Chondromyxoid Fibroma: An Updated Review.

Introduction
Chondromyxoid fibroma (CMF) is a rare benign cartilaginous tumor first described by Jaffe and Lichtenstein (1) in 1948. It belongs to the chondrogenic tumor group according to the latest World Health Organization (WHO) classification of soft tissue and bone tumors (2). The etiology of this peculiar tumor is unknown. The morphology of CMF can overlap with a variety of mesenchymal tumors such as chondroblastoma (CB), phosphaturic mesenchymal tumor (PMT), conventional chondrosarcoma (CS) and CMF-like osteosarcoma (CMF-OS). Advances in knowledge of the histopathological and molecular features of CMF are leading to more timely and accurate diagnosis and appropriate management. This review provides valuable insights into the clinical, radiological, histological, immunohistochemical and molecular genetic features of CMF. In addition, we discuss the differential diagnosis of this uncommon entity.

Clinical Features, Management and Prognosis
CMF accounts for less than 1% of all bone tumors and has a peak incidence in the second and third decades of life, with a slight male predominance (3-5). It most frequently occurs in the metaphysis of long bones, especially the proximal tibia and distal femur. The flat bones, mainly the ilium, are also relatively common sites (3). The clinical manifestations vary according to the size, location and extent of the lesion. Pain is the most common presenting symptom, and it may be noted for a considerable time (4). Local swelling may also be noted and is more common in tumors of the small bones. Limited range of motion in the adjacent joints and pathological fractures are rare. The natural history of CMF is not entirely understood; to date, there has been no evidence of potential spontaneous healing. Surgical management is the mainstay of treatment for patients with CMF. The accepted procedure is intralesional curettage and filling of the cavity with bone graft or bone cement (6-9). In general, curettage alone is not recommended due to its higher local recurrence rate (up to 80%) (6,7). Extensive curettage using a high-speed burr may be performed to reduce the local recurrence rate (10). En
bloc resection, when feasible, is another treatment option (4,11,12), although it may result in loss of function of the joint. Local recurrence occurs in 9-33% of cases (3,4,6-9,13-16). The risk of local recurrence appears to be higher in younger patients (8). Radiotherapy is not indicated except for surgically inaccessible lesions (4). Distant metastasis has not been documented; however, malignant transformation has been described in the literature (3,17).

Radiological Features
Radiographs typically show a well-defined, lobulated or oval eccentric lytic lesion with geographic bone destruction, sclerotic margins and internal trabeculations (18). A nearly hemispherical ‘bite’ from the cortical margin without periosteal reaction is the characteristic feature, particularly in long bones (19). In small bones, CMF tends to be central and shows fusiform expansion of the bone. In flat bones, the lesion may become large and markedly scalloped, producing a bubbly appearance. Computed tomography (CT) reveals a well-demarcated, expansile osteolytic lesion with a sclerotic rim (18). Unlike other cartilaginous tumors, intralesional calcification is unusual, occurring in 3.3-16% of cases (5,18-20). Interestingly, the incidence of calcification is higher in patients aged over 40 years and in tumors of the flat bones (20). On Magnetic resonance imaging (MRI), CMF usually exhibits low to intermediate signal intensity on T1-weighted sequences and heterogenous high signal intensity on T2-weighted sequences due to the different composition of fibrous, cartilaginous and myxoid components (18). Contrast-enhanced MRI demonstrates intense homogeneous, heterogeneous or peripheral enhancement (18). Positron emission tomography (PET) features of CMF have been described in the literature (21-24). Integrated PET/CT images show low to moderate fluorodeoxyglucose (FDG) uptake by the lesion, with a maximum range of standardized uptake value (SUVmax) of 3.2-5.9.
Less frequently, juxtacortical CMF has been documented in long and small bones (25-28). Ultrasonography may reveal a hypoechoic lesion with saucerization of the cortex and periosteal reaction (26). Radiographs and CT usually exhibit a radiolucent surface lesion with sclerotic margins and scalloping or saucerization of the underlying bone. Compared to conventional CMF, juxtacortical CMF tends to show extensive calcification (25). Like conventional CMF, MRI typically demonstrates a well-circumscribed lesion with intermediate signal intensity on T1-weighted sequences, heterogenous high signal intensity on T2-weighted sequences and peripheral or heterogeneous enhancement on contrast-enhanced sequences (28).

