Rare but Still There: An Interesting Case of Cytokeratin 20-Negative Merkel Cell Carcinoma.

Merkel cell carcinoma (MCC) of the skin is a rare and aggressive primary neuroendocrine tumor that mainly involves sun-exposed areas and can metastasize to other parts of the body. Due to varied clinical features and the sharing of similar histological features with other neuroendocrine tumors, diagnosis can be challenging. Therefore, immunohistochemistry plays an important role in diagnosis, and the characteristic perinuclear staining with cytokeratin 20 (CK 20) helps to differentiate it from other morphologically similar tumors, especially metastatic small cell carcinoma of the lung. We describe an interesting case of a 78-year-old female who was referred by a general practitioner (GP) with a few months' history of asymptomatic, rapidly enlarging, erythematous, nodular lesion on her left upper arm. Due to clinical findings and the location of the lesion on the sun-exposed area, wide differential diagnoses were considered. The lesion was excised for histological diagnosis. Surprisingly, morphological features favour the diagnosis of a neuroendocrine tumor. However, histological features including immunohistochemistry rendered it difficult to differentiate between primary cutaneous neuroendocrine carcinoma (Merkel cell CA) and metastatic small cell carcinoma of the lung due to the lack of specific and sensitive marker of CK 20 on immunohistochemistry. Subsequently, the patient had computer tomography of the chest/abdomen and pelvis (CTTAP) and positron emission tomography (PET) scans to rule out underlying primary malignancy. The case was also discussed at local and specialist skin multidisciplinary team meetings (MDT) including neuroendocrine MDT and a consensus diagnosis of Merkel cell carcinoma of the skin with negative CK 20 was established.

The difference between cytokeratin 20 expression in high- and low-grade urothelial bladder carcinomas: A cross-sectional study.

Background: Bladder cancer is one of the most common cancers worldwide. Expression of cytokeratin 20 (CK 20) could be used as a biomarker in different epithelia to determine malignancy, especially in gastrointestinal, urinary tract, and Merkel cells. CK 20 could be detected in several urothelial carcinomas and was associated with bladder cancer recurrence. The study aimed to assess the utility of CK 20 expression for bladder cancer grading. Materials and Methods: This was a retrospective study assessing CK 20 expression in 73 bladder cancer patients who had transurethral resection of bladder tumor or cystectomy. The data were then collected and analyzed with SPSS Statistics version 20.0. Results: Fifty-six (76.7%) cases of high- and 17 (23.3%) cases of low-grade urothelial bladder cancer were examined for CK 20 expression. Positive expression was present in 57 (78.1%) samples. A significant difference (P = 0.034) in CK 20 expression was observed between low-grade and high-grade urothelial carcinomas. Positive expression was seen in 44 (77.2%) high-grade cases and only 13 (22.8%) low-grade cases. Conclusion: The difference in the CK 20 expression was found to be statistically significant among different grades of bladder cancer but not to metastatic bladder cancer. Further, studies are required to establish CK 20 as a diagnostic tool. We suggest a combination with several markers to compare which is superior.

Propolis Has an Anticancer Effect on Early Stage Colorectal Cancer by Affecting Epithelial Differentiation and Gut Immunity in the Tumor Microenvironment.

