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AIMS: Salivary gland neoplasms (SGN) exhibiting the HMGA2::WIF1 fusion are recognized by their resemblance to histology found in canalicular adenoma. Recently, ~20% of cases among 28 HMGA2::WIF1-rearranged-SGN showed malignancy and adverse outcomes (recurrence, distant metastasis, and disease-specific mortality). Among them, MDM2/CDK4 amplifications were identified in one case. This outcome suggests that the MDM2/CDK4 amplifications could be useful to predict an aggressive course of carcinoma ex-pleomorphic adenoma (CEPA). METHODS AND RESULTS: We investigated the correlation between HMGA2 fusion and MDM2 amplification in four salivary gland neoplasms, providing detailed clinicopathological features and outcomes. Cases were selected from different institutions. Histological examination, immunohistochemistry, fluorescence in situ hybridization (FISH), RNA sequencing, and whole-exome capture were performed. The cohort included four CEPA cases, all female, aged between 32 and 89 years. Tumours arose from the parotid gland with an average size of 24.5 mm. None exhibited recurrence or distant metastases during the 4-5 months of follow-up. Pathologically, all cases displayed a peculiar atypical nuclei with 'gear-like appearance'. Immunohistochemically, tumours exhibited a biphasic pattern with myoepithelial and ductal differentiation markers. All cases showed HMGA2 overexpression and MDM2 amplification by FISH and RNA sequencing. In a control cohort of MDM2 nonamplified CEPA cases, not exhibiting the peculiar nuclear atypia. CONCLUSIONS: Our findings suggest a strong correlation between HMGA2 alteration/MDM2 amplification and a peculiar nuclear atypia, advocating for their evaluation in biphasic tumours to facilitate accurate diagnosis and tailored posttumour removal monitoring. Further studies are warranted to validate these observations and elucidate their prognostic implications.

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BACKGROUND: Liposarcomas are among the most common mesenchymal malignancies. However, the therapeutic options are still very limited and so far, targeted therapies had not yet been established. Immunotherapy, which has been a breakthrough in other oncological entities, seems to have no efficacy in liposarcoma. Complicating matters further, classification remains difficult due to the diversity of morphologies and nonspecific or absent markers in immunohistochemistry, leaving molecular pathology using FISH or sequencing as best options. Many liposarcomas harbor MDM2 gene amplifications. In close relation to the gene locus of MDM2, HER3 (ERBB3) gene is present and co-amplification could occur. Since the group of HER/EGFR receptor tyrosine kinases and its inhibitors/antibodies play a role in a broad spectrum of oncological diseases and treatments, and some HER3 inhibitors/antibodies are already under clinical investigation, we hypothesized that in case of HER3 co-amplifications a tumor might bear a further potential therapeutic target. METHODS: We performed FISH analysis (MDM2, DDIT3, HER3) in 56 archived cases and subsequently performed reclassification to confirm the diagnosis of liposarcoma. RESULTS: Next to 16 out of 56 cases needed to be re-classified, in 20 out of 54 cases, a cluster-amplification of HER3 could be detected, significantly correlating with MDM2 amplification. Our study shows that the entity of liposarcomas show specific molecular characteristics leading to reclassify archived cases by modern, established methodologies. Additionally, in 57.1% of these cases, HER3 was cluster-amplified profusely, presenting a putative therapeutic target for targeted therapy. CONCLUSION: Our study serves as the initial basis for further investigation of the HER3 gene as a putative therapeutic target in liposarcoma.

