Novel NCOA2/3-rearranged low-grade fibroblastic spindle cell tumors: A report of five cases.

Spindle cell mesenchymal neoplasms are a diverse and often challenging diagnostic group. While morphological impression is sufficient for some diagnoses, increasingly immunohistochemical and even molecular data is required to render an accurate diagnosis, which can lead to the characterization of new entities. We describe five cases of novel mesenchymal neoplasms with rearrangements in the NCOA2 and NCOA3 genes partnered with either CTCF or CRTC1. Three tumors occurred in the head and neck (palate, auditory canal), while the other two were in visceral organs (lung, urinary bladder). All cases occurred in adults (range 33-86) with a median age of 42 and fairly even sex distribution = (male-to-female = 3:2). Morphologically, they had similar features consisting of monotonous, bland spindle to ovoid cells with fascicular and reticular arrangements in a myxohyaline to collagenous stroma. However, immunophenotypically they had essentially a null phenotype, with only two tumors staining partially for CD34 and smooth muscle actin. Targeted RNA sequencing detected in-frame CTCF::NCOA2 (one case), CRTC1::NCOA2 (two cases), and CTCF::NCOA3 (two cases) fusions. Treatment was surgical resection in all cases. Local recurrence and/or distant metastases were not observed in any case (median follow-up, 7.5 months; range, 2-19 months). Given their morphologic, immunohistochemical, and molecular similarities, we believe that these cases may represent an emerging family of low-grade NCOA2/3-rearranged fibroblastic spindle cell neoplasms.

Clear Cell Stromal Tumor of Lung: A Clinicopathologic, Immunohistochemical, and Molecular Characterization of 8 Cases.

Clear cell stromal tumor (CCST) is a recently described mesenchymal neoplasm of the lung, characterized by spindle cells with variably clear-to-pale eosinophilic cytoplasm and prominent vascularity, as well as a recurrent YAP1::TFE3 gene fusion in most cases. Diagnosis can be challenging given its rarity and the lack of supportive immunohistochemical markers aside from TFE3. To date, less than 20 cases have been reported and data on clinical behavior are also limited. While most appear to be benign, aggressive behavior has been reported rarely. Here, we present the largest multi-institutional series of CCST to date, comprising a total of 8 cases and including 6 previously unpublished cases. We investigate its clinicopathologic and genomic features, while also assessing the diagnostic utility of immunohistochemistry (IHC) for YAP1 C-terminus (YAP-CT). Five patients were male and three were female. The median age was 59 years (range: 35 - 84). In all cases, a TFE3 rearrangement was demonstrated by either FISH or DNA / RNA sequencing. In 7 tumors, the YAP1::TFE3 fusion was identified by sequencing. We demonstrate that the combination of YAP1-CT loss and TFE3 overexpression by IHC reliably predicts an underlying YAP1::TFE3 fusion in these neoplasms and may be more sensitive than TFE3 FISH. Although the median follow-up time for our study was short (18 months, available in 7 cases), all cases pursued a benign clinical course, with no recurrences or metastases. Our study provides further characterization of this novel entity, supporting its wider recognition.

Novel PAX3::MAML3 Fusion Identified in Alveolar Rhabdomyosarcoma, Using DNA Methylation Profiling to Expand the Genetic Spectrum of "Fusion-Positive" Cases.

Alveolar rhabdomyosarcoma (ARMS) with FOXO1 gene rearrangements is an aggressive pediatric rhabdomyosarcoma subtype that is prognostically distinct from embryonal rhabdomyosarcoma and fusion-negative ARMS. Here, we report 2 cases of ARMS with PAX3::MAML3 fusions. The tumors arose in an infant and an adolescent as stage IV metastatic disease (by Children's Oncology Group staging system). Histologically, both cases were small round blue cell tumors arranged in vague nests and solid sheets that were diffusely positive for desmin and myogenin. By methylation profiling and unsupervised clustering analysis, the tumors clustered with ARMS with classic FOXO1 rearrangements and ARMS with variant PAX3::NCOA1/INO80D fusions, but not with biphenotypic sinonasal sarcoma (BSNS) with PAX3::MAML3/NCOA2/FOXO1/YAP1 fusions nor with other small round blue cell tumors, including embryonal rhabdomyosarcoma. The differentially methylated genes between ARMS and BSNS were highly enriched in genes involved in myogenesis, and 21% of these genes overlap with target genes of the PAX3::FOXO1 fusion transcription factor. On follow-up after initiation of vincristine/actinomycin/cyclophosphamide chemotherapy, the tumors showed partial and complete clinical responses, consistent with typical upfront chemotherapy responsiveness of ARMS with the classic FOXO1 rearrangement. We conclude that PAX3::MAML3 is a novel variant fusion of ARMS, which displays a methylation signature distinct from BSNS despite sharing similar PAX3 fusions. These findings highlight the utility of methylation profiling in classifying ARMS with noncanonical fusions.