Colorectal cancer (CRC) is one of the most common cancers and is the second leading cause of cancer-related death in the world. Due to the westernization of diets, young patients with CRC are often diagnosed at advanced stages with an associated poor prognosis. Improved lifestyle choices are one way to minimize CRC risk. Among diet choices is the inclusion of bee propolis, long recognized as a health supplement with anticancer activities. Understanding the effect of propolis on the gut environment is worth exploring, and especially its associated intratumoral immune changes and its anticancer effect on the occurrence and development of CRC. In this study, early stage CRC was induced with 1,2-dimethylhydrazine (DMH) and dextran sulfate sodium (DSS) for one month in an animal model, without and with propolis administration. The phenotypes of early stage CRC were evaluated by X-ray microcomputed tomography and histologic examination. The gut immunity of the tumor microenvironment was assessed by immunohistochemical staining for tumor-infiltrating lymphocytes (TILs) and further comparative quantification. We found that the characteristics of the CRC mice, including the body weight, tumor loading, and tumor dimensions, were significantly changed due to propolis administration. With further propolis administration, the CRC tissues of DMH/DSS-treated mice showed decreased cytokeratin 20 levels, a marker for intestinal epithelium differentiation. Additionally, the signal intensity and density of CD3+ and CD4+ TILs were significantly increased and fewer forkhead box protein P3 (FOXP3) lymphocytes were observed in the lamina propria. In conclusion, we found that propolis, a natural supplement, potentially prevented CRC progression by increasing CD3+ and CD4+ TILs and reducing FOXP3 lymphocytes in the tumor microenvironment of early stage CRC. Our study could suggest a promising role for propolis in complementary medicine as a food supplement to decrease or prevent CRC progression.

Diagnostic Significance of Cytokeratin 20 and p53 as Adjunct Immunocytological Markers in Urine Cytology to Detect Urothelial Carcinomas: A Systematic Review Study.

The progression and recurrence of urothelial carcinoma (UC) are correlated with carcinoma in situ and urothelial dysplasia. It is frequently challenging to distinguish dysplasia and carcinoma in situ from reactive atypia only based on histological characteristics. In daily practices, 2 of the adjunct immunohistochemistry markers (cytokeratin 20 (CK20) and p53) are used in addition to the histology to diagnose carcinoma in situ. This is accomplished by combining histological research results with immunohistochemistry. This systematic review summarizes the current findings on the diagnostic significance of p53 and CK20 as adjunct markers to urine cytology in the detection of UC. A systematic search of the relevant literature was conducted using PubMed, Wiley Online Library, and ScienceDirect databases. After screening for the eligibility criteria, a total of 14 selected articles were reviewed. Data extraction included a total number of samples, specimen samples, type of cells, and outcome parameters (mainly sensitivity and specificity). Urine cytology alone had a sensitivity of 75%-85% and specificity of 66%-95%. CK20 with urine cytology staining showed improved sensitivity and specificity in the range of 77%-94% and 71%-100%, respectively; p53 immunostaining with urine cytology showed a sensitivity of 52%-86% and specificity of 80%-98%. The dual staining in combination with urine cytology showed comparatively higher sensitivity and specificity in the range of 70%-90% and 74%-100%, respectively. This was more evident for high-grade UC (HGUC). Overall, single or dual staining combined with urine cytology was effective in this detection and can be applied as an adjunct marker in urine cytology.

Cytokeratin 20 expression is linked to stage progression and to poor prognosis in advanced (pT4) urothelial carcinoma of the bladder.

Cytokeratin 20 (CK20) expression is limited to umbrella cells in the normal urothelium. Since CK20 is often upregulated in neoplastic urothelial cells including dysplasia and carcinoma in situ, immunohistochemical CK20 analysis is often used for the assessment of bladder biopsies. CK20 expression is a feature of luminal bladder cancer subtype, but its prognostic relevance is disputed. In this study, we investigated CK20 on >2700 urothelial bladder carcinomas in a tissue microarray format by immunohistochemistry. Cytoplasmic and membranous CK20 staining was seen in 1319 (51.8%) cancers. The fraction of CK20 positive and especially strongly positive cases increased from pTaG2 low grade (44.5% strongly positive) and pTaG2 high grade (57.7%) to pTaG3 high grade (62.3%; p = 0.0006) but was lower in muscle-invasive (pT2-4) carcinomas (51.1% in all pTa vs. 29.6% in pT2-4; p < 0.0001). Within pT2-4 carcinomas, CK20 positivity was linked to nodal metastasis and lymphatic vessel invasion (p < 0.0001 each) and to venous invasion (p = 0.0177). CK20 staining was unrelated to overall patient survival if all 605 pT2-4 carcinomas were jointly analyzed but subgroup analyses revealed a significant association of CK20 positivity with favorable prognosis in 129 pT4 carcinomas (p = 0.0005). CK20 positivity was strongly linked to the expression of GATA3 (p < 0.0001), another feature of luminal bladder cancer. The combined analysis of both parameters showed best prognosis for luminal A (CK20+/GATA3+, CK20+/GATA3-) and worst outcome for luminal B (CK20-/GATA3+) and basal/squamous (CK20-/GATA3-) in pT4 urothelial carcinomas (p = 0.0005). In summary, the results of our study demonstrate a complex role of CK20 expression in urothelial neoplasms including neoexpression in pTa tumors, a subsequent loss of CK20 expression in a subset of tumors progressing to muscle-invasion, and a stage dependent prognostic role in muscle-invasive cancers.