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In the realm of rare cardiac tumors, intimal sarcoma presents a formidable challenge, often requiring innovative treatment approaches. This case report presents a unique instance of primary intimal sarcoma in the left atrium, underscoring the critical role of genomic profiling in guiding treatment. Initial genomic testing unveiled a somatic, active mutation in PDGFRß (PDGFRß N666K), accompanied by MDM2 and CDK4 amplifications. This discovery directed the treatment course toward pazopanib, a PDGFRß inhibitor, following irradiation. The patient's response was remarkable, with the therapeutic efficacy of pazopanib lasting for 16.3 months. However, the patient experienced a recurrence in the left atrium, where subsequent genomic analysis revealed the absence of the PDGFRß N666K mutation and a significant reduction in PDGFRß expression. This case report illustrates the complexities and evolving nature of cardiac intimal sarcoma treatment, emphasizing the potential of PDGFRß signaling as a strategic target and highlighting the importance of adapting treatment pathways in response to genetic shifts.

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Several high-grade pleomorphic sarcoma cases that cannot be classified into any existing established categories have been reported. These cases were provisionally classified into undifferentiated pleomorphic sarcoma (UPS). Some dedifferentiated liposarcoma (DDLS) cases may also have been classified into the UPS category due to the absence of MDM2 amplification or an atypical lipomatous tumor/well-differentiated liposarcoma component. We retrieved and reviewed 77 high-grade pleomorphic sarcoma cases, initially diagnosed as UPS in 66 cases and DDLS in 11 cases. Fluorescence in situ hybridization (FISH) analyses of DDIT3 and MDM2 were performed for available cases. Of the cases successfully subjected to DDIT3 FISH (n = 56), nine (7 UPS and 2 DDLS) showed DDIT3 amplification but no MDM2 amplification. Two UPS cases showed both telomeric (5') and centromeric (3') amplification of DDIT3 or low polysomy of chromosome 12, whereas 5 UPS and 2 DDLS cases showed 5'-predominant DDIT3 amplification. Histopathologically, all cases showed UPS-like proliferation of atypical pleomorphic tumor cells. Immunohistochemically, only one case showed focal nuclear positivity for DDIT3, supporting the previous finding that DDIT3 expression was not correlated with DDIT3 amplification. All three cases with focal MDM2 expression involved 5'-predominant amplification, two of which showed DDLS-like histological features. The majority of cases (7/9) showed decreased expression in p53 staining, suggesting that DDIT3 amplification regulates the expression of TP53 like MDM2. From a clinicopathological perspective, we hypothesize that DDIT3-amplified sarcoma, especially with 5'-predominant amplification, can be reclassified out of the UPS category.

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Myxofibrosarcomas (MFS) present as slowly enlarging superficial masses in elderly patients. Even though these tumors fail to exhibit a distinct immunophenotype, diagnosis is straightforward when they present in subcutaneous tissue. Intramuscular MFS, however, are more challenging to diagnose as the differential also includes dedifferentiated liposarcoma with myxoid features. The vast majority of dedifferentiated liposarcomas show MDM2 amplification, whereas limited data exists as to the MDM2 status of MFS. We sought to explore the rate of MDM2 amplification in cases of classic MFS. Our archives were searched for MFS; only subcutaneous well-sampled resections were included. FISH for MDM2 amplification was performed on each tumor. A cohort of myxoid dedifferentiated liposarcoma resections was studied for comparison. Twenty-two MFS arose in patients aged 44 to 85 years. All tumors contained an infiltrative population of atypical cells embedded in a myxoid stroma with curvilinear blood vessels. MDM2 amplification by FISH was identified in 3 (of 22; 14%) tumors. Available follow up on 17 patients (range 1-96 months; median 13 months) revealed 6 patients with local recurrence and 1 with distant metastasis. Of 3 patients with MDM2- amplified MFS, 1 experienced recurrence and died of unrelated causes, while the second was alive without disease 12 months after diagnosis. Even though the rate of MDM2 amplification by FISH in MFS appears to be low, a subset of cases may show this genetic alteration, which pathologists should be aware of to avoid misclassification as myxoid dedifferentiated liposarcomas. Further studies are necessary to determine if amplification status adds prognostic value.

