Advanced and widespread molecular techniques have deepened our understanding of mesenchymal lesions, revealing considerable overlap among morphologically defined entities now known to be related to protein kinases (PKs). This paradigm shift is important for understanding oncogenesis and also in terms of treatment options and prognosis. Therefore, it is preferable to stratify these tumors molecularly instead of morphologically, as the different categories have clinical implications. Molecular analyses are an essential and integrated part of the diagnostic workup of tissue specimens, especially those of young patients. Involved PKs range from receptor tyrosine kinases (neurotrophic tyrosine receptor kinase [NTRK]1, 2, 3; anaplastic lymphoma kinase [ALK]; proto-oncogene 1 [ROS1]; proto-oncogene [RET]; and proto-oncogene/hepatocyte growth factor receptor [MET]; etc.) to intracytoplasmic serine/threonine kinases (RAF proteins) activating the same pathways. Morphological patterns vary from infantile fibrosarcoma(-like) to lipofibromatosis(-like), dermatofibrosarcoma protuberans(-like), and malignant peripheral nerve sheath tumor-like. However, there is considerable overlap histopathologically and immunohistochemically. Most of the neoplasms are (myo)fibroblastic in type, consisting of monomorphic cells. Ahemangiopericytoma-like vasculature can be adiagnostic clue. The immunophenotype is characterized by variable expression of smooth muscle actin (SMA)/desmin/CD34 or CD34/S100. This review provides updates to understand the currently known spectrum of PK-related lesions, with emphasis on those occurring more rarely, to aid proper diagnoses and treatment. The aim is to contribute to abetter holistic classification.

Abstract Background: NTRK gene fusions are oncogenic drivers across various pediatric and adult tumor types. The prevalence of NTRK gene fusions varies widely, from high (up to 90%) in rare tumors such as infantile fibrosarcoma and secretory carcinoma of the breast to low (<0.5%) in common cancers like non-small cell lung and colorectal carcinoma. Larotrectinib is the first-in-class, highly selective, central nervous system (CNS)-active TRK inhibitor approved for tumor-agnostic use in patients with TRK fusion cancer based on a robust and durable objective response rate in both adult and pediatric patients with various tumor types. Here, we report updated data on larotrectinib-treated pediatric patients with TRK fusion non-primary CNS tumors. Methods: This analysis included patients from 2 clinical trials (NCT02637687 [SCOUT], NCT02576431 [NAVIGATE]). Responses were independent review committee-assessed (Response Evaluation Criteria in Solid Tumors [RECIST] v1.1). In SCOUT, patients could stop larotrectinib in the absence of on-treatment progression (“wait-and-see”). Responses in patients who were re-treated due to progression were assessed by investigators (RECIST v1.1). Results: Ninety-nine patients with non-primary CNS tumors were eligible for analysis as of July 2024, including 49% with infantile fibrosarcoma, 41% with soft tissue sarcoma, and 9% with other solid tumors. Overall response rate was 86% (95% confidence interval [CI] 77–92). In total, 53 patients had complete responses (CR; including 17 pathological CR), 32 had partial responses (PR), 9 had stable disease (SD), and 3 had progressive disease (PD); responses were undefined in 2 patients. Median time to response was 1.8 months (range 0.9–7.3). Median duration of response was 51 months (95% CI 31-not estimable [NE]). Median progression-free survival and overall survival (OS) were 49 months (95% CI 32–NE) and not reached (NR), respectively. The 5-year OS rate was 87% (95% CI 80–95). Median time to investigator-assessed treatment failure (from larotrectinib initiation to earliest documented on-treatment disease progression, start of other anticancer treatment, or death) was NR. Of 54 patients who entered a first “wait-and-see” period (median duration 33 months [range 1–72]), 18 resumed treatment due to PD. Of these, 11 had a response (6 CR and 5 PR [including 2 pending confirmation]), 5 had SD, 1 was not evaluable, and 1 was undefined. Most treatment-related adverse events (TRAEs) were Grade 1/2. Three patients (3%) discontinued due to a TRAE. Conclusions: Larotrectinib demonstrated rapid and durable responses, extended survival, and favorable safety in pediatric patients with TRK fusion cancer. This supports the wider adoption of next-generation sequencing panels that include NTRK gene fusions to identify pediatric patients who may benefit from targeted treatment. Citation Format: Leo Mascarenhas, Theodore W. Laetsch, Birgit Geoerger, Steven G. DuBois, Miranda P. Dierselhuis, Catherine M. Albert, Claudia Blattmann, Helen Toledano, Noah Federman, Ramamoorthy Nagasubramanian, Alberto Pappo, Tanya Watt, Domnita-Ileana Burcoveanu, Esther De La Cuesta, Natascha Neu, Daniel H. Orbach, Yizhuo Zhang. Larotrectinib long-term efficacy and safety in pediatric patients with TRK fusion non-primary CNS tumors: Analysis update [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Fusion-Positive Cancer: From Discovery to Therapy; 2026 Jan 13-15; Philadelphia PA. Philadelphia (PA): AACR; Cancer Res 2026;86(1_Suppl):Abstract nr A029.