Histological and Immunohistochemical Features
The definitive diagnosis of CMF is made based on histopathological examination. Grossly, curetted fragments appear firm or rubbery, blue-gray or semitranslucent (5). If the lesion is removed intact, it is multilobulated, well-circumscribed and sharply demarcated from the surrounding bone (4). Histologically, CMF shows a predominantly lobulated growth pattern with central hypocellularity and consists of stellate to spindle-shaped cells in a myxoid background. The cells frequently have abundant pink cytoplasm, producing an epithelioid appearance. Osteoclast-type giant cells may be seen at the periphery of the nodules. Enlarged, atypical cells, likely degenerative in nature, can occasionally be encountered. Hyaline cartilage is present in 19% of cases (3). Coarse calcification is observed in approximately one third of cases (3,20). Mitotic figures are uncommon. Immunohistochemically, the spindle cells are positive for smooth muscle actin (SMA) and muscle-specific actin (MSA) (29). Epithelial membrane antigen (EMA) may be focally positive in CMF (15). Chondroid areas are variably positive for S100 (29). Recently, nuclear expression of SRY-box transcription factor 9 (SOX9) and ETS transcription factor ERG (ERG) has also been demonstrated in CMF (30,31). Most notably, CMF typically shows strong and diffuse expression of glutamate metabotropic receptor 1 (GRM1) (32,33). Toland et
al. (32) suggested that GRM1 immunohistochemistry would be a useful adjunct tool for the diagnosis of CMF, especially in small biopsy specimens. Immunostaining for desmin and CD34 is typically negative (29).

Cytogenetic and Molecular Genetic Features
Cytogenetically, a diploid or near-diploid chromosomal complement has been observed in CMF. Chromosome 6 aberrations are prominent and have involved the following three recurrent regions: 6p23-25, 6q12-15 and 6q23-25 (34-44). Notably, an inv(6)(p25q13) has been described as the sole anomaly in a few cases (41). At the molecular level, GRM1 alterations including gene fusions and rearrangements have been identified in CMF (33,45). Torrence et
al. (33) reported that GRM1 gene rearrangements could be detected using fluorescence in
situ hybridization (FISH) in approximately 75% of cases. GRM1 gene is located on chromosome 6q24.3 and encodes a metabotropic glutamate receptor that functions by activating phospholipase C. It is recognized that GRM1 is activated through promotor swapping and gene fusion events in CMF (45). Interestingly, GRM1 expression is absent or very low in other cartilaginous tumors, strongly suggesting a key pathogenetic event in CMF (45). To date, several fusion partners of GRM1 have been detected in CMF, including BCL2 associated transcription factor 1 (BCLAF1), myocyte enhancer factor 2A (MEF2A) and PNN interacting serine and arginine rich protein (PNISR) (45,46). Moreover, recurrent rearrangements of collagen type XII alpha 1 chain (COL12A1) have been reported in a subset of CMFs (41,42,45). It has been shown that GRM1 upregulation is due to genomic rearrangements that place GRM1 under the influence of COL12A1 in some cases (45). COL12A1 gene is located on chromosome 6q13-14.1 and encodes the alpha chain of type XII collagen. This gene rearrangement has also been demonstrated in subungual exostosis with t(X;6)(q22;q13-14) (47).