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

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

Novel CRTC1::MRTFB(MKL2) Gene Fusion Detected in Myxoid Mesenchymal Neoplasms With Myogenic Differentiation Involving Bone and Soft Tissues.

Appropriate classification of fusion-driven bone and soft tissue neoplasms continues to evolve, often relying on the careful integration of morphologic findings with immunohistochemical, molecular, and clinical data. Herein, we present 3 cases of a morphologically distinct myxoid mesenchymal neoplasm with myogenic differentiation and novel CRTC1::MRTFB (formerly MKL2) gene fusion. Three tumors occurred in 1 male and 2 female patients with a median age of 72 years (range: 28-78). Tumors involved the left iliac bone, the right thigh, and the left perianal region with a median size of 4.0 cm (4.0-7.6 cm). Although 1 tumor presented as an incidental finding, the other 2 tumors were noted, given their persistent growth. At the time of the last follow-up, 1 patient was alive with unresected disease at 6 months, 1 patient was alive without evidence of disease at 12 months after surgery, and 1 patient died of disease 24 months after diagnosis. On histologic sections, the tumors showed multinodular growth and were composed of variably cellular spindle to round-shaped cells with distinct brightly eosinophilic cytoplasm embedded within a myxoid stroma. One tumor showed overt smooth muscle differentiation. Cytologic atypia and mitotic activity ranged from minimal (2 cases) to high (1 case). By immunohistochemistry, the neoplastic cells expressed focal smooth muscle actin, h-caldesmon, and desmin in all tested cases. Skeletal muscle markers were negative. Next-generation sequencing detected nearly identical CRTC1::MRTFB gene fusions in all cases. We suggest that myxoid mesenchymal tumors with myogenic differentiation harboring a CRTC1::MRTFB fusion may represent a previously unrecognized, distinctive entity that involves soft tissue and bone. Continued identification of these novel myxoid neoplasms with myogenic differentiation will be important in determining appropriate classification, understanding biologic potential, and creating treatment paradigms.

The utility of DNA methylation profiling in the diagnosis of un-, de- and trans-differentiated melanoma: a series of 11 cases.

AIMS: Melanomas are recognised for their remarkable morphological plasticity. Some tumours may lose conventional features and/or acquire non-melanocytic characteristics, referred to as undifferentiated, dedifferentiated and transdifferentiated melanoma. Despite this phenotypical variability, melanomas typically maintain their cancer driver aberrations, affecting genes such as BRAF, NRAS and NF1. Currently, little is known about whether the DNA methylation profile follows the loss or change of differentiation or is retained despite extensive morphological transformation. METHODS AND RESULTS: In this study we analysed 11 melanoma cases, comprising six males and five females, with a median age of 67 years, including five undifferentiated, four trans-differentiated and two de-differentiated melanomas. Undifferentiated and trans-differentiated tumours either arose in a patient with known melanoma and/or presented in the groin/axilla with molecular alterations consistent with melanoma. Cases with heterologous differentiation resembled chondrosarcoma, osteosarcoma, angiosarcoma and rhabdomyosarcoma both morphologically and immunohistochemically, while undifferentiated tumours resembled undifferentiated pleomorphic sarcoma. Methylome profiling was performed, and unsupervised clustering analysis revealed nine cases (five undifferentiated, three trans-differentiated and one de-differentiated) to cluster closely together with conventional melanomas from a reference set. Two cases clustered separately with a distinct group of conventional melanomas exhibiting H3K27me3 loss. CONCLUSIONS: Despite loss of differentiation and phenotypical plasticity, methylation patterns seem to be retained in undifferentiated, de-differentiated and trans-differentiated melanomas and represent useful diagnostic tools to enhance diagnostic precision in these diagnostically challenging cases.

