Germline predisposition to pediatric Ewing sarcoma is characterized by inherited pathogenic variants in DNA damage repair genes.

More knowledge is needed regarding germline predisposition to Ewing sarcoma to inform biological investigation and clinical practice. Here, we evaluated the enrichment of pathogenic germline variants in Ewing sarcoma relative to other pediatric sarcoma subtypes, as well as patterns of inheritance of these variants. We carried out European-focused and pan-ancestry case-control analyses to screen for enrichment of pathogenic germline variants in 141 established cancer predisposition genes in 1,147 individuals with pediatric sarcoma diagnoses (226 Ewing sarcoma, 438 osteosarcoma, 180 rhabdomyosarcoma, and 303 other sarcoma) relative to identically processed cancer-free control individuals. Findings in Ewing sarcoma were validated with an additional cohort of 430 individuals, and a subset of 301 Ewing sarcoma parent-proband trios was analyzed for inheritance patterns of identified pathogenic variants. A distinct pattern of pathogenic germline variants was seen in Ewing sarcoma relative to other sarcoma subtypes. FANCC was the only gene with an enrichment signal for heterozygous pathogenic variants in the European Ewing sarcoma discovery cohort (three individuals, OR 12.6, 95% CI 3.0-43.2, p = 0.003, FDR = 0.40). This enrichment in FANCC heterozygous pathogenic variants was again observed in the European Ewing sarcoma validation cohort (three individuals, OR 7.0, 95% CI 1.7-23.6, p = 0.014), representing a broader importance of genes involved in DNA damage repair, which were also nominally enriched in individuals with Ewing sarcoma. Pathogenic variants in DNA damage repair genes were acquired through autosomal inheritance. Our study provides new insight into germline risk factors contributing to Ewing sarcoma pathogenesis.

Phase II study of vemurafenib in children and young adults with tumors harboring BRAF V600 mutations: NCI-COG pediatric MATCH trial (APEC1621) Arm G.

BACKGROUND: This is a phase II subprotocol of the NCI-COG Pediatric MATCH study evaluating vemurafenib, a selective oral inhibitor of BRAF V600 mutated kinase, in patients with relapsed or refractory solid tumors harboring BRAF V600 mutations. METHODS: Patients received vemurafenib at 550 mg/m2 (maximum 960 mg/dose) orally twice daily for 28-day cycles until progression or intolerable toxicity. The primary aim was to determine the objective response rate and secondary objectives included estimating progression-free survival and assessing the tolerability of vemurafenib. RESULTS: Twenty-two patients matched to the subprotocol and 4 patients (18%) enrolled. Primary reasons for non-enrollment were ineligibility due to exclusions of low-grade glioma (nâ=â7) and prior BRAF inhibitor therapy (nâ=â7). Enrolled diagnoses were one each of histiocytosis, ameloblastoma, Ewing sarcoma, and high-grade glioma, all with BRAF V600E mutations. Treatment was overall tolerable with mostly expected grade 1/2 adverse events (AE). Grade 3 or 4 AE on treatment were acute kidney injury, hyperglycemia, and maculopapular rash. One patient came off therapy due to AE. One patient (glioma) had an objective partial response and remained on protocol therapy for 15 cycles. CONCLUSION: There was a low accrual rate on this MATCH subprotocol, with only 18% of those who matched with BRAFV600 mutations enrolling, resulting in early termination, and limiting study results (ClinicalTrials.gov Identifier: NCT03220035).

Olaparib for childhood tumors harboring defects in DNA damage repair genes: arm H of the NCI-COG Pediatric MATCH trial.

