Histomorphologic, Immunophenotypic, and Molecular Analysis
Macroscopic examination revealed a 130 mg spleen containing a well-circumscribed, moderately firm, red-pink mass. The hematoxylin and eosin sections demonstrated splenic parenchyma with a lesion that resembled splenic parenchyma with white and red pulp structures without overt morphologic atypia (Figure 1A). Also noted were somewhat spindled histiocytoid cell clusters that had a moderate amount of pale cytoplasm, oval vesicular nuclei, inconspicuous nucleoli, delicate fibrous bands, and increased vascularity (Figure 1B). In addition, there was a polymorphous proliferation of plasma cells and small lymphocytes admixed in the background. Ancillary immunohistochemical studies were performed on formalinfixed, paraffin-embedded (FFPE) tissue sections following routine procedures for CD20, CD3, CD5, smooth muscle actin (SMA), CD20, CD21, CD23, CD68, BLC6, IgG, IgG4, PAX-5 AE1/AE3, and Ki-67. Epstein Barr virus encoded small RNAs (EBER) in situ hybridization was performed with immunohistochemistry/EBER (please see Table I for additional antibody information). CD3, CD5, and CD43 were expressed in the background T-cell population while CD20 and PAX-5 highlighted the B-cells forming follicles in the white pulp with partial BCL6 expression.

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Table I: Antibodies used in this study

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Figure 1: A) Splenic parenchyma with partially altered architecture (H&E; x20) B) by the spindled cells in the background of chronic inflammatory cells (H&E; x200). The spindled cells show expression of C) CD35 (IHC; x200), D) CD23 (IHC; x200), E) SMA (IHC; x200), with numerous cells positive for F) EBER (IHC; x200).

Atypical spindled cells were positive for CD35 (Figure 1C), CD23 (Figure 1D), and CD21 (partial) as well as SMA (Figure 1E), consistent with a follicular dendritic cell and myofibroblastic origin, respectively. EBV-encoded small RNA was extensively positive using in situ hybridization (EBER) (Figure 1F).

Single Nucleotide Polymorphism Copy Number (SNP-CN) array analysis using the Thermo Fisher OncoScan FFPE Assay Kit (Thermo Fisher Scientific, Waltham, MA) was performed according to the manufacturer’s directions and revealed a copy neutral loss of heterozygosity of 5q in 100% of the specimens, which was considered constitutional. Also detected was a gain of the X chromosome that was clonal and the clinical significance of this finding was unclear (Figure 2A-B). To better characterize this tumor, somatic mutation detection by next-generation sequencing was performed (Myeloid 75-gene targeted sequencing panel). Two variants were detected by the Archer® VariantPlex Myeloid 75-gene panel BCORL1 c.3463C>A (p.P1155T) in approximately 56% of the alleles and JAK2 c.2852T>C (p.I951T) in approximately 47% of the alleles. These two variants are observed as germline variants in a minor proportion of the population with BCORL1 seen in 6 alleles and JAK2 seen in 10 alleles (both with minor allele frequencies of approximately 3.6x10^-5) ²⁶. In silico algorithms predict the variants to be tolerated/benign (provean.jcvi.org). Therefore, given the information currently available, the clinical significance of these variants is uncertain at best. Polymerase chain reaction did not detect rearrangement of the T-cell receptor or IgH.

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Figure 2: SNP-CN array analysis of chromosome 5 and chromosome X. A) Approximately 66.8-Mb (megabase) stretch of copy neutral loss of heterozygosity (CN-LOH) was detected interstitially on the long arm of chromosome 5. B) A clonal low-level loss of the entire X chromosome was detected in the splenic specimen.

A diagnosis of inflammatory pseudotumor-like follicular/ fibroblastic dendritic cell tumor (FDC/FRC) was rendered. The patient was followed up for 6 months after the surgery and no recurrent disease was identified. She remains in remission to our knowledge.