ALK-rearranged, CD34-positive spindle cell neoplasms resembling dermatofibrosarcoma protuberans: a study of seven cases.

AIMS: The majority of dermatofibrosarcoma protuberans (DFSP) harbour PDGFB or PDGFD rearrangements. We encountered ALK expression/rearrangement in a PDGFB/D-negative CD34-positive spindle cell neoplasm with features similar to DFSP, prompting evaluation of ALK-rearrangements in DFSP and plaque-like CD34-positive dermal fibroma (P-LDF). METHODS AND RESULTS: We searched the archives of academic institutions for cases previously coded as DFSP and P-LDF. NGS-naïve or PDGFB-negative DFSP were screened for ALK (clone D5F3) expression by immunohistochemistry. NGS or ALK FISH was performed on ALK-positive cases. Methylome profiling studies were performed and compared with conventional DFSP. One case of "DFSP" and two "P-LDF" with ALK expression were identified from the archives, while four cases were detected prospectively. These seven cases (6F:1M; 8 months to 76 years) arose in the dermis of the arm (two), scalp, eyelid, thigh, abdomen, and shoulder and ranged from 0.4 to 4.2 cm. Tumours were composed of spindled cells and displayed a storiform growth pattern. Cytologic atypia was absent, and mitotic figures were scarce (0-2/10 HPFs, high power fields). The lesional cells were diffusely positive for CD34 and ALK and negative for S100 protein. By NGS (n = 5), ALK fusion partners included DCTN1 (2), PLEKHH2, and CLIP2 in DFSP-like cases and FLNA in P-LDF-like lesions. ALK FISH was positive in one (of two) cases previously labelled P-LDF. Methylome profiling of two (of three) ALK-rearranged DFSP-like tumours showed clustering with conventional DFSP in the UMAP dimension reduction plot. To date, no tumour has recurred (n = 2; 26, 27 months). CONCLUSION: We describe a cohort of novel ALK-rearranged tumours with morphologic features similar to DFSP.

Nuclear DUX4 immunohistochemistry is a highly sensitive and specific marker for the presence of CIC::DUX4 fusion in CIC-rearranged sarcomas: a study of 48 molecularly confirmed cases.

AIMS: CIC-rearranged sarcomas (CRS) are clinically aggressive undifferentiated round cell sarcomas (URCS), commonly driven by CIC::DUX4. Due to the repetitive nature of DUX4 and the variability of the fusion breakpoints, CIC::DUX4 fusion may be missed by molecular testing. Immunohistochemical (IHC) stains have been studied as surrogates for the CIC::DUX4 fusion. We aim to assess the performance of DUX4 IHC in the work-up of CRS and its expression in non-CRS round cell or epithelioid neoplasms. METHODS AND RESULTS: Cases of molecularly confirmed CRS (n = 48) and non-CRS (n = 105) were included. CRS cases consisted of 35 females and 13 males, with ages ranging from less than 1 year to 67 years (median = 41 years). Among the molecularly confirmed non-CRS cases, C-terminal DUX4 expression was investigated in Ewing sarcomas (38 cases), alveolar rhabdomyosarcomas (18 cases), desmoplastic small round cell tumours (12 cases) and synovial sarcomas (n = five), as well as in non-mesenchymal neoplasms such as SMARCA4/SMARCB1-deficient tumours (n = five), carcinomas of unknown primary (n = three) and haematolymphoid neoplasms (four cases). DUX4 IHC was considered positive when strong nuclear expression was detected in more than 50% of neoplastic cells. When used as a surrogate for the diagnosis of CRS, the sensitivity and specificity of DUX4 IHC was 98 and 100%, respectively. Only one CRS case was negative for DUX4 IHC and harboured a CIC::FOXO4 fusion. CONCLUSIONS: DUX4 IHC is a highly sensitive and specific surrogate marker for the presence of CIC::DUX4 fusion, demonstrating its utility in establishing a diagnosis of CRS.