BACKGROUND: The National Cancer Institute-Children's Oncology Group Pediatric Molecular Analysis for Therapy Choice (MATCH) precision oncology platform trial enrolled children aged 1-21 years with treatment-refractory solid tumors and predefined actionable genetic alterations. Patients with tumors harboring alterations in DNA damage repair (DDR) genes were assigned to receive olaparib. METHODS: Tumor and blood samples were submitted for centralized molecular testing. Tumor and germline sequencing were conducted in parallel. Olaparib was given twice daily for 28-day cycles starting at a dose 30% lower than the adult recommended phase 2 dose (RP2D). The primary endpoint was the objective response. RESULTS: Eighteen patients matched (1.5% of those screened) based on the presence of a deleterious gene alteration in BRCA1/2, RAD51C/D, or ATM detected by tumor sequencing without germline subtraction or analysis of loss of heterozygosity (LOH). Eleven (61%) harbored a germline mutation, with only one exhibiting LOH. Six patients enrolled and received the olaparib starting dose of 135 mg/m2/dose. Two participants were fully evaluable; 4 were inevaluable because <85% of the prescribed dose was administered during cycle 1. There were no dose-limiting toxicities or responses. Minimal hematologic toxicity was observed. CONCLUSION: Most DDR gene alterations detected in Pediatric MATCH were germline, monoallelic, and unlikely to confer homologous recombination deficiency predicting sensitivity to olaparib monotherapy. The study closed due to poor accrual. CLINICALTRIALS.GOV IDENTIFIER: NCT03233204. IRB approved: initial July 24, 2017.

Belzutifan, a Potent HIF2α Inhibitor, in the Pacak-Zhuang Syndrome.

The integration of genomic testing into clinical care enables the use of individualized approaches to the management of rare diseases. We describe the use of belzutifan, a potent and selective small-molecule inhibitor of the protein hypoxia-inducible factor 2α (HIF2α), in a patient with polycythemia and multiple paragangliomas (the Pacak-Zhuang syndrome). The syndrome was caused in this patient by somatic mosaicism for an activating mutation in EPAS1. Treatment with belzutifan led to a rapid and sustained tumor response along with resolution of hypertension, headaches, and long-standing polycythemia. This case shows the application of a targeted therapy for the treatment of a patient with a rare tumor-predisposition syndrome. (Funded by the Morin Family Fund for Pediatric Cancer and Alex's Lemonade Stand Foundation.).

Evaluation of prevalence and outcomes of serial tyrosine kinase inhibitor use in pediatric patients with advanced solid tumors.

PURPOSE: Multitargeted tyrosine kinase inhibitors (mTKIs) are increasingly utilized in the treatment of pediatric sarcomas and other solid tumors. It is unknown whether serial treatment with multiple TKIs provides a benefit and which patients are most likely to benefit from mTKI rechallenge. METHODS: We performed a retrospective cohort study of pediatric cancer patients who received serial mTKI therapy off-study between 2007 and 2020 as either monotherapy or combination therapy. We report patient characteristics, clinical outcomes, dosing patterns, and treatment-associated toxicity. RESULTS: The study cohort included 25 patients. The overall prevalence of serial mTKI therapy among all patients treated for sarcoma at our institution was 3.7%, and the response rate to second mTKI was 9%. Median 6-month progression-free survival (PFS) and overall survival (OS) from start of second mTKI were 42.1% (95% CI: 20.4%-62.5%) and 79.1% (95% CI: 57.0%-90.8%), respectively. Patients who had received 4 months or more (n = 11) of therapy with first mTKI had significantly longer PFS versus those who received less than 4 months (n = 11; p = .001). Thirty-three percent of patients discontinued second mTKI due to toxicity. Six (40%) of 15 patients who discontinued the first mTKI due to progression had either a partial response or stable disease on the second mTKI. CONCLUSIONS: We observed a low response rate to mTKI rechallenge. However, we identified patients who had been treated with first mTKI for  ≥4 months as more likely to have prolonged stable disease with second mTKI. Several patients had a response or stable disease on the second mTKI despite having progressed on the first mTKI. Though toxicity was common, only a minority of patients discontinued the second mTKI due to toxicity.

Children's Oncology Group's 2023 blueprint for research: Bone tumors.

The Children's Oncology Group (COG) Bone Tumor Committee is responsible for clinical trials and biological research on localized, metastatic, and recurrent osteosarcoma and Ewing sarcoma (EWS). Results of clinical trials in localized disease completed and published in the past 10 years have led to international standard-of-care chemotherapy for osteosarcoma and EWS. A recent focus on identifying disease subgroups has led to the identification of biological features associated with poor outcomes including the presence of circulating tumor DNA (ctDNA) at diagnosis, and specific genomic alterations-MYC amplification for osteosarcoma and STAG2 and TP53 mutation for EWS. Studies validating these potential biomarkers are under way. Clinical trials evaluating the addition of multitargeted kinase inhibitors, which are active in relapsed bone sarcomas, to standard chemotherapy are under way in osteosarcoma and planned in EWS. In addition, the Committee has data analyses and a clinical trial under way to evaluate approaches to local management of the primary tumor and metastatic sites. Given the rarity of bone sarcomas, we have prioritized international interactions and are in the process of forming an international data-sharing consortium to facilitate refinement of risk stratification and study of rare disease subtypes.