A 3-month-old female infant was admitted for incessant crying for 3 days. Examination revealed a rapidly growing massive retroperitoneal mass that was difficult to resect. Needle biopsy confirmed infantile fibrosarcoma. Initial chemotherapy with the VAC regimen (vincristine, actinomycin D, and cyclophosphamide) was administered, but the response was poor. The common fusion in infantile fibrosarcoma is ETV6-NTRK3, and next-generation sequencing detected an RBPMS-NTRK3 gene fusion in this patient. To our knowledge, this is the first reported case of infantile fibrosarcoma with RBPMS-NTRK3 fusion in China. Treatment with larotrectinib resulted in marked tumor shrinkage.

Infantile fibrosarcoma (IF) is a rare soft-tissue tumor found in children and consisting of malignant fibroblasts. The incidence of IF in children during the first year of life is low compared with that of other types of malignant tumors and is estimated to be five cases per million infants. Most IF arise in the limbs and trunk, and only a small number of these tumors occur in the head and neck area. Objective: to demonstrate the opportunities of modern surgical oncology for malignant tumors of the hand in children of the first year of life. Case report. When the child M. was 2.5 months old, her parents found a lump at the base of the palmar surface of the right hand. After consultations with a pediatric surgeon and an orthopedic specialist at the place of residence, the patient was diagnosed with hygroma. An ultrasound examination of the soft tissues of the right hand performed on 11 August 2022 confirmed the diagnosis of hygroma (4.1 × 2.4 mm in size). A watch-and-wait strategy was recommended. After 2 months, the tumor increased 5-fold in size. At the end of October 2022, after magnetic resonance imaging, a soft-tissue tumor was suspected. Contrast-enhanced magnetic resonance imaging revealed a solid tumor, possibly malignant. The child was admitted to the Pediatric Oncology Department of the regional oncology hospital, where a biopsy of the neoplasm was performed. IF was diagnosed. Due to a high risk of hand amputation, the child was transferred to the V.F. Voyno-Yasenetsky Scientific and Practical Center of Specialized Healthcare for children of the Department of Health of Moscow for surgical treatment. When the child was admitted to the Department of Oncology, the tumor size was 4.5 × 4.0 × 3.0 cm. On 29 November 2022, an organ-preserving surgery was performed: removal of the soft-tissue tumor on the palmar surface of the right hand with grafting using local tissues. Histological and immunohistochemical conclusion: IF. After the surgery, the patient received 4 courses of adjuvant polychemotherapy. She tolerated the treatment satisfactorily. Two years after the end of specific therapy, the child remains healthy with no signs of disease relapse, and is under regular follow-up by a pediatrician and pediatric oncologist. The function of the operated hand has been fully restored. Conclusion. IF is a rare disease, registered mainly in children during the first year of life. In many cases, IF in children may mimic inflammatory processes and benign soft-tissue tumors. The primary treatment approach for IF is organ-preserving radical surgery. The results of treatment depend on the time of diagnosis, the extent of the tumor involvement and the radicality of the surgery performed.

