed that the smear was composed of three dimensional clusters and sheets. The tumor cells were round to ovoid with high nuclear : cytoplasmic ratios. The nuclei were vesicular with small nucleoli. There were some tumor cells showing keratinization. Some lymphocytes were found on the background and within clusters. The presence of poorly-differentiated tumor cells with a focal keratinization and a lymphocytic background on the FNA is suggestive of CASTLE.

Altered expression of microRNA miR-21, miR-155, and let-7a and their roles in pulmonary neuroendocrine tumors.

MicroRNA (miRNA) has a critical effect on tumorigenesis through post-transcriptional modification and is considered to be potential biomarkers for cancer diagnosis and treatment monitoring. We evaluated the expression pattern of three selected miRNAs (miR-21, miR-155, and let-7a) to evaluate their potential roles by quantitative reverse transcription-polymerase chain reaction using formalin-fixed and paraffin-embedded tissues of 63 surgically resected pulmonary neuroendocrine (NE) tumors (19 typical carcinoids (TCs), 6 atypical carcinoids (ACs), 19 large cell NE carcinomas (LCNECs), and 19 small cell lung carcinomas (SCLCs). Control amplification for U6 small nuclear RNA (U6) was performed in all samples. Normalized Ct values were calculated (Ct(Experimental miRNA) -Ct(U6) ) for each case and recorded. The expression levels of miR-21 and miR-155 were significantly higher in high-grade NE carcinomas (LCNECs and SCLCs) than in carcinoid tumors (TCs and ACs) (each P < 0.001). The expression level of miR-21 in carcinoid tumors with lymph node metastasis was significantly higher than in carcinoid tumors without lymph node metastasis (P= 0.010). To the best of our knowledge, the present study is the first to examine the expression patterns of miR-21 and miR-155 as an adjunctive diagnostic tool or clinically relevant biomarkers for pulmonary NE tumors.

Differential expression of forkhead box M1 and its downstream cyclin-dependent kinase inhibitors p27(kip1) and p21(waf1/cip1) in the diagnosis of pulmonary neuroendocrine tumours.

AIMS: Pulmonary neuroendocrine (NE) tumours represent a spectrum of phenotypically distinct entities with different biological behaviours. Difficulties in classifying these tumours are frequently encountered in clinical practice. Forkhead box M1 (FoxM1) is essential for the development of various cancers and is a proliferation-specific transcription factor that regulates transcription of cell cycle genes, including cyclin-dependent kinase inhibitors p27(kip1) and p21(waf1/cip1) . This study was performed to determine the utility of FoxM1, p27(kip1) and p21(waf1/cip1) as immunomarkers for subtyping pulmonary NE tumours. METHODS AND RESULTS: FoxM1, p27(kip1) and p21(waf1/cip1) expression was evaluated by immunohistochemistry in 60 pulmonary NE tumours [19 typical carcinoids (TCs), six atypical carcinoids (ACs), 17 large cell neuroendocrine carcinomas (LCNECs) and 18 small cell lung cancers (SCLCs)]. The frequencies of FoxM1 and p21(waf1/cip1) expression were significantly different between TCs and ACs (each P = 0.009), and those of FoxM1 and p27(kip1) expression were significantly different between LCNECs and SCLCs (P = 0.012 and P = 0.002, respectively). The combined FoxM1((-)) /p21(waf1/cip1(-)) and FoxM1((+)) /p27(kip1(high)) phenotypes had the best diagnostic accuracy for distinguishing TCs from ACs, and SCLCs from LCNECs, respectively. CONCLUSIONS: FoxM1, p27(kip1) and p21(waf1/cip1) showed distinct immunoreactivity according to histological subtype, which may be of value as an ancillary test in the differential diagnosis of pulmonary NE tumours.