Case Report: A 55-year-old gentleman with constitutional symptoms and lymphadenopathy. Imaging revealed a mass lesion in the right upper lobe of the lung. A biopsy of the cervical lymph node showed diffusely effaced architecture replaced by sheets of undifferentiated pleomorphic cells with vesicular nuclei, prominent nucleoli, eosinophilic cytoplasm, and multiple necrotic foci. An extensive immunohistochemistry (IHC) panel was applied, which showed positivity for synaptophysin, vimentin, and focal CD34 and EMA expression. Other markers like pan-cytokeratin, p40, TTF1, CD56, INSM1, calretinin, CD45, SOX10, S100, CD30, CD117, SMA, and Desmin were negative, with INI1 retained. The IHC panel excluded the morphological differentials of carcinoma, lymphoma, rhabdomyosarcoma, melanoma, and germ cell tumor. Further literature review led to the possibility of the SMARCA4-UT entity, which had a morphology and IHC profile similar to the present case. Testing for SMARCA4 (BRG-1) by IHC showed a complete loss in the tumor cells, favoring the diagnosis of Thoracic SMARCA4-deficient undifferentiated tumor (SMARCA4-UT).
Conclusion: SMARCA4-UTs are rare, highly aggressive, and poorly differentiated thoracic tumors. Recognizing them is vital as there is potential for therapeutic interventions such as immunotherapy and SMARCA4-targeted therapies, offering promising prospects for the future.
SMARCA4-UT predominantly affects young and middleaged individuals, with a slight male predominance. These tumors exclusively occur in the thoracic region and commonly present as masses in the mediastinum, lung, and/ or pleura. The clinical prognosis of this disease is known to be unfavorable[3]. Morphologically, the tumor displays an undifferentiated and/or rhabdoid phenotype and exhibits the expression of one or more stem cell markers, including CD34, SOX2, and SALL4. Thymic, lung, and mesothelial markers are absent, and there is a complete loss of BRG1 protein, as confirmed by immunohistochemistry (IHC)[4,5].
Herein, we present a case report of SMARCA4-deficient thoracic undifferentiated tumor (SMARCA4-UT) that posed challenges in its diagnosis based on examination of the lymph node biopsy specimen. The encountered difficulties in utilizing a panel of immunohistochemistry (IHC) markers and conducting a comprehensive literature review to establish an accurate diagnosis for this particular case of SMARCA4-UT are presented.
The diagnosis was confirmed by SMARCA4 (BRG-1) IHC, which showed a complete loss of SMARCA4 protein in the neoplastic cells. The treatment protocol included chemotherapeutic agents, paclitaxel, and carboplatin. CT scan done after three months of treatment revealed a moderate reduction in the size of the lung lesion and a slight decrease in lymph node volume with an increase in necrotic tissue.
As exemplified by the current case, the histomorphology of thoracic tumors, regardless of metastasis, exhibits a highgrade undifferentiated or epithelioid to rhabdoid cell phenotype, characterized by a relatively uniform dyscohesive arrangement in sheets, accompanied by brisk mitosis and necrosis. Regarding immunoprofiling, these tumors typically exhibit stem cell markers such as CD34, SALL4, and SOX2. There is usually either an absence or varying expression of PCK, EMA, and neuroendocrine markers, except for synaptophysin, which may show positivity. Additionally, focal expression of non-small cell lung cancer (NSCLC) markers like p63, p40, and TTF1 may be observed, while INI-1 expression remains intact. Immunonegativity is observed for calretinin, WT1, NUT, CD30, ALK, HMB- 45, Desmin, and LCA. The characteristic feature of these tumors is the complete loss or significant underexpression of SMARCA4 (BRG-1), along with SMARCA2 (BRM) loss in more than 95% of the cases[9].
Since the most common morphological presentation is as undifferentiated malignancy or with rhabdoid features, it warrants to rule out other mimics like carcinoma, lymphoma (large cell phenotype), germ cell tumor, melanoma, epithelioid sarcoma, and large cell neuroendocrine tumors. Differentiating these tumors on small biopsy is very challenging due to undifferentiated morphology and IHC promiscuity. As mentioned earlier, markers like CD34, PCK/EMA, and synaptophysin can be focally present and may not be helpful in small biopsies. Immunohistochemistry is of help if the before-mentioned markers are positive in a summative pattern. BRG1 IHC is helpful but it is not available at all centers. However, this antibody is now becoming essential in the repertoire of IHC panels to diagnose these neoplasms.
Another point to remember is distinguishing SMARCA4- UT from SMARCA4-deficient non-small cell lung cancer (SMARCA4-NSCLC), as the latter is relatively more prevalent. These two entities can be differentiated based on distinct histomorphological characteristics. SMARCA4- NSCLC typically presents with clear-cut adenocarcinoma (AdCC) features or, less frequently, squamous cell carcinoma (SCC). Additionally, the expression of CK7 and the absence or focal expression of CD34, SOX2, and SALL4 are helpful markers for differentiation[9,10]. The loss of SMARCA4 (BRG-1) can exceptionally occur in tumors such as SMARCB1/INI1-retained epithelioid sarcoma (ES); in this context, the absence of SOX2 and SALL4 expression aids in distinguishing ES from SMARCA4-UT[11].
More prevalent entities such as large cell neuroendocrine carcinoma and small cell carcinoma may initially elicit misdiagnosis due to crush artifacts and tissue necrosis, along with the expression of synaptophysin and a high ki-67 index[2]. These cases would derive an advantage from a composite approach involving a first-generation neuroendocrine marker (Synaptophysin and chromogranin) in conjunction with a second-generation neuroendocrine marker such as INSM1 and BRG-1 IHC loss to differentiate them from SMARCA4-UT.
Additional diagnostic techniques include next generation sequencing (NGS) in identifying SMARCA4 mutations. However, in some cases where the loss of BRG-1 expression was observed using IHC, it was not detected by NGS. This could be because of structural variations (translocations) involving the intronic regions. Fluorescence in-situ hybridization (FISH) also has a limited role in diagnosis due to the truncating mutations coupled with the loss of heterozygosity (LOH), which is frequently copy-neutral (accompanied by the duplication of the mutant allele)[2]. Therefore, it is crucial to use IHC to diagnose SMARCA4 mutations accurately.
SMARCA4-UT are aggressive and are associated with a poor prognosis, with median overall survival ranging from 4 to 7 months[3,12]. Improved outcomes have been reported in some instances of SMARCA4-UT treated with immunotherapy agents such as pembrolizumab[13], atezolizumab[14], and nivolumab[15]. Notably, one documented case demonstrated a remarkable survival period of up to 22 months[16].
Acknowledgement
We would like to express our gratitude to Dr. Sunil Pasricha, a Senior
Consultant at RGCI, for his valuable assistance with the BRG1 IHC
staining.
Conflict of Interest
The authors declare that they have no conflict of interest for this
article.
Authorship Contributions
Concept: MM, KAM, Design: MM, KAM, Data collection or
processing: KAM, Analysis or Interpretation: MM, FA, Literature
search: DV, Writing: DV, KAM, Approval: SK.
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