A case of FN1-fused calcified chondroid mesenchymal neoplasm of the hand with novel FGFR3 partner gene

Primary Cutaneous Spindle Cell Sarcoma With FN1::FGFR1 Fusion

We report a case of a primary cutaneous spindle cell sarcoma (SCS) with FN1::FGFR1 fusion. The tumor lacked the typical histologic and immunohistochemical features associated with other FN1-rearranged neoplasms, such as phosphaturic mesenchymal tumors (PMT) and calcified chondroid mesenchymal neoplasms (CCMN). Unlike PMTs, which often feature a cartilaginous matrix and are associated with tumor-induced osteomalacia (TIO), the present case lacked these characteristics and did not show FGF23 mRNA expression. Immunohistochemically, the tumor cells showed patchy staining for CD34 but were negative for markers such as ERG, desmin, S100, and pan-TRK. The fusion event in this case involves the loss of the FGFR1 Ig1 (D1) domain, a mechanism proposed to drive oncogenesis by releasing FGFR1 from autoinhibition. Despite the preservation of other FGFR1 domains, no evidence of FGF23 signaling was detected, and the patient had no clinical history of TIO. This case underscores the complexity of oncogenesis in FN1::FGFR1-rearranged neoplasms, a form of "promiscuous" gene fusion, where similar fusions lead to diverse tumor phenotypes. It emphasizes the importance of incorporating molecular testing in diagnosing spindle cell sarcomas, particularly those occurring in acral sites, to identify this underrecognized entity.

Tenosynovitis with psammomatous calcifications: Series of 18 cases and review of the literature emphasizing a common source of expert consultation and updated differential diagnosis

Tenosynovitis with psammomatous calcifications (TPC) is a rare, benign condition currently regarded as a pseudotumor possibly related to repetitive use and/or trauma with a predilection for females at acral sites. Thirty-five cases have been reported, with the largest series comprising 23 patients; yet, TPC remains poorly recognized by pathologists and clinicians alike. We report a series of eighteen additional cases along with radiology and clinical follow-up. Our cohort demonstrated a strong female sex predilection (14 females and 4 males), with ages ranging from 12 to 71 years (mean 50 years) and involved the hand/finger (10), toes/foot (5), wrist (2), or elbow (1). More than half (56%) were diagnosed in the expert consultation setting in which contributor suggested diagnoses (8) included: gout/pseudogout (n = 2), chondrosarcoma (N = 1), soft tissue chondroma (N = 1), calcified chondroid mesenchymal neoplasm (N = 1), calcifying aponeurotic fibroma (N = 1), giant cell tumor (N = 1), or "rule out malignancy" (n = 1). The majority of patients presented with painful masses and radiology showed indolent/benign features chiefly within the tendino-ligamentous tissues some with non-specific faint internal matrix/popcorn calcification pattern. None had known metabolic abnormalities and three (of 11) had a history of prior trauma/repetitive activity. The masses were generally small (mean 1.3 cm; range 0.4-2.4 cm) and composed of histiocytoid cells with variable amounts of grungy psammomatous round calcific debris located within tenosynovium. Occasional giant cells and admixed bland (myo)fibroblastic spindle cells were seen. Clinical follow-up (12 patients; mean 30 mos; range 2-61 mos) showed no local recurrences. Herein, we report a large series of well-characterized TPC, review the literature, and offer an updated differential diagnosis of this distinctive, rare, and under-recognized entity cured by simple excision. Increased awareness of TPC should allow confident distinction from morphologic mimics and avoidance of overtreatment.

Nosologic Reappraisal of the Recently Proposed Calcified Chondroid Mesenchymal Neoplasm Concept in a Series of 20 Cases