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Well-differentiated and dedifferentiated liposarcomas (WDLPS and DDLPS) are rare tumors that arise from lipocytes in soft tissue. There is a high unmet need in patients with these liposarcomas given poor outcomes, particularly for DDLPS. WDLPS and DDLPS share important genetic and histological characteristics - most notably, the amplification of the 2 genes MDM2 and CDK4. Both genes are considered oncogenes because of their ability to shut down tumor suppressor pathways. There are multiple therapeutic approaches that aim to target MDM2 and CDK4 activity for the purpose of restoring intrinsic tumor suppressor cellular response and terminating oncogenesis. However, current understanding of the molecular mechanisms involved in WDLPS and DDLPS pathology is limited. In recent years, significant efforts have been made to refine and implement targeted therapy for this patient population. The use of patient-derived cell and tumor xenograft models has been an important tool for recapitulating WDLPS and DDLPS biology. These models also offer valuable insights for drug development and drug combination studies. Here we offer a review of the current understanding of WDLPS and DDLPS biology and its therapeutic implications.

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The MDM2 proto-oncogene (MDM2) is a primary negative regulator of p53. The latter is frequently mutated in gastric cancer (GC). In the present study, we aimed to validate gene amplification, protein expression, and the putative tumor biological function of MDM2 in a well-characterized Western GC cohort. MDM2 amplification and protein expression were studied in a cohort of 327 GCs by fluorescence in situ hybridization (FISH) and immunohistochemistry. Gene amplification and protein expression were correlated with diverse clinicopathological patient characteristics including patient outcome. Immunohistochemically, 97 GCs (29.7%) were categorized as MDM2 positive and 230 GCs (70.3%) as negative. An amplification of MDM2 was found in 11 (3.4%) cases without evidence of intratumoral heterogeneity. Nine of these eleven (81.8%) cases showed MDM2 protein expression. MDM2 amplification correlated significantly with MDM2 protein expression (p < 0.001). On a case-by-case analysis, MDM2-amplified cases showed varied histological phenotypes and were most commonly microsatellite stable; EBV, HER2, and MET negative; and FGFR2 positive. A single case harbored both, MDM2 amplification and TP53 mutation. MDM2 amplification and MDM2 expression, respectively, did not correlate with overall or tumor-specific survival. Our targeted analysis of MDM2 in a well-characterized cohort of GC patients showed that MDM2 amplification is rare, of no specific histological phenotype, and may not be always mutually exclusive with TP53 mutations. Given the low number of cases, currently, no diagnostic or therapeutic recommendation related to MDM2 amplification can be given for GC of Western origin.

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Liposarcomas originating in the urinary bladder are extremely rare. Only six cases of bladder liposarcoma have been reported, and all have been described as myxoid liposarcomas. Notably, none of the patients underwent molecular testing. Here, we report a dedifferentiated liposarcoma (DDL) that occurred in the urinary bladder, primarily in a 69-year-old Chinese woman, with infrequent low-grade dedifferentiation. Computed tomography (CT) revealed an ill-defined solid mass in the anterior bladder wall. The patient underwent a partial bladder resection. Histologically, the tumor cells with mild-to-moderate nuclear atypia were arranged in fascicular and storiform patterns, mimicking a low-grade fibroblastic tumor. In addition, scattered small foci of typical lipoma-like well-differentiated components were identified. Immunohistochemically, the tumor tested positivity for MDM2, CDK4, and p16. Fluorescence in situ hybridization revealed MDM2 gene amplification in the neoplastic cells. Whole-exome sequencing showed that this tumor also harbored CDK4, TSPAN31, and JUN amplification. At the latest follow-up (85 months after surgery), the patient was alive, with no evidence of disease. To the best of our knowledge, this is the first example of a molecularly confirmed primary bladder liposarcoma and the first case of DDL at this site.

