The histologic spectrum of soft tissue spindle cell tumors with NTRK3 gene rearrangements

A morphological mimic: An NTRK3-rearranged spindle cell neoplasm presenting as a groin mass

Neurotrophic receptor tyrosine kinase (NTRK)-rearranged spindle cell neoplasms are a recently described group of soft tissue tumors. They commonly present as a painless mass on the extremities of children and young adults. They are characterized microscopically by a heterogeneous spectrum of infiltrative spindle cell proliferations, which can morphologically mimic several other spindle cell neoplasms. Their identification is vital, as they may be amenable to treatment with tyrosine kinase-targeted therapy. This case report describes a rare NTRK3-rearranged spindle cell neoplasm in the groin of a 29-year-old female and provides further clinical and morphological features of this entity.

Spindle Cell Lesions with Oncogenic EGFR Kinase Domain Aberrations: Expanding the Spectrum of Protein Kinase-Related Mesenchymal Tumors

EGFR aberrations are reported in a subset of myofibroblastic lesions with kinase domain duplication (EGFR-KDD) and exon 20 mutations being assigned to infantile fibrosarcomas (IFS), mesoblastic nephroma, and fibrous hamartoma of infancy (FHI), respectively. In this retrospective study, we correlated molecular findings with the histomorphology of 14 myofibroblastic lesions harboring such genetic changes identified by NGS. We additionally performed DNA methylation profiling (DNAmp) and immunohistochemistry. Lesions were from 10 males and 4 females with a mean age of 3 years (range, 0.3-14) and occurred subcutaneously in the upper limbs (n = 5), lower limbs (n = 3), back/thorax (n = 5), and the nasal cavity (n = 1). Eleven were cured by surgery, including 1 relapsed case. Two patients were lost to follow-up. One case was very recent, and the patient was biopsied. Histologically, the lesions showed a wide spectrum varying from classic FHI (n = 9) to IFS (n = 1) or lipofibromatosis-like tumors (LFT-like) (n = 2) or dermatofibrosarcoma protuberans-like (DFSP-like) (n = 1) to a predominantly myxoid spindle cell lesion (n = 1). Immunohistochemically, all neoplasms stained with CD34, whereas S100 was positive in 2/14. EGFR expression was observed in 9/10 cases. Molecularly, the IFS and 1 LFT-like harbored EGFR-KDD, whereas an exon 20 mutation was identified in all FHI, 1 LFT-like, the DFSP-like, and in predominant myxoid spindle cell lesion. By DNAmp, all but 2 cases formed a well-defined cluster, demonstrating that these lesions are also epigenetically related. In conclusion, EGFR kinase domain aberrations found in FHI, IFS, LFT-like, DFSP-like, and a spindle cell lesion with a predominant myxoid stroma of children and adolescents showed that these neoplasms with a broad morphologic spectrum belong to the group of protein kinase-related lesions with a distinct epigenetic signature. Molecular analyses, including DNAmp, help to identify and characterize this emerging category and become mandatory when targeted treatment is considered.

NTRK-rearranged spindle cell neoplasm: Initial observation of imaging appearance and clinicopathologic correlation

OBJECTIVE

To explore pre-treatment imaging findings of neurotrophic tyrosine receptor kinase (NTRK)-rearranged spindle cell neoplasm, an emerging group of molecularly defined soft tissue tumors and summarize the clinical course, including TRK inhibitor therapy response.

MATERIALS AND METHODS

This retrospective study included 8 women and 4 men with NTRK-rearranged spindle cell neoplasm (median age, 35.5 years, range, 0-66). Available pre-treatment MRI, CT, PET, and US imaging were reviewed. Tumor histology and the patients' clinical course were reviewed.

RESULTS

Primary tumors were located within the soft tissue, lungs, kidney, and breast with soft tissue being the most prevalent site (n = 6). Pre-treatment MRI (n = 4) revealed linear hypointense signal foci and contrast enhancement in all patients with hemorrhage in half of the tumors. A tail sign (n = 1) and fluid levels (n = 1) were less frequent. Ultrasound showed well-marginated hypoechoic masses with internal flow. Primary tumors were all non-calcified on CT (4/4). Metastases were FDG-avid (4/4). Among the 8 patients who developed metastasis, 7 developed pulmonary metastases. All four patients who received NTRK inhibitor therapy showed an initial decrease in tumor size or FDG uptake.

CONCLUSION

NTRK-rearranged neoplasms may occur as enhancing masses with linear hypointense signal foci on MRI and FDG avid metastases on PET. Pulmonary metastases were frequent in our study. Initial treatment response is observed in most patients.

NTRK3-EML4-rearranged spindle cell tumor with co-expression of S100 and CD34: an unusual mesenchymal tumor in the spectrum of the bland-looking spindle cell lesions of the oral cavity

A novel category of spindle cell tumors characterized by Neurotrophic Tyrosine Receptor Kinase (NTRK) rearrangements with a dual immunoreactivity for S-100 and CD34 has emerged in the last years as a distinct entity among soft tissue neoplasms. These genetic alterations lead to the continuous activation of NTRK genes, driving tumorigenesis and offering a unique prospect for targeted therapy. We herein present a rare case of NTRK3-rearranged spindle cell tumor with a hitherto unreported gene fusion involving the exon 14 of NTRK3 with the exon 2 of Echinoderm Microtubule-Associated Protein-Like 4, arising in the head and neck region. Tumor occurred in a 45-year-old patient who presented with a painful nodule in the oral mucosa. Due to the possibility of personalizing the treatment strategy for such tumors, pathologists should be aware of this emerging group of spindle cell tumors to promptly recognize them even when they occur in uncommon locations, including the oral cavity.

Back to the future! Selected bone and soft tissue neoplasms with shared genetic alterations but differing morphological and immunohistochemical phenotypes

Bone and soft tissue tumors (BST) are a highly heterogeneous group largely classified by their line of differentiation, based on their resemblance to their normal counterpart in adult tissue. Yet, rendering a specific diagnosis can be challenging, primarily due to their rarity and overlapping histopathologic features or clinical presentations. Over the past few decades, seemingly histogenetic-specific gene fusions/translocations and amplifications have been discovered, aiding in a more nuanced classification, leading to well-established objective diagnostic criteria and the development of specific surrogate ancillary tests targeting these genetic aberrations (e.g., immunohistochemistry). Ironically, the same research also has revealed that some specific tumor subtypes may be the result of differing and often multiple gene fusions/translocations, but, more interestingly, identical gene fusions may be present in more than one phenotypically and biologically distinct neoplasm, sometimes with entirely different clinical behavior. Prime examples include, EWSR1::ATF1 and, less commonly, EWSR1::CREB1 gene fusions present in both clear cell sarcoma, a malignant high-grade tumor with melanocytic differentiation, and angiomatoid fibrous histiocytoma, a mesenchymal neoplasm of intermediate malignancy with a generally indolent course. Similarly, MDM2 amplification, once deemed to be pathognomonic for atypical lipomatous tumor/well differentiated and dedifferentiated liposarcoma, has been documented in a range of additional distinct tumors, including low grade osteosarcomas (e.g. low grade central and surface parosteal) and high-grade intimal sarcomas, amongst others. Such findings reinforce the importance of careful attention to morphological and clinicoradiological features and correlation with molecular testing before rendering a specific diagnosis. Future classification systems in BST neoplasms cannot be solely based on molecular events and ideally will balance morphologic features with molecular analysis. Herein, we provide a narrative literature review of the more common BST neoplasms with shared genetic events but differing demographics, morphology, immunophenotype, and clinical behavior, re-emphasizing the importance of the hematoxylin and eosin slide and the "eye" of the practicing pathologist.