"Calcified chondroid mesenchymal neoplasm (CCMN)" is a recently proposed term for tumors with hypercellular chondroid histology and fusion genes. However, its impact on the present classification framework has not been extensively investigated. In this study, we analyzed 20 tumors with histology that would fit with that reported as "CCMN." With the combined use of RNA sequencing and fluorescence in situ hybridization, 15 tumors were found to have gene fusions, including FN1::FGFR2 (n = 4), FN1::TEK (n = 1), FN1::MERTK (n = 1), PDGFRA::USP8 (n = 2), FN1 rearrangements with undetermined non-FGFR2 partners (n = 4), and PDGFRA rearrangements with undetermined partners (n = 3). FN1 or PDGFRA rearrangement signals were restricted to epithelioid/polygonal cells, indicating that these were neoplastic elements mixed with abundant reactive cells. The fusion-positive tumors were found in 7 men and 8 women aged 29 to 82 years (median, 57 years), and most involved the temporomandibular joints or digits. Tumors showed a chondroid matrix with grungy calcification and cellular components with epithelioid/polygonal cells and were divided into 2 groups the histology of which corresponded to chondroid tenosynovial giant cell tumor or soft-tissue chondroma. Three fusion-positive tumors harbored calcium pyrophosphate dihydrate crystals. Nuclear atypia was frequent. Gene fusions were not detected in the remaining 5 tumors with abundant crystal/calcium deposition, and some may represent reactive conditions. All 20 tumors followed an indolent clinical course, and 4 patients with temporomandibular joint tumors were successfully followed up without surgery for 9 to 60 months. Using "CCMN" as an entity would entail mixed repercussions in diagnostic practice, and how and whether it should be used in diagnosis requires deliberate discussion. The concept may bring some benefits, but it may arguably cause more confusion because it would substantially restructure the conventional framework by drawing a boundary within the chondroma and conflating the chondroma with chondroid tenosynovial giant cell tumor.

Emerging fusion-associated mesenchymal tumours: a tabular guide and appraisal of five 'novel' entities

AIMS

The field of molecular pathology has undergone significant advancements in the clinical impact of sarcoma diagnosis, resulting in challenges to nosology of bone and soft tissue tumours. The surge in molecular data has led to the identification of novel fusions and description of new 'entities'. To illustrate this, we have selected five emerging entities with novel fusions: clear cell stromal tumour of the lung with YAP1::TFE3 fusion, GAB1::ABL1 fusion spindle cell neoplasm, NUTM1-rearranged sarcomas, NR1D1-rearranged sarcomas and calcified chondroid mesenchymal neoplasms.

METHODS

Literature for the relevant case reports and case series of these five entities were reviewed and clinicopathological data was collected. Additionally, this review includes a table format of recently described fusion-associated mesenchymal neoplasms.

RESULTS

The morphological and immunohistochemical features, along with diagnostic challenges, are discussed for each entity.

CONCLUSIONS

Here, we have provided a review of selected emerging mesenchymal neoplasms, which of these neoplasms will meet the threshold to be 'new entities' remains to be determined.

Chronic hearing loss turns out being a calcified chondroid mesenchymal neoplasm with FN1::FGFR2 fusion

A 53 year old female presented with a six-year history of right-sided slow deterioration in hearing and a feeling of pressure in the right ear. The patient had not experienced any pain but reported some paresthesia of the right half of the tongue, whereas no further other cranial nerve deficits were evident. The otoscopy was unremarkable as well as the rest of the clinical ENT examination except for a slight asymptomatic swelling of the right cheek. Imaging findings showed an expansive tumor infiltrating and destroying the right lateral skull base. The tumor was partially composed of cystic/regressive lesions with high contrast media uptake. The tumor had high-signal intensity with water-sensitive sequences (T2w) and was hypointense on T1w images. We performed a tumor resection via a transparotideal-infratemporal approach. Histologically, the tumor was composed of granular variably calcified chondroid matrix with extensive regressive changes and granulation-like tissue reaction associated with calcinosis and crystal deposition. Molecular analysis of the tumor via the TruSight- RNA-Fusion panel detected a fusion involving FN1::FGFR2, consistent with "calcified chondroid mesenchymal neoplasm" (CCMN), a rare tumor entity recently defined by Liu et al 2021. In regular follow-up care no residual tumor has been detected in imaging studies (MRI and CT) within 2 years and 4 months. The biology and consequently the radio sensitivity cannot be defined precisely since long term results are missing due to the first description of this entity in 2021. As a consequence, surgical resection is recommended as the treatment of choice. Thorough clinical and radiological follow-up is mandatory as local recurrences are to be expected due to the infiltrative behavior. In case of a loco regional recurrence the fusion with FGFR2 may represent a therapeutic option for a targeted therapy on molecular level.

Calcified chondroid mesenchymal neoplasm: a clinicopathological and molecular analysis

AIMS

Calcified chondroid mesenchymal neoplasm (CCMN) is a recently identified category of soft tissue neoplasms defined by cartilage or cartilaginous matrix formation and FN1 gene fusions. Its rarity and similarities to other soft tissue tumours pose diagnostic challenges. This study aims to deepen understanding of CCMN, highlighting molecular pathology's role in diagnosis to reduce misdiagnosis, overdiagnosis and overtreatment.

