Pathways Case Studies: October 2023

A 70-year-old woman presented with a left breast mass and left lung mass. The breast mass was excised. Computed tomography (CT) and mammographic images of the lungs and left breast, as well as hematoxylin and eosin and immunohistochemical stained slides of the breast mass, are shown in the figures below.

Figure 1: Mammogram
Figure 2: CT
Figure 3: H&E, 20x
Figure 4: H&E, 40x
Figure 5: TTF-1
Figure 6: Estrogen receptor (ER)

Based on the histologic morphology and immunophenotype, what is the correct diagnosis?

  • Metastatic urothelial carcinoma
  • Metastatic lung adenocarcinoma
  • Metastatic papillary thyroid carcinoma
  • Primary breast invasive ductal carcinoma

The correct answer is ...

Metastatic lung adenocarcinoma.

The histologic sections in this case demonstrate an infiltrative neoplasm composed of glands and papillae within a desmoplastic stroma. The cells show mild to moderate cytologic atypia, with nuclear pleomorphism, occasional prominent nucleoli, hyperchromasia, and rare intranuclear inclusions.

By immunohistochemistry, the neoplastic cells are strongly and diffusely positive for TTF-1, and are negative for ER, PR, and thyroglobulin. The overall morphology and immunophenotype are most consistent with an adenocarcinoma of lung metastatic to the breast.

Metastases to the breasts from extramammary sites are rare, and incidences of such have been reported as 0.4%-3%. The most common malignancies to secondarily involve the breasts are leukemia/lymphoma and malignant melanoma.1,2  Carcinomas may metastasize to the breast as well, including ovarian, lung, renal, and thyroid.2,3 Metastases to the breasts are most often encountered in patients with advanced and disseminated disease. In such settings, the patient’s extramammary primary malignancy is generally known to the care team. When the extramammary primary is occult, the distinction of a metastasis from a primary carcinoma of the breast is especially important for prognosis and therapeutic decisions.

The morphologic distinction between metastatic lung adenocarcinoma and primary breast carcinoma in the breast can be extremely difficult. In cases in which knowledge of a lung mass is unknown to the pathologist, metastatic lung carcinomas may be confused with triple negative primary breast carcinomas. Morphologic features that may suggest a primary breast carcinoma include the presence of an in-situ component and elastosis.1 Assessment of these helpful features may be limited in core biopsy specimens. 

In this case, a papillary architectural component was noted with positivity for TTF-1 confirmed. These findings were consistent with metastatic lung carcinoma but also raised the differential diagnosis of a metastatic thyroid carcinoma. Thyroglobulin IHC testing was also performed, with a negative result, making a metastatic thyroid carcinoma unlikely. 

Finally, while TTF-1 carries a high sensitivity and specificity for primary lung adenocarcinoma, aberrant TTF-1 expression has been described in a number of other entities, including extra-pulmonary neuroendocrine tumors and carcinomas arising from the bladder, prostate, gastrointestinal tract, and gynecologic organs.5,6 In the breast, carcinoma diagnoses, particularly those that are hormone receptor negative, may trigger expanded immunohistochemistry panels to confirm breast origin, including addition of markers such as mammaglobin, GATA3, or TRPS1.1-4

Radiology of the breast itself may not always be of help, as metastases to the breast may present as a solitary nodule or multiple nodules, with well- or ill-defined borders. More extensive imaging to include other sites may be useful, and other metastatic deposits may be identified.3,4 In this case, a single mass lesion was identified in the left breast, ipsilateral to the patient’s confirmed lung mass.