Primary NTRK -rearranged Spindle Cell Neoplasm of the Gastrointestinal Tract: A Clinicopathological and Molecular Analysis of 8 Cases

NTRK-rearranged spindle cell neoplasm occurs predominantly in the superficial or deep soft tissues of extremities or trunk. Occurrence in the visceral organs is extremely rare. Herein, we describe 8 cases of NTRK-rearranged spindle cell neoplasm that arose primarily in the gastrointestinal tract. Patients included 5 males and 3 females with age at presentation ranging from 6 to 63 years (median: 29.5 years). Tumors occurred in the colon (n=3), small intestine (n=2), rectum (n=2), and stomach (n=1). Tumor size ranged from 3.5 to 9 cm (median: 5 cm). Morphologically, 4 tumors were low-grade, composed of haphazard or intertwining fascicles of spindle cells, with prominent interstitial collagen fibers and ring-like perivascular hyalinization being present in 2 tumors. The other 4 tumors were histologically high-grade sarcomas, consisting of sweeping fascicles of atypical spindle cells showing increased cellularity and brisk mitotic activity. Immunohistochemically, 6/6 cases (100%) showed diffuse and strong cytoplasmic staining of pan-TRK. Variable expression of TrkA, CD34, and S100 was noted in 5/5 (100%), 5/8 (62.5%), and 4/7 (57.1%) cases, respectively. Fluorescence in situ hybridization analysis showed NTRK1 rearrangement (n=7) and NTRK2 rearrangement (n=1). In cases with available materials, RNA sequencing identified LMNA::NTRK1 (n=3), TPM3::NTRK1 (n=2), and STRN::NTRK2 (n=1) fusions. At follow-up (range: 4 to 30 months; median: 12.5 months), 6 of 7 patients who underwent surgery had no evidence of disease at last follow-up. One patient was succumbed to the disease at 12 months despite adjunctive treatment with TRK inhibitor larotrectinib after surgery. One patient was treated with larotrectinib alone. He showed significant response at 7 months after treatment. NTRK-rearranged spindle cell neoplasm represents an exceptionally rare entity in the gastrointestinal tract. The presence of interstitial collagen fibers and ring-like perivascular hyalinization and co-expression of CD34 and S100 are diagnostic clues to low-grade neoplasms. However, high-grade sarcomas pose a considerable diagnostic challenge to pathologists owing to the lack of specific features. The final diagnosis relies on molecular assays. Patients with advanced disease may benefit from TRK inhibitor treatment.

ETV6::NTRK3 Fusion-Positive Wild-Type Gastrointestinal Stromal Tumor (GIST) with Abundant Lymphoid Infiltration (TILs and Tertiary Lymphoid Structures): A Report on a New Case with Therapeutic Implications and a Literature Review

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract, with proto-oncogene, receptor tyrosine kinase (c-kit), or PDGFRα mutations detected in around 85% of cases. GISTs without c-kit or platelet-derived growth factor receptor alpha (PDGFRα) mutations are considered wild-type (WT), and their diverse molecular alterations and biological behaviors remain uncertain. They are usually not sensitive to tyrosine kinase inhibitors (TKIs). Recently, some molecular alterations, including neurotrophic tyrosine receptor kinase (NTRK) fusions, have been reported in very few cases of WT GISTs. This novel finding opens the window for the use of tropomyosin receptor kinase (TRK) inhibitor therapy in these subtypes of GIST. Herein, we report a new case of NTRK-fused WT high-risk GIST in a female patient with a large pelvic mass (large dimension of 20 cm). The tumor was removed, and the histopathology displayed spindle-predominant morphology with focal epithelioid areas, myxoid stromal tissue, and notable lymphoid infiltration with tertiary lymphoid structures. Ten mitoses were quantified in 50 high-power fields without nuclear pleomorphism. DOG1 showed strong and diffuse positivity, and CD117 showed moderate positivity. Succinate dehydrogenase subunit B (SDHB) was retained, Pan-TRK was focal positive (nuclear pattern), and the proliferation index Ki-67 was 7%. Next-generation sequencing (NGS) detected an ETV6::NTRK3 fusion, and this finding was confirmed by fluorescence in situ hybridization (FISH), which showed NTRK3 rearrangement. In addition, an RB1 mutation was found by NGS. The follow-up CT scan revealed peritoneal nodules suggestive of peritoneal dissemination, and Entrectinib (a TRK inhibitor) was administered. After 3 months of follow-up, a new CT scan showed a complete response. Based on our results and the cases from the literature, GISTs with NTRK fusions are very uncommon so far; hence, further screening studies, including more WT GIST cases, may increase the possibility of finding additional cases. The present case may offer new insights into the potential introduction of TRK inhibitors as treatments for GISTs with NTRK fusions. Additionally, the presence of abundant lymphoid infiltration in the present case may prompt further research into immunotherapy as a possible additional therapeutic option.

Case report: Adult NTRK-rearranged spindle cell neoplasms with TPM3-NTRK1 fusion in the pelvic

NTRK-rearranged spindle cell neoplasms (NTRK-RSCNs) are rare soft tissue tumor molecularly characterized by NTRK gene rearrangement, which occurs mostly in children and young adults, and rarely in adults. The abnormal tumor located in superficial or deep soft tissues of human extremities and trunk mostly, and rarely also involves abdominal organs. In this case, we report a malignant NTRK-RSCN that occurred in the pelvic region of an adult. The patient was found to have a large tumor in the pelvic region with a pathological diagnosis of infiltrative growth of short spindle-shaped tumor cells with marked heterogeneity. Immunohistochemistry of this patient showed positive vimentin, pan-TRK and Ki67 (approximately 60%) indicators with negative S100, Desmin and DOG1. Molecular diagnosis revealed c-KIT and PDGFRα wild type with TPM3-NTRK1 fusion, unfortunately this patient had a rapidly progressive disease and passed away. This case highlights the gene mutation in the molecular characteristics of NTRK-RSCNs, and the significance of accurate molecular typing for the diagnosis of difficult cases.

Kinase fusion positive intra-osseous spindle cell tumors: A series of eight cases with review of the literature

Mesenchymal spindle cell tumors with kinase fusions, often presenting in superficial or deep soft tissue locations, may rarely occur in bone. Herein, we describe the clinicopathologic and molecular data of eight bone tumors characterized by various kinase fusions from our files and incorporate the findings with the previously reported seven cases, mainly as single case reports. In the current series all but one of the patients were young children or teenagers, with an age range from newborn to 59 years (mean 19 years). Most tumors (n = 5) presented in the head and neck area (skull base, mastoid, maxilla, and mandible), and remaining three in the tibia, pelvic bone, and chest wall. The fusions included NTRK1 (n = 3), RET (n = 2), NTRK3 (n = 2), and BRAF (n = 1). In the combined series (n = 15), most tumors (73%) occurred in children and young adults (<30 years) and showed a predilection for jaw and skull bones (40%), followed by long and small tubular bones (33%). The fusions spanned a large spectrum of kinase genes, including in descending order NTRK3 (n = 6), NTRK1 (n = 4), RET (n = 2), BRAF (n = 2), and RAF1 (n = 1). All fusions confirmed by targeted RNA sequencing were in-frame and retained the kinase domain within the fusion oncoprotein. Similar to the soft tissue counterparts, most NTRK3-positive bone tumors in this series showed high-grade morphology (5/6), whereas the majority of NTRK1 tumors were low-grade (3/4). Notably, all four tumors presenting in the elderly were high-grade spindle cell sarcomas, with adult fibrosarcoma (FS)-like, malignant peripheral nerve sheath tumor (MPNST)-like and MPNST phenotypes. Overall, 10 tumors had high-grade morphology, ranging from infantile and adult-types FS, MPNST-like, and MPNST, whereas five showed benign/low-grade histology (MPNST-like and myxoma-like). Immunohistochemically (IHC), S100 and CD34 positivity was noted in 57% and 50%, respectively, while co-expression of S100 and CD34 in 43% of cases. One-third of tumors (4 high grade and the myxoma-like) were negative for both S100 and CD34. IHC for Pan-TRK was positive in all eight NTRK-fusion positive tumors tested and negative in two tumors with other kinase fusions. Clinical follow-up was too limited to allow general conclusions.

Complementary value of molecular analysis to expert review in refining classification of uncommon soft tissue tumors

The classification of many soft tissue tumors remains subjective due their rarity, significant overlap in microscopic features and often a non-specific immunohistochemical (IHC) profile. The application of molecular genetic tools, which leverage the underlying molecular pathogenesis of these neoplasms, have considerably improved the diagnostic abilities of pathologists and refined classification based on objective molecular markers. In this study, we describe the results of an international collaboration conducted over a 3-year period, assessing the added diagnostic value of applying molecular genetics to sarcoma expert pathologic review in a selected series of 84 uncommon, mostly unclassifiable mesenchymal tumors, 74 of which originated in soft tissues and 10 in bone. The case mix (71% historical, 29% contemporary) included mostly unusual and challenging soft tissue tumors, which remained unclassified even with the benefit of expert review and routine ancillary methods, including broad IHC panels and a limited number of commercially available fluorescence in situ hybridization (FISH) probes. All cases were further tested by FISH using a wide range of custom bacterial artificial chromosome probes covering most of known fusions in sarcomas, whereas targeted RNA sequencing was performed in 13 cases negative by FISH, for potential discovery of novel fusion genes. Tumor-defining molecular alterations were found in 48/84 tumors (57%). In 27 (32%) cases the tumor diagnosis was refined or revised by the additional molecular work-up, including five cases (6%), in which the updated diagnosis had clinical implications. Sarcoma classification is rapidly evolving due to an increased molecular characterization of these neoplasms, so unsurprisingly 17% of the tumors in this series harbored abnormalities only very recently described as defining novel molecularly defined soft tissue tumor subsets.