DNA Methylation Profiling Distinguishes Adamantinoma-Like Ewing Sarcoma From Conventional Ewing Sarcoma.

Adamantinoma-like Ewing sarcoma (ALES) has traditionally been considered a variant of Ewing sarcoma because it generally harbors EWSR1::FLI1 fusions despite showing diffuse positivity for keratins and p40. However, it has become increasingly recognized that different tumors can have identical translocations, including shared fusions between carcinomas and sarcomas, raising questions as to whether ALES might represent a separate entity. Using methylation profiling, we further explored the relationship between Ewing sarcoma and ALES. The archives of multiple institutions were searched for candidate cases of ALES. DNA methylation profiling was performed and results were compared to corresponding data from conventional Ewing sarcoma. Twelve cases of ALES (5 previously reported) were identified in 10 men and 2 women (aged 20-72 years; median age, 41.5 years). Cases included tumors arising in the parotid gland (3), sinonasal cavity (2), submandibular gland (2), thyroid gland (1), neck (1), gingiva (1), hypopharynx (1), and mandible (1). Histologic review consistently showed sheets and nests of basaloid cells within a fibromyxoid or hyalinized stroma. All tumors were positive for at least 1 keratin and CD99 expression, whereas all 10 cases tested were positive for p63 or p40; S100 protein expression was noted in 2 cases. Cases harbored either EWSR1::FLI1 fusions (n = 6), FUS::FLI1 fusions (n = 1), and/or EWSR1 rearrangements (n = 6). Methylation profiling was successful in 11/12 cases evaluated. Unsupervised clustering and dimensionality reduction (Uniform Manifold Approximation and Projection) of DNA methylation data revealed a distinct methylation cluster for all 11 cases, including the tumor with the FUS::FLI1 fusion, which clearly segregated them from the conventional Ewing sarcoma. Follow-up (n = 11, 1-154 months) revealed that 4 patients experienced recurrence and 6 developed metastatic disease. ALES demonstrates a distinct methylation signature from conventional Ewing sarcoma. This finding adds to the distinctive immunoprofile of ALES, suggesting that these 2 tumors should be considered distinct entities rather than histologic extremes of the same disease.

Novel EWSR1::GFI1B gene fusion in angiofibroma of soft tissue.

AIMS: Angiofibroma of soft tissue is a benign soft tissue tumour characterised by bland spindle cells and a distinct branching vascular network. The majority of soft tissue angiofibromas harbour AHRR::NCOA2 gene fusions. Here we present three cases of EWSR1::GFI1B-fused soft tissue tumours that are morphologically most reminiscent of soft tissue angiofibroma. METHODS AND RESULTS: All three cases presented in male patients with an age range of 35-78 years (median = 54 years). Two cases presented as subcutaneous nodules on the trunk (posterior neck and chest wall); one was an intramuscular foot mass. The tumours were unencapsulated nodules with infiltrative margins ranging from 2.2 to 3.4 cm in greatest dimension. Histologically, the tumours contained uniformly bland fibroblastic spindle cells with ovoid to fusiform nuclei and delicate cytoplasmic processes embedded in a myxoid to myxocollagenous stroma. All three cases were characterised by a thin-walled, branching vascular network evenly distributed throughout the tumour. Overt cytological atypia or conspicuous mitotic activity was absent. The spindle cells had an essentially null immunophenotype. By targeted RNA sequencing, an in-frame gene fusion between EWSR1 exons 1-7 and GFI1B exons 6-11 or 7-11 was detected in all three cases. The tumours were marginally excised. For all three cases, there were no documented local recurrence or distant metastases during a limited follow-up period of 6-10 months. CONCLUSIONS: We propose that EWSR1::GFI1B may represent a novel fusion variant of soft tissue angiofibroma.

Keratin-positive giant cell-rich tumors of soft tissue with HMGA2::NCOR2 fusions.