  1. Wang X, Luo Y, Liu L, Wei J, Lei H, Shi S, Yang L. Metastatic adenocarcinoma to the breast from the lung simulates primary breast carcinoma-a clinicopathologic study. Transl Cancer Res. 2021 Mar;10(3):1399-1409. doi:10.21037/tcr-20-2250. PMID: 35116465; PMCID: PMC8798917
  2. Sousaris N, Mendelsohn G, Barr RG. Lung cancer metastatic to breast: case report and review of the literature. Ultrasound Q. 2013 Sep;29(3):205-9. doi:10.1097/RUQ.0b013e3182a00fc4. PMID: 23975047.
  3. Ali RH, Taraboanta C, Mohammad T, Hayes MM, Ionescu DN. Metastatic non-small cell lung carcinoma a mimic of primary breast carcinoma-case series and literature review. Virchows Arch. 2018 May;472(5):771-777. doi:10.1007/s00428-017-2262-4. Epub 2017 Nov 5. PMID: 29105026.
  4. Noguera J, Martínez-Miravete P, Idoate F, Díaz L, Pina L, Zornoza G, Martínez-Regueira F. Metastases to the breast: a review of 33 cases. Australas Radiol. 2007 Apr;51(2):133-8. doi:10.1111/j.1440-1673.2007.01681.x. PMID: 17419856.
  5. Casteillo F, Fournel P, Da Cruz V, Karpathiou G, Boutet C, Jacquin JP, Tissot C, Meyer-Bisch V, Péoc'h M, Forest F. TTF-1-positive metastatic endometrioid carcinoma: a case report and review of literature of a potential diagnostic pitfall. Appl Immunohistochem Mol Morphol. 2020 Jan;28(1):e6-e9. doi:10.1097/PAI.0000000000000539. PMID: 28777147.
  6. Matoso A, Singh K, Jacob R, Greaves WO, Tavares R, Noble L, Resnick MB, Delellis RA, Wang LJ. Comparison of thyroid transcription factor-1 expression by 2 monoclonal antibodies in pulmonary and nonpulmonary primary tumors. Appl Immunohistochem Mol Morphol. 2010 Mar;18(2):142-9. doi:10.1097/PAI.0b013e3181bdf4e7. PMID: 19887917

Kaitlyn Nielson, M.D.

Fellow, Surgical Pathology
Mayo Clinic

Charles Sturgis, M.D.
Mayo Clinic
Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

A 76-year-old man admitted to the hospital for transurethral enucleation of the prostate to relieve symptoms due to benign prostatic hyperplasia. A peri-operative Foley catheter was inserted, and a urine specimen was collected. The patient remained afebrile throughout his hospital course, received continuous bladder irrigation with production of light red, clear to pink urine, and received cefdinir twice daily as part of his immediate post-operative management. The patient did not report any symptoms associated with a urinary tract infection, and there was no previous evidence of urinary tract infection. Peri-operative urine culture grew >100,000 colony forming units (CFU)/ml of Providencia rettgeri (Fig. 1A) and Alcaligenes faecalis (not pictured) between 10,000 and 100,000 CFU/ml. A. faecalis was susceptible to all antimicrobials tested except trimethoprim-sulfamethoxazole. The susceptibility profile of Providencia rettgeri is as shown in figure 1B.

Figure 1: (A-left) Sheep’s blood agar of isolated Providencia rettgeri showing large, creamy colonies from the urine specimen.
(B-right) Antimicrobial susceptibility profile of Providencia rettgeri from this patient.

What is the most likely mechanism of resistance for Providencia rettgeri in this patient?

  • Change in cell wall peptidoglycan architecture from D-Ala-D-Ala to D-Ala-D-Lac.
  • Induction of erm methyltransferase.
  • Production of a carbapenemase.
  • Modification in lipopolysaccharide (LPS) structure, altering cell envelope permeability.
  • Acquisition of mecA gene leading to production of altered penicillin binding protein (PBP2a).

The correct answer is ...

Production of a carbapenemase.

Acquisition of mecA gene leading to production of altered penicillin binding protein (PBP2a) is incorrect because acquisition of the gene mecA leads to the production of an altered penicillin binding protein (PBP2a), which is associated with methicillin resistant Staphylococcus aureus (MRSA). PBP2a binds less avidly to oxacillin (and methicillin), leading to resistance.