Long-Term Oncological Control by Repeated Minimally Invasive Hepatectomy for Intrahepatic Cholangiocarcinoma Exhibiting Atypical Marker Expression of CK7-CK20+: A Case Report.

There are few reports of repeated liver resections being performed multiple times for intrahepatic recurrence of intrahepatic cholangiocarcinoma (ICC). We performed five minimally invasive liver resections and two minimally invasive lung resections for ICC with metachronous intrahepatic recurrence and lung metastases. Pathological examination revealed that all resected tumors were moderately differentiated mass-forming ICC with immunohistochemical marker expression of CK7 negative and CK20 positive. We present this as a rare case of ICC with atypical marker expression in which long-term tumor control was achieved with multiple minimally invasive liver resections over 47 months from the initial diagnosis.

Role of Cytokeratin 20 as a Predictive and Prognostic Marker in Urothelial Neoplasms.

Introduction Several clinical peculiarities mark urothelial carcinomas and their biological behavior. Key in these are its relatively indolent course before manifestation and its high recurrence rate. So far, no biomarker has been identified as a predictor for these factors. The current study aims to evaluate the role of cytokeratin 20 (CK20) in non-invasive urothelial carcinomas (pTa and pT1) of the urinary bladder and its diagnostic and predictive role in tumor staging and recurrence. Materials and methods The study utilizes a retrospective, non-clinical approach via immunohistochemical marking of the paraffin-embedded tumor tissues for the initial diagnosis. Expression patterns were compared with tumor grade and stage, as well as the incidence of recurrence within a five-year follow-up period. Results A strong statistical correlation was established between expression and tumor grade, with high-grade tumors showing weak to moderate expression of CK20 while low-grade tumors showed an intensive expression pattern. No correlation was noted between the expression pattern, patient age and gender, tumor stage, and the likelihood of local recurrence. Conclusion While CK 20 is a reliable diagnostic marker when used together with other markers, its expression pattern in our study correlated only with bladder urothelial carcinoma grade.

Decreased Plasma Level of Cytokeratin 20 (KRT20) Is Indicative of the Emergence and Severity of Acute GvHD Irrespective to the Type of Organ Involvement.

Accurate risk prediction of acute graft versus host disease (aGvHD) is currently an unmet clinical need. This study sought to analyze whether three plasma proteins expressed in a largely skin- and gut-restricted manner would be affected by the development of acute cutaneous and gastrointestinal aGvHD. The diagnostic sensitivity, specificity, and prognostic value of plasma cytokeratin-15 (KRT15) cytokeratin-20 (KRT20), and occludin (OCLN) were evaluated in a discovery and a validation cohort using ELISA in comparison with elafin (PI3) and regenerating family member 3 alpha (REG3A), two established markers of skin- and gut aGvHD. The discovery cohort (n = 39) revealed that at the time of diagnosis, plasma KRT20 showed a progressive decrease from unaffected individuals to patients with single-, and patients with multi-organ aGvHD. KRT20 was affected by cutaneous (p = 0.0263) and gastrointestinal aGvHD (p = 0.0242) independently and in an additive manner. Sensitivity and specificity of KRT20 for aGvHD involving both target organs (AUC = 0.852) were comparable to that of PI3 for skin-aGvHD (AUC = 0.708) or that of REG3A for gut-aGvHD (AUC = 0.855). Patient follow-up in the validation cohort (n = 67) corroborated these observations (p < 0.001), and linked low KRT20 to grade 2+ disease (p < 0.001), but failed to confirm low KRT20 as an independent risk factor. These data established a link between low plasma KRT20 levels and moderate to severe aGvHD involving multiple target organs.

