S100 and CD34 positive spindle cell tumor with prominent perivascular hyalinization and a novel NCOA4-RET fusion

Update on cutaneous mesenchymal tumors in the 5th edition of WHO classification of skin tumors with an emphasis on new fusion-associated neoplasms

The section on mesenchymal tumors in the 5th edition of WHO classification of skin tumors has undergone several changes, the most important of which is the inclusion of newly identified tumor entities, which will be the main focus of this review article. These specifically include three novel cutaneous mesenchymal tumors with melanocytic differentiation, and rearrangements of the CRTC1::TRIM11, ACTIN::MITF, and MITF::CREM genes as well as EWSR1::SMAD3-rearranged fibroblastic tumors, superficial CD34-positive fibroblastic tumors, and NTRK-rearranged spindle cell neoplasms. Some of the other most important changes will be briefly mentioned as well.

Comprehensive clinicopathological, molecular, and methylation analysis of mesenchymal tumors with NTRK and other kinase gene aberrations

Alterations in kinase genes such as NTRK1/2/3, RET, and BRAF underlie infantile fibrosarcoma (IFS), the emerging entity 'NTRK-rearranged spindle cell neoplasms' included in the latest WHO classification, and a growing set of tumors with overlapping clinical and pathological features. In this study, we conducted a comprehensive clinicopathological and molecular analysis of 22 cases of IFS and other kinase gene-altered spindle cell neoplasms affecting both pediatric and adult patients. Follow-up periods for 16 patients ranged in length from 10 to 130 months (mean 38 months). Six patients were treated with targeted therapy, achieving a partial or complete response in five cases. Overall, three cases recurred and one metastasized. Eight patients were free of disease, five were alive with disease, and two patients died. All cases showed previously reported morphological patterns. Based on the cellularity and level of atypia, cases were divided into three morphological grade groups. S100 protein and CD34 were at least focally positive in 12/22 and 14/22 cases, respectively. Novel PWWP2A::RET, NUMA1::RET, ITSN1::RAF1, and CAPZA2::MET fusions, which we report herein in mesenchymal tumors for the first time, were detected by RNA sequencing. Additionally, the first uterine case with BRAF and EGFR mutations and CD34 and S100 co-expression is described. DNA sequencing performed in 13 cases uncovered very rare additional genetic aberrations. The CNV profiles showed that high-grade tumors demonstrate a significantly higher percentage of copy number gains and losses across the genome compared with low- and intermediate-grade tumors. Unsupervised clustering of the tumors' methylation profiles revealed that in 8/9 cases, the methylation profiles clustered with the IFS methylation class, irrespective of their clinicopathological or molecular features. © 2024 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

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

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

Retroperitoneal spindle cell tumor: a case report

Spindle cell tumors exhibit a relatively low occurrence rate and can manifest in various locations within the human body, including soft tissues and bones. The process of making a diagnosis is supported by conducting pathological and immunohistochemical tests. A 50-year-old female patient visited the hospital with abdominal pain that lasted about a week. Magnetic resonance imaging of the pelvis showed that this mass was independent and was not a lymph node mass, but a retroperitoneal sarcoma type mass. As part of the treatment, the mass was surgically excised, and a supracervical hysterectomy was carried out. The tumor was wrapped in a grayish-white capsule and showed a lobulating pattern. Retroperitoneal spindle cell tumors, particularly those occurring in abdominal soft tissues, are infrequently observed. Histopathological diagnosis is done in stages, and when cases are ambiguous, immunohistochemistry can provide valuable guidance in the right direction.

Recurrent RET fusions in fibrosarcoma-like neoplasms in adult viscera: expanding the clinicopathological and genetic spectrum

AIMS

RET-fused mesenchymal neoplasms mostly affect the soft tissue of paediatric patients. Given their responsiveness to selective RET inhibitors, it remains critical to identify those extraordinary cases occurring in the visceral organs of adults. In this study, we report three RET-rearranged spindle-cell tumours occurring in the visceral organs of adults.