Change in cell wall peptidoglycan architecture from D-Ala-D-Ala to D-Ala-D-Lac is incorrect because changes in cell wall peptidoglycan architecture from D-Ala-D-Ala to D-Ala-D-Lac are found in vancomycin-resistant Enterococcus faecium and are often due to mutations in the vanA gene cluster.

Induction of erm methyltransferase is incorrect, because induction of erm methyltransferase leads to methylation of RNA polymerase and macrolide resistance. This induction is associated with inducible clindamycin resistance in staphylococci and streptococci illustrated by the D-test.

Modification in lipopolysaccharide (LPS) structure, altering cell envelope permeability is incorrect because modifications in lipopolysaccharide (LPS) structure alter cell envelope permeability affecting a range of antimicrobial susceptibilities, particularly last resort antimicrobials such as polymyxins in gram-negative bacteria.

The prevalence of carbapenemases has been increasing with the rise in use of carbapenem antimicrobials (i.e., imipenem, ertapenem, meropenem). Carbapenems have become the drug of choice for treating bacteria causing multidrug-resistant gram-negative bacterial infections. Carbapenem resistance has been the result of rising prevalence of extended-spectrum β-lactamase (ESBL)-producing Enterobacterales. The first carbapenemase, NmcA, an Ambler class A carbapenemase (Naas & Nordmann, 1994), was reported in 1993 and since that time a wide array of carbapenemases have been identified in Enterobacterales and non-fermenting gram-negative bacteria, such as Acinetobacter and Pseudomonas aeruginosa. Carbapenemases are typically plasmid encoded and are easily transmitted; thus, patients with isolates positive for carbapenemases are placed on strict infection control precautions such as contact precautions when in hospitals or other healthcare settings (van Loon et al., 2018). Carbapenem-resistant Enterobacterales (CREs) are classified by the CDC as an “urgent” public health threat (CDC, 2019).

The most clinically significant carbapenemase is KPC (Ambler class A), which hydrolyzes a broad range of β-lactams while being inhibited by clavulanic acid and tazobactam (Nordmann, et al., 2012). Class B carbapenemases (e.g., IMP, NDM) exhibit a broad spectrum of activity against all penicillins, cephalosporins, and carbapenems, resistance to available β-lactam inhibitors, and susceptibility to aztreonam and metal ion chelators, as the mechanism of action is dependent on zinc ions in the active site. Class D carbapenemases (OXA) are penicillinases capable of hydrolyzing oxacillin that are poorly inhibited by clavulanic acid and ethylenediaminetetraacetic acid (EDTA) with variable carbapenemase activity. As an alternative to carbapenemases, carbapenem resistance can be due to changes in bacterial permeability or increased efflux (Li, et al., 2015; Szabó et al., 2006). Mechanistic differences in carbapenem resistance present challenges in patient management, infection control precautions, and diagnostic testing. Given the rise in carbapenem resistance, the Clinical and Laboratory Standards Institute (CLSI) lowered the breakpoints of carbapenem for better detection of carbapenemase-producing isolates in 2010 (CLSI, 2020; Hombach et al., 2012). However low-level resistance and even susceptibility to carbapenems have been observed for some producers of carbapenemases, although resistance against both carbapenems was seen in this case.

Carbapenemase activity is commonly detected using the CarbaNP test, which can rapidly detect carbapenem hydrolysis by carbapenemases within two hours. Hydrolysis of the carbapenem acidifies the medium, which results in a color change of the pH indicator. This method can detect any carbapenemases. CarbaNP testing for carbapenemases should be performed for any Enterobacterales or P. aeruginosa isolate with decreased carbapenem susceptibility. This phenotypic test only detects transmissible carbapenem resistance (plasmid mediated), and has a high specificity and sensitivity (>95%, Dortet et al., 2015; Poirel & Nordmann, 2015).