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Among sarcomas, MDM2 amplification is usually a molecular hallmark of well-differentiated liposarcoma and dedifferentiated liposarcoma (DDLPS) and occasionally a secondary genetic anomaly in other sarcomas. Histological evaluation and FISH analysis showing MDM2 amplification led to the diagnosis of DDLPS for a tumor located on the left arm of a 71-year-old patient. The patient was treated by adjuvant radiotherapy (RT) but the tumor recurred soon after. Array-comparative genomic hybridization and targeted RNA/DNA sequencing of the primary tumor and of four recurrences were done. Strikingly, the MDM2 amplification observed in the primary tumor had vanished in the recurrences. In contrast, other rearrangements, such as amplification of the genes TRIO and TERT as well as TRIO::TERT fusion were detected retrospectively in the primary tumor and in all the recurrences. The transitory nature of the MDM2 amplification raised the hypothesis that RT was active on cells that contained MDM2 amplification but not on other tumor cells with only the TERT and TRIO alterations. In contrast to MDM2 amplification, the TRIO::TERT amplified fusion was stable over time. The detection of this fusion was crucial in the analysis of the diagnostically challenging last tumor; it allowed to determine that it was a fourth recurrence, instead of a new independent tumor. It also suggested the diagnosis undifferentiated pleomorphic sarcoma rather than DDLPS. The TRIO::TERT fusion is not well explored. The current study shows that its role in sarcomas, with or without MDM2 amplification, should be more extensively researched.

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Detection of MDM2 gene amplification via fluorescence in situ hybridization (FISH) and MDM2 overexpression by immunohistochemistry (IHC) have been utilized for the diagnosis of low-grade osteosarcoma (LGOS). The aim of this study was to evaluate the diagnostic value of MDM2 RNA in situ hybridization (RNA-ISH) and compare this assay with MDM2 FISH and IHC in distinguishing LGOS from its histologic mimics. MDM2 RNA-ISH, FISH and IHC were performed on nondecalcified samples of 23 LGOSs and 52 control cases. Twenty (20/21, 95.2%) LGOSs were MDM2-amplified, and two cases failed in FISH. All control cases were MDM2-nonamplified. All 20 MDM2-amplified LGOSs and one MDM2-nonamplified LGOS harboring TP53 mutation and RB1 deletion showed positivity for RNA-ISH. Fifty of the 52 (96.2%) control cases were negative for RNA-ISH. The diagnostic sensitivity and specificity of MDM2 RNA-ISH were 100.0% and 96.2%, respectively. Nineteen of the 23 LGOSs were evaluated by MDM2 RNA-ISH and FISH in decalcified samples simultaneously. All decalcified LGOSs failed in FISH and most samples (18/19) were no staining in RNA-ISH. Fifteen (15/20, 75%) MDM2-amplified LGOSs were positive for IHC and 96.2% (50/52) of control cases were negative. The sensitivity of RNA-ISH (100%) was higher than that of IHC (75%). In conclusion, MDM2 RNA-ISH has great value for the diagnosis of LGOS, with excellent consistency with FISH and better sensitivity than IHC. Acid decalcification still has an adverse impact on RNA. Some MDM2-nonamplified tumors may show positivity for MDM2 RNA-ISH, which needs to be analyzed comprehensively in combination with clinicopathological features.

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Transcription factor EB (TFEB)-rearranged renal cell carcinoma (RCC) exhibits diverse gene fusion patterns and heterogeneous clinicopathologic features. Rare TFEB-amplified RCCs have been described recently and are associated with a more aggressive clinical course. Herein, we report a case of an 86-year-old man with a solid 9.2-cm kidney tumor that showed a diffuse high-grade sarcomatoid morphology. The tumor demonstrated a novel BYSL::TFEB fusion containing exons 1-2 of the BYSL gene fused to exons 3-10 of TFEB via next-generation sequencing by using NextSeq sequencer. Fluorescence in situ hybridization (FISH) studies displayed concurrent high-copy number TFEB amplification in two distinct patterns, a balanced increase of 5' and 3' copies, and solely increased 5' copies, and mouse double minute 2 (MDM2) gene amplification by using TFEB (6p21.1) dual-color break-apart probe and MDM2 FISH probe. Notably, the tumor showed a distinctive immunoprofile with overexpressions of TFEB, epithelial membrane antigen, Cathepsin K, and PDL-1 (SP263). FISH test for transcription factor binding to IGHM enhancer 3 (TFE3) was negative for rearrangement and corresponding immunonegativity of TFE3. These findings not only expand the repertoire of known TFEB fusion partners implicated in tumorigenesis, but also may provide novel information for target therapy.