Infantile fibrosarcoma is a rare fibroblastic tumor of intermediate malignancy predominantly seen in infants. We describe FDG PET/MRI and PET/CT findings in a pediatric patient with multiple lung metastases from infantile fibrosarcoma harboring LMNA-NTRK1 gene fusion 8 years after resection of the primary tumor in the left hand. The lung metastases appeared as multiple nodules with irregular or smooth margins in both lungs and showed variable FDG uptake, ranging from low-grade to high-grade activity, which may reflect different biologic behavior of these tumors. The lung tumors disappeared or decreased significantly in size after tropomyosin receptor kinase inhibitor therapy.

Background: Congenital infantile fibrosarcoma is a rare soft-tissue tumor in the pediatric age group and seldom involves the gastrointestinal tract. Methods: This study is a case report study done in the Raparin Pediatric Hospital in Erbil on 9 days old boy with signs of intestinal obstruction. After explorative laparotomy a solid mass involving duodenojejunal junction was found, resection of the mass was done with end-to-end anastomosis and approval was taken from patient’s parents. Results: Histopathologic examination showed extensive replacement and effacement of the small bowel architecture by a cellular neoplasm. The tumor was extended through muscularis propria to the serosa with histological features consistent with congenital infantile fibrosarcoma, the diagnosis was confirmed by immunohistochemistry and genetic analysis which demonstrated the t(12;15) translocation. The baby was discharged on the 5th hospital day and followed up for 2 years without recurrence. Conclusion: Here we reported a rare case of congenital infantile fibrosarcoma arising from the small bowel in a new born baby, operated on, totally resected and histopathology proved the diagnosis and the patient was disease free after 2 years follow up.

Definitive diagnosis of infantile fibrosarcoma (IFS) and related kinase-altered spindle cell neoplasms (SCN) requires immunohistochemistry (IHC) or molecular assays that are variable in performance or not widely available. Recent transcriptomic studies identified differential gene expression in IFS with ETV6::NTRK3 fusions, prompting investigation of SOX11 expression pattern by IHC in IFS/IFS-like SCN and morphologic mimics. SOX11 staining (MRQ-58) was performed on 201 spindle cell tumors, including 28 IFS and congenital mesoblastic nephroma (CMN) with ETV6::NTRK3 fusion, 44 IFS/IFS-like SCN with alternative kinase fusions, 15 inflammatory myofibroblastic tumor, 4 dermatofibrosarcoma protuberans, 6 myofibroma, and 104 mimics comprising 38 malignant peripheral nerve sheath tumor (MPNST), 24 synovial sarcoma, 19 desmoid fibromatosis, 7 neurofibroma, 4 schwannoma, 4 fibrous hamartoma of infancy, 4 spindle cell/sclerosing rhabdomyosarcoma (SRMS), and 4 clear cell sarcoma of kidney (CCSK). SOX11 was positive in 18/28 (64%) ETV6::NTRK3-rearranged IFS/CMN (21% with diffuse strong nuclear expression and 43% with multifocal moderate to strong positivity) and 3/69 (4%) other kinase-altered tumors, including one EGFR-altered tumor and two cellular myofibromas. SOX11 was positive in 5/24 (21%) synovial sarcoma, 6/38 (16%) MPNST, 2/4 (50%) SRMS, and all 4 CCSK. Overall, SOX11 was 64% sensitive for ETV6::NTRK3-fused IFS/CMN and 96% specific compared to kinase-altered tumors, with 88% specificity compared to all mimics. SOX11 is moderately sensitive but highly specific for IFS/CMN with ETV6::NTRK3 fusion. In the correct context, SOX11 shows utility as a screening adjunct for focused molecular testing.

ABSTRACTCongenital infantile fibrosarcoma is a rare cause of neonatal intestinal obstruction that can mimic intestinal atresia. This case underscores considering CIFS in the differential diagnosis of intestinal masses in infants and highlights the role of comprehensive histopathological and immunohistochemical analyses in ensuring accurate diagnosis and appropriate management.