Uterine Sarcoma With EML4::NTRK3 Fusion: A Spectrum of Mesenchymal Neoplasms Harboring Actionable Gene Fusions

NTRK gene fusions are part of a paradigm shift in oncology, arising as one of the main genomic alterations with actionability in the so-called "agnostic setting." In gynecologic pathology, the recent description of uterine sarcoma resembling fibrosarcoma and with NTRK rearrangements ( NTRK -rearranged uterine sarcoma) highlights the importance of recognizing clinicopathological cues that can lead to genomic profiling. Herein, we report the case of a 43-year-old woman presenting with vaginal bleeding and pelvic mass. Histopathology of the tumor showed moderately atypical spindle cells arranged in long fascicles reminiscent of fibrosarcoma, along with immunohistochemical positivity for S100, CD34, and pan-tropomyosin receptor kinase. This prompted RNA-sequencing and the finding of a rare EML4::NTRK3 fusion. Clinical, histologic, and molecular findings are described, in addition to discussions regarding differential diagnoses and possible implications of the findings in clinical practice.

[Receptor tyrosine kinase- fusions in paediatric spindle cell tumors]

Pediatric spindle cell tumors are rare and often difficult to diagnose due to a similar morphology and a non-specific immunohistochemical profile. Genetic characterization of these lesions has been constantly improving, which has led to the identification of new subgroups that were partly included in the WHO classification. Receptor tyrosine kinase fusions play a special role in these tumors and their verification has diagnostic relevance and can be an option for target-oriented therapies. In the case of pediatric spindle cell tumors, genetic fusions form especially with NTRK1‑3, ALK, RET, and ROS1. Overall, pediatric tumors with receptor tyrosine kinase fusions are predominantly low-grade tumors, which are often subdivided into the group of intermediate-malign tumors.

A spindle cell neoplasm with MYH9::EGFR fusion and co-expression of S100 and CD34, further expanding the family of kinase fusion positive spindle cell neoplasms

Soft tissue neoplasms displaying CD34 and S100 positivity with immunohistochemistry are rare with a wide morphological range and frequent neurotrophic tyrosine receptor kinase (NTRK) alterations. Recent reports describe fusions in other kinases besides NTRK in these tumors. In the present article, we report a case of a young male suffering from a soft tissue neoplasm in the lumbar region. At microscopic examination, it was a CD34 and S100-positive soft tissue tumor showing a multilobulated growth pattern composed of cells with pale cytoplasm and abundant normal smooth muscle stroma. The genetic profile showed two alterations affecting EGFR gene represented by a novel MYH9::EGFR fusion transcript and a p.K714N mutation.

Primary NTRK-rearranged spindle cell neoplasm of bone harboring an HMBOX1::NTRK3 gene fusion

The majority of neurotrophic tyrosine receptor kinase (NTRK) rearranged neoplasms occur either in the superficial or deep soft tissues of extremities or trunk. Occasionally, it arises in visceral organs. However, its occurrence as a primary osseous tumor has not been documented thus far. Herein, we describe a unique case of an NTRK rearranged neoplasm that presented as a primary bony lesion. The tumor occurred in a 21-year-old woman who presented with an increasing pain in the right lower extremity. Radiologic examinations revealed a destructive lytic lesion located in the lower portion of the right femur. Histologically, the tumor was composed of haphazard fascicles of monomorphic spindle cells displaying mild nuclear atypia and rare mitotic activity. Immunohistochemically, the tumor cells showed focal staining of pan-TRK and S100 protein. Fluorescence in situ hybridization analysis was performed with the utilization of break-apart probes for NTRK1/NTRK2/NTRK3 genes. An NTRK3 rearrangement was identified. Subsequent next-generation sequencing (RNA-seq) revealed HMBOX1exon6::NTRK3exon 14 fusion. Our study illustrates, albeit extremely rare, that NTRK-rearranged neoplasms can arise as a primary bone lesion. In addition, we describe a novel HMBOX1::NTRK3 fusion that has not been documented before.

GAB1::ABL1 fusions define a distinctive soft tissue neoplasm, with variable perineurial differentiation, and a predilection for children and young adults

Although well known as a fusion partner in hematological malignancies, fusion genes involving the ABL proto-oncogene 1 (ABL1), mapping to chromosomal region 9q34.12, have only been anecdotally reported in five soft tissue tumors. These neoplasms have been variously reported as perineurioma, angiofibroma, and solitary fibrous tumor, and all have harbored a GAB1::ABL1 gene fusion; however, the nosology and clinicopathological characteristics of soft tissue tumors carrying this rare fusion have not been delineated. We herein describe eight tumors containing the GAB1::ABL1 fusion and review previously reported cases in a series to define their morphological spectrum, address immunohistochemical evidence for a line of differentiation, with special reference to the presence or absence of a perineurial immunophenotype, and gather insight into their behavior. The patients included four females and four males, aged 13-37 years (median, 24 years). Two cases each originated in the shoulder area, trunk, hands, and lower extremities, with a size range of 1.5-8 cm (median, 3.4 cm). Four tumors were deep and four superficial. All tumors were morphologically similar, being composed of bland fibroblast-like spindle to ovoid cells diffusely arranged in a paucivascular fibrous to fibromyxoid stroma with variable resemblance to soft tissue perineurioma. Mitotic activity was generally low (0-8 mitoses in 10 high-power fields [HPFs]; median, 1). All lesions had at least focally infiltrative margins, but they otherwise lacked pleomorphism and necrosis. Immunohistochemistry showed focal reactivity for CD34 (5/7), epithelial membrane antigen (EMA) (3/8), claudin1 (2/3), GLUT1 (4/6), and S100 (2/7); other markers, including MUC4 (0/7), desmin (0/9), and smooth muscle actin (SMA) (0/4), were negative. RNA sequencing revealed a GAB1::ABL1 fusion in all cases with exon 6 of GAB1 fused to exon 2 of ABL1. Treatments included various forms of surgical intervention in seven cases; one tumor was biopsied only. Limited follow-up was available for five patients. One tumor regrew rapidly within 1 month to 1.5 cm after an initial marginal excision and was re-excised with close margins. Four patients were disease-free at 1, 3, 14, and 25 months of follow-up. Metastases have not, to date, been observed. This series characterizes "GAB1::ABL1 fusion-positive spindle cell neoplasm" as a distinct entity, with overlapping features with soft tissue perineurioma and predilection for children and young adults.

Fetal Lung Interstitial Tumor (FLIT): Review of The Literature

Fetal lung interstitial tumor (FLIT) is an extremely rare pediatric lung tumor that shares radiological features with congenital pulmonary malformations (cPAM) and other lung neoplasms. A review of the literature, together with the first European case, are herein reported. A systematic and manual search of the literature using the keyword "fetal lung interstitial tumor" was conducted on PUBMED, Scopus, and SCIE (Web of Science). Following the PRISMA guidelines, 12 articles were retrieved which describe a total of 21 cases of FLIT, and a new European case is presented. A prenatal diagnosis was reported in only 3 out of 22 (13%) cases. The mean age at surgery was 31 days of life (1-150); a lobectomy was performed in most of the cases. No complications or recurrence of disease were reported at a mean follow-up of 49 months. FLIT is rarely diagnosed during pregnancy, may present at birth with different levels of respiratory distress, and requires prompt surgical resection. Histology and immunohistochemistry allow for the differentiation of FLIT from cPAM and other lung tumors with poor prognosis, such as pleuropulmonary blastoma, congenital peri-bronchial myofibroblastic tumor, inflammatory myofibroblastic tumor, and congenital or infantile fibrosarcoma.