Phenotypic testing of the P. rettgeri was positive for carbapenemase by CarbaNP. P. rettgeri, of the order Enterobacterales, is a gram-negative bacterium that can produce inducible AmpC-β-lactamases, plasmid-mediated ESBLs, and IMP carbapenemases. Treatment regimens depend on the susceptibility of the isolate (Walters et al., 2018).

PCR for KPC, NDM, OXA-48-like, and VIM carbapenemases performed at Mayo Clinic was negative. Due to the discrepancy between phenotypic (CarbaNP) and genotypic tests for carbapenemases, the isolate was sent to the Minnesota Department of Health, where it was found to be positive by PCR for IMP carbapenemase. In part due to the rising incidence of IMP carbapenemases (Lowe et al., 2020; Woodworth et al., 2018), Mayo Clinic has begun offering the Cepheid Xpert Carba-R carbapenem resistance assay, a molecular PCR for the rapid detection and differentiation of bacteria producing the KPC, NDM, VIM, and OXA carbapenemases.

This patient remained asymptomatic during his hospitalization and was placed on contact precautions. His hospital course included 2 days of cefdinir followed by 2 days of cefepime, after which he was discharged on a 7-day course of trimethoprim-sulfamethoxazole. The A. faecalis isolate, which was resistant only to trimethoprim-sulfamethoxazole was covered by the cephalosporin therapy.


  1. CDC, A. (2019). Antibiotic resistance threats in the United States. US Department of Health and Human Services: Washington, DC, USA.
  2. CLSI. (2023). Performance Standards for Antimicrobial Susceptibility Testing. Clinical and Laboratory Standards Institute, 33rd ed. CLSI supplement M100. 
  3. Dortet L, Agathine A, Naas T, Cuzon G, Poirel L, Nordmann P. Evaluation of the RAPIDEC® CARBA NP, the Rapid CARB Screen® and the Carba NP test for biochemical detection of carbapenemase-producing Enterobacteriaceae. J Antimicrob Chemother. 2015;70(11):3014-3022. doi:10.1093/jac/dkv213
  4. Hombach M, Bloemberg GV, Böttger EC. Effects of clinical breakpoint changes in CLSI guidelines 2010/2011 and EUCAST guidelines 2011 on antibiotic susceptibility test reporting of Gram-negative bacilli. J Antimicrob Chemother. 2012;67(3):622-632. doi:10.1093/jac/dkr524
  5. Li XZ, Plésiat P, Nikaido H. The challenge of efflux-mediated antibiotic resistance in Gram-negative bacteria. Clin Microbiol Rev. 2015;28(2):337-418. doi:10.1128/CMR.00117-14
  6. Lowe CF, Matic N, Champagne S, Romney MG, Leung V, Ritchie G. (2020). The brief case: IMP, the uncommonly common Carbapenemase. J Clin Microbiol58(4), e01094-19.
  7. Naas T, Nordmann P. Analysis of a carbapenem-hydrolyzing class A beta-lactamase from Enterobacter cloacae and of its LysR-type regulatory protein. Proc Natl Acad Sci U S A. 1994 Aug 2;91(16):7693-7. doi:10.1073/pnas.91.16.7693. PMID: 8052644; PMCID: PMC44468.
  8. Nordmann P, Gniadkowski M, Giske CG, et al. Identification and screening of carbapenemase-producing Enterobacteriaceae. Clin Microbiol Infect. 2012;18(5):432-438. doi:10.1111/j.1469-0691.2012.03815.x
  9. Poirel L, Nordmann P. Rapidec Carba NP Test for Rapid Detection of Carbapenemase Producers. J Clin Microbiol. 2015;53(9):3003-3008. doi:10.1128/JCM.00977-15
  10. Szabó D, Silveira F, Hujer AM, et al. Outer membrane protein changes and efflux pump expression together may confer resistance to ertapenem in Enterobacter cloacae. Antimicrob Agents Chemother. 2006;50(8):2833-2835. doi:10.1128/AAC.01591-05
  11. van Loon K, Voor In 't Holt AF, Vos MC. A Systematic Review and Meta-analyses of the Clinical Epidemiology of Carbapenem-Resistant Enterobacteriaceae. Antimicrob Agents Chemother. 2017;62(1):e01730-17. Published 2017 Dec 21. doi:10.1128/AAC.01730-17
  12. Walters MS, Witwer M, Lee Y-K, Albrecht V, Lonsway D, Rasheed JK, Anacker M, Snippes-Vagnone P, Lynfield R, and Kallen AJ. (2018). Notes from the Field: Carbapenemase-producing carbapenem-resistant Enterobacteriaceae from less common Enterobacteriaceae genera—United States, 2014–2017. MMWR Morb Mortal Wkly Rep67(23), 668.
  13. Woodworth KR, Walters MS, Weiner LM, et al. Vital Signs: Containment of Novel Multidrug-Resistant Organisms and Resistance Mechanisms - United States, 2006-2017. MMWR Morb Mortal Wkly Rep. 2018;67(13):396-401. Published 2018 Apr 6. doi:10.15585/mmwr.mm6713e1