A pooled subgroup analysis of glucarpidase treatment in 86 pediatric, adolescent, and young adult patients receiving high-dose methotrexate therapy in open-label trials.

BACKGROUND: Delayed methotrexate elimination can occur in patients undergoing high-dose methotrexate cancer treatment. Effectiveness of glucarpidase for rapidly reducing methotrexate concentrations was shown in compassionate-use trials in patients aged 0-84 years. METHODS: We performed post hoc analyses of infants (≥28 days to <2 years), children (≥2 to <12 years), adolescents (≥12 to <15 years), and young adults (≥15 to <25 years) from four multicenter, open-label, single-arm, glucarpidase compassionate-use trials. Patients had toxic methotrexate levels due to delayed methotrexate elimination and/or renal dysfunction, and received glucarpidase (50 U/kg). The primary endpoint was clinically important reduction (CIR) in plasma methotrexate (methotrexate ≤1 µmol/L at all post-glucarpidase measurements) based on high-performance liquid chromatography. RESULTS: Among 86 patients included in efficacy analyses, CIR was achieved by zero of one infant (0.0%), five of 16 children (31.3%), seven of 24 adolescents (29.2%), and 26/45 young adults (57.8%). Median methotrexate reduction was 98.7% or higher in each group 15 minutes post-glucarpidase. Patients with pre-glucarpidase methotrexate less than 50 µmol/L (35/42, 83.3%) were more likely to achieve CIR than those with methotrexate 50 µmol/L or higher (1/37, 2.7%). The most common treatment-related adverse event was paresthesia, occurring in three adolescents (4.5%) and six young adults (5.2%). No other treatment-related adverse event occurred in 5% or higher of any age group. CONCLUSION: After accounting for pre-glucarpidase methotrexate levels, glucarpidase efficacy at inducing CIR in pediatric/young adult patients was consistent, with efficacy observed in the overall study population (i.e., patients aged 0-84), and no unexpected safety findings were observed. These findings demonstrate glucarpidase (50 U/kg) is an effective and well-tolerated dose for pediatric, adolescent, and young adult patients.

Loss of heterozygosity does not occur in BRCA1/2 mutant pediatric solid and central nervous system tumors.

Utilization of tumor-only sequencing has expanded in pediatric cancer patients, which can lead to identification of pathogenic variants in genes that may be germline and/or have uncertain relevance to the tumor in question, such as the homologous recombination (HR) pathway genes BRCA1/2. We identified patients with pathogenic BRCA1/2 mutations from somatic tumor sequencing, and performed additional germline sequencing to assess for the presence of loss of heterozygosity (LOH). Of seven patients identified, four (57.1%) mutations were found in the germline and none had associated LOH. Our data suggest that BRCA1/2 mutations identified in this context are likely incidental findings.

Phase 1 study of cabozantinib in combination with topotecan-cyclophosphamide for patients with relapsed Ewing sarcoma or osteosarcoma.

PURPOSE: Phase 1 study assessing the safety and toxicity of cabozantinib in combination with topotecan and cyclophosphamide for relapsed osteosarcoma and Ewing sarcoma. METHODS: Oral cabozantinib (25 mg/m2 ) was administered daily for 21 (dose level 1) or 14 (dose level -1B) days. Topotecan (0.75 mg/m2 ) and cyclophosphamide (250 mg/m2 ) were administered intravenously (IV) on days 1-5. A modified 3+3 design based upon first cycle dose-limiting toxicities (DLT) was used for dose escalation. RESULTS: Twelve patients with a median age of 15 (12.9-33.2) years were enrolled (seven with Ewing sarcoma; five with osteosarcoma); all were evaluable for toxicity. At dose level 1, three of six patients developed first cycle DLT: grade 3 epistaxis, grade 3 transaminitis, and prolonged grade 2 thrombocytopenia. Six patients were enrolled on dose level -1B (interrupted cabozantinib, given days 8-21), with one first cycle DLT (grade 3 pneumothorax) observed. Of the 10 response evaluable patients, one had partial response (Ewing sarcoma), seven had stable disease, and two had progressive disease. CONCLUSIONS: The recommended phase 2 doses and schedules for this combination are topotecan 0.75 mg/m2 IV days 1-5, cyclophosphamide 250 mg/m2 IV days 1-5, and cabozantinib 25 mg/m2 days 8-21. Non-concomitant administration of cabozantinib with cytotoxic therapy in this population has acceptable toxicity, while allowing for potential disease control.