Application of immunohistochemical studies in diagnosing emerging superficial mesenchymal neoplasms

Molecular diagnostics, with the subsequent development of novel immunohistochemical markers, continues to advance and expand the field of soft tissue pathology. As such, the ever-evolving molecular diagnostic landscape will continue to shape and refine our understanding and classification of neoplasms. This article reviews the current literature on various tumors of mesenchymal origin, including fibroblastic/fibrohistiocytic, adipocytic, vascular, and tumors of uncertain origin. We aim to give the reader a detailed understanding and pragmatic approach to various new and established immunohistochemical stains in diagnosing these neoplasms and also discuss various pitfalls with significant repercussions.

NTRK-rearranged spindle cell neoplasms: a clinicopathological and molecular study of 13 cases with peculiar characteristics at one of the largest institutions in China

NTRK-rearranged spindle cell neoplasms (NTRK-RSCNs) represent an emerging group of rare tumours defined using molecular means. To the best of our knowledge, there have been no large series of reports about this tumour in the Chinese population in English full-text articles. Herein, we present 13 NTRK-RSCNs with peculiar characteristics. Ten of the 13 (77%) patients were children without sex differences. The tumour locations included six trunks, four extremities, two recta, and one small bowel. The histological morphology included four lipofibromatosis-like neural tumour (LPF-NT)-like, eight malignant peripheral nerve sheath tumours (MPNST)/fibrosarcoma-like, and one extremely rare myxofibrosarcoma-like pattern. Immunohistochemically, all cases were CD34, pan-TRK and TRK-A positive, SOX-10 negative, and H3K27me3 intact. S-100 protein expression was identified in 11 of 13 (85%) cases. Genetically, NTRK1 rearrangements were considered positive (7/13, 54%) or suspicious for positivity (6/13, 46%) by fluorescence in situ hybridisation. Next-generation sequencing and Sanger sequencing confirmed NTRK1 fusions with a variety of partner genes, including five LMNA, three TPM3, one SQSTM1, three novel CPSF6, IGR (downstream PMVK), and GAS2L1 genes. Interestingly, the last tumour concurrently harboured a second EWSR1-PBX1 fusion, which has never been reported. Four patients developed local recurrence and two of them suffered metastasis. In our study, NTRK-RSCNs had peculiar fusions that displayed unusual or complicated clinicopathological features. Histological clues and IHC helped streamline a small subset of potential candidates. Although FISH is a powerful technology for identifying NTRK rearrangements, RNA-/DNA-based NGS is recommended for highly suspected cases in which FISH signal patterns are not discernible as classic positive patterns, particularly if targeted therapy is considered.

Frequent CD30 Expression in an Emerging Group of Mesenchymal Tumors With NTRK, BRAF, RAF1, or RET Fusions

Neurotrophic tyrosine receptor kinase (NTRK) fusions define infantile fibrosarcomas in young children and NTRK-rearranged spindle-cell tumors in older children and adults, which share characteristic spindle-cell histology and CD34 or S100 protein expression. Similar phenotypes were identified in tumors with BRAF, RAF1, or RET fusions, suggesting a unifying concept of "spindle-cell tumors with kinase gene fusions." In this study, we investigated CD30 expression in 38 mesenchymal tumors with kinase gene fusions using immunohistochemistry. CD30 was expressed in 15 of 22 NTRK-rearranged tumors and 12 of 16 tumors with BRAF, RAF1, or RET fusions. In total, CD30 was expressed in 27 of the 38 tumors (71%), with >50% CD30-positive cells in 21 tumors and predominantly moderate or strong staining in 24 tumors. CD34 and S100 protein were also expressed in 71% and 69% of the tumors, respectively. In contrast, CD30 was significantly less frequently expressed in other mesenchymal tumor types that histologically mimic kinase fusion-positive tumors (9 of 150 tumors, 6%), of which none showed >50% or predominantly strong staining. Among these mimicking tumors, malignant peripheral nerve sheath tumors occasionally (30%) expressed CD30, albeit in a weak focal manner in most positive cases. CD30 was also expressed in 3 of 15 separately analyzed ALK- or ROS1-positive inflammatory myofibroblastic tumors. Frequent expression of CD30 enhances the shared phenotype of spindle-cell tumors with NTRK and other kinase gene fusions, and its sensitivity seems similar to that of CD34 and S100 protein. Although moderate sensitivity hampers its use as a screening tool, CD30 expression could be valuable to rapidly identify high-yield candidates for molecular workup, particularly in communities that lack routine genetic analysis and/or for tumors with BRAF, RAF1, or RET fusions.

Recurrent RET fusions in fibrosarcoma-like neoplasms in adult viscera: expanding the clinicopathological and genetic spectrum

AIMS

RET-fused mesenchymal neoplasms mostly affect the soft tissue of paediatric patients. Given their responsiveness to selective RET inhibitors, it remains critical to identify those extraordinary cases occurring in the visceral organs of adults. In this study, we report three RET-rearranged spindle-cell tumours occurring in the visceral organs of adults.

METHODS AND RESULTS

Clinicopathological features were assessed and partner agnostic targeted next-generation sequencing on clinically validated platforms were performed. The patients were 18, 53, and 55 years old and included one male and two females. The tumours were located in the kidney (case 1), small intestine (case 2), and ureter (case 3), with maximum diameters of 14, 5, and 1 cm, respectively. Histologically, all tumours displayed a morphological spectrum typical of fibrosarcoma, including moderately to highly cellular, nonpleomorphic, ovoid to spindle-shaped cells arranged in long fascicles or haphazardly within collagenised to myxohyaline stroma. Foci of irregular alveolar oedema-like structures and areas with microcystic and reticular arrangements were identified in the renal tumour. Staghorn-type vessels and foci of band-like stromal hyalinisation were observed in the small intestine tumour. Cases 1 and 2 were high-grade and pursed a highly aggressive clinical course, while case 3 was of intermediate grade with no tumour recurrence or metastasis 14 years after surgery. All three tumours expressed CD34, which was coexpressed with S100 protein in cases 2 and 3. Molecular genetic testing revealed PRKAR1A::RET, KIF5B::RET, and SPECC1L::RET in-frame gene fusions.

CONCLUSION

Our study expands the clinicopathological and genetic spectrum of mesenchymal neoplasms associated with RET fusions.

Generating in vitro models of NTRK-fusion mesenchymal neoplasia as tools for investigating kinase oncogenic activation and response to targeted therapy

The discovery of neurotrophic tyrosine receptor kinase (NTRK) gene fusions as pan-tumor oncogenic drivers has led to new personalized therapies in oncology. Recent studies investigating NTRK fusions among mesenchymal neoplasms have identified several emerging soft tissue tumor entities displaying various phenotypes and clinical behaviors. Among them, tumors resembling lipofibromatosis or malignant peripheral nerve sheath tumors often harbor intra-chromosomal NTRK1 rearrangements, while most infantile fibrosarcomas are characterized by canonical ETV6::NTRK3 fusions. However, appropriate cellular models to investigate mechanisms of how kinase oncogenic activation through gene fusions drives such a wide spectrum of morphology and malignancy are lacking. Progress in genome editing has facilitated the efficient generation of chromosomal translocations in isogenic cell lines. In this study we employ various strategies to model NTRK fusions, including LMNA::NTRK1 (interstitial deletion) and ETV6::NTRK3 (reciprocal translocation) in human embryonic stem (hES) cells and mesenchymal progenitors (hES-MP). Here, we undertake various methods to model non-reciprocal, intrachromosomal deletions/translocations by induction of DNA double strand breaks (DSBs) exploiting either the repair mechanisms of homology directed repair (HDR) or non-homologous end joining (NHEJ). Expression of LMNA::NTRK1 or ETV6::NTRK3 fusions in either hES cells or hES-MP did not affect cell proliferation. However, the level of mRNA expression of the fusion transcripts was significantly upregulated in hES-MP, and phosphorylation of the LMNA::NTRK1 fusion oncoprotein was noted only in hES-MP but not in hES cells. Similarly, an NTRK1-driven transcriptional profile related to neuronal and neuroectodermal lineage was upregulated mainly in hES-MP, supporting the importance of appropriate cellular context in modeling cancer relevant aberrations. As proof of concept of the validity of our in vitro models, phosphorylation was depleted by two TRK inhibitors, Entrectinib and Larotrectinib, currently used as targeted therapy for tumors with NTRK fusions.