BACKGROUND: Giant cell tumor of soft tissue (GCT-ST) is a rare soft tissue neoplasm that is morphologically similar to but genetically distinct from giant cell tumor of bone. A novel keratin-positive GCT-ST (KPGCT-ST) harboring HMGA2::NCOR2 fusions was recently discovered. Fewer than 30 cases have been described; herein is reported an additional seven. METHODS: Cases diagnosed as GCT-ST were retrieved from institutional archives and consultation files. The histopathologic characteristics were assessed, and the electronic medical record was reviewed. RESULTS: Seven tumors were identified in six women and one man with a median age of 23 years. All patients underwent excision; no recurrences or metastases were noted during a median follow-up period of 7 months. Histopathologically, the tumors were characterized by a multinodular proliferation of keratin-positive mononuclear cells with evenly admixed osteoclast-like giant cells and absent neoplastic bone. A fibrous capsule with lymphoid cuffing was frequently seen. Foamy macrophages, inflammation, hemorrhage, and hemosiderin were variably present. The HMGA2::NCOR2 fusion was detected in all cases. CONCLUSIONS: Our findings support previously reported hypotheses that KPGCT-ST is a spectrum of the same entity as the recently described xanthogranulomatous epithelial tumor. Although follow-up data are limited, to date, KPGCT-ST appears to follow an indolent course.

The capillary lobule variant of radiation-associated angiosarcoma in the setting of breast cancer: A diagnostic pitfall.

AIM: Post-radiation angiosarcoma is an iatrogenic event seen in the setting of breast cancer treatment. Histopathologically, there are morphologic variants of angiosarcoma that mimic benign entities, including the capillary lobule variant of post-radiation angiosarcoma. We present the largest case series to date of this histopathologic variant of post-radiation angiosarcoma. METHODS AND RESULTS: Cases of the capillary lobule variant of post-radiation angiosarcoma from institutional/consultation archives from 2008 to June 2022 were reviewed. For inclusion, tumors had to occur in irradiated skin and exhibit a multi-lobular proliferation of tightly packed capillary-like vessels, as previously described in this variant. Prior ancillary studies were also reviewed. Eight cases met the criteria. All occurred in women treated with radiation for breast cancer (median age 75 years). All cases had similar findings, including a multi-lobular proliferation of tightly packed vessels, infiltrative cords, and atypical single endothelial cells. A conventional angiosarcoma pattern was also seen in five cases. All cases tested were positive for vascular markers (CD31, CD34, and/or ERG) and MYC. MYC amplification was shown by FISH in all cases tested. Smooth muscle actin (SMA) was positive in pericytes in the capillary lobules in all five cases tested and areas of conventional angiosarcoma in two of three cases. CONCLUSIONS: The capillary lobule variant of angiosarcoma is a rare and therefore potentially under-recognized variant of post-radiation angiosarcoma. The lobular architecture and SMA positivity may mimic benign vascular proliferations. Careful attention to histopathologic features and ancillary tests may facilitate accurate diagnosis.

Gene fusions in gastrointestinal tract cancers.

Fusion genes have been identified in a wide array of human neoplasms including hematologic and solid tumors, including gastrointestinal tract neoplasia. A fusion gene is the product of parts of two genes that are joined together following a deletion, translocation, or chromosomal inversion. Together with single nucleotide variants, insertions, deletions, and amplification, fusion genes represent one of the key genomic mechanisms for tumor development. Detecting fusions in the clinic is accomplished by a variety of techniques including break-apart fluorescence in situ hybridization, reverse transcription-polymerase chain reaction, and next-generation sequencing. Some recurrent gene fusions have been successfully targeted by small molecule or monoclonal antibody therapies (ie targeted therapies), while others are used as biomarkers for diagnostic and prognostic purposes. The purpose of this review article is to discuss the clinical utility of detection of gene fusions in carcinomas and neoplasms arising primarily in the digestive system.

YAP1-TFE3-fused hemangioendothelioma: a multi-institutional clinicopathologic study of 24 genetically-confirmed cases.