Casey Vieni, M.D., Ph.D.

Resident, Anatomic & Clinical Pathology
Mayo Clinic

Audrey Schuetz, M.D.

Consultant, Clinical Microbiology and Anatomic Pathology
Mayo Clinic
Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

A 31-year-old man who sustained a left hip fracture from a longboarding accident status-post uncomplicated surgical repair one month ago is presenting for hematology follow-up. During childhood, he had easy bruising and frequent 30-minute episodes of epistaxis which required four cauterizations, but these resolved in adulthood. Surgeries during adulthood, including recent hip fracture repair, tonsillectomy, and wisdom tooth extraction were not associated with bleeding complications. Laboratory findings at 1-month follow-up and at time of fracture are shown below.

Figure 1

Which etiology of diminished von Willebrand factor is most likely in this patient?

  • Nonsense-mediated decay of von Willebrand factor.
  • Markedly reduced Factor VIII binding by von Willebrand factor.
  • Type O blood.
  • Defective platelet binding by von Willebrand factor.

The correct answer is ...

Type O blood.

Primary hemostasis, namely platelet plug formation at sites of vascular injury, are significantly influenced by ABO group. Plasma VWF levels are ∼25% lower in healthy group O compared with healthy group non-O individuals. In addition, blood group O VWF demonstrates enhanced susceptibility to ADAMTS13 proteolysis. These effects on VWF may be mediated by the ABO(H) carbohydrate structures that are carried on both the N- and O-linked glycans of VWF and expressed on several different platelet surface glycoprotein receptors. Recent studies support the hypothesis that ABO group not only exerts major quantitative and qualitative effects on VWF, but also affect specific aspects of platelet function. 

This patient’s childhood history of severe epistaxis and easy bruising (i.e., pre-pubertal), as well as the improvement of symptoms with age can be explained by the hormonal increase associated with puberty. Increased testosterone as well as estrogen can increase the amount of von Willebrand factor production. Von Willebrand factor production is also increased in the setting of trauma and surgery, explaining why it was within normal limits peri-operatively but began to downtrend towards his baseline between 40%-50% one month after surgery. 

Type 2A vWD is characterized by defective platelet binding due to the absence of high molecular weight vWF multimers in both plasma and platelets. Type 2A vWD patients have a low vWF:RCo to vWF:Ag ratio (<0.6). 