ALK-rearranged Mesenchymal Neoplasms: A Report of 9 cases Further Expanding the Clinicopathologic Spectrum of Emerging Kinase Fusion Positive Group of Tumors

Anaplastic lymphoma kinase (ALK) fusions are oncogenic drivers in diverse cancer types. Although well established in inflammatory myofibroblastic tumor (IMT) and epithelioid fibrous histiocytoma (EFH), ALK rearrangements also occur in the emerging family of kinase fusion-positive mesenchymal neoplasms. We investigated 9 ALK-rearranged mesenchymal neoplasms (exclusive of IMT and EFH) arising in 6 males and 3 females with a wide age range of 10 to 78 years old (median 42 years). Tumors involved superficial and deep soft tissue (6) and viscera (3). Three were myxoid or collagenous low-grade paucicellular tumors with haphazardly arranged spindled cells. Three were cellular tumors with spindled cells in intersecting short fascicles or solid sheets. Three cases consisted of uniform epithelioid cells arranged in nests or solid sheets, with prominent mitotic activity and necrosis. Band-like stromal hyalinization was present in 6 cases. All tumors expressed ALK; four were positive for S100 and five were positive for CD34, while all were negative for SOX10. By targeted RNA sequencing, the breakpoints involved ALK exon 20; the 5' partners included KLC1, EML4, DCTN1, PLEKHH2, TIMP3, HMBOX1, and FMR1. All but two patients presented with localized disease. One patient had distant lung metastases; another had diffuse pleural involvement. Of the six cases with treatment information, five were surgically excised [one also received neoadjuvant radiation therapy (RT)], and one received RT and an ALK inhibitor. Of the four patients with follow-up (median 5.5 months), one remained alive with stable disease and three were alive without disease. We expand the clinicopathologic spectrum of ALK-fused mesenchymal neoplasms, including a low-grade malignant peripheral nerve sheath tumor-like subset and another subset characterized by epithelioid and high-grade morphology.

Head and Neck Mesenchymal Tumors with Kinase Fusions: A Report of 15 Cases With Emphasis on Wide Anatomic Distribution and Diverse Histologic Appearance

Mesenchymal tumors harboring various kinase fusions were recently recognized as emerging entities mainly in the soft tissues. We herein investigate the clinicopathologic and molecular characteristics of head and neck mesenchymal tumors harboring kinase fusions. The study cohort included 15 patients with a median age of 13 years (ranging from congenital to 63 y). The kinase genes involved in descending order were NTRK1 (n=6), NTRK3 (n=5), BRAF (n=2), and 1 each with MET, and RET. The anatomic locations were broad involving all tissue planes, including skin (n=4), intraosseous (n=4), major salivary glands (n=2), sinonasal tract (n=2), soft tissue of face or neck (n=2), and oral cavity (n=1). The histologic spectrum ranged from benign to high grade, in descending order including tumors resembling malignant peripheral nerve sheath tumor (MPNST)-like, fibrosarcoma (infantile or adult-type), lipofibromatosis-like neural tumor (LPFNT), inflammatory myofibroblastic tumor-like, and a novel phenotype resembling myxoma. Perivascular hyalinization/stromal keloid-like collagen bands and staghorn vasculature were common features in MPNST-like and LPFNT-like tumors. Two tumors (1 each with NTRK1 or BRAF rearrangement) were classified as high grade. By immunohistochemistry, S100 and CD34 positivity was noted in 71% and 60%, frequently in MPNST-like and LPFNT-like phenotypes. Pan-TRK was a sensitive marker for NTRK-translocated tumors but was negative in tumor with other kinase fusions. One patient with a high-grade tumor developed distant metastasis. Molecular testing for various kinase fusions should be considered for S100+/CD34+ spindle cell neoplasms with perivascular hyalinization and staghorn vessels, as pan-TRK positivity is seen only in NTRK fusions.

Imaging findings of NTRK‑rearranged spindle cell neoplasms: A case series

NTRK-rearranged spindle cell neoplasms (NTRK-RSCNs) are a new category of soft tissue tumors with NTRK gene fusions. The present study aimed to investigate the radiological features of NTRK-RSCNs and their association with histopathological findings. The present study included six patients with NTRK-RSCNs, whose fusion genes were confirmed using next-generation sequencing. All patients underwent surgery, and their diagnosis and clinical outcomes were investigated. In addition, the magnetic resonance imaging (MRI) features of all tumors and histopathological findings of the resected specimens were assessed. The present study included three women and three men, with a mean age of 22 years (range, 2-43 years). The NTRK gene fusions included four NTRK1 and two NTRK3 fusions. Three patients were preoperatively diagnosed with solitary fibrous tumors. One patient with NTRK3 fusion experienced local recurrence and distant metastases, whereas the other five patients had no local recurrence or metastasis. MRI revealed that all tumors were highly vascular with intra- and peritumoral flow voids of differing degrees. Furthermore, a partially ill-defined border, suggesting infiltration of tumors into the surrounding tissues, particularly fat tissue, was observed in five patients, which was confirmed by histopathological findings. In conclusion, NTRK-RSCNs are highly vascular tumors that can infiltrate the surrounding tissues. These findings suggested that NTRK-RSCNs should be considered in the differential diagnosis of highly vascular-rich mesenchymal tumors, including solitary fibrous tumors and alveolar soft part sarcomas. Furthermore, wide resection may be preferred to completely resect this type of tumor, considering its invasive nature.

Lipofibromatosis-like neural tumors: Report of a case and review of 73 reported cases

Lipofibromatosis-like neural tumors (LPF-NTs) are a recently discovered group of spindle cell tumors defined by the presence of a lipofibromatosis-like pattern, CD34 and/or S100 reactivity, and frequent neurotrophic receptor tyrosine kinase 1 (NTRK1) gene rearrangements. As new cases emerge, the spectrum of features observed in LPF-NTs continues to evolve. Here we describe the case of an 11-year-old with LPF-NT with a dermatofibrosarcoma protuberans-like honeycomb pattern, CD34 and S100 co-expression, and an NTRK1 rearrangement. We also review the clinical and molecular features of the 73 cases of LPF-NT previously described in the literature.

Dramatic response to entrectinib in a patient with malignant peripheral nerve sheath tumor harboring novel SNRNP70-NTRK3 fusion gene

Neurotropic tropomyosin receptor kinase (NTRK) gene rearrangements have been reported in limited cases of sarcomas; however, to date, there has been only one report of such rearrangements in malignant peripheral nerve sheath tumors (MPNSTs). Herein, we describe a 51-year-old male patient with a buttock tumor arising from the sciatic nerve, which was diagnosed as MPNST with positive S-100 staining, negative SOX10 staining, and loss of trimethylation at lysine 27 of histone H3 (H3K27me3) confirmed by immunohistochemistry. Soon after the resection of the primary tumor, the patient was found to have pulmonary and lymph node metastases. Chemotherapy with eribulin and trabectedin showed limited effects. However, the patient responded rapidly to pazopanib, but severe side effects caused discontinuation of the treatment. RNA panel testing revealed a novel fusion gene between Small Nuclear Ribonucleoprotein U1 Subunit 70 (SNRNP70) gene and NTRK3 gene. Furthermore, loss of NF1, SUZ12, and CDKN2A genes was confirmed by DNA panel testing, which is compatible with a histological diagnosis of MPNST. SNRNP70 possesses a coiled-coiled domain and seems to induce constitutive activation of NTRK3 through dimerization. In fact, immunohistochemistry revealed diffuse staining of pan-TRK within tumor cells. Treatment with entrectinib, which is an NTRK inhibitor, showed a quick and durable response for 10 months. Although NTRK rearrangements are very rare in MPNST, this case highlights the importance of genetic testing in MPNST, especially using an RNA panel for the detection of rare fusion genes.

Clinicopathological features and genomic profiles of a group of secretory breast carcinomas in which progressive cases have more complex genomic features

BACKGROUND

Secretory breast carcinoma (SBC) is a rare malignant breast neoplasm with distinct histological features, including solid, microcystic, tubular, and rarely papillary structures, traditionally characterized by a t (12;15) (p13:q25) translocation, which usually leads to ETV6-NTRK3 fusion, suggesting an early event in tumorigenesis. Due to the rarity of this disease, very few genome sequencing studies have been performed on a series of cases, especially progressive cases.