YAP1-TFE3-fused hemangioendothelioma is an extremely rare malignant vascular tumor. We present the largest multi-institutional clinicopathologic study of YAP1-TFE3-fused hemangioendothelioma to date. The 24 cases of YAP1-TFE3-fused hemangioendothelioma showed a female predominance (17 female, 7 male) across a wide age range (20-78 years old, median 44). Tumors were most commonly located in soft tissue (50%), followed by bone (29%), lung (13%), and liver (8%), ranging from 3 to 115 mm in size (median 40 mm). About two-thirds presented with multifocal disease, including 7 cases with distant organ metastasis. Histopathologically, we describe three dominant architectural patterns: solid sheets of coalescing nests, pseudoalveolar and (pseudo)vasoformative pattern, and discohesive strands and clusters of cells set in a myxoid to myxohyaline stroma. These patterns were present in variable proportions across different tumors and often coexisted within the same tumor. The dominant cytomorphology (88%) was large epithelioid cells with abundant, glassy eosinophilic to vacuolated cytoplasm, prominent nucleoli and well-demarcated cell borders. Multinucleated or binucleated cells, prominent admixed erythrocytic and lymphocytic infiltrates, and intratumoral fat were frequently present. Immunohistochemically, ERG, CD31, and TFE3 were consistently expressed, while expression of CD34 (83%) and cytokeratin AE1/AE3 (20%) was variable. CAMTA1 was negative in all but one case. All cases were confirmed by molecular testing to harbor YAP1-TFE3 gene fusions: majority with YAP1 exon 1 fused to TFE3 exon 4 (88%), or less commonly, TFE3 exon 6 (12%). Most patients (88%) were treated with primary surgical resection. Over a follow-up period of 4-360 months (median 36 months) in 17 cases, 35% of patients remained alive without disease, and 47% survived many years with stable, albeit multifocal and/or metastatic disease. Five-year progression-free survival probability was 88%. We propose categorizing YAP1-TFE3-fused hemangioendothelioma as a distinct disease entity given its unique clinical and histopathologic characteristics in comparison to conventional epithelioid hemangioendothelioma.

PRRX1-NCOA1-rearranged fibroblastic tumour: a clinicopathological, immunohistochemical and molecular genetic study of six cases of a potentially under-recognised, distinctive mesenchymal tumour.

AIMS: PRRX1-NCOA1-rearranged fibroblastic tumour is a recently described, rare mesenchymal tumour. Only four cases have been previously reported. The aim of this article is to report six additional cases of this unusual mesenchymal neoplasm, with an emphasis on its differential diagnosis. METHODS AND RESULTS: The six cases were from three females and three males (age, 20-49 years; median, 42 years). Three tumours were located on the abdominal wall; two from the shoulder/axillary areas, and one on the lateral hip. All presented as slow-growing subcutaneous nodules, ranging from 26 to 55 mm (median, 40 mm). The tumours consisted of circumscribed, variably cellular nodules composed of relatively bland plump spindled to epithelioid cells arranged singly, in cords, and occasionally in nests, embedded in hyalinised and collagenous stroma. Small hypocellular myxoid zones with ropey collagen fibres were present, as were irregularly dilated, gaping, crescent-shaped or staghorn-like thin-walled vessels, best appreciated at the periphery. Immunohistochemistry for CD34, S100, MUC4 and STAT6 was consistently negative. RNA-sequencing revealed PRRX1-NCOA1 fusions in all cases. Of the four cases with limited follow-up (1.5-4 months), none recurred following local surgical excision. CONCLUSIONS: The morphological features of PRRX1-NCOA1-rearranged fibroblastic tumour overlap with those of RB1-deficient soft-tissue tumours, solitary fibrous tumour, and low-grade fibromyxoid sarcoma/sclerosing epithelioid fibrosarcoma. This differential diagnosis can be resolved with a combination of careful morphological study and the application of a panel of immunostains, although molecular genetic study is most definitive. The natural history of PRRX1-NCOA1-rearranged fibroblastic tumour appears to be quite favourable, although longer-term study of a larger number of cases is warranted.

Novel PPP1CB-ALK fusion in spindle cell tumor defined by S100 and CD34 coexpression and distinctive stromal and perivascular hyalinization.