Background:Fibrosarcoma is a rare malignant neoplasm representing 10% of musculoskeletal sarcomas. It is classified as adult-type and infantile fibrosarcoma (IFS). Second primary malignancies (SPMs) may occur in fibrosarcoma survivors. As there are no studies that have analyzed this issue, we aimed to assess the risk of SPMs following fibrosarcoma diagnosis.Methods:Patients diagnosed with fibrosarcoma from 2000 to 2021 were selected using SEER*stat software. We used an MP-SIR session to calculate the standardized incidence ratio (SIR) as Observed/Expected (O/E), and the excess risk (ER) was per 10 000. Patients were subgrouped histologically: IFS, fibromyxosarcoma, periosteal fibrosarcoma, facial fibrosarcoma, dermatofibrosarcoma (DFS), central odontogenic fibrosarcoma, and ameloblastic fibrosarcoma.Results:Of 10 420 fibrosarcoma patients, 1048 developed SPMs. The majority were Caucasians (71.1%), and 50.2% were females. A significant risk of SPMs was observed in all sites (O/E = 1.46, P < 0.05, ER = 35.21). IFS had no risk to develop SPMs (Observed = 1.00, O/E = 1.85, P > 0.05). DFS had a high SPMs risk (O/E = 1.52, P < 0.05, 95% CI: 1.41–1.64). Fibromyxosarcoma SPMs had an alarming risk if they originated in the heart and soft tissue (O/E = 32.38, P < 0.05, 95% CI: 23.2–43.9).Conclusions:There is a significantly high risk of multisystem SPMs following primary fibrosarcoma, especially for Caucasians and females. IFS had an insignificant risk of SPMs compared to adult-type fibrosarcomas (AFS). The common sites for SPMs were the heart and soft tissue. The results highlight the necessity of SPMs screening after AFS diagnosis.

Assessing targetable NTRK1/2/3 fusions in mesenchymal tumors via whole-exome RNA sequencing.

To present a rare case of congenital infantile fibrosarcoma (IFS) mimicking a vascular anomaly and to emphasize the diagnostic value of color Doppler and contrast-enhanced ultrasound (CEUS) in differentiating IFS from benign vascular tumors such as congenital hemangioma. We present the case of a 4-month-old male with a congenital lumbosacral mass initially suggestive of congenital hemangioma. Ultrasound and Doppler examination demonstrated a highly vascularized lesion; however, the imaging findings remained equivocal for a definitive diagnosis of hemangioma. MRI confirmed a solid, hypervascular lesion without spinal involvement. Histopathology identified a spindle cell neoplasm consistent with IFS, likely driven by an NTRK rearrangement. The mass was surgically excised with a good postoperative recovery. Doppler and CEUS findings showed vascular features atypical for congenital hemangiomas, which generally present with diffuse, centripetal enhancement and sustained contrast uptake. In contrast, this lesion exhibited an organized radial vascular pattern with early unidirectional enhancement and rapid washout. However, imaging findings remained equivocal for a definitive diagnosis of hemangioma, necessitating further histopathological evaluation, leading to timely surgical management. This case underscores the diagnostic challenge in distinguishing IFS from benign vascular tumors and highlights the importance of histological evaluation. Early recognition is essential, as IFS, though locally aggressive, has a favorable prognosis when promptly and properly managed.