METHODS

We conducted a clinicopathological analysis of five newly identified CCMN cases and reviewed 87 cases documented in PubMed. Next-generation sequencing was used to detect molecular alterations, while clinical, radiological and histopathological features were extensively reviewed.

RESULTS

CCMN typically affects adults, presenting as a slow-growing, painless mass in soft tissue. Histologically, CCMN exhibits a chondroid matrix with variable calcification. Molecular analyses in our cases identified FN1::FGFR1, FN1::FGFR2 and FN1::TEK fusions. Review of the 87 cases revealed consistent clinical, imaging and molecular profiles, underscoring CCMN's distinct characteristics.

CONCLUSIONS

CCMN should be considered in the differential diagnosis of soft tissue tumours with chondroid and calcified components. Detecting FN1 gene fusions aids in distinguishing CCMN from morphologically similar tumours.

Acral Fibrochondromyxoid Tumor: A Clinicopathologic and Molecular Genetic Study of 37 Cases

Acral fibrochondromyxoid tumor (AFCMT) is a recently described likely benign mesenchymal neoplasm arising in the distal extremities with distinctive histologic features and a recurrent THBS1::ADGRF5 fusion. We studied an additional 37 cases of AFCMT and expanded on the so-far reported clinicopathologic and molecular findings. Tumors occurred in 21 females and 16 males, ranging in age from 17 to 78 years (median age: 47), and solely involved the hands (24/37, 65%) or feet (13/37, 35%). Histologic examination revealed well-delineated uni- or multinodular tumors with prominent vasculature-rich septa and bland, chondrocyte-like tumor cells set within abundant chondromyxoid stroma. Immunohistochemical studies showed that tumor cells were positive for CD34 (25/27; 93%) and ERG (27/27; 100%), whereas negative for S100 protein (0/31). Molecular analysis revealed evidence of a THBS1::ADGRF5 fusion in 17 of 19 (89%) successfully tested tumors. Clinical follow-up was available in 8 cases (median: 97 months), with multiple local recurrences in 1 case at 276, 312, and 360 months. We conclude that AFCMT is a distinct entity with reproducible morphologic, immunohistochemical, and molecular genetic features that should be differentiated from other similar appearing acral mesenchymal neoplasms.

Chondroid Synoviocytic Neoplasm: A Clinicopathologic, Immunohistochemical, and Molecular Genetic Study of a Distinctive Tumor of Synoviocytes

Tumors resembling tenosynovial giant cell tumor (TGCT) but additionally forming chondroid matrix are rare and most often involve the temporomandibular joint (TMJ). We studied 21 tumors consisting of synoviocytes (large, eosinophilic mononuclear cells containing hemosiderin) and chondroid matrix to better understand these unusual neoplasms. The tumors occurred in 10 males and 11 females, in the age group of 31 to 80 years (median, 50 years) and involved the TMJ region (16), extremities (4), and spine (1). As in conventional TGCT, all were composed of synoviocytes, small histiocytes, foamy macrophages, siderophages, and osteoclast-like giant cells in variably hyalinized background. Expansile nodules of large, moderately atypical synoviocytes were present, in addition to "chondroblastoma-like," "chondroma-like," or "phosphaturic mesenchymal tumor-like" calcified matrix. The synoviocytes expressed clusterin (17/19) and less often desmin (3/15). The tumors were frequently CSF1 positive by chromogenic in situ hybridization (8/13) but at best weakly positive for CSF1 by immunohistochemistry (0/3). Background small histiocytes were CD163 positive (12/12). All were FGF23 negative (0/10). Cells within lacunae showed a synoviocytic phenotype (clusterin positive; S100 protein and ERG negative). RNA-Seq was successful in 13 cases; fusions were present in 7 tumors, including FN1::TEK (5 cases); FN1::PRG4 (2 cases); and MALAT1::FN1, PDGFRA::USP35, and TIMP3::ZCCHC7 (1 case each). Three tumors contained more than 1 fusion (FN1::PRG4 with TIMP3::ZCCHC7, FN1::TEK with FN1::PRG4, and FN1::TEK with MALAT1::FN1). Clinical follow-up (17 patients; median follow-up duration 38 months; range 4-173 months) showed 13 (76%) to be alive without evidence of disease and 4 (24%) to be alive with persistent/recurrent local disease. No metastases or deaths from disease were observed. We conclude that these unusual tumors represent a distinct category of synoviocytic neoplasia, which we term "chondroid synoviocytic neoplasm," rather than simply ordinary TGCT with cartilage. Despite potentially worrisome morphologic features, they appear to behave in at most a locally aggressive fashion.