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MDM2 amplification represents the leading oncogenic pathway and diagnostic hallmark of liposarcoma, whose assessment is based on Fluorescence In Situ Hybridization (FISH) analysis. Despite its diagnostic relevance, no univocal interpretation criteria regarding FISH assessments of MDM2 amplification have been established so far, leading to several different approaches and potential diagnostic misinterpretations. This study aims to address the most common issues and proposes troubleshooting guidelines for MDM2 amplification assessments by FISH. We retrospectively retrieved 51 liposarcomas, 25 Lipomas, 5 Spindle Cell Lipoma/Pleomorphic Lipomas, and 2 Atypical Spindle Cell Lipomatous Tumors and the corresponding MDM2 FISH analysis. We observed MDM2 amplification in liposarcomas cases only (43 out of 51 cases) and identified three MDM2-amplified patterns (scattered (50% of cases), clustered (14% of cases), and mixed (36% of cases)) and two nonamplified patterns (low number of signals (82% of cases) and polysomic (18% of cases)). Based on these data and published evidence in the literature, we propose a set of criteria to guide MDM2 amplification analysis in liposarcoma. Kindled by the compelling importance of MDM2 assessments to improve diagnostic and therapeutic liposarcoma management, these suggestions could represent the first step to develop a univocal interpretation model and consensus guidelines.

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BACKGROUND: Murine double minute 2 (MDM2) is an oncogene that inhibits p53, leading to decreased apoptosis. Sarcomas showing MDM2 amplification are rare among pediatric patients. CASE: A 14-year-old boy presented with pleomorphic sarcoma of the head showing MDM2 amplification without a well-differentiated liposarcoma component. Although chemotherapy was initially performed to reduce the tumor size before surgery, the tumor did not shrink. The patient underwent complete surgical resection. Microscopic examination revealed a positive surgical margin; thus, postoperative proton-beam radiotherapy was performed. 3 years after the therapy, no sign of recurrence was observed. CONCLUSION: Macroscopic surgical resection combined with adjuvant postoperative radiotherapy was effective against MDM2-amplified pleomorphic sarcoma refractory to neoadjuvant chemotherapy in a pediatric patient.

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Despite recent advances in treatment and surveillance, metastatic melanoma still carries a poor prognosis. Large/giant congenital melanocytic nevi (CMNs) constitute a known risk factor for the condition, with the greatest risk for malignant transformation thought to be during childhood (median age at diagnosis of 3 years in a previous cohort). Herein, we present the case of a 30-year-old male who, after undergoing multiple excision/grafting procedures for a giant CMN as a child, was diagnosed with an NRAS-mutant, MDM2-amplified metastatic melanoma more than 20 years later. Response to ipilimumab/nivolumab immunotherapy, cisplatin/vinblastine/temozolomide chemotherapy, and nivolumab/relatlimab immunotherapy was poor. This case highlights the importance of lifetime monitoring with once-yearly dermatological examination (including lymph node palpation) in large/giant CMN patients, as well as the need for further clinical trials evaluating novel therapies for NRAS-mutant melanoma.