Primary Transitional Cell Carcinoma of the Endometrium: A Case Report and Review of Literature.

Transitional cell carcinoma of the endometrium is a rare cancer. We present a 58-year-old Caucasian female was diagnosed with high-grade polyploid transitional cell carcinoma of the endometrium. The pathological findings included a dense serosal adhesion over the fundus and the posterior wall, necrotic and polyploid papillary mass in the endometrial cavity, serosal adhesions and unremarkable parenchyma in the fallopian tubes. Others were the presence of cancer antigen 125 and cytokeratin 7 and absence of cytokeratin 20, actin, estrogen receptor, soluble protein 100, vimentin, and cytokeratin high molecular weight. The case is relevant to practice as it identifies the histological patterns for primary transitional cell carcinoma of the endometrium, including expressing cytokeratin 7 instead of cytokeratin 20. The tumor showed a papillary and solid architecture and composed of undifferentiated to poorly differentiated cells with no defined glandular nor squamous differentiation.

Immunoexpression of SDHB, FH, and CK20 among eosinophilic renal tumors: A tissue microarray study.

BACKGROUND: Differential diagnosis can be a challenge for eosinophilic subtypes of renal cell tumors due to their overlapping histomorphological and immunohistochemical features. We aimed to investigate the frequency of rare variants of renal cell carcinomas (RCCs) such as succinate dehydrogenase-deficient RCC (SDDRCC), hereditary leiomyomatosis and RCC (HLRCC)-associated RCC, and eosinophilic, solid, and cystic RCC (ESCRCC) in our population. MATERIALS AND METHODS: Renal tumors which could be considered in the eosinophilic tumor category were included: 91 conventional clear cell RCCs with eosinophilic cytoplasm, 72 papillary RCCs, 74 chromophobe RCCs, 88 oncocytomas, and 37 other rare subtypes. Using the tissue microarray method, succinate dehydrogenase B (SDHB), fumarate hydratase (FH), and cytokeratin 20 (CK20) antibodies were performed by immunohistochemistry. Immunohistochemistry was repeated on whole block sections for selected cases. The utility of these antibodies in the differential diagnosis was also investigated. RESULTS: Loss of SDHB expression was detected in three tumors, two of which showed typical morphology for SDDRCC. In additional two tumors, SDHB showed weak cytoplasmic expression without a mitochondrial pattern (possible-SDHB deficient). None of the tumors showed loss of FH expression. Heterogeneous reactions were observed with SDHB and FH antibodies. Only one ESCRCC was detected with diffuse CK20 positivity. CONCLUSION: SDDRCCs, HLRCC-associated RCCs, and ESCRCCs are very rare tumors depending on the population. Possible weak staining and focal loss of SDHB and FH expression should be kept in mind and genetic testing must be included for equivocal results.

Comprehensive Gene Expression Analyses of Immunohistochemically Defined Subgroups of Muscle-Invasive Urinary Bladder Urothelial Carcinoma.