10048 Background: NTRK gene fusions are significant oncogenic drivers in pediatric tumors (e.g. infantile fibrosarcoma). Zurletrectinib is a highly selective next-generation TRK inhibitor. Preclinical data of zurletrectinib showed strong activity against resistant mutations, e.g., G595R. Promising efficacy was observed in a phase I/II clinical trial (NCT04685226). The pivotal phase II clinical trial (NCT05745623) is currently ongoing. Here we report an integrated analysis by combining pediatric and adolescent pts from the two clinical trials. Methods: Eligible pts with locally advanced or metastatic solid tumor harboring NTRK fusions, who failed from standard of care or for whom there was currently no effective therapy were included in the efficacy analysis. Adolescent pts (12-18 years) received zurletrectinib tablet at fixed dose, and pediatric pts ( &lt; 12 years) received zurletrectinib orally disintegrating tablet (ODT) based on body surface area (BSA). The primary endpoint was confirmed objective response rate (ORR) per independent review committee (IRC). Tumor responses were assessed by IRC and investigators per RECSIT1.1 and RANO (BM) criteria. Treatment-emergent adverse events (TEAEs) were evaluated and graded according to CTCAE v5.0. Results: As of 23 Nov 2024, 18 pts in total were enrolled, including 8 pediatric pts and 10 adolescent pts. Median age was 5.0 (range: 3-9) and 13.5 (range: 12-15) respectively. ECOG performance status was between 0-1. Among the 18 pts, 6 TRK inhibitor treatment-naïve pts with central lab confirmed NTRK+ were efficacy evaluable. The confirmed ORR assessed by IRC was 100% (95% CI 54.1, 100.0). All of the pts achieved partial response (PR) at the 1 st tumor assessment and maintained the remission as of the cutoff date. Median time to response were 1.0 month (95% CI: 0.99, NE) in adolescent pts and 0.9 (95% CI: 0.89, NE) month in pediatric pts. It is worth noting that one pediatric patient who progressed on prior first-generation TRK inhibitor achieved complete response after receiving zurletrectinib. The most common treatment related adverse events (TRAEs) were ALT increased (n = 8) and anemia (n = 6), the majority of which were Gr 1 or 2. There were no TRAEs leading to dose reduction or discontinuation, and no serious TRAEs were reported. PK results indicated that zurletrectinib PK profiles in pediatrics and adolescents at the recommended phase 2 dose (RP2D) were similar to that in adults. Conclusions: The integrated analysis demonstrated that zurletrectinib had significant efficacy and good safety profile in pediatric and adolescent pts with NTRK+ solid tumors. Zurletrectinib also showed the potential to overcome the resistance to 1 st generation TRK inhibitors. These findings support zurletrectinib is a better treatment option for NTRK+ pediatric and adolescent pts. Clinical trial information: NCT04685226 .

3066 Background: Larotrectinib (laro) is the first-in-class, highly selective, TRK inhibitor approved for tumor- and age-agnostic use in TRK fusion cancers. This is the seminal report of primary and secondary laro resistance based on an analysis of the regulatory dataset that supported drug approval across multiple countries. Methods: Genomic data from patients (pts) with non-primary CNS TRK fusion cancer enrolled in a global, prospective, multicenter database of three laro clinical trials including adult and pediatric pts were analyzed. Tumor DNA (Illumina TruSight Oncology [TSO] Comprehensive, TSO 500, or FoundationOne CDx) or circulating tumor DNA (Guardant360 or GuardantOMNI) NGS was performed pre-laro (baseline; BL) and post-laro initiation. On-target NTRK (solvent front [SF], gatekeeper [GK], xDFG) mutations and COSMIC-classified tier 1/2 off-target alterations were identified. Primary laro resistance analysis set included pts with no meaningful clinical benefit (PD/SD &lt; 4 months). Secondary (acquired) laro resistance analysis set included pts who developed resistance after meaningful clinical benefit (CR/PR/SD ≥4 months). Data cutoff: July 20, 2024. Results: Of 304 adult and pediatric pts enrolled, 216 had BL genomic data. Primary laro resistance was observed in 24 pts. Only 1 pt had an on-target mutation ( NTRK3 G623R), likely attributable to prior crizotinib; 9 pts (38%) had off-target alterations involving AKT, BRAF, FGFR1, GNAS, KRAS, NRAS, and PIK3CA . Secondary laro resistance was observed in 55 pts with valid post-BL ctDNA (the most common of these TRK fusion cancers were infantile fibrosarcoma [22%], other soft tissue sarcoma [18%], thyroid [11%], lung and salivary gland [9% each]); acquired alterations were identified in 16 of these pts. On-target resistance alone was observed in 5 of 16 pts (31%) and were mainly SF or GK single or double mutation-mediated ( NTRK1 F589L, NTRK1 G595R, NTRK3 G623R [n = 2], NTRK3 G623R/G696A). One xDFG mutation was identified. Off-target resistance alone was observed in 7 of 16 pts (44%) and included hotspot KRAS G12D/A/S/V or G13D, PIK3CA E545K or E542A, BRAF V600E, and GNAS R844H/C mutations. Complex, combined on-target and off-target resistance was observed in 4 of 16 pts (25%): on-target SF or GK alterations ( NTRK1 G595R, NTRK1 F589L/G595R, NTRK3 G623R, NTRK3 G623R/F617L) co-occurred with KRAS G12D or G12D/G13D, and NRAS G12D or Q61H. An analysis of resistance profiles by cancer type and age will be presented. Conclusions: In this analysis, on-target resistance to laro, including potential double NTRK resistance mutations, was commonly observed. Off-target, largely MAPK or PI3K/AKT pathway reactivating resistance, also occurred. In select cases, complex and likely polyclonal resistance including both on-target and off-target alterations were identified. These observations impact novel therapy development for TRK fusion cancers. Clinical trial information: NCT02637687 , NCT02576431 , NCT02122913 .