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We report the histological, immunohistochemical, and molecular findings of a dedifferentiated liposarcoma with inflammatory myofibroblastic tumor-like features occurring in the paratesticular region. Histologically, the dedifferentiated component closely resembled an inflammatory myofibroblastic tumor. The neoplastic cells were positive for smooth muscle actin with focal CD56, CD99, Bcl2 and EMA expression. WT1, calretinin, myogenin, CK(AE1/AE3), desmin, H-caldesmon, CD34, ALK, CKIT, DOG1, MUC4 and STAT6 were negative. MDM2 showed diffuse and strong nuclear positivity in neoplastic cells and fluorescence in situ hybridization (FISH) revealed amplified MDM2 (high level) but no SYT rearrangement. Although a lipomatous component was evident macroscopically, well-differentiated liposarcomatous components were not evident in the section examined. Dedifferentiated liposarcoma can have prominent inflammatory myofibroblastic tumor-like features. Pathologists should be aware of this histological variant in order to avoid misdiagnosing dedifferentiated liposarcoma as inflammatory myofibroblastic tumor or other spindle cell tumors which have different behavioral patterns and treatment requirements.

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Se presenta un estudio histológico, inmunohistoquímico (IHQ) y molecular de un liposarcoma desdiferenciado paratesticular remitido a nuestro centro, con hallazgos histológicos similares a un tumor miofibroblástico inflamatorio. Las células tumorales fueron positivas para actina músculo liso (AML) y focalmente positivas para CD56, CD99, Bcl2 y EMA. La expresión de WT1, calretinina, miogenina, CK(AE1/AE3), desmina, H-caldesmona, CD34, ALK, CKIT, DOG1, MUC4 y STAT6 fue negativa. MDM2 mostró positividad nuclear intensa y difusa por IHQ y alto nivel de amplificación génica mediante hibridación fluorescente in situ (FISH). La FISH no reveló reordenamiento del gen SYT. En el estudio histológico del corte remitido no encontramos evidencias de componente liposarcomatoso bien diferenciado, aunque el aspecto macroscópico de la pieza lo sugería. El liposarcoma desdiferenciado puede presentar hallazgos histológicos que recuerdan a un tumor miofibroblástico inflamatorio y que expanden el espectro histológico de esta variante de liposarcoma. El conocimiento de la existencia de esta variante de liposarcoma es de crucial importancia para no confundirla con otras neoplasias que, aunque histológicamente similares, difieren en la evolución clínica y/o tratamiento.(AU)

We report the histological, immunohistochemical, and molecular findings of a dedifferentiated liposarcoma with inflammatory myofibroblastic tumor-like features occurring in the paratesticular region. Histologically, the dedifferentiated component closely resembled an inflammatory myofibroblastic tumor. The neoplastic cells were positive for smooth muscle actin with focal CD56, CD99, Bcl2 and EMA expression. WT1, calretinin, myogenin, CK(AE1/AE3), desmin, H-caldesmon, CD34, ALK, CKIT, DOG1, MUC4 and STAT6 were negative. MDM2 showed diffuse and strong nuclear positivity in neoplastic cells and fluorescence in situ hybridization (FISH) revealed amplified MDM2 (high level) but no SYT rearrangement. Although a lipomatous component was evident macroscopically, well-differentiated liposarcomatous components were not evident in the section examined. Dedifferentiated liposarcoma can have prominent inflammatory myofibroblastic tumor-like features. Pathologists should be aware of this histological variant in order to avoid misdiagnosing dedifferentiated liposarcoma as inflammatory myofibroblastic tumor or other spindle cell tumors which have different behavioral patterns and treatment requirements.(AU)

