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

Superficial ALK-rearranged myxoid spindle cell neoplasms: Clinicopathologic and molecular analysis of two cases and a review of the literature

Superficial anaplastic lymphoma kinase (ALK)-rearranged myxoid spindle cell neoplasms are a recently identified subtype of cutaneous soft tissue tumors, distinct for their co-expression of CD34 and S100 and characterized by ALK gene rearrangements. Although 72 cases have been reported primarily as isolated case reports, this tumor subtype has yet to be included in the WHO classification of soft tissue tumors, underscoring the need for further study. In this study, we diagnosed two additional cases, both arising in the dermis and subcutaneous tissue. These tumors exhibited characteristic pathological features, including linear or concentric whorl patterns, prominent myxoid and collagenized stroma, mild cellular atypia, and rare mitotic activity. The presence of infiltrative margins and the potential for recurrence after surgery suggest at least locally aggressive clinical behavior. Immunohistochemically, the tumors diffusely expressed S100 and CD34, with strong ALK-D5F3 positivity, confirmed by ALK gene rearrangement. These findings further expand the clinical and pathological spectrum of ALK-rearranged neoplasms and highlight the need for continued research on their biological behavior and classification.

ALK -rearranged Mesenchymal Neoplasms With Prominent Foamy/Pseudolipogenic Cell Morphology : Expanding the Phenotypic Spectrum of ALK Fusion Neoplasms and Report of Novel Fusion Partners

The category of ALK -rearranged mesenchymal neoplasms has been evolving rapidly, with reports of morphologically diverse lesions of cutaneous, soft tissue, and visceral origin. While some of these represent morphologically defined entities harboring recurrent ALK fusions (inflammatory myofibroblastic tumor and epithelioid fibrous histiocytoma), others are unclassified by morphology with variable overlap with the tyrosine kinase family of neoplasia and their underlying ALK fusions cannot be suspected based on morphology. We herein report 3 cases that expand the anatomic, morphologic, and genotypic spectrum of ALK -rearranged unclassified neoplasms. Patients were all adults aged 46 to 69 (median: 63) who presented with a mass located in the gingiva, subcutis of the back, and submucosal posterior pharyngeal wall. The tumor size ranged from 1 to 2.7 cm (median: 1.6). Conservative surgery was the treatment in all patients. Follow-up was available for one patient who remained disease-free at 14 months. Histologically, all tumors displayed large polygonal cells with foamy to granular and lipogenic-like microvacuolated copious cytoplasm and medium-sized round nuclei with 1 or 2 prominent nucleoli. Mitoses and necrosis were not seen. The initial diagnostic impression was PEComa, inflammatory rhabdomyoblastic tumor and unclassified pseudolipogenic neoplasm. Strong cytoplasmic ALK was detected by immunohistochemistry in all cases. Other positive markers include Cathepsin K (2/2), desmin (1/3), focal MyoD1 (1/1), focal SMA (1/3), and focal EMA (1/2). Targeted RNA sequencing revealed ALK fusions with exon 20 (2 cases) and exon 19 (one case) of ALK fused to RND3 (exon 3), SQSTM1 (exon 6), and desmin (intron 6). Methylation profiling in the desmin-fused case (initially diagnosed as inflammatory rhabdomyoblastic tumor) revealed an inflammatory myofibroblastic tumor match with a low confidence score of 0.5 and a flat copy number variation (CNV) profile. No NF1 mutation was detected in this case, altogether excluding an inflammatory rhabdomyoblastic tumor. Our study highlights and expands the morphologic and anatomic diversity of ALK- fused neoplasms and documents novel fusion partners ( RND3 and desmin).