A number of urinary bladder urothelial carcinoma (UB UC) mRNA-based classification systems have been reported. It also has been observed that treatment response and prognosis are different for each molecular subtype. In this study, cytokeratin (CK)5/6 and CK20 immunohistochemistry (IHC) were performed, and IHC-based subgroup classification was applied. UB UC was classified into CK5/6 single-positive (SP), CK20 SP, double-positive (DP) and double-negative (DN) subgroups, and transcriptional analysis was performed. The results of gene ontology (GO) terms and functional analysis using differentially expressed genes indicate that, CK5/6 SP and DP subgroups were enriched in cell migration, immune activation, interleukin 6-Janus kinase-signal transducer and activator of transcription 3 (IL6-JAK-STAT3) signaling pathway and tumor necrosis factor-α signaling via the nuclear factor-κB (NF-κB) signaling pathway signature gene. In addition, compared with the other subgroups, the DN subgroup showed inhibited cell movement, cell migration, and cell activation. Furthermore, in survival analysis, the CK5/6 SP subgroup was significantly associated with poor progression-free survival (p = 0.008). The results of our study indicate that the CK5/6 positive subgroup exhibited high gene expression signature related to aggressive behavior and exhibited worse clinical outcome.

Comparative Analysis of Blood and Bone Marrow for the Detection of Circulating and Disseminated Tumor Cells and Their Prognostic and Predictive Value in Esophageal Cancer Patients.

Hematogenic tumor cell spread is a key event in metastasis. However, the clinical significance of circulating tumor cells (CTC) in the blood and disseminated tumor cells (DTC) in bone marrow is still not fully understood. Here, the presence of DTC and CTC in esophageal cancer (EC) patients and its correlation with clinical parameters was investigated to evaluate the CTC/DTC prognostic value in EC. This study included 77 EC patients with complete surgical tumor resection. CTC and DTC were analyzed in blood and bone marrow using nested CK20 reverse transcription-nested polymerase chain reaction (RT-PCR) and findings were correlated with clinical data. Twenty-seven of 76 patients (36.5%) showed CK20 positivity in the blood, 19 of 61 patients (31.1%) in bone marrow, and 40 (51.9%) of 77 patients were positive in either blood or bone marrow or both. In multivariate analyses, only the DTC status emerged as independent predictor of overall and tumor specific survival. Our study revealed that, while the presence of CTC in blood is not associated with a worse prognosis, DTC detection in the bone marrow is a highly specific and independent prognostic marker in EC patients. Larger cohort studies could unravel how this finding can be translated into improved therapy management in EC.

CK20 versus AMACR and p53 immunostains in evaluation of Urothelial Carcinoma in Situ and Reactive Atypia.

Ancillary testing with immunohistochemistry has shown recent promise in the workup of equivocal bladder lesions. We read with interest the recent findings of Alston et al., who assessed the diagnostic utility of alpha-methylacyl-CoA racemase (AMACR) in comparison to cytokeratin 20 (CK20) in evaluation of atypia in challenging flat urothelial lesions in the differential between carcinoma in situ (CIS) and reactive atypia. AMACR was reported to be a somewhat more specific but less sensitive marker for CIS than CK20, though showing weaker intensity. Spurred by their report, with the knowledge that we had consistently and consecutively performed AMACR, CK20, and p53 on flat urothelial lesions challenging enough to reach intradepartmental consensus, we performed a retrospective review. Similarly, we found that AMACR was less sensitive (80%) and more specific (100%) than CK20, with the same caveat of less staining intensity. Additionally, our p53 review identified a significant rate (~ 27%) of equivocal/non-informative findings. Taken together, our experience in this consecutive cohort confirms the impression of Alston et al. regarding the utility and challenges of AMACR use, while highlighting challenges with p53, which we plan to use more sparingly prospectively.

Diagnostic significance of dual immunocytochemical staining of p53/cytokeratin20 on liquid-based urine cytology to detect urothelial carcinoma.

