Diagnostic challenges in primary pulmonary follicular dendritic cell sarcoma: 2 case reports.

Introduction
Follicular dendritic cell sarcoma (FDCS) is a malignant tumour composed of spindle-shaped, or oval cells with a follicular dendritic cell (FDC) morphology and an FDC immunophenotype. FDCS was first reported in lymph nodes by Monda et al. in 1986 [1] and is more commonly found in the lymph nodes, particularly in the neck and armpit. The liver and spleen are also common sites of FDCS occurrence. FDCS can also occur in the gastrointestinal, tract, tonsil, mediastinum, nasopharynx, thyroid and retroperitoneum.Its common metastatic sites include the liver, lung and lymph nodes. Pathologists remain relatively unfamiliar with extranodal FDCS, particularly primary pulmonary FDCS, which is rare and may be misdiagnosed as lung cancer in biopsy specimens. In this paper, we analysed 2 cases of primary lung FDCS and summarized their clinicopathological features, differential diagnosis and research progress in conjunction with a literature review to improve the understanding of this tumour by pathologists. We also review how to identify diagnostic clues in biopsy specimens to avoid misdiagnosis.

Materials and methods

Case selection
We analysed 2 cases of primary pulmonary FDCS diagnosed at the Pathology Departments of the First Hospital of Nanchang and the Jiangxi Provincial People’s Hospital in 2022. Both cases were reviewed and confirmed by the authors. Both patients were males, and were aged 71 and 56 years, respectively. We gathered clinical and pathological data from the patients. Follow-up information was obtained via telephone, with the follow-up commencing on the date of diagnosis and ending on 31 January, 2024. This study was been approved by the Medical Ethics Committee of the First Hospital of Nanchang, and consent was obtained from each participant. Additionally, we performed a literature search of FDCS cases reported in PubMed (http://www.ncbi.nlm.nih.gov/pubmed/), encompassing 11 reported cases of primary pulmonary FDCS, to gather relevant clinicopathological data for discussion alongside our two cases.

Morphological observation
The specimens were fixed with 10% neutral formalin, embedded in paraffin, and sectioned continuously at a thickness 4 μm. H&E staining was performed to observe the histological morphology.

Immunohistochemistry
Antibodies pan-CK, vimentin, CD21, CD23, CD35, CXCL13, D2-40, fascin, WT-1, CD34, p63, p40, TTF1, syn, BRG1, ALK, SMA, desmin, HMB45, S100, CD68, Ki67 were purchased from MaiXin Company, Fuzhou, China. IHC staining was performed on paraffin sections in accordance with the kit protocols.

In situ hybridization
An ISH kit with an EBV probe was acquired from Zhongshan Golden Bridge Biotechnology Co., Ltd., Beijing, China. ISH for Epstein-Barr virus-encoded small RNA (EBER) was carried out in accordance with the manufacturer’s instructions. Positive signals manifested as brownish-yellow and were located within the cell nuclei.

Results

Clinical features
The lung masses of the two patients were displayed on the computed tomography (CT) scans. No masses were found in other organs, no lymph node enlargement was observed, and there was no history of cancer. In Patient 1, a 6.5-cm mass was detected, but no abnormalities were observed in the surrounding lung tissue (Fig. 1A). The mass was located in close proximity to the pleura, and exhibited a grey-white, firm cut surface with areas of focal necrosis (Fig. 2A). Patient 2 had a 6-cm mass located at the hilum of the lung (Fig. 3A). The mass was closely associated with the main pulmonary artery, making separation difficult and resulting in a fragmented cut surface that appeared grey-white and firm. No abnormalities were observed in the other examinations.

Histological features
In the biopsy specimen from patient 1, the cells in certain areas formed nest-like structures (Fig. 1B), which were initially misdiagnosed as lung cancer. In some areas, the cells were diffusely distributed, with ovoid nuclei, unclear cell boundaries, and pairs of cells present, which were accompanied by numerous lymphocytes in the background (Fig. 1C). The characteristics of FDCS were confirmed through the application of IHC, a technique widely used in medical diagnosis. In subsequent lung excision specimens, the tumour cells exhibited a diffuse pattern, presenting as flaky, fascicular, Schiff-like, and swirling structures, similar to those in meningiomas (Fig. 2B). The cells appeared ovoid or epithelial-like, with indistinct cell borders, an eosinophilic cytoplasm, finely stained characteristics, and small, bubble-like nuclei (Fig. 2C). In Patient 2, the tumour cells similary displayed a diffuse and clustered arrangement; however, the specimen exhibited a graterer incidence of multinucleated, degenerated or giant cells (Fig. 3B, C). Another important histological feature was the abundant infiltration of small lymphocytes between tumor cells.

Immunophenotype and in situ hybridization features
FDC-specific markers were positively expressed in the tumour cells. In Case 1, the biopsy and excised specimens exhibited diffuse staining for CD21 (Figs. 1D and 2D), CD23, CD35, fascin and vimentin, with focal staining for podoplanin (D2-40) and CKpan. In Case 2, diffusey positivity was observed for podoplanin (D2-40) (Fig. 3D), facsin and SMA, with a few cells expressing p63; however, CK and P40 were negative. The Ki-67 proliferation index ranged between 35% and 60%. BRG1 and INI-1 were posutive. Tests with other antibodies, including those against CXCL13, WT-1, TTF-1, Syn, S100, ALK, CD34, desmin, and CD68 were negative. The EBER ISH results were negative.

Treatment and outcomes
Both patients underwent surgery and adjuvant chemotherapy, with follow-up periods of 14 and 15 months, respectively. They showed no evidence of disease, and no recurrence or metastasis was observed.