Differential Diagnosis
The differential diagnosis of CMF includes CB, PMT, conventional CS and CMF-OS. The corresponding clinicopathological and molecular characteristics are summarized in Table I.
CB is a relatively rare benign chondrogenic tumor that frequently occurs in the epiphysis of long bones, particularly the femur, proximal tibia and proximal humerus. It has a peak incidence in the second to early third decades of life, with a male predominance (48). The most common symptom is mild to significant pain. Surgery is the treatment of choice for CB and includes extensive intralesional curettage with local adjuvants and en
bloc resection (49-51). Local recurrence occurs in 4.3-39.5% of cases (49-57). Notably, Xu et
al. (55) reported that the local recurrence rate was 0% in the extremities after resection. Predisposing factors for local recurrence remain controversial, but younger age, larger tumor size and tumor location may increase the risk (51,55,57). Although rare, malignant transformation and distant metastasis have been documented in the literature (52,53,56,58). The most common site of distant metastasis is lung (58). Radiographs usually show a well-demarcated, round or oval lytic lesion with geographic bone destruction and thin sclerotic margins. CT may reveal a well-defined, osteolytic lesion with a thin sclerotic rim and stippled or patchy calcifications (59). Unlike CMF, intralesional calcification is frequently found in CB. On MRI, CB usually exhibits intermediate signal intensity on T1-weighted sequences and variable signal intensity on T2-weighted sequences. Peritumoral edema or aneurysmal bone cyst (ABC)-like changes can be seen (48,60). Contrast-enhanced MRI demonstrates heterogeneous lobular or septal enhancement (60). Integrated PET/CT images show varying degrees of FDG uptake by the lesion, with a mean SUVmax of 7.2 (60). Histologically, CB is characterized by a sheet-like proliferation of ovoid to polygonal cells with grooved, reniform nuclei and eosinophilic cytoplasm. Pericellular lace-like or chicken-wire calcification is the hallmark of CB (48). Osteoclast-like giant cells are often distributed irregularly, and ABC-like changes are common. Mitotic activity is generally low. Recently, histologically malignant CB has been described in the literature (61,62). In contrast to conventional CB, malignant CB exhibits permeative growth and significant cytologic atypia. Several immunohistochemical studies have demonstrated that H3.3 K36M mutant antibody is a highly sensitive and specific marker for the diagnosis of CB (63-66). Additionally, SOX9, S100, cytokeratin and discovered on GIST1 (DOG1) may be focally positive in CB (30,66,67). Cytogenetic studies have shown heterogenous rearrangements of chromosomes 5, 8 or 17 (68-70), but no consistent chromosomal alterations have been found in CB. At the molecular level, point mutations of H3-3B (H3.3 histone B), located on chromosome 17q25.1, have been identified in 70-95% of CBs (71-73). The discovery of H3-3B mutations has led to more precise diagnostics of CB.
PMT is a rare soft-tissue or bone neoplasm that most frequently occurs in middle-aged adults, with no sex predominance (74). It is usually associated with tumor-induced osteomalacia (TIO). Most PMT-associated TIOs are mediated through overproduction of fibroblast growth factor 23 (FGF23) (74). PMT most often affects the extremities and acral sites in the soft-tissues, whereas bone tumors commonly occur in the appendicular skeleton, cranial bones and paranasal sinuses (75). Clinical symptoms are mostly related to TIO and include muscle pain, bone pain, pathological fractures, progressive muscle weakness and gait disturbance (76). Surgery remains the definitive treatment for PMT. Following complete resection of the tumor, patients will make a dramatic recovery with disappearance of symptoms and return of laboratory values to normal. Local recurrence occurs in 14.2% of cases (77). Although histologically benign, rare cases of malignant transformation and distant metastasis have been described (78-80). There are currently no definite clinical prognostic factors in PMT. Radiographs may be useful in displaying