Clear cell tumor with melanocytic differentiation and MITF::CREM translocation.

In addition to melanoma, a large and diverse family of tumors shows melanocytic differentiation. The best characterized member of this family is clear cell sarcoma, which is characterized by EWSR1::ATF1 and EWSR1::CREB1 fusions. These fusions drive the transcription of MITF, the master regulator of melanocytic differentiation. Clear cell tumor with melanocytic differentiation and MITF::CREM translocation is a recently described tumor with some similarities to clear cell sarcoma. However, only a single case has been reported. Here, we describe a second molecularly proven case that arose on the scalp of a newborn baby. In contrast to the prior reported case, the current case showed predominantly high-grade cytomorphologic features with only focal clear cell areas. Similar to the prior case, the tumor showed immunohistochemical evidence of neural crest origin/differentiation with prominent melanocytic differentiation. The fusion breakpoints were also similar and preserved the transcriptional activation domain of CREM, suggesting that CREM hyperactivity is a major feature of this tumor type. The current tumor showed a short-interval recurrence. These results expand the clinical and pathologic spectrum of this potentially new entity.

Adverse prognostic impact of the loss of STAG2 protein expression in patients with newly diagnosed localised Ewing sarcoma: A report from the Children's Oncology Group.

BACKGROUND: Ewing sarcoma (EWS) is an aggressive sarcoma with no validated molecular biomarkers. We aimed to determine the frequency of STAG2 protein loss by immunohistochemistry (IHC) and whether loss of expression is associated with outcome. METHODS: We performed a retrospective cohort study of patients with EWS enrolled to Children's Oncology Group studies. We obtained unstained slides from 235 patients and DNA for sequencing from 75 patients. STAG2 expression was tested for association with clinical features and survival was estimated using Kaplan-Meier methods with log-rank tests. RESULTS: In total, 155 cases passed quality control for STAG2 IHC. STAG2 expression in 20/155 cases could not be categorised with the limited available tissue, leaving 135 patients with definitive STAG2 IHC. In localised and metastatic disease, STAG2 was lost in 29/108 and 6/27 cases, respectively. Among patients with IHC and sequencing, 0/17 STAG2 expressing cases had STAG2 mutations, and 2/7 cases with STAG2 loss had STAG2 mutations. Among patients with localised disease, 5-year event-free survival was 54% (95% CI 34-70%) and 75% (95% CI 63-84%) for patients with STAG2 loss vs. expression (P = 0.0034). CONCLUSION: STAG2 loss of expression is identified in a population of patients without identifiable STAG2 mutations and carries a poor prognosis.

Rapid and highly sensitive approach for multiplexed somatic fusion detection.

Somatic gene translocations are key to making an accurate diagnosis in many cancers including many pediatric sarcomas. Currently available molecular diagnostic approaches to identifying somatic pathognomonic translocations have limitations such as minimal multiplexing, high cost, complex computational requirements, or slow turnaround times. We sought to develop a new fusion-detection assay optimized to mitigate these challenges. To accomplish this goal, we developed a highly sensitive multiplexed digital PCR-based approach that can identify the gene partners of multiple somatic fusion transcripts. This assay was validated for specificity with cell lines and synthetized DNA fragments. Assay sensitivity was optimized using a tiered amplification approach for fusion detection from low input and/or degraded RNA. The assay was then tested for the potential application of fusion detection from FFPE tissue and liquid biopsy samples. We found that this multiplexed PCR approach was able to accurately identify the presence of seven different targeted fusion transcripts with a turnaround time of 1 to 2 days. The addition of a tiered amplification step allowed the detection of targeted fusions from as little as 1 pg of RNA input. We also identified fusions from as little as two unstained slides of FFPE tumor biopsy tissue, from circulating tumor cells collected from tumor-bearing mice, and from liquid biopsy samples from patients with known fusion-positive cancers. We also demonstrated that the assay could be easily adapted for additional fusion targets. In summary, this novel assay detects multiple somatic fusion partners in biologic samples with low tumor content and low-quality RNA in less than two days. The assay is inexpensive and could be applied to surgical and liquid biopsies, particularly in places with inadequate resources for more expensive and expertise-dependent assays such as next-generation sequencing.