Retrospective analysis of 13 FDCS cases
In a comprehensive review of the English-language publication on FDCS, 11 cases of primary pulmonary FDCS were identified and included for analysis, and their clinicopathological data were extracted in detail. Our evaluation of the 13 patients revealed a predominance of males (male-to-female ratio of 10:3), and a median age of 51 years (range 33–76 years). The main symptom was coughing, and the prognosis was relatively favourable. Among the 13 patients, only 2 died from FDCS, while no recurrence was observed in the remaining patients. Tumour cells were typically not stained for all the markers of FDCs, thus necessitating the development of a set of markers. Table 1 provides a brief overview of the clinical and pathological features of the primary pulmonary FDCS cases from this study and the literature.

Discussion
FDCS was excluded from histiocytic and dendritic cell tumours according to the 5th edition of the World Health Organization Classification of Haematolymphoid Tumours. FDCS is included in the new classification of tumours of lymphatic mesenchymal origin, because FDCs are thought to originate from mesenchymal cells rather than haematopoietic stem cells [11]. Primary pulmonary FDCS is extremely rare. To date, only 11 cases of primary pulmonary FDCS have been reported in the literature [10]. There are only 13 cases in total (Table 1), including the two described in this article. We found that pulmonary FDCS was more common in young and middle-aged men. The pathogenesis of FDCS is unclear, but a small number of cases (approximately 10–20%) have been reported to be associated with hyaline vascular Castleman disease [12], which may precede or occur simultaneously with FDCS. Neither of the 2 patients in this study had a history of Castleman disease, wich often presents as a painless, slow-growing lung mass. No characteristic findings were observed on imaging. CT scans revealed a solid mass of uneven density, which was challenging to differentiate from lung cancer. Most FDCSs present as well-defined masses, with diameters ranging from 1 to 11 cm, and those with large deep body cavity volumes may exhibit bleeding, necrosis and infiltration of adjacent tissues.
The tumour cells showed a variety of histological patterns, such as sheets, bundles and Schiff-like structures; in addition, most of them presented vortex-like morphologies similar to those of meningiomas. Follicular dendritic cells (FDCs) exhibit bidirectionality and often present as spindle-shaped, oval-shaped, or epithelioid cells, wich are characterized by indistinct boundaries, an eosinophilic cytoplasm, finely stained features, and small nuclei. Multinucleated or giant cells can be seen within the tumour. Another important histological feature, of these tumours is the abundant infiltration of small lymphocytes, which are composed of B or T lymphocytes between tumour cells. IHC serves as a reliable basis for the diagnosis of dendritic cell and histiocytic tumours. Tumour cells express at least one FDC marker, namely CD21, CD23, CD35, podoplanin (D2-40), clusterin [13] or CXCL13 [14], and often express vimentin and fascin. It is preferable to use a panel of markers for combined detection, because one or more antibodies may be lost.
Particular attention should be given to distinguishing FDCS from lung cancer when FDCS arises in the lung and when a biopsy specimen is obtained. The histological features of FDCS are not fully represented in biopsy specimens. In surgical specimens, FDCS needs to be distinguished from the following tumours: inflammatory myofibroblastic tumours, interdigitating dendritic cell sarcoma, meningioma, and metastatic melanoma. FDCS tumour cells exhibit variable positive expression of FDC markers. The two cases in this article are consistent with the literature cases. Fortunately, except for FDCS, none of the above tumours expresses FDC markers.
In recent years, preliminary research has been conducted on the molecular pathological and genetic characteristics of FDCS. These studies indicate that FDCS typically exhibit few mutations in programmed cell death ligand-1 (PD-L1), suggesting the potential for immunotherapy in the treatment of FDCS [15]. Additionally, a small subset of FDCS patients have rearrangements of immunoglobulin (Ig) or the BRAF V600E mutation, but lack mutations in KRAS, NRAS, MAP2K1 and other genes [12, 16]. These preliminary research findings provide important information for further understanding of the therapeutic mechanisms and development of FDCS.
At present, there is no unified standard or guideline for the treatment of primary pulmonary FDCS. Surgical resection is preferred and effective treatment method. However, depending on the location and growth pattern of the tumour, adjuvant therapies such as radiotherapy, chemotherapy, and tyrosine kinase inhibitors may be considered [12]. Primary pulmonary FDCS generally has a favourable prognosis, with a local recurrence rate of approximately 40% and a distant metastasis rate of approximately 25%, with primary metastases commonly observed in the lungs, liver, lymph nodes, and bones [12]. A poor prognosis is associated with FDCS presenting significant nuclear pleomorphism or polymorphism, tumours with a diameter of 6 cm, more than 5 mitoses/10 high-power fields, or the presence of coagulative necrosis [17, 18]. In this study, the two patients presented with tumours measuring ≥ 6 cm in diameter, which, according to the literature, correlates with a moderate to high risk of FDCS recurrence.

Conclusion
FDCSs exhibit unique histological morphology and immunohistochemical phenotypes. However, diagnosing primary pulmonary FDCS remains challenging, particularly when lung biopsy specimens are being evaluated. FDCS is characterized by cohesive cellularity and a stroma rich in lymphocytes, with immunohistochemical studies revealing positive staining for CD21, CD23 or CD35, and negative staining for cytokeratin and S100. Therefore, when a pulmonary mass is encountered, in addition to lung cancer, the possibility of FDCS should be considered. To ensure accurate diagnosis and prevent misdiagnosis, it is crucial to decte specific immunohistochemical markers such as CD21, CD35, and CD23, which are commonly positive in FDCS. It is hoped that through this study, we can deepen our understanding of FDCS and thus better manage this rare disease.

Supplementary Information