Solitary fibrous tumor (SFT) is an uncommon fibroblastic neoplasm. Although histologic characteristics and frequent CD34 expression allow for an accurate diagnosis in the majority of SFT cases, a wide histologic spectrum and an occasional unexpected immunophenotype may pose diagnostic challenges. Molecular analyses have discovered that almost all SFTs harbor an NAB2-STAT6 fusion gene, which is considered specific to this tumor type. Recent studies have suggested that STAT6 immunohistochemistry is a reliable surrogate for detection of the fusion gene. Our aim was to validate these findings by examining a large number of SFT cases and a broad array of 30 different types of non-SFT tumors. A total of 49 SFTs with a range of histologic characteristics and 159 benign or malignant tumors that can mimic SFTs were retrieved and stained for STAT6. All 49 SFTs (100%) showed STAT6 expression that was restricted in the nucleus, mostly in a diffuse and strong manner, irrespective of the tumor sites and histologic patterns. The staining was uniform in most cases but was heterogenous in about 20% of the cases in which zonal staining attenuation was observed, likely reflecting variability in fixation or tissue ischemia. In contrast, only 4 non-SFT tumors (2.5%) exhibited weak nuclear STAT6 expression, whereas the remaining 155 cases showed no staining or often weak reactivity in both the cytoplasm and the nucleus. Therefore, nuclear STAT6 immunoreactivity is a highly sensitive and specific marker of SFTs and can be helpful when diagnosis is inconclusive by conventional methods.

Tertiary lymphoid structures (TLS) are transient ectopic lymphoid aggregates whose formation might be caused by chronic inflammation states, such as cancer. However, how TLS are induced in the tumor microenvironment (TME) and how they affect patient survival are not well understood. We investigated TLS distribution in relation to tumor infiltrating lymphocytes (TILs) and related gene expression in high-grade serous ovarian cancer (HGSC) specimens. CXCL13 gene expression correlated with TLS presence and the infiltration of T cells and B cells, and it was a favorable prognostic factor for patients with HGSC. Coexistence of CD8 ⁺ T cells and B cell lineages in the TME significantly improved the prognosis of HGSC and was correlated with the presence of TLS. CXCL13 expression was predominantly coincident with CD4 ⁺ T cells in TLS and CD8 ⁺ T cells in TILs, and it shifted from CD4 ⁺ T cells to CD21 ⁺ follicular DCs as TLS matured. In a mouse ovarian cancer model, recombinant CXCL13 induced TLS and enhanced survival by the infiltration of CD8 ⁺ T cells. These results suggest that TLS formation was associated with CXCL13-producing CD4 ⁺ T cells and that TLS facilitated the coordinated antitumor response of cellular and humoral immunity in ovarian cancer.

A growing number of studies suggest critical tumor suppressor roles of the SWI/SNF chromatin remodeling complex in a variety of human cancers. The recent discovery of SMARCA4-deficient thoracic sarcomas has added to the list of tumor groups with the SMARCA4 inactivating mutation. To better characterize these tumors and establish their nosological status, we undertook a clinicopathological and molecular analysis of 12 SMARCA4-deficient thoracic sarcomas and compared them with three potentially related disease entities. Eleven men and one woman with SMARCA4-deficient thoracic sarcomas (aged 27–82 years, median 39 years) were included in the study. Most of the patients had heavy smoking exposure and pulmonary emphysema/bullae. The primary tumors were large and involved the thoracic region in all cases and simultaneously affected the abdominal cavity in 3 cases. The patients followed a rapid course, with a median survival of 7 months. Histologically, all tumors showed diffuse sheets of mildly dyscohesive, relatively monotonous, and undifferentiated epithelioid cells with prominent nucleoli. Immunohistochemically, all tumors demonstrated a complete absence (8 cases) or diffuse severe reduction (4 cases) of SMARCA4 expression. Cytokeratin, CD34, SOX2, SALL4, and p53 were expressed in 6/12, 10/12, 10/12, 10/12, and 7/10 cases, respectively. SMARCA2 expression was deficient in 11/12 cases, and none (0/8) expressed claudin-4. Targeted sequencing was performed in 5 cases and demonstrated the inactivating SMARCA4 mutation in each case and uncovered alterations in TP53 (5/5), NF1 (2/5), CDKN2A (2/5), KRAS (1/5), and KEAP1 (1/5), among others. Comparative analysis supported the distinctiveness of SMARCA4-deficient thoracic sarcomas as they were distinguishable from 13 malignant rhabdoid tumors, 15 epithelioid sarcomas, and 12 SMARCA4-deficient lung carcinomas based on clinicopathological and immunohistochemical grounds. SMARCA4-deficient thoracic sarcomas constitute a unique, highly lethal entity that requires full recognition and differentiation from other epithelioid malignancies involving the thoracic region.