METHODS AND RESULTS

Clinicopathological features were assessed and partner agnostic targeted next-generation sequencing on clinically validated platforms were performed. The patients were 18, 53, and 55 years old and included one male and two females. The tumours were located in the kidney (case 1), small intestine (case 2), and ureter (case 3), with maximum diameters of 14, 5, and 1 cm, respectively. Histologically, all tumours displayed a morphological spectrum typical of fibrosarcoma, including moderately to highly cellular, nonpleomorphic, ovoid to spindle-shaped cells arranged in long fascicles or haphazardly within collagenised to myxohyaline stroma. Foci of irregular alveolar oedema-like structures and areas with microcystic and reticular arrangements were identified in the renal tumour. Staghorn-type vessels and foci of band-like stromal hyalinisation were observed in the small intestine tumour. Cases 1 and 2 were high-grade and pursed a highly aggressive clinical course, while case 3 was of intermediate grade with no tumour recurrence or metastasis 14 years after surgery. All three tumours expressed CD34, which was coexpressed with S100 protein in cases 2 and 3. Molecular genetic testing revealed PRKAR1A::RET, KIF5B::RET, and SPECC1L::RET in-frame gene fusions.

CONCLUSION

Our study expands the clinicopathological and genetic spectrum of mesenchymal neoplasms associated with RET fusions.

ALK-rearranged Mesenchymal Neoplasms: A Report of 9 cases Further Expanding the Clinicopathologic Spectrum of Emerging Kinase Fusion Positive Group of Tumors

Anaplastic lymphoma kinase (ALK) fusions are oncogenic drivers in diverse cancer types. Although well established in inflammatory myofibroblastic tumor (IMT) and epithelioid fibrous histiocytoma (EFH), ALK rearrangements also occur in the emerging family of kinase fusion-positive mesenchymal neoplasms. We investigated 9 ALK-rearranged mesenchymal neoplasms (exclusive of IMT and EFH) arising in 6 males and 3 females with a wide age range of 10 to 78 years old (median 42 years). Tumors involved superficial and deep soft tissue (6) and viscera (3). Three were myxoid or collagenous low-grade paucicellular tumors with haphazardly arranged spindled cells. Three were cellular tumors with spindled cells in intersecting short fascicles or solid sheets. Three cases consisted of uniform epithelioid cells arranged in nests or solid sheets, with prominent mitotic activity and necrosis. Band-like stromal hyalinization was present in 6 cases. All tumors expressed ALK; four were positive for S100 and five were positive for CD34, while all were negative for SOX10. By targeted RNA sequencing, the breakpoints involved ALK exon 20; the 5' partners included KLC1, EML4, DCTN1, PLEKHH2, TIMP3, HMBOX1, and FMR1. All but two patients presented with localized disease. One patient had distant lung metastases; another had diffuse pleural involvement. Of the six cases with treatment information, five were surgically excised [one also received neoadjuvant radiation therapy (RT)], and one received RT and an ALK inhibitor. Of the four patients with follow-up (median 5.5 months), one remained alive with stable disease and three were alive without disease. We expand the clinicopathologic spectrum of ALK-fused mesenchymal neoplasms, including a low-grade malignant peripheral nerve sheath tumor-like subset and another subset characterized by epithelioid and high-grade morphology.

Case report: ALK-rearranged spindle and epithelioid cell neoplasms with S100 and CD34 co-expression: Additional evidence of kinase fusion-positive soft tissue tumors

ALK rearrangements have rarely been reported in S100- and CD34-co-expressing soft tissue neoplasms with lipofibromatosis-like neural tumor (LPFNT) pattern or stromal and perivascular hyalinization, mimicking NTRK-rearranged spindle cell tumors. Here, we reported ALK fusions involving related partner genes in two adult soft tissue tumors with S100 and CD34 co-expression, and conducted a literature review of mesenchymal tumors harboring ALK or other kinase fusions. Case 1 was a 25-year-old female who underwent excision of a soft tissue mass in the anterior thigh region. Morphologically, the tumor was composed of spindle cells adjacent to epithelioid cells embedded in myxedematous and hyalinized stroma, with infiltrative boundary. Spindle cells mixed with inflammatory infiltration resembling inflammatory myofibroblastic tumor (IMT) were seen sporadically. However, brisk mitosis and focal necrosis was also observed, indicating an intermediate-grade sarcoma. In case 2, the left side of the neck of a 34-year-old man was affected. The tumor was composed of monomorphic spindle cells arranged in fascicular growth or patternless pattern, with stromal and perivascular hyalinization. Sparse inflammatory cell infiltration was also observed. Both tumors showed CD34, S100, and ALK-D5F3 immunoreactivity. Next generation sequencing (NGS) test identified a PLEKHH2::ALK fusion in case 1, which was confirmed by RT-PCR and Sanger sequencing, whereas the RT-PCR (ARMS method) test detected an EML4::ALK fusion in case 2. In conclusion, this study expands the morphological and genetic landscape of tumors with S100 and CD34 co-expression harboring kinase fusions, and suggests that kinase fusion-positive mesenchymal neoplasms are becoming an enlarging entity with a variety of morphological patterns.