A novel group of S100- and CD34-positive spindle cell tumors with distinctive stromal and perivascular hyalinization harboring recurrent gene fusions involving kinases including RAF1, BRAF, NTRK1/2/3, and RET have been recently reported. To our knowledge, no such cases harboring ALK rearrangements have been identified. We report a previously healthy 41-year-old male with a 12-cm intramuscular shoulder mass. The tumor was composed of bland-appearing spindled to epithelioid cells, arranged in a patternless pattern in a background of loose myxoid stroma containing striking amianthoid-like stromal collagen and perivascular rings. In accordance with the previously reported tumors, the tumor cells showed diffuse immunopositivity with S100 and CD34, while lacking SOX10 expression. Targeted RNA-based next-generation sequencing identified a novel serine/threonine-protein phosphatase PP1-beta-catalytic subunit (PPP1CB)-ALK fusion gene. Although ALK break-apart was not detected by FISH, likely due to a paracentric inversion of chromosome 2, the presence of the fusion was confirmed by Sanger sequencing showing a 10-bp linker between exon 6 of PPP1CB and intron 19 of ALK while maintaining reading frame. Subsequent ALK-1 immunostain exhibited diffuse cytoplasmic staining in the tumor cells. Our case expands the molecular genetic spectrum of the distinctive group of spindle cell tumors with CD34/S100+ immunophenotype, supporting the important role of various kinases as drivers of oncogenesis. Awareness of this entity including its unique morphologic and immunophenotypic features as well as its interchangeable kinase gene fusions is crucial for correct classification and potential targeted therapy, particularly in aggressive subsets.

Xanthogranulomatous epithelial tumor: report of 6 cases of a novel, potentially deceptive lesion with a predilection for young women.

Epithelial marker expression and/or epithelial differentiation, as well as "anomalous" expression of keratins, are features of some soft tissue tumors. Recently, we have encountered an unusual mesenchymal tumor composed of bland, distinctly eosinophilic, keratin-positive epithelial cells, which were almost entirely obscured by xanthogranulomatous inflammation. Six cases were identified (5 F, 1 M; 16-62 years (median 21 years)) arising in soft tissue (n = 4) and bone (n = 2) and ranging in size from 2 to 7 cm. The tumors were generally circumscribed, with a fibrous capsule containing lymphoid aggregates, and consisted in large part of a sheet-like proliferation of foamy histiocytes, Touton-type and osteoclast-type giant cells, and chronic inflammatory cells. Closer inspection, however, disclosed a distinct population of uniform, cytologically bland mononuclear cells with brightly eosinophilic cytoplasm arranged singly and in small nests and cords. Overt squamous and/or glandular differentiation was absent. By immunohistochemistry, these cells were diffusely positive with the OSCAR and AE1/AE3 keratin antibodies, and focally positive for high-molecular weight keratins; endothelial and myoid markers were negative and SMARCB1 was retained. RNA-seq identified a PLEKHM1 variant of undetermined significance in one case, likely related to this patient's underlying osteopetrosis. Follow-up to date has been benign. In summary, we have identified a novel tumor of soft tissue and bone with a predilection for young females, provisionally termed "xanthogranulomatous epithelial tumor". These unusual lesions do not appear to arise from adnexa, or represent known keratin-positive soft tissue tumors, and the origin of their constituent epithelial cells is obscure. The natural history of this distinctive lesion appears indolent, although study of additional cases and longer term follow-up are needed.

Chondroblastomas presenting in adulthood: a study of 39 patients with emphasis on histological features and skeletal distribution.

AIMS: Chondroblastomas (CB) are rare bone tumours that typically arise in the epiphysis/apophysis of long bones in skeletally immature patients. We explore the clinicopathological features of CB presenting in adults. METHODS AND RESULTS: CB in patients ≥20 years of age were retrieved from our institutional archives. Thirty-nine CB were identified (29 male/10 female; aged 20-54 years). Twenty (51%) cases occurred in long tubular bones, 10 (26%) in small bones of the feet, five (13%) in flat bones and four (10%) in the patella. All cases showed classic cytological features of CB, and chondroid matrix was universally present. Calcification was identified in 10 cases (26%), including various combinations of serpiginous (n = 7), punctate (n = 6), classic chicken-wire (n = 4) and psammomatous (n = 2) patterns. Haemosiderin (n = 19), woven bone (n = 13), secondary aneurysmal bone cyst formation (n = 8), foamy macrophages (n = 4), hyalinised vascular spaces (n = 2) and cholesterol clefts (n = 2) were noted. Follow-up information (n = 32, 1-452 months) revealed local recurrence in three patients, all >40 years of age with flat bone origin, one of which developed pulmonary metastases 132 months after initial diagnosis. CONCLUSIONS: CB in patients >20 years of age more frequently involves the short bones of the hands/feet and flat bones compared to those arising in their younger counterparts. A subset may harbour extensive serpiginous or psammomatous calcification rather than the classic chicken-wire pattern. Although the overall local recurrence rate in adulthood is approximately 10%, all three patients with recurrent disease had tumours involving flat bones, suggesting that tumours arising in these sites may behave more aggressively.