3148 Background: NTRK gene fusions are oncogenic drivers in various tumor types. Larotrectinib (laro) is the first-in-class, highly selective, central nervous system (CNS)-active TRK inhibitor approved for tumor-agnostic use in patients (pts) with TRK fusion cancer based on a robust and durable objective response rate in pts with various cancers. Here, we report updated long-term efficacy and safety data in adult and pediatric pts with non-primary CNS TRK fusion cancer treated with laro. Methods: Pts with TRK fusion cancer enrolled in 3 laro clinical trials (NCT02637687 [SCOUT], NCT02576431 [NAVIGATE], NCT02122913) were included. Laro was administered at 100 mg twice daily (BID) and 100 mg/m 2 BID in most adult and pediatric pts, respectively. Responses were independent review committee (IRC)-assessed per RECIST v1.1. Pts enrolled in SCOUT were permitted to stop laro in the absence of on-treatment progression (“wait-and-see”). The data cutoff was July 20, 2024. Results: At data cutoff,304 pts were eligible for efficacy assessment by IRC; 25 pts had known CNS metastases at baseline. Median age was 45 years (range 0–90). There were 28 different tumor types, including soft tissue sarcoma (24%), infantile fibrosarcoma (16%), lung (11%), and thyroid (10%). A total of 101 pts (33%) received no prior systemic therapies in the metastatic/unresectable setting; 115 (38%) received 2 or more. NTRK gene fusions were detected by next-generation sequencing (NGS) in 267 (88%) pts. The overall response rate was 65% (95% confidence interval [CI] 59–70): 66 (22%) complete responses (CR), 20 (7%) pathological CR, 112 (37%) partial responses, 56 (18%) stable disease, 32 (11%) progressive disease, and 18 (6%) not evaluable/undefined. Median time to response was 1.8 months (mo; range 0.9–22.9). Median duration of response (DoR), progression-free survival (PFS), and overall survival (OS) were 43 mo (95% CI 34–not estimable), 28 mo (95% CI 22–38), and not reached, respectively, at median follow-ups of 45, 42, and 57 mo. The 4-year rates for DoR, PFS, and OS were 48% (95% CI 40–57), 39% (95% CI 32–46), and 63% (95% CI 57–68), respectively. Median duration of treatment was 19 mo (range 0–100+). Fifty-five of 99 pediatric pts in SCOUT had participated in “wait-and-see”; the median duration of the first “wait-and-see” period was 33 mo (range 1–72). At data cutoff, 83 pts (27%) remained on trial (either on treatment or in “wait-and-see”). Treatment-related adverse events (TRAEs) were mainly Grade 1/2 (n = 189; 62%). Grade 3/4 TRAEs occurred in 71 (23%) pts. Five (2%) pts discontinued due to TRAEs. Conclusions: Laro continues to demonstrate rapid and durable responses, extended survival, clinical benefit, and a favorable safety profile in pts with TRK fusion cancer. This data supports the wider adoption of NGS panels that include NTRK gene fusions to identify pts who may benefit from treatment with TRK inhibitors. Clinical trial information: NCT02637687 , NCT02576431 , NCT02122913 .