Ewing sarcoma is a high-grade round cell sarcoma that affects bones and soft tissues in children and young adults. Its diagnosis can be challenging, and the differential diagnoses include a wide variety of small round cell tumors. CD99 and FLI-1 are the currently accepted immunohistochemical markers for Ewing sarcoma, but their accuracy has been controversial. NKX2.2 is a homeodomain-containing transcription factor that plays a critical role in neuroendocrine/glial differentiation. The NKX2.2 gene was recently identified as a target of EWS-FLI-1, the fusion protein specific to Ewing sarcoma, and was shown to be differentially upregulated in Ewing sarcoma on the basis of array-based gene expression analysis. However, the immunohistochemical diagnostic potential of this marker has not been tested. We immunostained representative sections of 30 genetically confirmed Ewing sarcomas and 130 non-Ewing small round cell tumors by using an antibody to NKX2.2. Nuclear staining in at least 5% of the cells was deemed positive. Twenty-eight (93%) of the 30 Ewing sarcomas were positive for NKX2.2. The staining was diffuse (>50%) in all the positive cases and was moderate or strong in intensity for most cases (25 of 28). NKX2.2 was also positive in 14 non-Ewing tumors, including all the olfactory neuroblastomas and a minor subset of small cell carcinomas, synovial sarcomas, mesenchymal chondrosarcomas, and malignant melanomas. All the other non-Ewing tumors tested were negative for this marker. NKX2.2 is a valuable marker for Ewing sarcoma, with a sensitivity of 93% and a specificity of 89%, and aids in the differential diagnosis of small round cell tumors.

Recent discovery of ROS1 gene fusion in a subset of lung cancers has raised clinical interest, because ROS1 fusion–positive cancers are reportedly sensitive to kinase inhibitors. To better understand these tumors, we examined 799 surgically resected non–small cell lung cancers by reverse transcriptase polymerase chain reaction and identified 15 tumors harboring ROS1 fusion transcripts (2.5% of adenocarcinomas). The most frequent fusion partner was CD74 followed by EZR. The affected patients were often younger nonsmoking female individuals, and they had overall survival rates similar to those of the ROS1 fusion–negative cancer patients. All the ROS1 fusion–positive tumors were adenocarcinomas except 1, which was an adenosquamous carcinoma. Histologic examination identified an at least focal presence of either solid growth with signet-ring cells or cribriform architecture with abundant extracellular mucus in 53% of the cases. These 2 patterns are reportedly also characteristic of anaplastic lymphoma kinase (ALK)-rearranged lung cancers, and our data suggest a phenotypic resemblance between the ROS1-rearranged and ALK-rearranged tumors. All tumors except 1 were immunoreactive to thyroid transcription factor-1. Fluorescence in situ hybridization using ROS1 break-apart probes revealed positive rearrangement signals in 23% to 93% of the tumor cells in ROS1 fusion–positive cancers, which were readily distinguished using a 15% cutoff value from 50 ROS1 fusion–negative tumors tested, which showed 0% to 6% rearrangement signals. However, this perfect test performance was achieved only when isolated 3′ signals were included along with classic split signals in the definition of rearrangement positivity. Fluorescence in situ hybridization signal patterns were unrelated to 5′ fusion partner genes. All ROS1 fusion–positive tumors lacked alteration of EGFR, KRAS, HER2, ALK, and RET genes.

The recent discovery and characterization of an oncogenic ROS1 gene fusion in a subset of lung cancers has raised significant clinical interest because small molecule inhibitors may be effective to these tumors. As lung cancers with ROS1 rearrangements comprise only 1–3% of lung adenocarcinomas, patients with such tumors must be identified to gain optimal benefit from molecular therapy. Recently, immunohistochemical analyses using a novel anti-ROS1 rabbit monoclonal antibody (D4D6) have shown promise for accurate identification of ROS1-rearranged cancers. To validate this finding, we compared the immunostaining results of tissue microarrays (TMAs) containing 17 ROS1-rearranged and 253 ROS1-non-rearranged lung carcinomas. All 17 ROS1-rearranged cancers showed ROS1 immunoreactivity mostly in a diffuse and moderate-to-strong manner with an H-score range of 5–300 (median, 260). In contrast, 69% of ROS1-non-rearranged cancers lacked detectable immunoreactivity, whereas the remaining 31% showed reactivity mainly in a weak or focal manner. The H-score for the entire ROS1-non-rearranged group ranged from 0 to 240 (median, 0). The difference in H-score between the two cohorts was statistically significant, and the H-score cutoff (≥150) allowed optimal discrimination (94% sensitivity and 98% specificity). Similar but slightly less-specific performance was achieved using the extent of diffuse (≥75%) staining or ≥2+ staining intensity as cutoffs. CD74-ROS1 and EZR-ROS1 fusions were significantly associated with at least focal globular immunoreactivity and plasma membranous accentuation, respectively, and these patterns were specific to ROS1-rearranged cases. Although full-length ROS1 is expressed in some ROS1-non-rearranged cases, we showed that establishment of an optimal set of interpretative criteria makes ROS1 immunohistochemistry a valuable method to rapidly and accurately screen lung cancer patients for appropriate molecular therapy.