ALK-Rearranged Epithelioid and Spindle Cell Neoplasm of the Sinonasal Tract

Anaplastic lymphoma kinase (ALK)-rearranged mesenchymal neoplasms (non-inflammatory myofibroblastic tumor and non-epithelioid fibrous histiocytoma) have been recently described which tend to occur in the superficial and deep soft tissues. Occurrence as a primary sinonasal neoplasm has not been reported thus far. Herein, we describe the first case of sinonasal ALK-rearranged mesenchymal tumor that harbored remarkable epithelioid and spindle cell morphology. The tumor affected a 40-year-old man who presented with flu-like symptoms and was thought to have influenza A. However, computed tomography demonstrated a nasal polypoid lesion causing curvature of the nasal septum. Histological examination revealed a heterogeneous tumor composed of round to epithelioid cells with foci of spindle cells. The tumor cells exhibited moderate pleomorphism and mitotic activity. By immunohistochemistry, they showed diffuse staining of CD34, S100, ALK (D5F3) and CD30. Fluorescence in situ hybridization analysis demonstrated ALK rearrangement. Subsequent next-generation sequencing (RNA-seq) identified a rare PLEKHH2exon6::ALKexon20 fusion. This study further demonstrates the importance of molecular profiling in identifying kinase fusion-positive soft tissue tumors, particularly for those that arise at unusual sites and display atypical cytomorphology.

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.

EMA-Positive Superficial ALK-Rearranged Myxoid Spindle Cell Neoplasm Masquerading as Perineurioma/Hybrid Nerve Sheath Tumor

ABSTRACT

Superficial anaplastic lymphoma kinase (ALK)-rearranged myxoid spindle cell neoplasm (SAMS) is a recently described entity which coexpresses ALK, CD34, and commonly S100. These neoplasms are characterized morphologically by concentric spindle cell whorls and cords and are commonly set in an abundant myxoid to myxocollagenous stroma, thus mimicking perineurioma or hybrid nerve sheath tumor. EMA immunostain has been reported to be negative in SAMS which helps in excluding the latter entities. Herein, we report the first EMA-positive SAMS of the right leg in a 37-year-old female patient masquerading as perineurioma/hybrid nerve sheath tumor. The tumor morphologically was comprised of spindle cells arranged in loose whorls and short fascicles set in myxoid to collagenous stroma and coexpressed CD34 and EMA, reminiscent of perineurioma. S100 showed focal staining. ALK immunostain was subsequently performed and was positive. ALK gene rearrangement was identified by fluorescence in situ hybridization break-apart assay and was further confirmed by next-generation sequencing-based RNA sequencing demonstrating FLNA::ALK fusion, thus supporting the diagnosis of SAMS. In conclusion, EMA can be expressed in SAMS, thus posing as a diagnostic pitfall. ALK immunostain and molecular studies are essential for confirming the diagnosis of SAMS and excluding potential mimickers, particularly perineurioma or hybrid nerve sheath tumor.

ALK-rearranged mesenchymal neoplasms: a clinicopathological and molecular study of eight additional cases of an emerging group of tyrosine kinase fusion mesenchymal tumours

AIMS

Mesenchymal neoplasms characterised by ALK fusions mainly include inflammatory myofibroblastic tumour (IMT) and epithelioid fibrous histiocytoma (EFH). Most recently, ALK-rearranged mesenchymal tumours that are not IMT or EFH have been reported. Our aim is to further characterise eight such neoplasms, with a detailed clinicopathological, immunohistochemical and molecular analysis.

METHODS

Clinicopathological features were assessed and partner agnostic targeted RNA-sequencing on clinically validated platforms was performed.

RESULTS

The patients consisted of seven males and one female with a median age of 47 years (28 -59 years). The tumours ranged in size from 2.0 to 10.0 cm (mean=3.0 cm) and involved superficial and deep soft tissue (n=6) and visceral locations (n=2). Of the seven patients with follow-up (9-130 months), two developed distant metastases and five had no disease recurrence or metastasis. The tumours demonstrated diverse architectures and variable cellularity and cellular morphologies. The main constitutive cells appeared in elongated spindled in three, primitive to ovoid in two and round to epithelioid in three cases. We expanded the histopathological spectrum to include mildly to moderately cellular spindled to stellate cells in a multinodular growth in a prominent myxoid and vascularised stroma (n=2). All tumours expressed ALK(D5F3); seven were positive for S100 protein and six were positive for CD34. By fluorescence in situ hybridisation, ALK rearrangement was identified in all eight tumours. ALK fusion partners were identified by RNA-sequencing in all cases, including previously reported: EML4 (n=3), DCTN (n=1), CLIP1 (n=1) and PLEKHH2 (n=1), and also two novel fusion partners: TKT (n=1) and MMP2 (n=1).

