Primary cutaneous Ewing sarcoma with diffuse S100/SOX10 positivity and pseudoalveolar pattern: An extraordinarily rare case highlighting a potential pitfall with significant repercussions

Primary cutaneous/subcutaneous Ewings sarcoma

Amongst Ewing sarcoma family of tumours, (EFST), cutaneous/subcutaneous Ewing sarcoma are defined as tumours arising from cutaneous or subcutaneous tissue, not invading the underlying aponeurosis. They are rare tumours, with less than 200 patients published. They are typically small tumours (less than 5cm), and can arise at any anatomical location, with a particular tropism for distal, truncal and head/neck locations, compared to classical Ewing sarcoma. Like other conjunctive tumours, they have to be treated in specialized centers, with a diagnostic procedure following ESMO guidelines about soft-tissue tumours, favoring a core needle biopsy in most cases. They share classical pathological and molecular features of EFST (including EWSR1 rearrangement). Metastatic presentation is rare (less than 5% at diagnosis), but must be carefully searched using appropriated imaging considered the bad prognosis of these patients. Treatment strategy relies on neoadjuvant and adjuvant chemotherapy, surrounding the local treatment. Patients with localized disease have good prognosis and have to be treated with the dual objective of curability, and of minimizing acute and late toxicity. That is why in case of small tumours (<200mL), patients can be treated with less intensive protocols, as Saint Jude's (low-dose semi-continuous cyclophosphamide/doxorubicin regimen as induction chemotherapy and vincristine/actinomycin courses as maintenance therapy), setting aside the option of classical VDC/IE protocol for larger tumors. Local treatment must rely on carcinologic surgery, with the aim to avoid radiotherapy when possible. Patients with metastatic disease have bad prognosis resemble classical Ewing sarcoma, and have to be treated accordingly.

A MAZ::NCOA2 Subcutaneous Small Round Cell Sarcoma of Infancy With Diffuse S100/SOX10 Positivity: A Novel Entity

Small round cell sarcomas (SRCSs) constitute a heterogeneous group of high-grade tumors with a poor prognosis, predominantly affecting children and young adults. According to the 2020 WHO Soft Tissue Tumor classification, SRCSs are categorized into Ewing sarcoma, round cell sarcoma with EWSR1-non-ETS fusions, CIC-rearranged sarcoma, and sarcoma with BCOR genetic alterations. Herein, we report a case of a 10-month-old boy presenting with a progressively enlarging left lumbar mass. Histopathological examination revealed a well-demarcated lesion composed of small, round to oval cells with scant cytoplasm and mildly irregular nuclei. Immunohistochemical staining demonstrated strong, diffuse positivity for S100 and SOX10, indicating neurocristic differentiation. Next-generation sequencing identified an in-frame fusion between MAZ exon 3 on chromosome 16 and NCOA2 exon 12 on chromosome 8. Fluorescence in situ hybridization (FISH) confirmed a break-apart signal at the NCOA2 locus. To the best of our knowledge, this represents the first documented instance of an NCOA2 rearrangement involving MAZ in SRCSs. This case broadens the molecular spectrum of SRCSs, highlights the importance of incorporating molecular techniques into diagnostic workflows, and may have implications for future diagnostic and therapeutic strategies.

Superficial Neurocristic FET::ETS Fusion Tumor: Expanding the Clinicopathological and Molecular Genetic Spectrum of a Recently Described Entity

Superficial neurocristic EWSR1::FLI1 fusion tumor is a very recently described, clinically indolent tumor of the skin and superficial soft tissues, which differs in essentially all ways from Ewing sarcoma, despite harboring an identical fusion event. The EWSR1 and FLI1 genes are members of the FET and ETS gene family, respectively, and very rare examples of Ewing sarcoma harbor alternative FET::ETS fusion events, such as EWSR1::ERG, FUS::FLI1, FUS::ERG, EWSR1::ETV4, and others. We report 5 new cases of this very rare entity, harboring in 3 cases alternative FET::ETS fusion events. The tumors occurred in 2 males and 3 females (median age, 14 years, range, 8-69 years) and presented as solitary dermal/subcutaneous masses of the thigh, foot, shoulder, arm, and back (median size, 1.8 cm; range, 1-2 cm). All patients underwent wide excisions; one received adjuvant chemotherapy. Clinical follow-up on 3 patients (median, 24 months; range, 18-31 months) showed all to be without disease. Morphologically, all tumors displayed typical features of this entity as described, with nests of cytologically bland, diffusely S100 protein/SOX10-positive round cells without mitotic activity, surrounded by fibrous bands containing spindled cells with similar nuclear features. The tumors also showed membranous CD99 (4/5) and nuclear NKX2.2 (3/3) expression. RNA sequencing (5 cases) demonstrated FUS::FLI1, FUS::ERG, EWSR1::FLI1, EWSR1::ERG, and a novel FUS::ETV5. Methylation profiling (4 cases) showed all to cluster with previously reported superficial neurocristic EWSR1::FLI1 fusion tumors and apart from conventional and "adamantinoma-like" Ewing sarcoma. Our findings confirm the distinctive clinicopathological features of this very rare, recently described entity and expand its molecular genetic spectrum. Reflecting on these findings, we propose modifying the name of this entity to "superficial neurocristic FET::ETS fusion tumor."

