Intraoperative cytology of low-grade fetal adenocarcinoma: Critical morphological features for distinction from carcinoid tumor

INTRODUCTION

Low-grade fetal adenocarcinoma (L-FLAC), an exceedingly rare subtype of lung adenocarcinoma, accounting for 0.3% of all pulmonary adenocarcinomas, predominantly affects young-to-middle-aged adults, with predisposition in females.^[1] L-FLAC demonstrates a relatively favorable prognosis compared with other subtypes, with surgical resection being the mainstay of treatment. However, its accurate diagnosis poses substantial challenges, particularly in intraoperative settings.

Histologically, L-FLAC recapitulates fetal lung tissue in the pseudoglandular developmental stage (8–16 weeks), exhibiting characteristic tubular glands lined by glycogen-rich, non-ciliated columnar cells with subnuclear vacuolation.^[1,2] A pathognomonic feature is the presence of morules, which are squamoid-appearing structures without keratinization. This presence demonstrates nuclear/cytoplasmic b-catenin expression, implicating aberrant activation of the wingless/Int-1 (Wnt) signaling pathway in L-FLAC pathogenesis.^[3,4]

Differentiating L-FLAC from neuroendocrine tumors, such as carcinoid tumors, remains difficult due to shared cytomorphological features. Both entities exhibit tumor cells with finely granular chromatin arranged in planar and rosette-like configurations, leading to considerably different management strategies. While typical carcinoid tumors may be amenable to limited resection in selected cases, atypical carcinoid tumors and adenocarcinomas generally warrant more aggressive surgical approaches with consideration of adjuvant therapy based on staging.^[5-7] Therefore, an accurate intraoperative diagnosis is crucial for determining the appropriate surgical extent.

Intraoperative cytology cannot evaluate tissue architecture but enables histological subtype estimation. Pulmonary neoplastic lesions often exhibit characteristic cytological features that facilitate intraoperative diagnosis without information on tissue architecture. For lung tumors, rapid intraoperative cytology optimizes specimen preparation time and minimizes deformation of resected specimens due to sampling. Thus, understanding the range of cytological features of lung tumors and potential diagnostic pitfalls in rapid intraoperative cytology is crucial.

Despite the increasing recognition of L-FLAC as a distinct clinicopathological entity, literature documenting its cytomorphological characteristics remains limited, particularly regarding intraoperative diagnostic challenges. Herein, we present a case of L-FLAC that was initially misinterpreted as an atypical carcinoid tumor on intraoperative cytology. The present case highlights the subtle yet critical cytomorphological features that can facilitate the accurate diagnosis and appropriate clinical management of this uncommon neoplasm.

CASE REPORT

This report describes a single patient who underwent intraoperative fine-needle aspiration cytology using standard techniques with Papanicolaou staining. Cytological examination was performed by board-certified cytopathologists. Papanicolaou staining was performed using a commercial staining kit (Muto Pure Chemicals Co., Ltd., Tokyo, Japan).

A 47-year-old female non-smoker with a history of sick sinus syndrome requiring pacemaker implantation presented with an incidental pulmonary nodule discovered during a routine workplace health examination. The patient had no history of respiratory disease. She underwent surgery without prior bronchoscopic examination, and the surgical approach was determined based on intraoperative findings. Pre-operative bronchoscopy was omitted due to the tumor’s challenging location across the incomplete interlobar fissure (S³–S⁵) and concerns regarding bronchoscopic risks due to the implanted pacemaker for sick sinus syndrome. Intraoperative needle aspiration cytology yielded a preliminary diagnosis of a suspected atypical carcinoid tumor; however, a definitive diagnosis could not be established. Subsequently, the patient underwent a lobectomy with lymph node dissection.

Computed tomography (CT) findings

CT revealed a 21-mm nodule with heterogeneous enhancement spanning segments S³–S⁵ of the right lung, traversing the interlobar fissure [Figure 1]. Peripherally located carcinoid tumors characteristically present as well-circumscribed, round nodules with homogeneous internal density and indolent growth patterns – radiological features remarkably similar to those observed in L-FLAC. The patient was then referred to our hospital for surgery.

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Figure 1:

Computed tomography findings of low-grade fetal adenocarcinoma. A patient with low-grade fetal adenocarcinoma presented with an incidental pulmonary nodule. Computed tomography images demonstrate a well-circumscribed nodule measuring approximately 20 × 15 mm traversing the right upper and middle lobes of the lung. (a) Axial view showing the nodule (arrowhead). (b) Sagittal view with the nodule (red circled).

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Cytological findings

Intraoperative cytological specimens revealed two distinct cellular patterns that created diagnostic difficulties [Figure 2]. The predominant pattern consisted of planar atypical cells with loose cohesion, observed on a clean background. These cells exhibited round nuclei with high N/C ratios and fine granular chromatin with focal rosette-like formations – features strongly suggestive of neuroendocrine differentiation [Figure 2a-c].