Comprehensive Molecular Characterization of a Large Series of Calcified Chondroid Mesenchymal Neoplasms Widening Their Morphologic Spectrum

Recently, FN1 fusions to receptor tyrosine kinase genes have been identified in soft tissue tumors with calcified chondroid matrix named calcifying chondroid mesenchymal neoplasms (CCMNs). We collected 33 cases of CCMN from the French network for soft tissue and bone tumors. We performed whole-exome RNA sequencing, expression analysis, and genome-wide DNA methylation profiling in 33, 30, and 20 cases of CCMN compared with a control group of tumors, including noncalcified tenosynovial giant cell tumor (TGCT). Among them, 15 cases showed morphologic overlap with soft tissue chondroma, 8 cases with tophaceous pseudogout, and 10 cases with chondroid TGCT. RNA-sequencing revealed a fusion of FN1 in 76% of cases (25/33) with different 5' partners, including most frequently FGFR2 (14 cases), TEK or FGFR1. Among CCMN associated with FGFR1 fusions, 2 cases had overexpression of FGF23 without tumor-induced osteomalacia. Four CCMN had PDGFRA::USP8 fusions; 3 of which had histologic features of TGCT and were located in the hip, foot, and temporomandibular joint (TMJ). All cases with FN1::TEK fusion were located at TMJ and had histologic features of TGCT with or without chondroid matrix. They formed a distinct cluster on unsupervised clustering analyses based on whole transcriptome and genome-wide methylome data. Our study confirms the high prevalence of FN1 fusions in CCMN. In addition, through transcriptome and methylome analyses, we have identified a novel subgroup of tumors located at the TMJ, exhibiting TGCT-like features and FN1::TEK fusions.

Calcified chondroid mesenchymal neoplasm: report of a case involving the temporomandibular joint region and review of the literature

The calcified chondroid mesenchymal neoplasm (CCMN) represents a recently recognized tumor type with only 50 well-documented cases in the English-language literature. Herein we report an additional case of CCMN presenting as a large mass in the temporomandibular joint region of a 41-year-old female. A review of previously reported cases and the current case of CCMN shows the following features: 1) average age 52 years (range 14-87 years) and an approximately even sex distribution; 2) most frequently involved sites: distal extremities (including foot, hand, wrist, forearm) (n=41) and temporomandibular joint/temporal/parotid region (n=9); 3) multilobular soft tissue tumor with chondroid to cartilaginous matrix, often grungy or lace-like calcifications, and variable cytologic atypia; 4) frequently detected FN1 rearrangement (n=15), including FN1 fusion with FGFR2 (n=7) or other receptor tyrosine kinases; 5) 2 reported local recurrences (after incomplete excision); 6) no reports of malignant biologic behavior.

Superficial acral calcified chondroid mesenchymal neoplasm harboring an FN1::FGFR2 fusion and review of the literature

Calcified chondroid mesenchymal neoplasm is a recently recognized bone and soft tissue entity primarily found in the extremities and the temporomandibular joint. This neoplasm is typically driven by the fusion of the FN1 gene with a kinase. In this case report, we provide a detailed account of a rare superficial calcified chondroid mesenchymal neoplasm located on the left big toe, characterized by an FN1::FGFR2 fusion. The tumor exhibited a peripheral collarette and consisted of large intradermal histiocytoid to epithelioid cells with no mitotic activity. These cells displayed fine chromatin and abundant pale eosinophilic cytoplasm, forming a swirling syncytium. They were interspersed with localized areas of glassy chondromyxoid matrix containing randomly mineralized calcific material and isolated osteoclast-like giant cells. RNA sequencing confirmed the presence of an FN1 (exon 29)::FGFR2 (exon 7) gene fusion. Our report emphasizes the importance for dermatopathologists to consider this entity when evaluating superficial lesions displaying mesenchymal, chondroid, and calcified attributes.