BACKGROUND: Urine cytology is a noninvasive and inexpensive method; however, it is limited in low sensitivity for detecting and monitoring urothelial carcinoma (UC). To overcome limitation of cytology, several tests using urine samples have been attempted that immunocytochemical staining is an inexpensive and easy to perform ancillary technique. Dual immunocytochemical staining for p53 and cytokeratin 20 (CK20) is assessed in liquid-based urine cytology slides. MATERIALS AND METHODS: Liquid-based urine cytology samples collected between 2008 and 2013 and matched follow-up biopsy samples of high-grade UC (HGUC) (n = 44) and low-grade UC (LGUC) (n = 14) were analyzed. RESULTS: Urine cytology showing atypical cells was subjected to dual-color immunostaining for p53 and CK20. The sensitivity of urine cytology combined with p53 and CK20 immunostaining was 77.3% in HGUC and 52.9% in LGUC. Of 20 cases diagnosed with atypia by urine cytology, 13 (65%) were positive for p53 or CK20. Dual immunocytochemical staining for p53/CK20 improved the diagnostic accuracy of urine cytology. CONCLUSIONS: The present results indicate that cytomorphology combined with p53/CK20 immunostaining is useful for the detection of HGUC and LGUC.

Primary Cutaneous Neuroendocrine Carcinoma with Diffuse Expression of Thyroid Transcription Factor-1: Report of Two Cases.

Primary cutaneous neuroendocrine carcinoma (PCNC), previously known as Merkel cell carcinoma (MCC), is a rare tumor of the skin with aggressive behavior and poor prognosis. Typically, PCNC is positive for Cytokeratin-20 (CK20) and negative for Thyroid Transcription Factor-1 (TTF-1). Rarely, CK-20 negative and TTF-1 positive PCNC have been described. We present the case of two patients with skin lesions histologically compatible with MCCs and a behavior characteristic of this disease, but with expression of TTF-1 instead of CK-20. In conclusion, there are increasing reports of TTF1+ CK20- skin lesions without signs of systemic disease which behave clinically and prognostically like a PCNC. The origin of these TTF1 tumors are, to date, unknown.

Diagnostic and prognostic roles of CK20 in the pathology of urothelial lesions. A systematic review.

Cytokeratin 20 (CK20) is one of the most common immunohistochemical markers in the routine practice of a pathology lab, as biopsies from the urinary tract encompass a wide spectrum of lesions which may pose issues in their detection and classification. In this review, we aim to outline the diagnostic accuracy and prognostic value of CK20 in flat urothelial lesions, papillary non-invasive and invasive urothelial carcinoma, molecular subgroups and variant histology, and we briefly discuss its limitations and potential pitfalls.

Usefulness of large-section cytokeratin 20 in the detection of intestinal wall infiltration and mesangial metastasis in patients with middle and lower rectal cancer.

OBJECTIVE: The objective of the study was to evaluate the usefulness of large-section cytokeratin 20 (CK20) staining technique in the detection of infiltration on the distal wall and mesangial metastasis in patients with middle and lower rectal cancer. MATERIALS AND METHODS: A total of 62 patients with rectal cancer in the middle and lower segment were studied on large slices stained with CK20. Logistic regression was used to analyze the clinicopathologic factors related to distal low and middle rectal cancer metastasis to the mesorectum and rectal wall. RESULTS: Two types of distal metastasis of the tumor were observed in the rectal wall in 18% (11/62) of the patients: submucosal invasion and muscularis propria invasion. The extent of distal metastasis to the rectal wall was around 0.5-1.0 cm. Four types of distal metastasis occurred in the mesorectum: lymph node invasion, blood and lymphatic vessel invasion, perineural invasion, and isolated neoplastic microfoci. Distal metastasis to the mesorectum was observed in 24% (15/62) of the patients. The extent of metastasis to the mesorectum was around 0.5-4.0 cm. Another three patients with microcapillary invasion in the distal mesorectum were observed by immunohistochemistry, as it was difficult to determine the spread by conventional hematoxylin and eosin staining. CONCLUSION: The large-section CK20 staining technique is useful for the detection of infiltration on the distal wall and mesangial metastasis in patients with middle and lower rectal cancer.

Immunohistochemistry of cytokeratin (CK) 5/6, CD44 and CK20 as prognostic biomarkers of non-muscle-invasive papillary upper tract urothelial carcinoma.

