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CIC rearranged sarcoma (CRS) is a newly distinct entity within the category of undifferentiated small round cell sarcomas, distinguished from Ewing sarcoma (ES) by recent molecular advances.
These tumours show morphological overlap with ES but lack the characteristic EWSR1/FUS-ETS gene fusion and have been found to have a distinct clinicopathological profile. CRS represents up to 68% of EWSR1/FUS negative small round cell sarcomas, with DUX4 being the most common gene fusion partner.
We describe a case of a 68-year-old woman presenting with a submandibular soft tissue mass found to harbour CIC-DUX4 translocation. This case highlights the difficulties faced with the accurate classification of this tumour, confounded by an unusual location and uncommon histopathological features, which has significant clinical and prognostic implications.
A 68-year-old woman presented with a one-month history of an enlarging painless left submandibular mass. Fine needle aspiration showed atypical cells suspicious for malignancy and positron emission tomography/computed tomography (PET/CT) demonstrated an FDG-avid centrally necrotic mass which appeared to arise from the anterolateral aspect of the submandibular gland. Neck dissection revealed a 35 mm mass affixed to the submandibular gland which was not involved on microscopy. The tumour had a multinodular growth pattern (Fig. 1A) with geographic necrosis (Fig. 1B) and invaded the surrounding fibroadipose tissue and an adjacent lymph node. It was composed of sheets of ovoid to epithelioid cells with strand formation in some areas associated with oedematous stroma (Fig. 1C,D). The cells had large ovoid to irregular nuclei, coarse chromatin and small nucleoli with a small to moderate amount of pale eosinophilic cytoplasm (Fig. 1E,F). There were occasional interspersed rhabdoid-like cells with eccentric nuclei and abundant dense eosinophilic cytoplasm (Fig. 1D,E). Frequent mitoses (56/10 HPF) (Fig. 1F) and lymphovascular invasion were identified and the tumour focally abutted the resection margin.
Fig. 1Histological features of CIC-DUX4 sarcoma occurring in the soft tissues of the neck. (A) The tumour has a multinodular growth pattern separated by fibrous bands. (B) Extensive areas of geographic necrosis. (C) Sheets of tumour cells with an area showing strand formation and oedematous stroma. (D) Sheets of medium-sized round cells with scattered rhabdoid-like cells (arrow). (E) High power examination reveals ovoid to epithelioid cells with mildly pleomorphic nuclei, coarse chromatin, small nucleoli and pale eosinophilic cytoplasm. Interspersed rhabdoid-like cells with eccentric nuclei and abundant dense eosinophilic cytoplasm (E, arrows) and frequent mitoses (F, arrows) are identified.
Immunohistochemistry showed positive staining for p16, FLI-1 and nuclear reactivity for WT1 (Fig. 2A–C). There was patchy membranous staining for CD99 (Fig. 2D), patchy reactivity for GATA3 and ERG (Fig. 2E) and focal staining for calretinin (Fig. 2F), CD56, synaptophysin and CAM5.2. INI-1 and BRG1 showed retained expression. The tumour was negative for p63, p40, ER, TTF-1, CDX2, S100, SOX10, Melan A, chromogranin, SMA, SMMS, CD34, D2-40, CD45, CD20, CD3, CD30, NUT, EMA and a range of cytokeratins. The Ki-67 index was approximately 70%. Fluorescence in situ hybridisation (FISH) was negative for EWSR1 and CIC rearrangement. Given a high degree of clinicopathological suspicion, whole genome/transcriptome sequencing was performed and demonstrated a CIC-DUX4 gene fusion which led to the diagnosis of CRS. Three months after adjuvant radiotherapy, subcarinal lymph node, pulmonary and pleural metastases were discovered on restaging PET/CT. The patient declined adjuvant chemotherapy due to unlikely meaningful benefit and succumbed to disease 12 months after initial presentation.
Fig. 2Immunohistochemical features. The tumour cells show diffuse reactivity for (A) p16, (B) FLI-1 and (C) WT1. (D) Patchy membranous staining for CD99. (E) Patchy expression of ERG. (F) A focal area of staining for calretinin.
The majority arise in the deep soft tissues of the extremities and trunk but can uncommonly affect other sites including viscera (10%) and bone (<5%). These tumours are composed of sheets of medium-sized round cells, often with a lobulated growth pattern separated by fibrous bands. Minor components of epithelioid, rhabdoid or spindled cell morphology are often present. Interestingly, whereas most of the literature describes these heterogenous elements in focal aggregations, Rekhi et al. reported a case with ‘rhabdoid-like’ cells interspersed throughout round to polygonal cells as was observed in this case.
Clinicopathological features of undifferentiated round cell sarcomas of bone and soft tissues: an attempt to unravel the BCOR-CCNB3- & CIC-DUX4-positive sarcomas.
Tumour nuclei typically show mild pleomorphism with vesicular chromatin and prominent nucleoli. Mitotic activity is high (mean 30/10 HPF) and geographic necrosis is common. Focal myxoid stromal changes can be seen. Immunohistochemically, CRS usually shows variable CD99 staining, most often in a focal patchy pattern, and nuclear reactivity for ETV4 and WT1. These tumours can also show staining for calretinin, ERG and FLI-1. NKX2-2, keratin, S100 and myogenic markers are typically negative.
CIC-DUX4 is the most common gene fusion in CRS resulting from the chromosomal translocations t(4; 19) (q35; q13) or t(10; 19) (q26; q13).