RNA sequencing identifies a novel USP9X-USP6 promoter swap gene fusion in a primary aneurysmal bone cyst.

Primary aneurysmal bone cyst (ABC) is a benign multiloculated cystic lesion of bone that is defined cytogenetically by USP6 gene rearrangements. Rearrangements involving USP6 are promoter swaps, usually generated by fusion of the noncoding upstream exons of different partner genes with exon 1 or 2 of USP6, thus leading to transcriptional upregulation of full-length USP6 coding sequence. Testing for USP6 rearrangements is used diagnostically to distinguish it from secondary ABC and other giant cell-rich primary bone tumors. In this report, we present a case of a 16-year-old male with a primary ABC of the left distal femur. USP6 break apart fluorescence in situ hybridization was positive for a rearrangement and conventional chromosome analysis identified a reciprocal X;17 translocation. In order to identify the putative USP6 fusion partner, we performed RNA sequencing and uncovered a novel USP9X-USP6 promoter swap fusion. This result was confirmed by reverse transcription-polymerase chain reaction (RT-PCR) and by mate pair sequencing thus showing the utility of these alternative methodologies in identifying novel fusion candidates. Ubiquitin-specific protease 9X (USP9X), like USP6, encodes a highly conserved substrate-specific deubiquitylating enzyme. USP9X is highly expressed in a number of tissue types and acts as both an oncogene and tumor suppressor in several human cancers. We conclude that oncogenic activation of USP6 via USP9X promoter exchange represents a novel driver of primary ABC formation.

Histiocyte-rich rhabdomyoblastic tumor: rhabdomyosarcoma, rhabdomyoma, or rhabdomyoblastic tumor of uncertain malignant potential? A histologically distinctive rhabdomyoblastic tumor in search of a place in the classification of skeletal muscle neoplasms.

Skeletal muscle tumors are traditionally classified as rhabdomyoma or rhabdomyosarcoma. We have identified an unusual adult rhabdomyoblastic tumor not clearly corresponding to a previously described variant of rhabdomyoma or rhabdomyosarcoma, characterized by a very striking proliferation of non-neoplastic histiocytes, obscuring the underlying tumor. Ten cases were identified in nine males and one female with a median age of 43 years (range 23-69 years). Tumors involved the deep soft tissues of the trunk (N = 4), lower limbs (N = 4), and neck (N = 2). Tumors were well-circumscribed, nodular masses, frequently surrounded by a fibrous capsule containing lymphoid aggregates and sometimes calcifications. Numerous foamy macrophages, multinucleated Touton-type giant cells, and sheets/fascicles of smaller, often spindled macrophages largely obscured the underlying desmin, MyoD1, and myogenin-positive rhabdomyoblastic tumor. Cases were wild type for MYOD1 and no other mutations or rearrangements characteristic of a known subtype of rhabdomyoma or rhabdomyosarcoma were identified. Two of four cases successfully analyzed using a next-generation sequencing panel of 170 common cancer-related genes harbored inactivating NF1 mutations. Next-generation sequencing showed no gene fusions. Clinical follow (nine patients; median 9 months; mean 23 months; range 3-124 months) showed all patients received wide excision; four patients also received adjuvant radiotherapy and none received chemotherapy. At the time of last follow-up, all patients were alive and without disease; no local recurrences or distant metastases occurred. We hypothesize that these unusual tumors represent rhabdomyoblastic tumors of uncertain malignant potential. Possibly over time they should be relegated to a new category of skeletal muscle tumors of intermediate (borderline) malignancy.