fractures due to osteomalacia. CT usually reveals an osteolytic lesion with a narrow zone of transition and internal matrix in intraosseous PMT (81). On MRI, PMT typically exhibits intermediate signal intensity on T1-weighted sequences and high signal intensity with dark foci on T2-weighted sequences (82). Contrast-enhanced MRI demonstrates homogeneous enhancement (81). PET/CT may be useful in identifying an occult lesion and frequently shows moderate FDG uptake by the lesion, with a mean SUVmax of 4.1 (82). Histologically, PMT is composed of bland, spindle to stellae cells in a smudgy or grungy calcified matrix. A very well-developed capillary network is typically present (74). Osteoclast-like giant cells and mature adipose tissue may be found. Mitotic activity and necrosis are usually absent. Notably, CMF-like PMT has been described (83-85), posing a great diagnostic challenge. Moreover, a small number of histologically malignant PMTs have been reported in both bone and soft-tissue (76); these lesions show high cellularity, high nuclear atypia, increased mitotic activity and tumor necrosis (86). By immunohistochemistry, PMT often expresses CD56, ERG, fibroblast growth factor receptor 1 (FGFR1), special AT-rich sequence-binding protein 2 (SATB2) and somatostatin receptor 2A (SSTR2A) (83,87). Expression of FGF23 protein has been shown in some PMTs (86). Cytogenetic abnormalities have been reported in only two cases, with no shared alterations (88). At the molecular level, nearly half of PMTs harbor fibronectin 1 (FN1)-FGFR1 or, more rarely, FN1-fibroblast growth factor 1 (FGF1) fusions (89,90), implicating activation of FGFR1 signaling pathway. However, fusion-negative PMTs frequently exhibit overexpression of klotho (KL) (91,92). These fusion-negative cases with high KL expression may show distinct clinicopathological features such as a predilection for skeletal or sinonasal locations and histological resemblance to cellular solitary fibrous tumor (92).
Conventional CS is the most frequent subtype of CS, accounting for approximately 83% of all cases (93). It has a peak incidence in the third to six decades of life, with no significant sex predominance (94,95). Conventional CS predominantly occurs in metaphysis of long bones (especially the proximal femur, proximal humerus and distal femur) and the flat bones (especially the ilium and ribs) (94,95). In addition to arising de
novo, conventional CS can arise secondary to a benign pre-existing lesion such as osteochondroma or enchondroma. Currently, conventional CS is divided into three histological grades comprising atypical cartilaginous tumor/CS grade 1 (ACT/CS1), CS grade 2 (CS2: intermediate-grade) and CS grade 3 (CS3: high-grade) (94,95). Clinically, pain and local swelling are the most common presenting symptoms. Surgery remains the mainstay of curative treatment for conventional CS and includes extensive intralesional curettage with local adjuvants for ACT in the long bones and wide en
bloc resection for CS1 in the axial skeleton, CS2 and CS3 (96,97). Conventional CS does not respond to chemotherapy or radiation. Local recurrence rates are 7.5-11% for ACT/CS1, 19% for CS2 and 26% for CS3 (94,95). ACT/CS1 rarely metastasize. Metastatic rates are 10-30% for CS2 and 32-71% for CS3 (94,95). The 10-year overall survival rates are 88-95% for ACT/CS1, 58-86% for CS2 and 26-55% for CS3 (94,95). Radiographs typically reveal an ill-defined, mixed lytic and sclerotic lesion with geographic bone destruction, endosteal scalloping and ring and arc-type calcifications (98). In high-grade CSs, radiographs may reveal a larger lesion with motheaten or permeative bone destruction, cortical destruction and less matrix mineralization (99). CT is optimal to detect subtle matrix mineralization, which is demonstrated in 94% of cases (98). On MRI, conventional CS usually exhibits low to intermediate signal intensity on T1-weighted sequences and high signal intensity on T2-weighted sequences. Areas of matrix mineralization show low signal intensity on all MR pulse sequences. Contrast-enhanced MRI demonstrates mild peripheral and septal enhancement (98). On PET/CT, conventional CS shows varying degrees of