These involve the CIC gene on chromosome 19 and the double homeobox 4 (DUX4) gene, or its paralog DUX4L (DUX4-like), on chromosome 4 or 10 respectively. CIC is a human homologue of the Drosophila capicua gene which encodes a high mobility group (HMG)-box transcriptional repressor. It is expressed in the developing cerebellum and involved in ErbB signalling which plays a role in cell growth and survival. CIC gene abnormalities have been implicated in some central nervous system malignancies. DUX4 is a subtelomeric gene that encodes a double-homeobox transcription factor expressed in germ cells but epigenetically silenced in differentiated tissues. Aberrant DUX4 expression has been linked to fascioscapulohumeral muscular dystrophy. The oncogenic chimeric protein is composed of most of the preserved CIC protein, including the HMG-box, with loss of a large portion of the DUX4 N-terminal region, including the double-homeobox domain. CIC-DUX4 has modified CIC transcriptional activity due to fusion of the DUX4 C-terminus and directly upregulates members of the polyoma enhancer activator 3 (PEA3) subgroup of the ETS transcription factor superfamily, ETV4, ETV1 and ETV5. Overexpression of PEA3 genes has been implicated in the carcinogenesis and tumour progression of multiple cancers.
CRS arising in the head and neck is rare with a large seminal study reporting 12 of 111 CRS cases in this location.
This complicates the diagnosis of small round cell tumours in this region which have a wide range of differential diagnoses. Owosho et al. studied 16 patients with head and neck CRS and found that they occurred most commonly in the soft tissues of the neck.
Interestingly, the submandibular region was a less common site with only two cases reported. This series was also compared to cases of ES in the head and neck, including extraskeletal sites, which were found to be more monotonous and lacked the mild nuclear pleomorphism, conspicuous nucleoli and focal component of heterogenous cell types characteristic of CRS. CRS was also shown to have diffuse nuclear WT1 reactivity and a lack of strong diffuse membranous CD99 staining in contrast to ES, as was observed in this case.
Small round cell tumours in the head and neck present a diagnostic challenge and a wide range of differential diagnoses with overlapping histopathological features needs to be considered.
CRS can be difficult to distinguish from undifferentiated carcinomas which are more common at this site, particularly in the minority of cases that show focal positivity for pan-cytokeratin.
CIC-DUX4 rearranged uterine cervix round-cell sarcoma exhibiting near-complete pathologic response following radiation and neoadjuvant chemotherapy: a case report.
however other epithelial markers including pan-cytokeratin were negative. The role of p16 immunohistochemistry, which is overexpressed in multiple high grade malignancies, has not been previously studied in CRS and warrants investigation. The solid variant of alveolar rhabdomyosarcoma could also be considered at this age and location, however the lack of staining with myogenic markers excludes this diagnosis. The involvement of an adjacent lymph node raised the possibility of lymphoma which was inconsistent with negative lymphoid markers. Myoepithelial carcinoma and poorly differentiated synovial sarcoma can show morphological overlap with CRS but have contradictory immunohistochemical profiles.
Ancillary genetic testing is required to confirm the diagnosis of CRS, however histopathological features are essential to guide such investigations. Despite a high degree of suspicion based on morphology and immunophenotype, FISH was negative for CIC as well as EWSR1 rearrangement in this case. CIC-DUX4 gene fusion was instead confirmed by whole genome/transcriptome sequencing. The rate of false negative FISH results with CIC break apart probes has been reported as 14%.
The causative mechanism is unclear but cryptic insertions undetectable by FISH assays have been suggested. The combination of classic histological and immunohistochemical features should prompt further genetic testing in this context. Novel biomarkers have been identified to aid in the recognition of CRS including nuclear ETV4 expression which has proven to be highly sensitive and specific.
DUX4 immunohistochemistry and RNA in situ hybridisation for the detection of upregulated co-expression of ETV1, ETV4 and ETV5 have been shown to be promising diagnostic adjuncts.
This is complicated by the limitations of wide surgical resection in the head and neck due to complex anatomy. The rapid development of thoracic metastases only 3 months post-adjuvant radiotherapy in this case demonstrates the aggressive nature of this tumour. Most patients are treated with an ES-style chemotherapy regime and demonstrate a poor pathological response.
This emphasises the importance of accurate diagnosis of CRS for future advances in treatment.
Our case contributes to the distinctive pathological profile of CRS as described in the literature and highlights several uncommon features that expand the disease spectrum in accordance with other case reports. CRS should be included in the list of differential diagnoses for undifferentiated tumours arising in the soft tissues of the head and neck that are difficult to characterise. An integrated approach utilising morphological, immunohistochemical and molecular techniques is essential for the accurate diagnosis of CRS which has significant clinical implications.
Acknowledgements
We thank Dr Catherine Mitchell at the Peter MacCallum Cancer Centre for review of this case. Support from the Head and Neck Surgical Unit and Multidisciplinary Team at University Hospital Geelong, Barwon Health is also greatly appreciated.
Conflicts of interest and sources of funding
The authors state that there are no conflicts of interest to disclose.
References
World Health Organization Classification of Tumours Editorial Board
World Health Organization Classification of Tumours of Soft Tissue and Bone.
5th ed. IARC Press,
Lyon2020 (Chapter 3, Undifferentiated small round cell sarcomas of bone and soft tissue)
Clinicopathological features of undifferentiated round cell sarcomas of bone and soft tissues: an attempt to unravel the BCOR-CCNB3- & CIC-DUX4-positive sarcomas.
CIC-DUX4 rearranged uterine cervix round-cell sarcoma exhibiting near-complete pathologic response following radiation and neoadjuvant chemotherapy: a case report.
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