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Figure 2:

Intraoperative fine-needle aspiration cytology findings. (a) Low-power view reveals planar atypical cells with diminished cohesion in a clean background without necrosis; Papanicolaou stain, ×200. (b and c) Medium- and higher-power views reveal round cells exhibiting high N/C ratios and finely granular chromatin, with focal rosette-like formations reminiscent of neuroendocrine tumors (boxed area); Papanicolaou stain, ×400 and ×1000, respectively. (d) Area of cohesive cell clusters displays three-dimensional architecture and cribriform patterns (arrowheads), characteristic of L-FLAC; Papanicolaou stain, ×400. (e and f) Focal cell aggregates with ground-glass nuclei and intranuclear inclusions (boxed area), corresponding to morula-like structures diagnostic of L-FLAC; Papanicolaou stain, ×400 and ×1000, respectively. Scale bars: (a) 50 μm; (b) 20 μm; (c) 10 μm; (d) 20 μm; (e) 20 μm; (f) 10 μm. L-FLAC, Low-grade fetal adenocarcinoma; N/C, nuclear-to-cytoplasmic ratio.

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The second, less-prominent pattern consisted of densely nucleated, strongly cohesive cell clusters with a three-dimensional architecture resembling endometrial tissue. These clusters exhibited characteristic cribriform structures [Figure 2d]. Focal cell aggregates with ground-glass nuclei and intranuclear inclusions corresponding to morule-like structures in histological sections were identified, although they were initially overlooked [Figure 2e and f]. When recognized collectively, these cytomorphological features provide the basis for diagnosing L-FLAC rather than neuroendocrine tumors.

Histopathological findings

Pathological examination revealed a 1.8 × 1.6 × 1.6 cm solid, grayish-white tumor located in an incompletely lobulated area spanning the right upper and middle lobes [Figure 3a and b]. Histologically, the tumor displayed proliferation of atypical epithelial cells, forming distinctive cribriform and solid growth patterns with focal rosette-like arrangements [Figure 3c and d]. Neoplastic cells contained a clear cytoplasm with subnuclear vacuoles, characteristic of fetal adenocarcinoma [Figure 3d]. Diagnostic morule-like structures composed of squamoid cells with a mildly eosinophilic cytoplasm were readily identified, with some cells displaying intranuclear inclusions [Figure 3e]. Immunohistochemical staining revealed strong nuclear and cytoplasmic b-catenin positivity (clone 14, Sigma-Aldrich, St. Louis, MO, USA), specifically in these morule-like structures, confirming the diagnosis of L-FLAC [Figure 3f]. Adjacent non-neoplastic lung tissue within the specimen showed no nuclear b-catenin positivity, confirming the reliability and specificity of the nuclear staining observed in the morule-like structures. The final pathological stages were pT2a, pN0, cM0, and p Stage IB (UICC 9^th).

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Figure 3:

Gross and histopathological findings of the resected specimen. (a and b) Macroscopic appearance shows a well-circumscribed, grayish-white solid tumor (a, fresh specimen; b, after formalin fixation). (c) Low-power histological view demonstrates a well-delineated neoplastic lesion characterized by proliferating atypical glands with focal confluent growth; hematoxylin-eosin stain, ×10. (d) Medium-power view exhibits cribriform and solid growth patterns with focal rosette-like arrangements (boxed area). Neoplastic cells contain clear cytoplasm with subnuclear vacuolation; hematoxylin-eosin stain, ×40. (e) Characteristic morule-like structures composed of squamoid cells with mild eosinophilic cytoplasm are observed (arrowheads), with some cells displaying intranuclear inclusions (arrow); hematoxylin-eosin stain, ×40. (f) Immunohistochemistry for b-catenin shows strong nuclear positivity in morule-like structures, confirming the diagnosis of L-FLAC; ×40. Scale bars: (a) 15 mm; (c) 100 μm; (d) 20 μm; (e) 20 μm; (f) 20 μm. L-FLAC, Low-grade fetal adenocarcinoma.

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DISCUSSION

This report illustrates the critical diagnostic challenges associated with intraoperative cytological diagnosis of L-FLAC. The initial interpretation as an atypical carcinoid tumor highlights the importance of recognizing specific cytomorphological features that distinguish these entities in time-sensitive intraoperative settings.