Histologic characterization of paediatric mesenchymal neoplasms treated with kinase-targeted therapy.

AIMS: Recurrent alterations involving receptor tyrosine or cytoplasmic kinase genes have been described in soft-tissue neoplasms such as infantile fibrosarcoma (IFS) and inflammatory myofibroblastic tumour (IMT). Recent trials and regulatory approvals for targeted inhibitors against the kinase domains of these oncoproteins have allowed for increased use of targeted therapies. We aimed to characterize the histologic features of paediatric mesenchymal neoplasms with kinase alterations treated with targeted inhibitors. METHODS AND RESULTS: Eight patients with tyrosine kinase-altered mesenchymal neoplasms with pre- and posttreatment samples were identified. Tumours occurred in five females and three males with a median age at presentation of 6.5 years. Tumour sites were bone/somatic soft-tissue (n = 5) and viscera (n = 3). Pretreatment diagnoses were: IMT (n = 3), epithelioid inflammatory myofibroblastic sarcoma (n = 1), and descriptive diagnoses (n = 4) such as "kinase-driven spindle cell tumor." Fusions identified were ETV6::NTRK3 (n = 2), TPM3::NTRK1, SEPT7::BRAF, TFG::ROS1, KLC1::ALK, RANBP2::ALK, and MAP4::RAF1. Patients were treated with larotrectinib (n = 3), ALK or ALK/ROS1 inhibitors (n = 3), and MEK inhibitors (n = 2). Posttreatment tumours exhibited a striking decrease in cellularity (7/8) and the presence of collagenous stroma (7/8) with extensive glassy hyalinization (5/8). In two cases, abundant coarse or psammomatous calcifications were seen and in one case prominent perivascular hyalinization was noted. Residual viable tumour was seen in 3/8 cases (<5% in one case, and >75% in 2/8 cases). CONCLUSION: Mesenchymal neoplasms with tyrosine kinase alterations treated with targeted inhibitors show a pathologic response, which includes decreased cellularity and stromal hyalinization. The presence of these features may be helpful in assessing tumour response after targeted therapy.

68 Ga-DOTATATE PET and functional imaging in pediatric pheochromocytoma and paraganglioma.

Pheochromocytoma and paraganglioma (PPGL) are rare neuroendocrine tumors in childhood. Up to 40% of PPGL are currently thought to be associated with a hereditary predisposition. Nuclear medicine imaging modalities such as fluorodeoxyglucose positron emission tomography (18 F-FDG PET), 68 Ga-DOTATATE PET, and 123 I-metaiodobenzylguanidine (123 I-MIBG) scintigraphy play an essential role in the staging, response assessment, and determination of suitability for targeted radiotherapy in patients with PPGL. Each of these functional imaging modalities targets a different cellular characteristic and as such can be complementary to anatomic imaging and to each other. With the recent US Food and Drug Administration approval and increasing use of 68 Ga-DOTATATE for imaging in children, the purpose of this article is to use a case-based approach to highlight both the advantages and limitations of DOTATATE imaging as it is compared to current radiologic imaging techniques in the staging and response assessment of pediatric PPGL, as well as other neuroendocrine malignancies.

Derivation and validation of risk groups in patients with osteosarcoma utilizing regression tree analysis.

BACKGROUND: For patients with osteosarcoma, apart from stage and primary site, we lack reliable prognostic factors for risk stratification at diagnosis. There is a need for further defined, discrete prognostic groups using presenting clinical features. METHODS: We analyzed a cohort of 3069 patients less than 50 years of age, diagnosed with primary osteosarcoma of the bone between 1986 and 2013 from the Surveillance, Epidemiology, and End Results (SEER) database. Patients were randomly split into test and validation cohorts. Optimal cut points for age, tumor size, and grade were identified using classification and regression tree analysis. Manual recursive partitioning was used to identify discrete prognostic groups within the test cohort. These groups were applied to the validation cohort, and overall survival was analyzed using Cox models, Kaplan Meier methods, and log-rank tests. RESULTS: After applying recursive partitioning to the test cohort, our initial model included six groups. Application of these groups to the validation cohort resulted in four final groups. Key risk factors included presence of metastases, tumor site, tumor grade, age, and tumor size. Patients with localized axial tumors were identified as having similar outcomes to patients with metastases. Age and tumor size were only prognostically important in patients with extremity tumors when assessed in the validation cohort. CONCLUSIONS: This analysis supports prior reports that patients with axial tumors are a high-risk group, and demonstrates the importance of age and tumor size in patients with appendicular tumors. Biologic and genetic markers are needed to further define subgroups in osteosarcoma.