[Undifferentiated small round cell sarcomas of bone and soft tissue]

In the 2020 5th edition of the World Health Organization classification of soft tissue and bone tumours a major reorganization of Undifferentiated Small Round Cell Sarcomas (USRCS) took place based on the underlying molecular features. The classification now recognizes Ewing sarcoma, round cell sarcoma with EWSR1-non-ETS fusions, CIC-rearranged sarcoma and sarcoma with BCOR alterations. The focus on these genetic alterations highlights the importance of molecular techniques in the diagnosis of these entities. Knowledge of these features can drastically reduce the time to diagnosis and avoid potential misdiagnosis. Molecular diagnostic capabilities should not be limited to an overall small number of centres worldwide as is reflected by the WHO's recognition of 'essential' and 'desirable' diagnostic criteria. A good knowledge of the usual histomorphology, uncommon variants and diagnostic pitfalls remains essential even in centres with access to a full molecular testing arsenal. This review aims to give an overview of the current classification of USRCS not by going over each entity, but instead going over the molecular, morphological, immunophenotypic and clinical features step by step to allow easy comparison of these features between the separate entities.

Superficial Neurocristic EWSR1::FLI1 Fusion Tumor: A Distinctive, Clinically Indolent, S100 Protein/SOX10-Positive Neoplasm

It is now understood that identical gene fusions may be shared by different entities. We report a distinctive neoplasm of the skin and subcutis, harboring the Ewing sarcoma-associated EWSR1::FLI1 fusion but differing otherwise from Ewing sarcoma. Slides and blocks for 5 cutaneous neoplasms coded as other than Ewing sarcoma and harboring EWSR1::FLI1 were retrieved. Immunohistochemical and molecular genetic results were abstracted from reports. Methylation profiling was performed. Clinical information was obtained. The tumors occurred in 4 men and 1 woman (median: 25 years of age; range: 19-69 years) and involved the skin/subcutis of the back (2), thigh, buttock, and chest wall (median: 2.4 cm; range: 1-11 cm). Two tumors were present "years" before coming to clinical attention. The lesions were multinodular and circumscribed and consisted of nests of bland, round cells admixed with hyalinized collagenous bands containing spindled cells. Hemorrhage and cystic change were often present; necrosis was absent. All were diffusely S100 protein/SOX10-positive; 4 of 5 were CD99-negative. One tested case was strongly positive for NKX2.2. A variety of other tested markers were either focally positive (glial fibrillary acidic protein, p63) or negative. Molecular genetic results were as follows: EWSR1 exon 7::FLI1 exon 8, EWSR1 exon 11::FLI1 exon 5, EWSR1 exon 11::FLI1 exon 6, EWSR1 exon 7::FLI1 exon 6, and EWSR1 exon 10::FLI1 exon 6. Methylation profiling (3 cases) showed these to form a unique cluster, distinct from Ewing sarcoma. All patients underwent excision with negative margins; one received 1 cycle of chemotherapy. Clinical follow-up showed all patients to be alive without disease (median: 17 months; range: 11-62 months). Despite similar gene fusions, the morphologic, immunohistochemical, epigenetic, and clinical features of these unique EWSR1::FLI1-fused neoplasms of the skin and subcutis differ substantially from Ewing sarcoma. Interestingly, EWSR1 rearrangements involved exons 10 or 11, only rarely seen in Ewing sarcoma, in a majority of cases. Superficial neurocristic EWSR1::FLI1 fusion tumors should be rigorously distinguished from true cutaneous Ewing sarcomas.

Primary Cutaneous Ewing Sarcoma of the Scalp With Metastasis to the Lung: An Unusual Manifestation During Pregnancy

ABSTRACT

A 32-year-old G2P1L1 (5 months pregnant) woman presented with a 3-month history of a slow-growing cystic lesion on her scalp vertex. Similar lesions in the exact location were excised twice in the past with a diagnosis of trichilemmal carcinoma (TC). A biopsy of the scalp lesion showed morphology and immunoprofile consistent with previously diagnosed TC. Staging PET/CT demonstrated a 4.7 cm right upper lobe lung, and a subsequent lung biopsy showed a small, round blue-cell tumor with necrosis, morphologically identical to the prior biopsies from the scalp. Considering the unusual clinical course of TC, a lung biopsy was sent for next-generation sequencing that showed EWSR1-FLI1 (type1) fusion. Additionally, CD99 immunostaining revealed uniform cytoplasmic and membranous staining in the tumor cells. The previous scalp excision specimen was also sent for mutation analysis, which showed EWSR1-FLI1 fusion. In conjunction with clinical history and histological and molecular findings, a definitive diagnosis of primary cutaneous Ewing sarcoma (PCES) with local recurrence and metastasis to the lung was made. We present a case of PCES, which was previously misdiagnosed and treated as TC. This case emphasizes the importance of CD99 in the initial screening of cutaneous small round blue-cell tumors to avoid misdiagnosis from other morphological overlaps. Also, despite its rarity, PCES should be included in the differential diagnosis of small, round, blue cell tumors at cutaneous sites. Our case also exemplifies common biases in medical decision-making, including premature closure and anchoring bias which can result in misdiagnosis or diagnostic delay and associated delay in appropriate management.

Adding Perplexity to Rarity: Diffuse S100-Protein and SOX10 Expression in a Molecularly Confirmed PAX7-Positive Primary Cutaneous Ewing Sarcoma

ABSTRACT

Primary cutaneous Ewing sarcoma (EWS) is a very rare neoplasm that shares similar morphologic, immunohistochemical, and molecular features with its osseous counterpart. Herein, we present an extraordinarily rare case of PAX7-positive cutaneous EWS in a 9-year-old girl that was also diffusely positive for SOX10 and S100-protein. Next generation sequencing detected the EWSR1-FLI1 fusion supporting the diagnosis, which was further validated by break-apart EWSR1 fluorescence in situ hybridization. Diffuse S100-protein and SOX10 expression has been reported only in a handful of cases of EWS and may pose significant diagnostic challenges for dermatopathologists. PAX7 is a recently introduced marker, which is highly sensitive for EWS and can potentially have discriminatory power in the differential diagnosis of cutaneous undifferentiated round blue cell tumors.