Retroperitoneal Spindle Cell Tumor: A Case Report

Spindle cell tumor is very rare. Herein, we report a case of retroperitoneal spindle cell tumor in a 52-year-old female. The patient first presented with a complaint of persistent pain in the right upper abdomen. In the follow-up, a CT scan was performed and showed a retroperitoneal soft tissue density mass measuring 11 cm in diameter. Then, a subsequent operation was performed, and we completely removed the tumor and partially invaded lesions. The tumor was histologically diagnosed as a spindle cell tumor. Therefore, it is imperative for us to enhance the understanding of this seldom found tumor. Surgery remains the best option for treatment.

ALK rearrangements in infantile fibrosarcoma-like spindle cell tumours of soft tissue and kidney

AIMS

Recurrent alterations in receptor tyrosine kinase (RTK) and downstream effectors are described in infantile fibrosarcoma (IFS)/cellular congenital mesoblastic nephroma (cCMN) and a subset of spindle cell sarcomas, provisionally designated 'NTRK-rearranged' spindle cell neoplasms. These two groups of tumours demonstrate overlapping morphologies and harbour alterations in NTRK1/2/3, RET, MET, ABL1, ROS1, RAF1 and BRAF, although their relationship is not fully elucidated. We describe herein a cohort of paediatric tumours with clinicopathological features not typical for inflammatory myofibroblastic tumour, but rather with similarities to cCMN/IFS harbouring ALK fusions.

METHODS AND RESULTS

Clinicopathological features were assessed and partner agnostic targeted RNA sequencing on clinically validated platforms were performed. Tumours occurred in patients aged from 2 to 10 years (median age 2 years) with a 2:2 male to female ratio and an average size of 8.4 cm. Two tumours arose in soft tissues and two in the kidney. Morphological features included spindle to ovoid cells arranged in long fascicles or haphazardly within a myxoid to collagenised stroma; a subset of cases had either dilated, ectatic vessels or focal perivascular hyalinosis. By immunohistochemistry, all cases tested showed cytoplasmic expression of anaplastic lymphoma kinase (ALK) and one case demonstrated co-expression of CD34 and S100.

CONCLUSIONS

This series of ALK-rearranged IFS-like tumours expands the spectrum of targetable kinases altered in these tumours and reinforces the potential overlap between IFS/cCMN-like tumours and the provisional entity of 'NTRK-rearranged' spindle cell neoplasms.

Novel BRAF gene fusions and activating point mutations in spindle cell sarcomas with histologic overlap with infantile fibrosarcoma

Infantile fibrosarcoma (IFS)/cellular congenital mesoblastic nephroma (cCMN) commonly harbors the classic ETV6-NTRK3 translocation. However, there are recent reports of mesenchymal tumors with IFS-like morphology harboring fusions of other receptor tyrosine kinases or downstream effectors, including NTRK1/2/3, MET, RET, and RAF1 fusions as well as one prior series with BRAF fusions. Discovery of these additional molecular drivers contributes to a more integrated diagnostic approach and presents important targets for therapy. Here we report the clinicopathologic and molecular features of 14 BRAF-altered tumors, of which 5 had BRAF point mutations and 10 harbored one or more BRAF fusions. Of the BRAF fusion-positive tumors, one harbored two BRAF fusions (FOXN3-BRAF, TRIP11-BRAF) and another harbored three unique alternative splice variants of EPB41L2-BRAF. Tumors occurred in ten males and four females, aged from birth to 32 years (median 6 months). Twelve were soft tissue based; two were visceral including one located in the kidney (cCMN). All neoplasms demonstrated ovoid to short spindle cells most frequently arranged haphazardly or in intersecting fascicles, often with collagenized stroma and a chronic inflammatory infiltrate. No specific immunophenotype was observed; expression of CD34, S100, and SMA was variable. To date, this is the largest cohort of BRAF-altered spindle cell neoplasms with IFS-like morphology, including not only seven novel BRAF fusion partners but also the first description of oncogenic BRAF point mutations in these tumors.