The histomorphologic and immunophenotypic features of FDC/FRC resemble other spindled cell tumors that are more aggressive and potentially require more aggressive treatments. Therefore, it is crucial to perform a thorough diagnostic work-up to render an accurate diagnosis. FDC/ FRC sarcoma harbors mesenchymal and inflammatory components and a wide range of immunophenotypic profiles causing a diagnostic challenge in differentiating it from other spindle cell tumors of the spleen including follicular dendritic cell sarcoma, interdigitating dendritic cell sarcoma arising in or involving the spleen, fibroblastic reticular cell tumor, and reactive processes such as splenic inflammatory pseudotumor also known as inflammatory myofibroblastic tumor (Table II). Ge et al. have authored a review manuscript and described the histomorphologic characteristics of FDC/FRC sarcoma in detail, including proliferation of neoplastic spindled cells with varying degree of nuclear atypia, a vesicular chromatin pattern, and conspicuous nucleoli in a background of lymphoplasmacytic infiltrate that may be accompanied by eosinophils, a granulomatous reaction, necrosis, and hemorrhage ⁹. The neoplastic cells are often of follicular dendritic cell origin with expression of CD21, CD23, or CD35, similar to follicular dendritic cell sarcoma (FDCS). However, although the spindled cell morphology and expression of dendritic cell markers are often shared by these two tumors, there are several differences: FDC/FRC sarcoma is often an isolated lesion and almost exclusively associated with EBV. FDC may occur in multiple sites and, in addition to FDC markers, Clusterin, a glycoprotein associated with apoptosis, is frequently expressed in FDCS ^27, although not reported in FDC/FRC sarcoma. On the other hand, fibroblastic reticular cell differentiation and SMA expression are not commonly observed in FDCS ²⁸. The differentiation between FDC/FRC sarcoma and FDCs is crucial given the biological behavior of inflammatory FDC/FRC sarcoma is more indolent than a true intraabdominal FDC. Interdigitating dendritic cell sarcoma (IDCS) is another dendritic cell tumor characterized by neoplastic proliferation of spindled to ovoid cells arranged in fascicles or whorls or in a storiform pattern ¹. Tumor cells are consistently positive with S100 and Vimentin, which are mostly negative in FDC/FRC sarcoma. IDCS has an aggressive clinical course in the majority of the cases. Fibroblastic reticular cell tumor (FRCT) is a rare neoplasm that can be seen in the spleen. FRCT has a similar histomorphology to other dendritic cell tumors with a rather distinct immunophenotype including expression of SMA, desmin, and CD68 as well as cytokeratin, which separates it from the others. Our case was also negative with AE1/AE3. Splenic inflammatory pseudotumor (IPT) is a rare benign reactive process resulting from an infectious, autoimmune or reparative process, with subsets expressing EBV ²⁹. EBV+ IPT often has a similar histomorphology to FDC/ FRC sarcoma including isolated nodules formed by a bland spindled myofibroblastic proliferation and a polymorphic inflammatory infiltrate that can be accompanied by hemorrhage. The myofibroblasts are characteristically positive for SMA, Vimentin and, in subsets, CD68 without expression of FDC markers (30).

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Table II: Clinicopathologic and immunophenotypic features of spindled cell tumors of the spleen

The molecular pathogenesis of the disease is poorly understood. Gain of chromosome X was detected by SNP array in our case and was considered clonal; however, the association of such an anomaly with FDC/FRC sarcoma has not been reported to our knowledge. Gain of the X chromosome in the current case may suggest chromosome X mosaicism, which is usually age-related and preferentially affects the inactivated X chromosome ³¹. The existing data from cancer genomes indicate that the female X chromosome, particularly inactive X, usually has a higher mutation burden of point mutations and that this is a frequent event in cancer ³². Bruehl et al. recently reported the results of comprehensive mutation analysis using NGS in two cases of FDC/FRC sarcoma and found no variants of strong or potential clinical significance ³³. The results of NGS in the current case were similar; variants were detected in BCORL1 and JAK2. However, the clinical significance of BCORL1 and JAK2, if any, is uncertain given their presence in a very minor fraction of the population database. Furthermore, whether these germline variants have susceptibility or predispose to hematologic malignancies are unknown at this time. BCORL1 and JAK2 gene alterations have been demonstrated in various tumors including myeloid neoplasms. Additionally, BCORL1 has been shown in medulloblastoma, retinoblastoma, and uterine sarcoma ^34,35 and might potentially be a pathogenic gene in the development of FDC/FRC. Although these variants may represent heterozygous germline sequence variants, the clinical significance of these variants in the context of this case is undetermined at this time and larger scale case series and studies are warranted to better understand the biology and clinical implications of this rare disease.

AUTHORSHIP CONTRIBUTIONS
Concept: DP, Design: BMV, DP, Data collection or processing: BMV, KD, DP, Analysis or Interpretation: BMV, KD, DP, Literature search: BMV, DP, Writing: BMV, DP, Approval: DP.

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