Outcome of patients with relapsed or progressive Ewing sarcoma enrolled on cooperative group phase 2 clinical trials: A report from the Children's Oncology Group.

SevenChildren's Oncology Group phase 2 trials for patients with relapsed/progressive solid tumors were analyzed to estimate the event-free survival (EFS) for relapsed/progressive Ewing sarcoma. One hundred twenty-eight Ewing sarcoma patients were enrolled and 124 events occurred. The 6-month EFS was 12.7%, demonstrating the poor outcome of these patients. Only docetaxel achieved its protocol-specified radiographic response rate for activity; however, the EFS for docetaxel was similar to other agents, indicating that a higher radiographic response rate may not translate into superior disease control. This EFS benchmark could be utilized as an additional endpoint in trials for recurrent Ewing sarcoma.

Charting a path for prioritization of novel agents for clinical trials in osteosarcoma: A report from the Children's Oncology Group New Agents for Osteosarcoma Task Force.

Osteosarcoma is the most common bone tumor in children and young adults. Metastatic and relapsed disease confer poor prognosis, and there have been no improvements in outcomes for several decades. The disease's biological complexity, lack of drugs developed specifically for osteosarcoma, imperfect preclinical models, and limits of existing clinical trial designs have contributed to lack of progress. The Children's Oncology Group Bone Tumor Committee established the New Agents for Osteosarcoma Task Force to identify and prioritize agents for inclusion in clinical trials. The group identified multitargeted tyrosine kinase inhibitors, immunotherapies targeting B7-H3, CD47-SIRPα inhibitors, telaglenastat, and epigenetic modifiers as the top agents of interest. Only multitargeted tyrosine kinase inhibitors met all criteria for frontline evaluation and have already been incorporated into an upcoming phase III study concept. The task force will continue to reassess identified agents of interest as new data become available and evaluate novel agents using this method.

The Pan-Cancer Landscape of Coamplification of the Tyrosine Kinases KIT, KDR, and PDGFRA.

PURPOSE: Amplifications of receptor tyrosine kinases (RTKS) are therapeutic targets in multiple tumor types (e.g. HER2 in breast cancer), and amplification of the chromosome 4 segment harboring the three RTKs KIT, PDGFRA, and KDR (4q12amp) may be similarly targetable. The presence of 4q12amp has been sporadically reported in small tumor specific series but a large-scale analysis is lacking. We assess the pan-cancer landscape of 4q12amp and provide early clinical support for the feasibility of targeting this amplicon. EXPERIMENTAL DESIGN: Tumor specimens from 132,872 patients with advanced cancer were assayed with hybrid capture based comprehensive genomic profiling which assays 186-315 genes for all classes of genomic alterations, including amplifications. Baseline demographic data were abstracted, and presence of 4q12amp was defined as 6 or more copies of KIT/KDR/PDGFRA. Concurrent alterations and treatment outcomes with matched therapies were explored in a subset of cases. RESULTS: Overall 0.65% of cases harbored 4q12amp at a median copy number of 10 (range 6-344). Among cancers with >100 cases in this series, glioblastomas, angiosarcomas, and osteosarcomas were enriched for 4q12amp at 4.7%, 4.8%, and 6.4%, respectively (all p < 0.001), giving an overall sarcoma (n = 6,885) incidence of 1.9%. Among 99 pulmonary adenocarcinoma cases harboring 4q12amp, 50 (50%) lacked any other known driver of NSLCC. Four index cases plus a previously reported case on treatment with empirical TKIs monotherapy had stable disease on average exceeding 20 months. CONCLUSION: We define 4q12amp as a significant event across the pan-cancer landscape, comparable to known pan-cancer targets such as NTRK and microsatellite instability, with notable enrichment in several cancers such as osteosarcoma where standard treatment is limited. The responses to available TKIs observed in index cases strongly suggest 4q12amp is a druggable oncogenic target across cancers that warrants a focused drug development strategy. IMPLICATIONS FOR PRACTICE: Coamplification of the receptor tyrosine kinases (rtks) KIT/KDR/PDGFRA (4q12amp) is present broadly across cancers (0.65%), with enrichment in osteosarcoma and gliomas. Evidence for this amplicon having an oncogenic role is the mutual exclusivity of 4q12amp to other known drivers in 50% of pulmonary adenocarcinoma cases. Furthermore, preliminary clinical evidence for driver status comes from four index cases of patients empirically treated with commercially available tyrosine kinase inhibitors with activity against KIT/KDR/PDGFRA who had stable disease for 20 months on average. The sum of these lines of evidence suggests further clinical and preclinical investigation of 4q12amp is warranted as the possible basis for a pan-cancer drug development strategy.