AIMS: Immunohistochemical (IHC) staining for cytokeratin (CK) 5/6, CD44 and CK20 has been significantly associated with the prognosis of urinary bladder urothelial carcinoma, and probably reflects its molecular characteristics. We aimed to investigate the IHC-based subgroups and their prognostic effects on non-muscle-invasive papillary upper tract urothelial carcinoma (UTUC). METHODS AND RESULTS: IHC staining for CK5/6, CK20 and CD44 was analysed in 211 patients with non-muscle-invasive papillary UTUC. Staining was classified as showing a negative, positive or normal pattern. We found that CK5/6-negative, CD44-negative and CK20-positive tumours were distinctly high-risk subgroups that were associated with high grade (CK5/6-negative, P < 0.001; CD44-negative, P < 0.001; CK20-positive, P = 0.017) and frequent intravesical recurrence (CK5/6-negative, P = 0.002). Using survival analysis with Kaplan-Meier and log-rank tests, we found that these IHC subgroups were correlated with poor progression-free (CK5/6-negative, P = 0.001; CD44-negative, P = 0.009; CK20-positive, P = 0.031) and cancer-specific (CK5/6-negative, P = 0.009) survival. Furthermore, CK5/6 negativity was an independent prognostic factor for shorter progression-free (P = 0.009) and cancer-specific (P = 0.045) survival. CK5/6 improved Harrell's C-indices for progression-free (0.68-0.77, P = 0.029) and cancer-specific (0.59-0.77, P < 0.001) survival. When markers were combined, luminal-like subtypes showed poor prognoses. CONCLUSIONS: We demonstrated that IHC staining for CK5/6, CD44 and CK20 was significantly associated with the clinicopathological characteristics and prognoses of patients with non-muscle-invasive papillary UTUC. The IHC subgroups may be correlated with the molecular characteristics of non-muscle-invasive papillary UTUC.

Neurofilament is superior to cytokeratin 20 in supporting cutaneous origin for neuroendocrine carcinoma.

AIM: Primary cutaneous neuroendocrine carcinoma, or Merkel cell carcinoma (MCC), cannot be distinguished morphologically from small-cell neuroendocrine carcinomas (SmCC) from other sites. Immunohistochemistry is required to confirm cutaneous origin, and is also used for detection of sentinel lymph node (SLN) metastases of MCC. Cytokeratin 20 (CK20) expression is commonly used for these purposes, but is negative in some MCC cases, and has unclear specificity. We evaluated immunohistochemistry for neurofilament and CK20 in MCC compared with SmCC from other sites. METHODS AND RESULTS: We evaluated neurofilament expression in 55 MCC specimens from 39 unique patients, including nine CK20-negative MCC tumours. Neurofilament expression was observed in 42 of 55 (76.4%) MCC cases, including seven of nine (77.8%) CK20-negative MCC cases. Neurofilament was expressed in nine of 12 (75%) Merkel cell polyomavirus-positive tumours and five of 10 (50%) virus-negative tumours. Compared to a standard immunohistochemical panel (cytokeratin cocktail and CK20), neurofilament was 87.5% sensitive for detecting SLN metastases. Neurofilament and CK20 expression was also assessed in 61 extracutaneous SmCC from 60 unique patients, with primary sites including lung (27), bladder (18), cervix (3), gastrointestinal tract (3), sinonasal tract (2) and other sites (7). The specificity of neurofilament and CK20 for MCC versus non-cutaneous SmCC was 96.7% and 59.0%, respectively. CONCLUSIONS: Neurofilament has superior specificity to CK20 in distinguishing MCC from non-cutaneous SmCC. Neurofilament is frequently expressed in CK20- and virus-negative MCC tumours. Limitations of neurofilament immunohistochemistry include lower sensitivity than CK20 and subtle staining in some tumours. However, our findings indicate that neurofilament is useful for excluding non-cutaneous SmCC.