METHODS

Seven lesions from 5 patients diagnosed at the Third Affiliated Hospital of Soochow University from 2007 to 2021 were included. Clinicopathological features and prognosis/survival data were collected. Next-generation DNA sequencing was performed on six of the seven lesions.

RESULTS

In total, 3/7 (42.9%) lesions demonstrated estrogen receptor (ER) expression, including weak, moderate to strong staining, and no lesion demonstrated progesterone receptor (PR) expression. There were no cases of human epidermal growth factor (HER2) overexpression, and the Ki-67 index was low. S-100 and pan-TRK protein were diffusely positively expressed in all cases. All lesions were characterized by a t(12;15) (p13:q25) translocation, leading to ETV6-NTRK3 fusion confirmed by fluorescence in situ hybridization (FISH). The sequencing results showed that ETV6-NTRK3 fusion was the main driver of early tumorigenesis, while SBC with invasive biological behavior had more complex genomic variation in which TERT promoter mutation was detected.

CONCLUSIONS

Immunohistochemical staining of a biomarker panel, including ER, PR, HER2, Ki-67, S-100 and pan-TRK, can be used as an auxiliary diagnostic tool, and FISH detection can be used as a diagnostic tool. ETV6-NTRK3 gene fusion involving multiple sites may drive tumorigenesis, while mutations in the TERT promoter region may be a factor driving tumor progression.

GISTs with NTRK Gene Fusions: A Clinicopathological, Immunophenotypic, and Molecular Study

The most common mutations in gastrointestinal stromal tumors (GISTs) are KIT or PDGFRA mutations. Recently, neurotrophic tyrosine receptor kinase (NTRK) fusions have been reported in WT GISTs, which increased interest in introducing tropomyosin receptor kinase (TRK) inhibitors as treatments for GISTs with NTRK fusions. Hence, we aimed to screen NTRK fusions in WT GISTs; we used fluorescence in situ hybridization (FISH), next-generation sequencing (NGS), and immunohistochemistry (IHC) to screen NTRK fusions in 46 WT GISTs and evaluate each method. We further reviewed NTRK fusion-positive GISTs from the literature and performed clinical and pathological analyses; two GISTs with an ETV6-NTRK3 fusion (5%) were identified, while only one (50%) was positive for Pan-TRK expression. On the other hand, among the six GISTs with Pan-TRK-positive expression, only one (17%) harbored NTRK fusion. The literature review revealed the strong consistency between FISH and NGS and the limited value of Pan-TRK IHC in screening NTRK fusions in GISTs. In addition, the clinical and pathological analysis showed that GISTs with NTRK rearrangement occurred less frequently in the stomach, were more frequently larger in size, and the epithelioid type presented with a higher risk of recurrence. The NTRK3 fusion has been more common than the NTRK1 fusion in GISTs to date; our study identified two ETV6-NTRK3 fusions in 46 WT GISTs. Compared with FISH and IHC, NGS is preferred for screening WT GISTs, including NTRK rearrangements. However, since GISTs with NTRK fusions are rare, further studies including more samples and mechanistic investigations should be conducted in the future.

Expanding the Spectrum of the Soft Tissue Tumors with Kinase Gene Fusions: A Pediatric Spindle Cell Neoplasm Harboring MTAP-RAF1 Fusion

Introduction: NTRK-rearranged spindled cell tumors have been increasingly recognized with the widespread use of molecular studies. We describe a pediatric spindle cell neoplasm with MTAP-RAF1 gene fusion that fits into this group. Case report: An 8-year-old girl presented with mandibular mass. Histopathologically, it was a moderate to increased cellular spindle cell tumor with mild-to-moderate nuclear pleomorphism, focal perivascular keloid-like collagen, that was positive for S-100 and CD34. MTAP-RAF1 fusion was detected by next generation sequencing, confirming a low-grade sarcoma with MTAP-RAF1 fusion that is presently included in the category of NTRK-rearranged spindled cell tumors. Discussion: MTAP-RAF1 fusion, in the spectrum of spindle cell neoplasms with kinase gene rearrangements, can occur in the pediatric age group.

Infantile Fibrosarcoma and Other Spindle Cell Neoplasms of Infancy. A Review of Morphologically Overlapping yet Molecularly Distinctive Entities

BACKGROUND

Regardless of age at presentation, many soft tissue neoplasms have overlapping histopathologic and immunophenotypic features to serve as a diagnostic challenge.

CASE REPORT

We reported a case of a spindle cell neoplasm in an infant, which was initially considered a vascular anomaly clinically and an eventual biopsy revealed marked inflammation with a spindle cell component that was resolved as an infantile fibrosarcoma with an ETV6 break-apart.

CONCLUSION

The context of this case lead to a further consideration of various other spindle cell neoplasms arising predominantly in the soft tissues during the infancy period as defined by the first two years of age. Though sharing similar morphologic features, these tumors can be categorized into several molecular genetic groups, which have provided both diagnostic and pathogenetic insights as well as treatment options in some cases.

IntestinalLMNA::NTRK1-fused spindle cell neoplasm with S100 and CD34 coexpression: a new case

Recurrent fusions involving neurotrophin tyrosine receptor kinase (NTRK) genes have been increasingly recognised in spindle cell tumours of somatic soft tissues due to the widespread use of RNA-based sequencing techniques. This heterogeneous group of neoplasms is included as an emerging entity in the currentWHO Classification of Soft Tissue and Bone Tumors. A subset of these tumours, associated with NTRK1 fusions, displays a distinctive phenotype in the form of monomorphic cytomorphology, patternless arrangement, perivascular and stromal hyalinisation, and CD34+/S100+/SOX10− immunoprofile. Gastrointestinal tract counterparts have been recently described with emphasis on distinction fromKIT/PDGFRA/BRAF/RASwild-type gastrointestinal stromal tumours (GIST). Here, we present a recently encountered intestinal spindle cell neoplasm harbouring anLMNA::NTRK1gene fusion in a woman in her early 20s, which was initially thought to represent a GIST or a solitary fibrous tumour. Awareness of this emerging tumour type in the gastrointestinal tract is important due to treatment implications.

Case report: ALK-rearranged spindle and epithelioid cell neoplasms with S100 and CD34 co-expression: Additional evidence of kinase fusion-positive soft tissue tumors

ALK rearrangements have rarely been reported in S100- and CD34-co-expressing soft tissue neoplasms with lipofibromatosis-like neural tumor (LPFNT) pattern or stromal and perivascular hyalinization, mimicking NTRK-rearranged spindle cell tumors. Here, we reported ALK fusions involving related partner genes in two adult soft tissue tumors with S100 and CD34 co-expression, and conducted a literature review of mesenchymal tumors harboring ALK or other kinase fusions. Case 1 was a 25-year-old female who underwent excision of a soft tissue mass in the anterior thigh region. Morphologically, the tumor was composed of spindle cells adjacent to epithelioid cells embedded in myxedematous and hyalinized stroma, with infiltrative boundary. Spindle cells mixed with inflammatory infiltration resembling inflammatory myofibroblastic tumor (IMT) were seen sporadically. However, brisk mitosis and focal necrosis was also observed, indicating an intermediate-grade sarcoma. In case 2, the left side of the neck of a 34-year-old man was affected. The tumor was composed of monomorphic spindle cells arranged in fascicular growth or patternless pattern, with stromal and perivascular hyalinization. Sparse inflammatory cell infiltration was also observed. Both tumors showed CD34, S100, and ALK-D5F3 immunoreactivity. Next generation sequencing (NGS) test identified a PLEKHH2::ALK fusion in case 1, which was confirmed by RT-PCR and Sanger sequencing, whereas the RT-PCR (ARMS method) test detected an EML4::ALK fusion in case 2. In conclusion, this study expands the morphological and genetic landscape of tumors with S100 and CD34 co-expression harboring kinase fusions, and suggests that kinase fusion-positive mesenchymal neoplasms are becoming an enlarging entity with a variety of morphological patterns.

