Primary Clear Cell Sarcoma of Bone: Report of a Case Arising from Distal Femur in a Pediatric Patient and Review of the Literature

Clear cell sarcoma is a rare mesenchymal tumor with melanocytic differentiation, firstly described by Enziger in 1965. It accounts for less than 1% of all soft tissue sarcomas and occurs predominantly in the extremities of adults. Primary clear cell sarcoma of bone is even rarer; current literature consists of only 13 cases with a mean age of presentation of 45-year-old, ranging from 18 to 81. Because of the overlapping expression of melanocytic markers, the differential diagnosis should be addressed with metastatic malignant melanoma and metastasis from clear cell sarcoma of soft parts. Essential for the diagnosis is the evidence of translocations t (12;22) or t (2;22) resulting respectively in EWSR1/ATF1 and EWSR1-CREB1 gene fusion. Clinical history and radiological assessment could help discriminate between bone or soft parts origin. We report an unusual case of clear cell sarcoma of distal femur involving a 13-year-old patient. Histological sections showed a malignant neoplasm composed of nests of uniform cells separated by fibrous septa, showing strong immunoreactivity for melanoma triple cocktail. Real Time-Polymerase Chain Reaction revealed the presence of the fusion product EWSR1-ATF1, confirming the diagnosis of clear cell sarcoma of bone. Our purpose was to revise the current literature in order to find common clinicalradiological features to this rare entity. To the best of our knowledge, our case is the first bone clear cell sarcoma arising from the distal femur in a pediatric patient described in the literature: it highlights the importance of considering this rare entity in the differential diagnosis of primary bone tumor, although extremely rare and of the use of molecular means to confirm the diagnosis. © 2020 Debora Lana. Hosting by Science Repository. All rights reserved.


Introduction
Clear cell sarcoma (CCS) is a rare mesenchymal tumor accounting for less than 1% of all sarcomas, mainly arising from deep soft tissues near tendons and aponeurosis of the extremities, above all foot and ankle, with a peak of incidence in young adults, between 20-40 years of age. Enziger described it first in 1965 [1]. Due to its characteristic expression of melanocytic markers (HMB-45, S-100, MelanA), overlapping with malignant melanoma (MM) immunological profile, it was previously known as MM of soft parts. Nevertheless, the presence of chromosomal translocations t (12;22) in about 90% of CCS and less frequently t (2;22), resulting respectively in EWSR1-ATF1 and EWSR1-CREB1 genes fusions, confirms the diagnosis of CCS, since these translocations are absent in MM and other mimickers [2][3][4][5].
Primary CCS of bone is its rarer form originating from the bone, which shares the same morphological, immunohistochemical, and molecular features. Due to its extreme rarity, imaging appearance, and histological pattern similar to other entities (like soft tissue CCS and MM), differential diagnosis is challenging and must include mainly a Surgical Case Reports doi: 10.31487/j.SCR.2020.09.10 Volume 3(9): 2-5 metastatic MM and a bone metastasis or secondary bone involvement from soft tissue CCS. The mainstay in the differential diagnosis of CCS of the bone with other bone entities and MM is based on the positivity of rearrangement EWSR1-ATF1, whereas with soft tissues CCS only on clinical and radiological features. From a radiological point of view, the majority of cases reported shared a pure lytic bone lesion with cortical disruption and secondary soft tissue involvement. Bone scan and 18 Flabeled fluoro-2-deoxyglucose positron emission tomography/computed tomography ( 18 F-FDG PET/CT) are essential to pathological staging and detection of distant locations when ruling out metastatic MM or soft tissue CCS. To date, only 13 cases described as primary CCS of bone have been reported in literature: in 1996, first described a case in the ulna; other presented cases were from ribs, sternum, metatarsal bone, radius, scapula, humerus, sacrum, pubis and calcaneus [6][7][8][9][10][11][12][13][14][15][16][17][18]. No cases have been reported before in long bones of the lower limb or in pediatric age and upon critical review, only 3 of 13 were shown to harbour t (12;22) translocation, while the remaining cases were not confirmed by molecular analysis. Furthermore, in our opinion, one case reported as CCS arising from third metatarsus did not really originate from bone, as shown by the diagnostic imaging reported by authors (Table 1) [12].