GAB1-ABL1 fusions in tumors that have histologic overlap with NTRK-rearranged spindle cell tumors

Fibroblastic spindle cell tumors are a heterogeneous group of rare soft tissue tumors that are increasingly recognized as associated with a variety of kinase gene fusions. We report two cases of GAB1-ABL1 fusions in spindle cell tumors that histologically overlap with neurotrophic tyrosine receptor kinase (NTRK)-rearranged spindle cell tumors. The first case occurred in a 76-year-old female who had a large deep-seated spindle cell tumor composed of monotonous ovoid to spindle cells in a background of thick stromal collagen bands with prominent hyalinized vessels and inconspicuous mitoses (<1/10 HPF). Immunohistochemical stains showed co-expression of S100 and CD34. A GAB1-ABL1 fusion was detected by whole transcriptome RNA sequencing. The patient had a partial response to imatinib. The second case was previously described as a solitary fibrous tumor, occurring in a 9-year-old female with a cellular spindle cell tumor with patchy CD34 immunoexpression but no expression of S100. Upon clinicopathologic re-review, including anchored multiplex next-generation sequencing, a GAB1-ABL1 fusion was identified. In summary, we report the first two cases of spindle cell tumors with variable expression of CD34 and/or S100, driven by GAB1-ABL1 gene fusions with histologic overlap with NTRK-rearranged spindle cell tumors, suggesting that ABL-fusions may also be oncogenic drivers within this spectrum of tumors. These cases highlight the evolving understanding of fibroblastic spindle cell tumor biology and the utility of sequencing in identifying a targetable alteration.

Spindle cell tumor with CD34 and S100 co-expression and distinctive stromal and perivascular hyalinization showing EML4-ALK fusion

Recently, a novel group of CD34+ and S100+ spindle cell tumors with distinctive stromal and perivascular hyalinization showing recurrent gene fusions involving RAF1, BRAF, NTRK1/2/3, and RET has been identified. ALK rearrangements have been rarely reported in this group of tumors. We report a 24-year-old woman with a 1.5-cm pink mass of the scalp. The tumor was made of spindle cells organized in fascicles or haphazardly arranged in a patternless architecture, with areas of stromal and perivascular hyalinization. The tumor cells diffusely expressed CD34 and S100, without SOX-10 expression. The tumor showed diffuse immunopositivity for ALK. RNA sequencing using next-generation sequencing (NGS) detected an EML4-ALK fusion. This case extends the spectrum of this newly described group of CD34+/S100+ spindle cell tumors at the molecular-genetic level. Dermatopathologists should be aware of this recent entity, as it may fall in the differential diagnosis of many other spindle cell tumors with CD34 expression. NGS-based techniques should be performed when facing spindle cell tumors with similar morphology and immunophenotype. Identification of kinase fusions is essential for the precise classification and better knowledge of these tumors, and for targeted therapy in rare aggressive cases.

NTRK and other recently described kinase fusion positive uterine sarcomas: A review of a group of rare neoplasms

The landscape of uterine sarcomas has greatly expanded in recent years to include neoplasms with recurrent gene fusions, such as BCOR and YWHAE translocated high-grade endometrial stromal sarcomas. Sophisticated molecular techniques have also resulted in the description of "new" entities associated with recurrent kinase fusions involving NTRK and RET as well as COL1A1-PDGFB rearranged uterine sarcomas. These rare neoplasms will be discussed in this review, highlighting that some of the underlying molecular events are clinically actionable and potentially susceptible to targeted therapy. While relatively few of these neoplasms have been described to date, likely being previously lumped under the spectrum of undifferentiated uterine sarcoma, the number of cases will expand in the future given their recognition and the increasing availability of molecular testing. These neoplasms have overlapping morphology (often with a "fibrosarcoma-like" appearance) and immunohistochemical features, and are characterized by variable clinical outcomes. Although immunohistochemistry may assist in some cases, a definitive subclassification requires confirmatory molecular studies. As these molecular assays may not be routinely available in most laboratories, referral to reference centers may be needed. In order to assist the pathologist, we suggest a diagnostic algorithm for routine practice when dealing with a malignant or potentially malignant uterine spindle cell neoplasm.