CONCLUSIONS

Our study expands the clinicopathological and molecular spectrum of ALK-rearranged mesenchymal neoplasms.

CD30 expression in an emerging group of mesenchymal spindle cell neoplasms with ALK fusion detected by flow cytometry and immunohistochemistry

An emerging group of spindle cell neoplasms harboring fusions involving NTRK or non-NTRK kinase genes often share characteristic S100 and/or CD34 expression; however, the diagnostic utility of immunohistochemical stains is not well established in this family owing to their lack of specificity. Recently, CD30 expression in spindle cell neoplasms with kinase gene fusions, such as NTRK, BRAF, RAF1, and RET, has been increasingly identified. We herein report a 10-year-old girl with high-grade spindle cell sarcoma of the neck. Prior to histopathological evaluation, flow cytometry (FCM) analysis and touch smear cytology of the tumor tissue revealed CD34+ and dimCD30+ spindle cell populations. Histopathologically, the case was characterized by monomorphic spindle-shaped cytomorphology with CD30, S100, and CD34 positivity and harbored close similarities with spindle cell neoplasms with NTRK or non-NTRK gene fusions. Subsequently, a comprehensive next-generation sequencing sarcoma panel identified a rare PLEKHH2::ALK fusion, and a diagnosis of ALK-rearranged spindle cell neoplasm was made. The patient showed significant tumor response to single-agent treatment with alectinib, an ALK-tyrosine kinase inhibitor. This case supports that CD30 is expressed in an ALK-rearranged mesenchymal neoplasm. The benefit of the early detection of CD30 expression by FCM for a prompt diagnosis and treatment is highlighted in the context of an aggressive clinical course. This case represents a learning experience regarding the need to the check the status of CD30 expression in these tumors and suggests the potential clinical benefits of CD30-targeted therapy.

ALK-Rearranged Epithelioid Mesenchymal Neoplasm: Expanding the Spectrum of Tyrosine Kinase-Altered Mesenchymal Tumors

The anaplastic lymphoma kinase (ALK) gene encodes a receptor tyrosine kinase, and fusions involving this gene have been reported in a variety of mesenchymal neoplasms. ALK-altered tumors with epithelioid morphology have been described in epithelioid inflammatory myofibroblastic sarcoma and epithelioid fibrous histiocytoma. Herein, we describe the clinicopathologic features of 7 ALK-rearranged mesenchymal tumors with epithelioid morphology occurring predominately in the pediatric population. Tumors occurred in 4 females and 3 males with an age ranging from 1 month to 28 years. Five tumors were superficial and solitary, while 1 presented with multiple peritoneal/omental nodules, and 1 presented as a large mediastinal mass. Morphologically, all tumors comprised epithelioid cells arranged in sheets, anastomosing cords, or small clusters embedded in a myxohyaline stroma. The cells had slightly variably sized ovoid nuclei with moderately prominent nucleoli and abundant eosinophilic cytoplasm. Four cases had sparse mitotic figures without necrosis. The remaining 3 tumors (2 deep and 1 superficial) had more than 10 mitoses per 10 high-power fields as well as foci of necrosis. ALK fusions were identified in all cases. The fusion partners included HMBOX1 (n = 1), VCL (n = 1), PRRC2B (n = 1), MYH10 (n = 1), STRN (n = 1), and EML4 (n = 2). One tumor recurred locally 2 years after initial resection; 1 patient had widely metastatic disease (mediastinal tumor). At the time of last follow-up (n = 6), 4 patients were alive without evidence of disease, 1 died due to complications of therapy (peritoneal tumor), and 1 was alive with disease. Our findings expand the spectrum of ALK-rearranged mesenchymal tumors. Our cases predominately occurred in older children and mainly exhibited epithelioid to round cell morphology, as opposed to spindle cell morphology. We also show that tumors in a deep location with higher-grade features follow a more aggressive clinical course.