NTRK -Rearranged Uterine Sarcomas: Clinicopathologic Features of 15 Cases, Literature Review, and Risk Stratification

NTRK -rearranged uterine sarcomas are rare spindle cell neoplasms that typically arise in the uterine cervix of young women. Some tumors recur or metastasize, but features which predict behavior have not been identified to date. Distinguishing these tumors from morphologic mimics is significant because patients with advanced stage disease may be treated with TRK inhibitors. Herein, we present 15 cases of NTRK- rearranged uterine sarcomas, the largest series to date. Median patient age was 35 years (range: 16 to 61). The majority arose in the uterine cervix (n=14) and all but 2 were organ-confined at diagnosis. Tumors were composed of an infiltrative, fascicular proliferation of spindle cells and most showed mild-to-moderate cytologic atypia. All were pan-TRK positive by immunohistochemistry (13/13); S100 (11/13) and CD34 (6/10) were usually positive. RNA or DNA sequencing found NTRK1 (10/13) and NTRK3 (3/13) fusions with partners TPR , TPM3 , EML4 , TFG , SPECC1L , C16orf72 , and IRF2BP2 . Unusual morphology was seen in 2 tumors which were originally diagnosed as unclassifiable uterine sarcomas, 1 of which also harbored TP53 mutations. Follow up was available for 9 patients, of whom 3 died of disease. By incorporating outcome data of previously reported tumors, adverse prognostic features were identified, including a mitotic index ≥8 per 10 high-power fields, lymphovascular invasion, necrosis, and NTRK3 fusion. Patients with tumors which lacked any of these 4 features had an excellent prognosis. This study expands the morphologic spectrum of NTRK -rearranged uterine sarcomas and identifies features which can be used for risk stratification.

NTRK-rearranged spindle cell neoplasm of the lower extremity: radiologic-pathologic correlation

Neurotrophic tyrosine receptor kinase (NTRK)-rearranged spindle cell neoplasm is a recently characterized soft tissue tumor and has been classified as provisional by the World Health Organization. Detection of the genetic rearrangement is important because these tumors are amenable to targeted tyrosine kinase inhibitor therapy, which can play a key role in patients with unresectable or advanced disease. Although the spectrum of histopathology associated with this entity is broad, one notable feature is the infiltrative growth pattern, which is most reminiscent of lipofibromatosis-like neural tumor. Description of their diverse histologic attributes has aided recognition, but so far little attention has been paid to correlating the gross appearance and imaging features of these lesions. In this report, we describe the clinical, imaging, histopathological, and genetic features of a soft tissue NTRK-rearranged spindle cell neoplasm. Inclusion of this more recently identified entity into the imaging differential of tumors with intratumoral relatively hypovascular nodules and infiltrative margins is important because testing for NTRK rearrangement is not routinely performed.

New molecular entities of soft tissue and bone tumors

PURPOSE OF REVIEW

The advances of molecular techniques have led to the refinement of the classification of mesenchymal tumors, leading to newly introduced entities in the recently published fifth edition of the WHO Classification of Soft Tissue and Bone Tumors, which are discussed in this review.

RECENT FINDINGS

For the first time, entities are included of which the name refers to the underlying molecular alteration including round cell sarcoma with EWSR1 -non-ETS fusions, CIC -rearranged sarcoma, and sarcoma with BCOR genetic alteration. EWSR1-SMAD3 -positive fibroblastic tumor and NTRK -rearranged spindle cell neoplasm are provisionally included as 'emerging' entities based on the underlying molecular alteration, though the entity still needs to be better defined. Other newly recognized entities are not named after their molecular change, but the molecular alteration helped to delineate them from others: atypical spindle cell/pleomorphic lipomatous tumor, anastomosing hemangioma, angiofibroma of soft tissue, myxoid pleomorphic liposarcoma, and poorly differentiated chordoma.

SUMMARY

Classification of mesenchymal tumors is increasingly based on the underlying molecular changes, although this cannot be interpreted separately from clinical, morphological, and immunohistochemical characteristics.

Adult NTRK-rearranged spindle cell neoplasms of the viscera: with an emphasis on rare locations and heterologous elements

NTRK-rearranged mesenchymal neoplasms mostly affect the soft tissues of pediatric patients. Given the responsiveness to selective NTRK inhibitors, it remains critical to identify those ultra-rare cases occurring in the viscera of adults. In five females and two males aged 18-53 years, we characterized visceral mesenchymal tumors harboring TPM3-NTRK1 [uterine cervix (N = 2), pleura, prostate], LMNA-NTRK1 (lung), SQSTM1-NTRK3 (heart), and NTRK3 rearrangement with unknown fusion partner (colon/mesocolon) with RNA sequencing, FISH, RT-PCR, and immunohistochemistry. The tumors exhibited spindled to ovoid/epithelioid or pleomorphic cells, often arranged in fascicles, and were low-to-intermediate-grade and high-grade in three and four cases, respectively. Keloid-like stromal collagen and perivascular hyalinization was noted in five. Adenosarcoma-like appearances were observed in two, manifesting frond-like protrusions in one cervical tumor and phyllodes-like architecture in the prostatic tumor. Abrupt high-grade transformation into pleomorphic liposarcoma was found in another cervical tumor, while the pleural tumor contained intermixed rhabdomyoblasts. Pan-TRK immunostaining was positive in all cases. All cases expressed CD34, while five were S100-positive. CDKN2A homozygous deletion with concomitant p16 loss occurred in 4/7. Whole-exome sequencing identified TP53 mutation (c.672+2T>C, involving a splice site, with concomitant protein loss) in a cervical sarcoma, limited to its heterologous liposarcomatous component. At least moderate pan-TRK immunoreactivity was present in varying proportions of potential pathologic mimics, with BCOR-positive sarcoma (56%, 5/9), undifferentiated uterine sarcoma (50%, 3/6), and spindle cell/sclerosing rhabdomyosarcoma (33%, 2/6) being among the most frequent. This underscored the unsatisfactory specificity of pan-TRK immunohistochemistry and warranted molecular confirmation in the diagnosis of adult NTRK-rearranged visceral mesenchymal neoplasms. The current report highlights the ever-expanding clinicopathologic and genetic spectrum of this entity by describing the unprecedented cardiac and pleural locations and heterologous differentiation, as well as the second NTRK-rearranged "prostatic stromal sarcoma," while substantiating CDKN2A deletion as a frequent occurrence.

Tyrosine kinase altered spindle cell neoplasms with EGFR internal tandem duplications

In this study, we present two extra-renal pediatric spindle cell neoplasms with EGFR internal tandem duplications (ITD). Histologically, these tumors demonstrated the same histologic features seen in other tyrosine kinase altered spindle cell neoplasms, with one case showing abundant adipose tissue with cellular fibrous septae resembling lipofibromatosis and the other case showing fascicles of spindled cells resembling infantile fibrosarcoma. There was variable expression of CD34, S100 and SMA, and all cases were negative for panTRK. This case series adds to our molecular understanding of the spectrum of tyrosine kinase altered spindle cell neoplasms and represents the first reported examples of EGFR ITDs in extra-renal tumors. The presence of EGFR alterations in the absence of gene fusions represents a potential therapeutic target and necessitates a broader testing panel for this group of tumors. This article is protected by copyright. All rights reserved.

Pediatric Sarcomas: The Next Generation of Molecular Studies

Pediatric sarcomas constitute one of the largest groups of childhood cancers, following hematopoietic, neural, and renal lesions. Partly because of their diversity, they continue to offer challenges in diagnosis and treatment. In spite of the diagnostic, nosologic, and therapeutic gains made with genetic technology, newer means for investigation are needed. This article reviews emerging technology being used to study human neoplasia and how these methods might be applicable to pediatric sarcomas. Methods reviewed include single cell RNA sequencing (scRNAseq), spatial multi-omics, high-throughput functional genomics, and clustered regularly interspersed short palindromic sequence-Cas9 (CRISPR-Cas9) technology. In spite of these advances, the field continues to be challenged by a dearth of properly annotated materials, particularly from recurrences and metastases and pre- and post-treatment samples.

NTRK Fusions in a Sarcomas Series: Pathology, Molecular and Clinical Aspects

Targeting molecular alterations has been proven to be an inflecting point in tumor treatment. Especially in recent years, inhibitors that target the tyrosine receptor kinase show excellent response rates and durable effects in all kind of tumors that harbor fusions of one of the three neurotrophic tyrosine receptor kinase genes (NTRK1, NTRK2 and NTRK3). Today, the therapeutic options in most metastatic sarcomas are rather limited. Therefore, identifying which sarcoma types are more likely to harbor these targetable NTRK fusions is of paramount importance. At the moment, identification of these fusions is solely based on immunohistochemistry and confirmed by molecular techniques. However, a first attempt has been made to describe the histomorphology of NTRK-fusion positive sarcomas, in order to pinpoint which of these tumors are the best candidates for testing. In this study, we investigate the immunohistochemical expression of pan-TRK in 70 soft tissue and bone sarcomas. The pan-TRK positive cases were further investigated with molecular techniques for the presence of a NTRK fusion. Seven out of the 70 cases showed positivity for pan-TRK, whereas two of these seven cases presented an NTRK3 fusion. Further analysis of the fused sarcomas revealed some unique histological, molecular and clinical findings. The goal of this study is to expand the histomorphological spectrum of the NTRK-fused sarcomas, to identify their fusion partners and to correlate these parameters with the clinical outcome of the disease. In addition, we evaluated the immunohistochemical expression pattern of the pan-TRK and its correlation with the involved NTRK gene.