A low-grade malignant soft tissue tumor with S100 and CD34 co-expression showing novel CDC42SE2-BRAF fusion with distinct features

Recently, a novel group of spindle cell tumors defined by S100 and CD34 co-expression harboring recurrent fusions involving RET, RAF1, BRAF, and NTRK1/2 gene has been identified. Morphologically, they are characterized by monomorphic neoplasm cells, "patternless" growth pattern, stromal, and perivascular hyalinization, lacked necrosis. We reported a 52-year-old Chinese female patient with a S100 and CD34 co-expression sarcoma presenting in the right proximal forearm. The forearm mass initially emerged 19 months ago when it was misdiagnosed as a solitary fibrous tumor and was surgically removed without further treatment. Microscopically, the primary and the recurred tumors share the same features, resembling the morphology of the recently characterized group. Nevertheless, some distinct features, such as predominantly epithelioid tumor cells and focally staghorn vessels, were also present in our case. Genomic profiling with clinical next-generation sequencing was performed and revealed CDC42SE2-BRAF gene fusion, MET amplification, and CDKN2A/B deletion. Both FISH and nested RT-PCR were performed to confirm the gene fusion. The patient was treated with crizotinib for two cycles but showed no obvious benefit. The presented case adds to the spectrum of the novel, characterized solid tumors, and provides suggestions for emerging therapeutic strategies for precision medicine involving targeted kinase inhibitors.

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.

Emerging soft tissue tumors with kinase fusions: An overview of the recent literature with an emphasis on diagnostic criteria

A recent breakthrough in the classification of soft tissue tumors (STT) has been a significant expansion in the number of neoplasms associated with NTRK and other kinase related fusions. This is important not only for diagnostic purposes, but also because it opens new avenues for targeted therapy. Indeed, recent clincal trials have shown significant benefit across multiple tumor-types, prompting approval of NTRK inhibitors for clinical use in the setting of advanced/metastatic NTRK-rearranged neoplasms. Despite these therapeutic oportunities, diagnostic challenges have transpired in recognizing these emerging new histologic subtypes of kinase fusions positive-STT prospectively. This, in part, is attributable to their wide morphologic spectrum, variable risk of malignancy, and non-specific immunoprofile. As such, recommendations for pathologic criteria and immunohistochemical testing are needed to improve classification and streamline the small subset of potential candidates for further molecular validation. This overview summarizes the key histologic features of various STT associated with NTRK and other kinase fusions, which appear to share a similar morphologic spectrum. Immunohistochemically, many of these tumors, regardless of the kinase fusion type, notably show co-expression of S100 and CD34; issues related to the utility of pan-NTRK and NTRK1 immunostaining are therefore summarized. Finally, I discuss the role of confirmatory molecular testing and how, in some instances, this may also be of prognostic value. This review is intended as a critical summary of the current literature to emphasize pathologic criteria for improving recognition of this emerging and complex group of kinase fusion associated STT.

RET Gene Fusions in Malignancies of the Thyroid and Other Tissues

Following the identification of the BCR-ABL1 (Breakpoint Cluster Region-ABelson murine Leukemia) fusion in chronic myelogenous leukemia, gene fusions generating chimeric oncoproteins have been recognized as common genomic structural variations in human malignancies. This is, in particular, a frequent mechanism in the oncogenic conversion of protein kinases. Gene fusion was the first mechanism identified for the oncogenic activation of the receptor tyrosine kinase RET (REarranged during Transfection), initially discovered in papillary thyroid carcinoma (PTC). More recently, the advent of highly sensitive massive parallel (next generation sequencing, NGS) sequencing of tumor DNA or cell-free (cfDNA) circulating tumor DNA, allowed for the detection of RET fusions in many other solid and hematopoietic malignancies. This review summarizes the role of RET fusions in the pathogenesis of human cancer.

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.