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Ossifying fibroma of the craniofacial bones is a fibro-osseous lesion characterized by varied patterns of bone formation in a fibroblastic stroma. Ossifying fibroma is a putatively benign lesion with no reports of malignant transformation or metastasis. Differentiation from other fibro-osseous lesions can be challenging necessitating synthesis of clinical, radiological and pathological findings. The molecular pathogenesis of ossifying fibroma is poorly understood but recent studies have reported MDM2 gene amplification and chromosomal copy number changes in a subset of ossifying fibromas. MDM2 amplification in ossifying fibroma, if true, presents a diagnostic problem because this genetic event, at least among craniofacial fibro-osseous lesions, was previously considered specific for low-grade osteosarcoma. In the present study, we investigated the utility of MDM2 and CDK4 immunohistochemistry, and fluorescence in situ hybridization for MDM2 gene amplification, in the diagnosis of 44 craniofacial bone ossifying fibromas. Focal MDM2 and CDK4 nuclear immunoreactivity was found in 11 and 1 ossifying fibromas, respectively, but none demonstrated MDM2 amplification by fluorescence in situ hybridization. A single tumor displayed MDM2 amplification without nuclear immunoreactivity to either MDM2 or CDK4. Our data suggest that while focal MDM2 and CDK4 nuclear expression may be detected in a minority of ossifying fibromas, this expression does not correlate with MDM2 amplification. In addition, MDM2 amplification is extremely rare in ossifying fibroma so the detection of this genetic abnormality should continue to raise concern for osteosarcoma.

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Dermatofibrosarcoma protuberans (DFSP) is characterized by collagen type I alpha 1 chain-platelet-derived growth factor B chain (COL1A1-PDGFB) fusion. We present a case of fibrosarcomatous DFSP with lung metastasis in a 53-year-old man. Histologically, the primary and metastatic tumors were composed of high-grade fibrosarcomatous component with varying myxoid changes, while only a small focus of the classic DFSP element was identified in the primary lesion. No evidence of COL1A1-PDGFB fusion was identified by routine fluorescence in situ hybridization (FISH). Subsequent next-generation sequencing and COL1A1 break-apart FISH identified the fusion. In addition, coamplification of 12q15 and 12p13, along with CDKN2A/2B deletion, was confirmed to be limited to the fibrosarcomatous component. The current case is a novel FS-DFSP with cryptic COL1A1-PDGFB fusion. This is the first published example of DFSP harboring coamplification of 12q and 12p sequences. More importantly, the genetic aberrations restricted to the fibrosarcomatous component indicated a synergistic role of higher-grade progression.

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A 76-year-old woman was referred for 6 months of progressively worsening dysphagia and unintentional weight loss. An esophagogastroduodenoscopy demonstrated an area of extrinsic compression in the lower esophagus measuring 7 cm in greatest dimension. Contrast-enhanced computed tomography revealed a solid homogeneous mass in the lower middle/posterior mediastinum, laterally displacing the esophagus. Endoscopic ultrasound-guided fine-needle biopsy showed a hypocellular infiltrate of pleomorphic cells in a loose collagenous matrix. By immunohistochemistry, neoplastic cells were negative for epithelial, vascular, neural, and melanocytic markers. Fluorescent in situ hybridization detected MDM2 amplification, compatible with a diagnosis of dedifferentiated liposarcoma.

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Ameloblastic fibrosarcoma (AFS) is the most common odontogenic sarcoma, but the incidence is relatively low, and its molecular biology is poorly understood. We experienced a young female patient with a rapidly growing soft tissue tumor of the left maxilla, which eventually occupied the left side of the oral cavity. Histologically, the tumor mainly consisted of a proliferation of atypical spindle to polygonal cells without any specific differentiation, but a small number of benign odontogenic epithelial foci mainly in the tumor periphery were also noted; thus, a diagnosis of AFS was made. We performed whole-exome sequencing (WES) on the tumor to investigate its molecular features and identify therapeutic options. We found that the tumor harbored EGFR exon 20 insertions and MDM2 amplification; the former may be a target for newly developed tyrosine kinase inhibitors in case of recurrence. To the best of our knowledge, this is the first case of AFS for which WES was performed and with EGFR mutation. Our case provides new genetic information on AFS and suggests that comprehensive genetic analysis can clarify the molecular biology in rare cancers, potentially leading to the proposal of therapeutic strategies.

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