DICER1-associated central nervous system sarcoma in children: comprehensive clinicopathologic and genetic analysis of a newly described rare tumor.

The spectrum of neoplasms associated with DICER1 variants continues to expand, with the recent addition of primary "DICER1-associated central nervous system sarcoma" (DCS). DCS is a high-grade malignancy predominantly affecting pediatric patients. Six pediatric DCS were identified through a combination of clinical diagnostic studies, archival inquiry, and interinstitutional collaboration. Clinical, histologic, immunohistologic, and molecular features were examined. Genomic findings in the 6 DCS were compared with those in 14 additional DICER1-associated tumors sequenced with the same assay. The six patients presented at ages 3-15 years with CNS tumors located in the temporal (n = 2), parietal (n = 1), fronto-parietal (n = 1), and frontal (n = 2) lobes. All underwent surgical resection. Histologic examination demonstrated high-grade malignant spindle cell tumors with pleuropulmonary blastoma-like embryonic "organoid" features and focal rhabdomyoblastic differentiation; immature cartilage was seen in one case. Immunohistochemically, there was patchy desmin and myogenin staining, and patchy loss of H3K27me3, and within eosinophilic cytoplasmic globules, alfa-fetoprotein staining. Biallelic DICER1 variants were identified in all cases, with germline variants in two of five patients tested. DCS demonstrated genomic alterations enriched for Ras pathway activation and TP53 inactivation. Tumor mutational burden was significantly higher in the 6 DCS tumors than in 14 other DICER1-associated tumors examined (mean 12.9 vs. 6.8 mutations/Mb, p = 0.035). Postoperative care included radiation (n = 5) and chemotherapy (n = 3); at the last follow-up, three patients were alive without DCS, and three had died of disease. Our analysis expands the clinical, histologic, immunohistological, and molecular spectrum of DCS, identifying distinctive features that can aid in the diagnosis, multidisciplinary evaluation, and treatment of DCS.

Recurrent RET gene fusions in paediatric spindle mesenchymal neoplasms.

AIMS: The classification of paediatric spindle mesenchymal tumours is evolving, and the spectrum of so-called 'infantile fibrosarcoma' has expanded to include tumours with NTRK, BRAF and MET gene fusions. RET-rearranged paediatric spindle cell neoplasms are an emerging group; there is sparse literature on their clinical, pathological and genetic features, and their nosological place in the canon of soft tissue tumours is uncertain. In this study, we report five RET-rearranged paediatric spindle cell tumours with fusion partners MYH10, KIAA1217 and CLIP2. METHODS AND RESULTS: The tumours occurred in the pelvic region, paraspinal region, kidney and subcutaneous tissue of hand and abdomen. The patients' ages ranged from 6 months to 13 years (median 1 year). The tumours were composed of monomorphic spindle cells arranged in a fascicular pattern. Lesional cells had minimally atypical ovoid or tapered nuclei and pale cytoplasm with indistinct borders. Necrosis was not identified. Mitoses numbered three to 12 per 10 high-power field. Cases showed inconsistent and variable expression of S100, CD34 and SMA. Clinical behaviour ranged from small lesions potentially cured by simple resection to large lesions exhibiting metastasis, but responsive to kinase inhibitor therapy. CONCLUSIONS: Our findings help to define RET-rearranged spindle cell tumours. Although it is likely that these tumours comprise part of the morphological and clinical spectrum of infantile fibrosarcoma (IFS), identification of RET gene alteration is important for its unique therapeutic implications.