Case Report
We report the case of a 13-year-old Caucasian patient with 2 months history of right knee pain not associated with previous trauma, who was referred to our institution from another hospital. The patient and his family were informed that data from the case would be submitted for publication and gave their consent. Clinically he presented a hard swelling at the medial condyle of the right femur. The range of motion was normal and there were no vascular or neurological deficits. Radiographs and computed tomography (CT) of the right knee put in evidence an ill-defined osteolytic lesion of the medial femoral condyle which spreads over the growth plate with a geographic pattern cortical disruption and limited extra-compartmental involvement (Figures 1-3). A CT guided core needle biopsy was performed and the 18 Figures 6a, 6b &  6c). At the light of the melanocytic immunophenotype of the neoplasm, Real Time-Polymerase Chain Reaction (RT-PCR) for the research of the fusion product EWSR1-ATF1 resulting from the reciprocal translocation t (12;22) (q13;q12) was performed and resulted positive, confirming a diagnosis of CCS (Figure 6e). Taking into account the histological, immunohistochemical and molecular pattern of the neoplasm, its location and radiological aspect, we concluded that the neoplasm should be regarded as a rare case of primary CCS of bone. The young patient underwent wide resection of the distal right femur and reconstruction with a modular prosthesis (Figures 7a & 7b).  The surgical specimen of distal femur resection showed areas similar to those found in the biopsy, but also others of spindling with monomorphic neoplastic cells exhibiting clear cytoplasm and arranged in a nested pattern (Figures 6a & 6b). There was limited necrosis (<10%) and the mitotic count was low. Interspersed wreath-like giant cells were conspicuous. Extensive permeation of malignant cells through trabecular bone and cortical breaching with peri-osseous soft tissue infiltration was evident (Figure 6d) surgery, the CT scan put in evidence multiple lung and bone metastasis, the latter located in thoracic and upper lumbar vertebral bodies: consequently, the patient has been addressed to adjuvant chemotherapy.

Discussion
Primary CCS of bone is an extremely rare primary tumor of uncertain histological origin. The literature on bone CCS consists to date of only 13 single case reports with a mean age of presentation of 45-year-old, ranging from 18 to 81. The sites of presentation vary from axial to appendicular skeleton, but in the cases, which involved long bones, the tumor seemed to occur most frequently in the meta-epiphyseal region. Only 3 cases have molecular confirmation with positivity to translocation t (12;22) (q13; q12); on the contrary, 4 patients were negative and in 6 cases this information lacked, undermining the veracity of the diagnosis. Furthermore, from our critical review one case seemed to originate from soft tissues rather than from bone [12] ( Table 1).
As for soft tissue CCS, histologically exhibits round or spindle neoplastic cells with clear to moderate eosinophilic cytoplasm, organized in nests or short fascicles separated by dense collagenous septa. Their nuclei are round and uniform, often with large nucleoli [4].  [19].
Genetically, is characterized by the presence of t (12; 22) (q13; q12) translocation, that leads to the fusion of activating transcription factor1 (ATF1) oncogene on chromosome 12q13 and Ewing sarcoma oncogene (EWSR1) on chromosome 22q12, resulting in the EWSR1-ATF1 fusion protein [4,5]. It is not known, due to the poor number of cases described in the literature, if translocation t (2;22) (q34; q12), resulting in EWSR1-CREB1 gene fusion, reported in a minority of soft tissue CCS, is expressed in CCS of bone as well [4,18,19]. From a radiological point of view, all the cases reported of CCS of bone shared the same radiological aspects of pure lytic bone lesion, sometimes breaking through the cortex in a geographic pattern and infiltrating the surrounding soft tissues, like in the case we describe.
According to our retrospective analysis of the cases reported so far and including ours, the most frequent treatment was wide surgical resection, as the mainstay of treatment for the vast majority of primary bone sarcomas, except for the cases of foot bone involvement in which amputation was performed. Clinical course is unfavorable with progression in almost 50% of the cases, with lung being the most frequent site of metastases. Contrary to soft tissue CCS, nodal involvement is not frequent (7,1% vs. 29%) ( Table 1) [4]. Overall, since no effective chemotherapy is feasible, prognosis of patients affected by CCS of bone is still poor and, with the limit of a retrospective analysis, 42,8 of the patients died at follow-up or were alive with disease at a mean follow-up time of respectively 15 and 7,6 months (Table 1).
Currently, with the lack of a wider clinical experience on a larger cohort of patients, surgery appears to be the only beneficial treatment, with the hope that new therapeutic agents and targeted molecular therapy will be implemented to improve the oncologic outcome in both early and late stages disease. To the best of our knowledge, the present case is the first CCS of bone described in the pediatric age and the first one arising from Surgical Case Reports doi: 10.31487/j.SCR.2020.09.10 Volume 3(9): 5-5 femur. Although extremely rare, CCS of bone should be considered in the differential diagnosis of primary bone tumors.

Declarations
Written informed consent was obtained from the subject and his parents before publishing the study. We the authors of this submission confirm that we have not published the same or a very similar study with the same or very similar results and major conclusions in any other journals. These include English or non-English language journals and journals that are indexed or not indexed in PubMed, regardless of different words being used in the article titles, introduction, and discussion.