Type 2B vWD is characterized by spontaneous and increased binding of vWF to GpIbα receptors on platelets due to dominant gain-of-function A1 domain mutations and the absence of HMW-vWF multimers in plasma. Due to spontaneous platelet binding HMW-vWF multimers in plasma are proteolyzed by ADAMTS-13. Patients have a low VWF:RCo to VWF:Ag ratio (<0.6) and an increase in ristocetin-induced platelet aggregation (RIPA) at low dose ristocetin. Thrombocytopenia can also be observed in some type 2B vWD patients under stress conditions, such as pregnancy or infection, and after DDAVP use. This patient has a vWF:RCo to vWF:Ag ratio >0.6 and normal multimers, making these two possibilities less likely.

Type 2N vWD is characterized by markedly reduced or lack of vWF affinity FVIII binding. Recessive mutations in the vWF-FVIII binding domain result in the lack of FVIII binding and a disproportionate decrease in FVIII:C level to between 0.05 and 0.30 IU/mL. Patients can be homozygous or compound heterozygous for FVIII binding mutations or be compound heterozygous a FVIII binding mutation and a vWF null allele. This patient has normal Factor VIII activity, making this possibility less likely. Type 3 vWD is characterized by the absence of plasma vWF and a consequent decrease in the FVIII level to 10%. Patients are homozygous or compound heterozygous for the vWF null mutations. The most common nonsense mutation is R1659X in exon 28. Nonsense-mediated decay of the allele-specific mRNA is thought to be the molecular mechanism of the nonsense mutations. This patient has detectable vWF antigen and activity, as well as normal Factor VIII activity, making this possibility less likely.


  1. Ward SE, O'Sullivan JM, O'Donnell JS. The relationship between ABO blood group, von Willebrand factor, and primary hemostasis. Blood. 2020;136(25):2864-2874. doi:10.1182/blood.2020005843
  2. Berber E. The molecular genetics of von Willebrand disease. Turk J Haematol. 2012 Dec;29(4):313-24. doi:10.5505/tjh.2012.39205. Epub 2012 Dec 5. PMID: 24385719; PMCID: PMC3781629.

Edwin Lin, M.D., Ph.D.

Fellow, Anatomic & Clinical Pathology
Mayo Clinic

Aneel Ashrani, M.D., M.S.

Consultant, Hematology
Mayo Clinic
Assistant Professor of Laboratory Medicine and Pathology
Associate Professor of Medicine
Mayo Clinic College of Medicine and Science

A 41-year-old woman presents with worsening abnormal uterine bleeding. An endometrial biopsy returns as hyperplasia with atypia, and the patient undergoes hysterectomy. A uterine mass is identified by gross examination. Representative photomicrographs of the lesion on frozen (figures 1 and 2) and permanent sections (figure 3) are shown.

Figure 1: Frozen section – toluidine blue
Figure 2: Frozen section – toluidine blue
Figure 3: Permanent section – H&E

Which of the following immunohistochemical stains would you expect to show diffuse positivity in this case?

  • Beta-catenin (nuclear and membranous staining)
  • P16
  • P53 (mutant pattern)
  • Inhibin

The correct answer is ...

Beta-catenin (nuclear and membranous staining).

This is an example of a corded and hyalinized variant of endometrioid endometrial carcinoma (CHEC), in which neoplastic cells are arranged in cords or small clusters and set within a hyalinized stromal background. Other morphologic clues to this variant include extensive squamous metaplasia, osseous metaplasia, or spindle cell proliferations. Patients with CHEC also tend to be younger than those with conventional endometrioid adenocarcinoma, with a mean age of less than 50 years at presentation.1 This pattern is also clinically important, as CHEC tend to have low-grade morphology and overall favorable outcomes, though some high-grade CHEC have been reported.2,3 Furthermore, CHEC-pattern changes have also been reported in endometrial hyperplasia and extrauterine sites such as ovarian primary endometrioid carcinoma, where they may cause diagnostic confusion.2,4 CHEC characteristically harbor a mutation in CTNNB1, which is immunohistochemically manifested as diffuse, nuclear and membranous reactivity for beta-catenin.