Abstract Introduction: Oncogenic fusions involving the neurotrophic receptor tyrosine kinase genes, NTRK1, NTRK2, or NTRK3 are detected in both adult and pediatric cancers, including in 1% of a broad range of common cancer types such as lung, colorectal, and breast, 25% of thyroid and gastrointestinal stromal tumors, and &amp;gt;90% of several rare tumor types, such as congenital infantile fibrosarcoma (1). Fusions arise by intra- or extrachromosomal rearrangement, fusing the NTRK kinase domain to a 5’ upstream gene partner, leading to constitutive kinase activation, thereby driving oncogenesis. This seems to occur primarily via activation of PI3K, MAPK and/or PLCg signaling, however our understanding of NTRK fusion biology is limited by a scarcity of disease models (1). Aim: The aim of this project is to generate additional preclinical models to further our understanding of NTRK fusion-driven oncogenesis and response to treatment. Methods and Results: We identified a novel SGK3-NTRK3 fusion in a patient, with breast cancer and brain metastases, treated at our center. We generated cellular models in HEK293 (human embryonic kidney cell line) and MCF7 (breast adenocarcinoma cell line) by transfecting with a plasmid encoding the fusion construct, with and without a CMV promoter, and demonstrated that the fusion induces AKT activation. To employ more physiologically relevant models, we additionally generated the fusion in these cell lines by CRISPR-gene editing, utilizing sgRNAs that induce double stranded breaks in the relevant genomic loci for this specific fusion. A genetically engineered mouse model of the fusion was further developed in a C57BL/6J background by intracranial injection of an adeno-associated viral construct (GFP/Cre) with sgRNAs targeting SGK3, NTRK3, and TP53. Tumors developed in 50% of the mice within four months, suggesting that this novel fusion acts as an oncogenic driver. Fluorescence In Situ Hybridization to further validate and localize the fusion, gene expression analysis to further characterize the fusion’s biological impact, and viability assays to assess its sensitivity to NTRK inhibition are underway. Conclusions: These findings suggest that the SGK3-NTRK3 fusion is a strong oncogenic driver in a glioma setting, and we are currently extending these investigations to a breast cancer setting. Through the integration of these novel models, our research seeks to advance our understanding of NTRK fusion-driven cancer biology and to improve therapeutic strategies for these cancers.

Sarcomas are a rare and heterogeneous group of mesenchymal malignant tumors and account for approximately 1% of all adult cancers and around 20% of all pediatric solid tumors in Europe. Technology advances have enabled a more accurate and efficient characterization of the molecular mechanisms underlying the pathogenesis of sarcoma subtypes and revealed novel and unexpected therapeutic targets with prognostic/predictive biomarkers, namely the neurotrophic tyrosine receptor kinase (NTRK) gene fusion. The NTRK fusion assessment has recently become a standard part of management for patients with unresectable locally advanced or metastatic cancers and has been identified in various tumor types. In the more prevalent adult and pediatric sarcomas, NTRK fusions are present in 1% and 20%, respectively, and in more than 90% of very rare subsets of tumors. The inhibition of TRK activity with first-generation TRK inhibitors has been found to be effective and well tolerated in adult and pediatric patients, independently of the tumor type. Overall, the therapeutic benefit to those patients compensates for the difficulties of identifying NTRK gene fusions. However, the rarity and diagnostic complexity of NTRK gene fusions raise several questions and challenges for clinicians. To address these issues, an expert panel of medical and pediatric oncologists, radiologists, surgeons, orthopedists, and pathologists reviewed the recent literature and discussed the current status and challenges, proposing a diagnostic algorithm for identifying NTRK fusion sarcomas. The aim of this article is to review the updated information on this issue and to provide the experts' recommendations and practical guidance on the optimal management of patients with soft tissue sarcomas, infantile fibrosarcoma, gastrointestinal stromal tumors, and osteosarcoma.