Expanding the Spectrum of Adult NTRK3-Rearranged Spindle Cell Neoplasms: A Recurrent NTRK3-SQSTM1 Fusion Spindle Cell Tumor With Deceptively Bland Morphology

OBJECTIVES

NTRK-rearranged spindle cell neoplasms (other than infantile fibrosarcoma) are an emerging entity of tumors that demonstrate wide variation in clinical and histopathologic features. We report a case of an NTRK-rearranged spindle cell neoplasm bearing a deceptively bland morphology.

METHODS

We performed histopathologic, immunohistochemical, and molecular evaluation on resection tissue. We also conducted a literature review on adult NTRK3-rearranged spindle cell neoplasms.

RESULTS

The tumor presented as a recurrent ankle mass in an elderly patient. Histologically, it was composed of bland spindle cells set in a fibrous to edematous stroma. Blood vessels were interspersed with subtle perivascular hyalinization and scattered lymphoid aggregates. Immunohistochemically, the spindle cells expressed CD34 and S100 while being negative for SOX10. The tumor also showed cytoplasmic reactivity for pan-tyrosine receptor kinase immunohistochemistry. Next-generation sequencing identified an NTRK3-SQSTM1 fusion. To the best of our knowledge, this fusion pair has not been previously reported in adult NTRK-rearranged mesenchymal tumors.

CONCLUSIONS

Altogether, this rare and diagnostically challenging case of an NTRK3-rearranged spindle cell tumor with low-grade morphology is in contrast to many of the reported adult NTRK3-rearranged mesenchymal tumors. Recognition of low-grade NTRK-rearranged tumors demands close attention to clues in morphology and immunoprofiles.

NTRK Fusions Detection in Paediatric Sarcomas to Expand the Morphological Spectrum and Clinical Relevance of Selected Entities

Undifferentiated round cell sarcomas (URCS) of soft tissue and bone and tumours of uncertain differentiation (TUD) are commonly ascribed to a subset of neoplasms with low frequency of NTRK gene fusions. However, more recently NTRK-rearranged round and spindle cell tumours have been noted in case reports and in limited or heterogeneous cohorts. The aim of our study was to investigate the presence of NTRK gene fusions in a large retrospective cohort of paediatric URCS and TUD after a systematic review of the diagnosis, according to the recently updated WHO classification scheme. One-hundred and five patients with diagnosis of URCS or TUD, involving the bone or soft tissue, were retrospectively evaluated. After the case selection and the histopathological review of the case cohort, pan-Trk immunohistochemistry (IHC) testing was performed on formalin-fixed paraffin-embedded (FFPE) tissues. Tumour RNA was extracted from FFPE tissue and subjected to next-generation sequencing (NGS) library preparation, using a 10-gene NGS fusion panel, sequenced on an Illumina MiSeq. The NGS-positive cases were further confirmed by real-time PCR. On immunohistochemical screening, 12/105 (11.4%) cases were positive using the pan-Trk antibody, showing three different staining patterns with the cytoplasmic distribution being most common. Molecular analysis using NGS and confirmed by the real-rime PCR detected two positive cases for the ETV6-NTRK3 fusion. The histological pattern of the two positive cases, together with the demonstration of the NTRK rearrangement, leaded to re-classify these previously not otherwise specified sarcomas with uncertain differentiation into the emerging category of NTRK-rearranged neoplasms. In addition, we found the two NTRK fused neoplasms showing a clinical indolent course, in contrast with literature.

Pediatric Non-Myofibroblastic Primitive Spindle Cell Tumors with ALK Gene Rearrangements and Response to Crizotinib

We describe two poorly differentiated, non-myofibroblastic (SMA-, S100+, CD34±), spindle cell neoplasms with immunohistochemical positivity for ALK and with ALK gene rearrangements leading to PLEKHH2::ALK and CLTC::ALK fusions, respectively. ALK protein overexpression and/or gene fusions should be evaluated in poorly differentiated spindle cell neoplasms, even when there is an absence of a myofibroblastic phenotype. A positive ALK evaluation has therapeutic implications as both tumors responded to single-agent treatment with the tyrosine kinase inhibitor crizotinib.

Case Report: Adult NTRK-Rearranged Spindle Cell Neoplasm: Early Tumor Shrinkage in a Case With Bone and Visceral Metastases Treated With Targeted Therapy

Background

NTRK (neurotrophic tyrosine receptor kinase)-rearranged spindle cell neoplasms are a new group of tumors included in the new 5^th edition of the World Health Organization (WHO) classification of soft Tissue and Bone Sarcomas. These tumors are characterized by NTRK gene fusions and show a wide spectrum of histologies and clinical behavior. Several targeted therapies have recently been approved for tumors harboring NTRK fusions, including STS.

Case Presentation

A 26-year-old male with advanced, pretreated NTRK rearranged spindle cell neoplasm and liver, lung and bone metastases was treated with larotrectinib on a continuous 28-day schedule, at a dose of 100 mg twice daily. An ¹⁸FDG-PET/CT scan performed after 7 days of treatment showed tumor shrinkage in both visceral and bone lesions. There was no drug-related toxicity. Subsequent evaluations confirmed continued tumor regression in disease sites. The patient is well and continues treatment.

Conclusion

The clinical and radiological response of our patient with an uncommon TPM4 (exon 7)-NTRK1 (exon 12) gene fusion tumor treated with a first-generation TRK inhibitor could contribute to a better understanding of the biology of this new STS entity and help to improve patient management.

Recent Advances on Immunohistochemistry and Molecular Biology for the Diagnosis of Adnexal Sweat Gland Tumors

Simple Summary

Cutaneous sweat gland tumors form an extremely diverse and heterogeneous group of neoplasms that show histological differentiation to the sweat apparatus. Due to their rarity, wide diagnostic range, and significant morphological overlap between entities, their accurate diagnosis remains challenging for pathologists. Until recently, little was known about the molecular pathogenesis of adnexal tumors. Recent findings have revealed a wide range of gene fusions and other oncogenic factors that can be used for diagnostic purposes and, for some, can be detected by immunohistochemistry. Among other organs containing exocrine glands, such as salivary glands, breasts, and bronchi, most of these biomarkers have been reported in homologous neoplasms that share morphological features with their cutaneous counterparts. This review aims to describe these recent molecular and immunohistochemical biomarkers in the field of sweat gland tumors.

Abstract

Cutaneous sweat gland tumors are a subset of adnexal neoplasms that derive or differentiate into the sweat apparatus. Their great diversity, rarity, and complex terminology make their pathological diagnosis challenging. Recent findings have revealed a wide spectrum of oncogenic drivers, several of which are of diagnostic interest for pathologists. Most of these molecular alterations are represented by gene fusions, which are shared with other homologous neoplasms occurring in organs containing exocrine glands, such as salivary and breast glands, which show similarities to the sweat apparatus. This review aims to provide a synthesis of the most recent immunohistochemical and molecular markers used for the diagnosis of sweat gland tumors and to highlight their relationship with similar tumors in other organs. It will cover adenoid cystic carcinoma (NFIB, MYB, and MYBL1 fusion), cutaneous mixed tumor (PLAG1 fusion), cylindroma and spiradenoma and their carcinomas thereof (NF-κB activation through CYLD inactivation or ALKP1 hotspot mutation), hidradenoma and hidradenocarcinoma (MAML2 fusion), myoepithelioma (EWSR1 and FUS fusion), poroma and porocarcinoma (YAP1, MAML2, and NUTM1 fusion), secretory carcinoma (ETV6, NTRK3 fusion), tubular adenoma and syringo-cystadenoma papilliferum (HRAS and BRAF activating mutations). Sweat gland tumors for which there are no known molecular abnormalities will also be briefly discussed, as well as potential future developments.