Diagnostic difficulty stems primarily from the predominance of planar cell arrangements with neuroendocrine-like features that closely mimic the characteristic pattern of carcinoid tumors. Cells with uniform nuclei containing fine granular chromatin and focal rosette-like formations strongly suggest neuroendocrine differentiation.^[8] The clear cytoplasm with vacuoles, characteristic of L-FLAC in histological specimens, often becomes indistinct in cytological preparations, causing both glandular and rosette-like structures to appear morphologically similar to cytological smears. This cytological transformation may obscure the distinction between true glandular lumina and rosette formations, further complicating the differential diagnosis. However, careful examination revealed several key cytomorphological features that ultimately enabled accurate diagnosis. Critical features for L-FLAC diagnosis include “dual cell populations, ground-glass nuclei with intranuclear inclusions in morules, and cribriform architecture – features absent in carcinoids.”

Dual cell populations – loose cohesive planar cells and strongly cohesive cell clusters with cribriform architecture – are characteristic of L-FLAC.^[9] This bidirectional differentiation pattern differs from the more monomorphic cellular presentation typically observed in carcinoid tumors. Morule-like structures with ground-glass nuclei represent a pathognomonic diagnostic clue for L-FLAC.^[4,9] These structures, which correspond to the characteristic morules seen on histological examination, are not observed in carcinoid tumors. Careful attention to these focal cell aggregates can provide essential diagnostic differentiation, even when initially obscured by predominant neuroendocrine features.

Accurately distinguishing L-FLACs from carcinoid tumors is clinically significant, particularly in intraoperative settings where surgical decisions directly impact patient management. While typical carcinoid tumors may be candidates for limited resection in selected cases, L-FLACs generally require lobectomy with lymph node dissection for adequate oncological control.^[10] In our case, the initial interpretation of atypical carcinoid tumors led to appropriate surgical management, as atypical carcinoids also typically require lobectomy with lymph node dissection, similar to L-FLACs. However, because the lesion was misinterpreted as a typical carcinoid tumor, a more limited resection might have been performed, potentially compromising the oncological outcomes.

The immunohistochemical detection of strongly nuclear b-catenin expression in morule-like structures provided molecular confirmation of the diagnosis. This finding correlates with the activated Wnt/b-catenin pathway characteristic of L-FLAC, further distinguishing it from carcinoid tumors.^[3,4]

Although immunocytochemical stains theoretically aid diagnosis, their intraoperative application is limited by protocol differences in immunohistochemistry, restricted antibody availability, and time constraints of rapid diagnosis. The absence of nuclear heaviness and the presence of intranuclear inclusions or ground-glass nuclei differentiate L-FLAC from carcinoid tumors, whereas the degree of nuclear atypia distinguishes it from conventional pulmonary adenocarcinomas.

From an educational diagnostic perspective, this case underscores four critical learning points for cytopathologists evaluating intraoperative lung specimens:

1. Include L-FLAC in the differential diagnosis when neuroendocrine features are observed, particularly in young, female patients

2. Identify dual cell populations and cribriform architecture as distinguishing features of L-FLAC

3. Recognizing morule-like structures with ground-glass nuclei and intranuclear inclusions as pathognomonic for L-FLAC; and

4. Focusing exclusively on neuroendocrine-like features (round cells with high N/C ratios and fine granular chromatin) can lead to misdiagnosis as carcinoid tumors.

The favorable clinical outcome in our case, with no recurrence at 2-year post-surgery, aligns with the generally good prognosis reported for L-FLAC. An accurate diagnosis is essential for appropriate surgical planning, proper prognostication, and post-operative management decisions.

As our findings are based on a single case, confirmation through larger studies is required before establishing definitive diagnostic criteria.

SUMMARY

Differentiating L-FLAC from carcinoid tumors under time constraints presents an intraoperative diagnostic challenge. Three critical cytomorphological features – dual cell populations, cribriform architecture, and morule-like structures with ground-glass nuclei – enable rapid and reliable distinction. Vigilance should be increased in young female patients with pulmonary lesions showing neuroendocrine-like features; misclassification could lead to suboptimal surgical management. Providing cytopathologists with practical diagnostic criteria applicable in challenging intraoperative settings impacts surgical approach and patient outcomes.

AVAILABILITY OF DATA AND MATERIALS

The availability of the data used in this case is subject to confirmation by the journal or the authors. For more information on data availability and access procedures, please contact the journal or corresponding author.

ABBREVIATIONS

CT: Computed tomography

L-FLAC: Low-grade fetal adenocarcinoma

N/C: Nuclear-to-cytoplasmic ratio

UICC: Union for International Cancer Control

WHO: World Health Organization

AUTHOR CONTRIBUTIONS

HS: Conceived the study, analyzed data, and drafted the manuscript; AR: Contributed to data analysis and interpretation. YM, ST, KY, ST, and KH: Contributed to data acquisition. All the authors critically reviewed the manuscript for important intellectual content. All authors are responsible for all aspects of the work and ensure that issues related to the accuracy or completeness of any part of the work are properly investigated and resolved. All the authors have read and approved the final manuscript. All authors are eligible for ICMJE authorship.