1659 SOX-11 Expression in ETV6::NTRK3-Rearranged Infantile Fibrosarcoma / Spindle Cell Neoplasms is Frequent and Highly Specific

Objective: To investigate the clinicopathological and genetic features of infantile rhabdomyofibrosarcoma (IRFS) with EGFR kinase domain duplication (EGFR-KDD). Methods: The clinical, morphological and immunohistochemical features of three IRFS with EGFR-KDD diagnosed from January 2022 to January 2024 at Department of Pathology, Foshan Traditional Chinese Medicine Hospital, Foshan, China were retrospectively analyzed using PCR or next generation sequencing technique; and related literature was reviewed. Results: There were 1 male and 2 females, aged at presentation ranging from 1 to 4 years. The tumor occurred in the left thigh, right maxillofacial region, and right popliteal space. The presenting symptom was a painless mass which was accidentally discovered. The maximum diameter of tumors ranged from 3 to 5 cm. Microscopically, the tumors were poorly defined and composed of relatively monomorphic spindle cells, arranged in diffuse, fascicular growth patterns, with moderate pale eosinophilic cytoplasm. Mitoses were abundant. A few round rhabdomyoblastic tumor cells with abundant eosinophilic cytoplasm were found. There was no evidence of hemorrhage or necrosis. The tumor cells expressed vimentin, SMA, MSA, desmin, MyoD1 and myogenin; and the Ki-67 proliferation index was 10%-60%. RT-PCR showed EGFR-KDD in all three cases. Gene fusion was detected in three cases based on next generation sequencing, but only one case had EGFR-KDD. Follow-up data for 12 to 36 months showed two patients died of the disease and one patient was alive without recurrences and metastasis. Conclusions: IRFS is a rare soft tissue tumor that resembles infantile fibrosarcoma but has immunohistochemical evidence of rhabdomyoblastic differentiation. It more commonly occurs in infants and tends to appear in limbs and torso with poor prognosis. Aggressive multimodality treatment is recommended for these patients. EGFR-KDD may be a genetic driver to IRFS. Clinical response to EGFR targeted therapy might be promising in the future.

Infantile Fibrosarcoma is a malignant tumor of fibroblastic origin, typically found in early childhood, locally aggressive, and characterized by molecular alterations that activate tyrosine kinase signaling, primarily the ETV6::NTRK3 fusion. In recent years, a series of fusions different from the classic one have been described, including NTRK1, RAF1, and BRAF. In this paper, we present a case of IFS with a novel EVI5::BRAF fusion. We observed a spindle cell neoplasm growing in intertwined fascicles within a fibrous stroma, without the formation of an immature osteoid matrix. Weak and focal immunoreactivity for S100 was observed. SATB2 exhibited diffuse and intense staining, with focal expression of osteonectin and negativity for caldesmon, Smooth Muscle Actin, desmin, GFAP, SOX10, MelanA, panTRK, and HMB45. The Ki67 index was 7%, and the tumor harbored an EVI5::BRAF genetic fusion. To the best of our knowledge, the EVI5::BRAF fusion has not yet been described in BRAF fusions in IFS. Nevertheless, further studies are needed to define the prognostic features of these emerging BRAF sarcomas, along with new anti-BRAF therapeutic approaches.

Larotrectinib is a highly selective tropomyosin receptor kinase (TRK) inhibitor with efficacy in children with TRK fusion tumors. We evaluated patient outcomes after elective discontinuation of larotrectinib in the absence of disease progression in a protocol-defined wait-and-see subset analysis of eligible patients where treatment resumption with larotrectinib was allowed if disease progressed. We also assessed the safety and efficacy of larotrectinib in all pediatric patients with sarcoma. This cohort included 91 patients (younger than 18 years) from two clinical trials: infantile fibrosarcoma (49), other soft tissue sarcomas or related mesenchymal tumors (41), and bone sarcoma (1). Treatment-related adverse events were of maximum grade 1 or 2 in 25% and 25% of patients, respectively. The overall response rate was 87% (95% CI, 78 to 93). In the wait-and-see analysis, 47 patients discontinued larotrectinib. Median time from discontinuation to disease progression was not reached. Sixteen patients had tumor progression during the wait-and-see period. All 16 patients resumed larotrectinib, and 15 (94%) achieved disease control, with 11 objective responses. Larotrectinib continues to demonstrate durable responses with favorable safety in children with TRK fusion sarcomas. Treatment discontinuation is feasible in select patients with objective response and clinical benefit noted in those who have disease progression after elective treatment discontinuation.