FDG uptake by the lesion (100,101). Histologically, ACT/CS1 shows a lobular growth pattern, abundant hyaline cartilage matrix with sometimes myxoid changes and low cellularity. Binucleated cells are frequently observed, but mitoses are absent (94). In CS2, cellularity is higher than ACT/CS1; additionally, extensive myxoid matrix changes may be present, mimicking CMF. Nuclear atypia and mitoses can be seen. In CS3, nuclei are larger and more hyperchromatic, and mitoses are more easily found (95). Focal spindle cell changes may be present at the periphery of lobules. Necrosis can be seen regardless of tumor grade. Immunohistochemically, isocitrate dehydrogenase (NADP(+)) 1 (IDH1) expression via the R132H mutation-specific antibody can be observed in a subset of conventional CSs (102). Cytogenetically, conventional CSs are heterogeneous, with karyotypic complexity ranging from simple numerical changes to abundant numerical and structural abnormalities (103). At the molecular level, approximately 50% of conventional CSs harbor heterozygous mutations in IDH1 R132 and isocitrate dehydrogenase (NADP(+)) 2 (IDH2) R172 (102). Moreover, mutations of collagen type II alpha 1 chain (COL2A1) have been reported in approximately 37% of CSs (104). Additional genomic alterations include tumor protein p53 (TP53) mutations, telomerase reverse transcriptase (TERT) mutations and cyclin dependent kinase inhibitor 2A/B (CDKN2A/B) deletions in high-grade CS (104-107).
CMF-OS is an extremely rare variant of low-grade OS (108). It has a peak incidence in the third to fourth decades of life, with no significant sex predominance (109). Unlike CMF, CMF-OS more commonly occurs in the craniofacial region (109). Common presenting symptoms include pain, local swelling or mass. Surgery is the treatment of choice for CMF-OS, and wide resection is always recommended whenever feasible. CMF-OS has a high risk of local recurrence and distant metastasis despite its low-grade morphology (109). The role of chemotherapy or radiotherapy in the management of CMF-OS remains unknown. Radiographs and CT may show an ill-defined, expansive osteolytic lesion with incomplete trabeculations, cortical destruction and soft-tissue extension (108-110). A periosteal reaction is also frequently observed (110,111). On MRI, CMF-OS usually exhibits low to intermediate signal intensity on T1-weighted sequences and high signal intensity on T2-weighted sequences. Contrast-enhanced MRI demonstrates heterogeneous enhancement (109,110,112). Histologically, CMF-OS is composed of lobules of spindle, stellate or polygonal cells with moderate nuclear pleomorphism and hyperchromasia embedded in a highly myxoid stroma (108). The most distinctive feature is the presence of direct production of osteoid by the neoplastic cells. Immunohistochemically, unlike CMF, S100 expression is absent in CMF-OS. No cytogenetic and molecular genetic alterations have been reported thus far.

Conclusion
CMF is a rare cartilaginous tumor with recurrent but no metastatic potential that primarily occurs in the long bones of adolescents and young adults. Unlike other cartilaginous tumors, intralesional calcification in CMF is unusual by imaging. En bloc resection is favored over curettage to decrease likelihood of local recurrence. CMF is histologically characterized by a lobular pattern with stellate to spindle-shaped cells in a myxoid background. GRM1 alteration is highly characteristic of CMF. The use of GRM1 immunostaining is a valuable adjunct in the diagnosis of CMF. Further investigations are required to delineate the correlation between genetic alterations and clinicopathological features.

Conflicts of Interest
The Authors declare no conflicts of interest associated with this article.

Authors’ Contributions
JN conceived and designed the study, searched the literature and drafted the article. YS, YC and MA collected the data and reviewed the article.

Artificial Intelligence (AI) Disclosure
No artificial intelligence (AI) tools, including large language models or machine learning software, were used in the preparation, analysis, or presentation of this article.

Funding
No funding was received for conducting this study.