The corded growth pattern in CHEC could raise the possibility of sex cord-like tumors of the uterus, such as uterine tumor resembling ovarian sex cord tumor (UTROSCT). UTROSCT can show positivity for sex cord markers such as inhibin or calretinin, whereas CHEC should not have such expression.

In any neoplasm of the gynecologic tract with a squamoid component, cervical carcinomas should also at least be considered. Given that many of them are HPV-driven, they classically would show diffuse, “block” positivity with p16 staining.

Lastly, carcinosarcoma represents a major pitfall in the diagnosis of CHEC, as the spindled or metaplastic components in CHEC might resemble a malignant mesenchymal component of carcinosarcoma. However, carcinosarcomas have both a high-grade epithelial and a high-grade mesenchymal component, as opposed to the low-grade morphology typically seen in CHEC.1 Carcinosarcoma patients tend to be of older age—presenting in their 8th decade-and nearly all contain TP53 mutations, which can be reflected in diffusely positive or completely negative immunohistochemical staining for p53.5


  1. Travaglino et al. Corded and hyalinized endometrioid carcinoma: Summary of clinical, histological, immunohistochemical and molecular data. Pathol Res Pract. 2023 Jul;247:154515. doi:10.1016/j.prp.2023.154515. Epub 2023 May 8. PMID: 37209572.
  2. Pors et al. The Evolving Spectrum of Endometrial Glandular Proliferations With Corded and Hyalinized Features. Am J Surg Pathol. 2023 Sep 1;47(9):1067-1076. doi:10.1097/PAS.0000000000002078. Epub 2023 Jul 27. PMID: 37493099.
  3. Ladwig et al. Corded and Hyalinized Endometrioid Adenocarcinoma (CHEC) of the Uterine Corpus are Characterized by CTNNB1 Mutations and Can Show Adverse Clinical Outcomes. Int J Gynecol Pathol. 2021 Mar 1;40(2):103-115. doi:10.1097/PGP.0000000000000671. PMID: 32909971.
  4. Talia and McCluggage. The diverse morphology and immunophenotype of ovarian endometrioid carcinomas. Pathology. 2023 Apr;55(3):269-286. doi:10.1016/j.pathol.2023.01.003. Epub 2023 Jan 21. PMID: 36759286.
  5. WHO Classification of Tumours Editorial Board. Female genital tumours [Internet]. Lyon (France): International Agency for Research on Cancer; 2020 [cited 2023 Aug 23]. (WHO classification of tumours series, 5th ed.; vol. 4). Available from:
Photo of Ryan W. Kendziora, M.D.

Ryan Kendziora, M.D. 

Fellow, Pathology
Mayo Clinic

Gary Keeney, M.D.

Consultant, Anatomic Pathology
Mayo Clinic
Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

A 65-year-old woman with a history of resected breast cancer 10 years ago and resected lung cancer 3 years ago presented to hospital services feeling unwell. CT imaging demonstrated a new 2.8 cm right upper-lobe lung mass with hilar and mediastinal adenopathy, as well as multiple liver masses. She underwent ultrasound-guided biopsy of one of the liver lesions (shown below). A broad panel of immunohistochemical stains was performed.

Positive stains: CAM5.2, AE1/3, CK7, POU2F3, Ki-67: 75%
Negative stains: TTF1 (clone SPT24 as well as 8G7G3/1), p40, Chromogranin, Synaptophysin, GATA3, GCDFP1, ER, PAX8, WT1, CDX2, HepPar1, Arginase

Figure 1: Low Power
Figure 2: High Power
Figure 3: POU2F3

What is your diagnosis?

  • Metastatic breast carcinoma
  • Metastatic non-small cell lung cancer
  • Small cell carcinoma
  • Hepatocellular carcinoma

The correct answer is ...

Small cell carcinoma.

The tumor shows an organoid arrangement of abnormal cells with increased N:C ratio, nuclear molding, and finely granular chromatin. There is abundant mitotic activity, as well as both punctate single cell and geographic tumor necrosis. Even without stains, the morphologic features demonstrate a high-grade, poorly differentiated neoplasm. 