FGFR-targeted therapeutics: clinical activity, mechanisms of resistance and new directions

Fibroblast growth factor (FGF) signalling via FGF receptors (FGFR1-4) orchestrates fetal development and contributes to tissue and whole-body homeostasis, but can also promote tumorigenesis. Various agents, including pan-FGFR inhibitors (erdafitinib and futibatinib), FGFR1/2/3 inhibitors (infigratinib and pemigatinib), as well as a range of more-specific agents, have been developed and several have entered clinical use. Erdafitinib is approved for patients with urothelial carcinoma harbouring FGFR2/3 alterations, and futibatinib and pemigatinib are approved for patients with cholangiocarcinoma harbouring FGFR2 fusions and/or rearrangements. Clinical benefit from these agents is in part limited by hyperphosphataemia owing to off-target inhibition of FGFR1 as well as the emergence of resistance mutations in FGFR genes, activation of bypass signalling pathways, concurrent TP53 alterations and possibly epithelial-mesenchymal transition-related isoform switching. The next generation of small-molecule inhibitors, such as lirafugratinib and LOXO-435, and the FGFR2-specific antibody bemarituzumab are expected to have a reduced risk of hyperphosphataemia and the ability to overcome certain resistance mutations. In this Review, we describe the development and current clinical role of FGFR inhibitors and provide perspective on future research directions including expansion of the therapeutic indications for use of FGFR inhibitors, combination of these agents with immune-checkpoint inhibitors and the application of novel technologies, such as artificial intelligence.

FGFR1 gene fusions in a subset of pediatric mesenchymal tumors: Expanding the genetic spectrum of tumors sharing histologic overlap with infantile fibrosarcoma and "NTRK-rearranged" spindle cell neoplasms

As the classification of kinase-driven spindle cell tumors continues to evolve, we describe the first series of pediatric mesenchymal tumors harboring FGFR1 gene fusions that share histologic overlap with infantile fibrosarcoma and "NTRK-rearranged" spindle cell neoplasms. Herein, we present three cases of FGFR1-rearranged pediatric mesenchymal tumors, including one case with FGFR1::PARD6B gene fusion and two cases with FGFR1::EBF2 gene fusion. The tumors involved infants ranging from 3 to 9 months in age with a male-to-female ratio of 2:1. All tumors involved the deep soft tissue of the gluteal, pelvic, or perirectal region. Histologically, the tumors comprised a cellular spindle cell neoplasm with primitive stellate cells, focal myxoid stroma, focal epithelioid features, no necrosis, and occasional mitotic figures (2-6 per 10 high-power field). By immunohistochemistry, the neoplastic cells focally expressed CD34 but lacked expression of S100 protein, SMA, desmin, myogenin, MyoD1, pan-TRK, and ALK. These three cases, including a case with long-term clinical follow-up, demonstrate that FGFR1 fusions occur in a subset of newly described pediatric kinase-driven mesenchymal tumors with locally aggressive behavior. Importantly, knowledge of these genetic alterations in this spectrum of pediatric tumors is key for diagnostic and targeted therapeutic purposes.

Decoding Oncofusions: Unveiling Mechanisms, Clinical Impact, and Prospects for Personalized Cancer Therapies

Cancer-associated gene fusions, also known as oncofusions, have emerged as influential drivers of oncogenesis across a diverse range of cancer types. These genetic events occur via chromosomal translocations, deletions, and inversions, leading to the fusion of previously separate genes. Due to the drastic nature of these mutations, they often result in profound alterations of cellular behavior. The identification of oncofusions has revolutionized cancer research, with advancements in sequencing technologies facilitating the discovery of novel fusion events at an accelerated pace. Oncofusions exert their effects through the manipulation of critical cellular signaling pathways that regulate processes such as proliferation, differentiation, and survival. Extensive investigations have been conducted to understand the roles of oncofusions in solid tumors, leukemias, and lymphomas. Large-scale initiatives, including the Cancer Genome Atlas, have played a pivotal role in unraveling the landscape of oncofusions by characterizing a vast number of cancer samples across different tumor types. While validating the functional relevance of oncofusions remains a challenge, even non-driver mutations can hold significance in cancer treatment. Oncofusions have demonstrated potential value in the context of immunotherapy through the production of neoantigens. Their clinical importance has been observed in both treatment and diagnostic settings, with specific fusion events serving as therapeutic targets or diagnostic markers. However, despite the progress made, there is still considerable untapped potential within the field of oncofusions. Further research and validation efforts are necessary to understand their effects on a functional basis and to exploit the new targeted treatment avenues offered by oncofusions. Through further functional and clinical studies, oncofusions will enable the advancement of precision medicine and the drive towards more effective and specific treatments for cancer patients.