Historically, in the setting of a lung mass with hilar adenopathy, a diagnosis of small cell carcinoma (SCLC) could be made on morphology alone.1 However, given the history of previous malignancy in this patient, additional studies are prudent. 

The extensive immunohistochemical work-up in this patient yields some interesting results. Whilst diffuse keratin expression confirms epithelial origin, additional markers are largely negative. Stains for mammary origin (GCDFP1, ER, and GATA3) are non-reactive. Additional less likely considerations, such as squamous cell, ovarian, hepatocellular, and gastrointestinal carcinoma, are effectively ruled out by negative staining in the remainder of the panel. 

Most SCLC (about 80%) show strong nuclear positivity for TTF1, and show at least focal expression of classic neuroendocrine markers such as synaptophysin, chromogranin, and CD56 (the latter not performed in this case). However, up to 20% of SCLC may be negative for both synaptophysin and chromogranin, and even with the introduction of INSM1, they may be completely negative for neuroendocrine expression.2 

Recent studies exploring the molecular landscape of SCLC has revealed insight into why some tumors lack neuroendocrine expression and defined a distinct subtype of SLCL with a “tuft cell” phenotype.3 These are characterized by the expression of POU2F3, a recently identified marker expressed in 10%-12% of SCLC. Interestingly, POU2F3-positive cases show low or negative expression of standard neuroendocrine markers. Future studies into the now well-defined molecular subtypes of SCLC, including the tuft cell type, may yield important prognostic and treatment implications for patients. 

It is important to note that the use of POU2F3 requires the appropriate clinical and histopathologic context, as it is not specific for SCLC. This marker has been documented to stain large cell neuroendocrine carcinoma and squamous cell carcinomas.4 


  1. WHO Classification of Tumours Editorial Board. Thoracic tumours [Internet]. Lyon (France): International Agency for Research on Cancer; 2021 [cited 2023 Aug 25]. (WHO classification of tumours series, 5th ed.; vol. 5). Available from:
  2. Rekhtman N. Lung neuroendocrine neoplasms: recent progress and persistent challenges. Mod Pathol. 2022 Jan;35(Suppl 1):36-50. doi:10.1038/s41379-021-00943-2. Epub 2021 Oct 18. PMID: 34663914; PMCID: PMC8695375.
  3. Gay CM, Stewart CA, Park EM, Diao L, Groves SM, Heeke S, Nabet BY, Fujimoto J, Solis LM, Lu W, Xi Y, Cardnell RJ, Wang Q, Fabbri G, Cargill KR, Vokes NI, Ramkumar K, Zhang B, Della Corte CM, Robson P, Swisher SG, Roth JA, Glisson BS, Shames DS, Wistuba II, Wang J, Quaranta V, Minna J, Heymach JV, Byers LA. Patterns of transcription factor programs and immune pathway activation define four major subtypes of SCLC with distinct therapeutic vulnerabilities. Cancer Cell. 2021 Mar 8;39(3):346-360.e7. doi:10.1016/j.ccell.2020.12.014. Epub 2021 Jan 21. PMID: 33482121; PMCID: PMC8143037.
  4. Wang Y, Jin Y, Shen X, Zheng Q, Xue Q, Chen L, Lin Y, Li Y. POU2F3: A Sensitive and Specific Diagnostic Marker for Neuroendocrine-low/negative Small Cell Lung Cancer. Am J Surg Pathol. 2023 Sep 1;47(9):1059-1066. doi:10.1097/PAS.0000000000002081. Epub 2023 Jun 26. PMID: 37357936.

Ariel Sandhu, M.B., B.Ch.

Fellow, Pulmonary Pathology
Mayo Clinic

Melanie Bois, M.D.

Consultant, Anatomic Pathology
Mayo Clinic
Associate Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

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