Overview and definition
Fibrocartilaginous mesenchymoma (FM) is a rare, locally aggressive intraosseous neoplasm classified under "other mesenchymal tumours of bone" in the 2020 WHO Classification of Soft Tissue and Bone Tumours.[1] First described by Dahlin et al. in 1984, the lesion is composed of mildly atypical spindle cells admixed with discrete nodules of hyaline cartilage that recapitulate the architecture of the epiphyseal growth plate, undergoing endochondral ossification.[2][3] Pathogenesis remains uncertain; the growth-plate–like cartilage was initially hypothesized to represent displaced physeal remnants, but occurrence in bones lacking an epiphyseal plate (vertebrae, pubis) argues against this theory.[2] On imaging, FM characteristically presents as a lytic lesion with cartilaginous calcifications and cortical disruption. Despite its locally aggressive behavior and propensity for recurrence after incomplete excision, no metastases or tumor-related deaths have been documented; en-bloc excision with tumor-free margins is the treatment of choice.[2]
Terminology and classification
- Current accepted name: Fibrocartilaginous mesenchymoma (FM). Recognized as a distinct entity in the 2020 WHO Classification of Soft Tissue and Bone Tumours, 5th edition, under "other mesenchymal tumours of bone".[1]
- Abbreviation: FCM is used interchangeably with FM in the literature.[4][5]
- Important distinction from fibrocartilaginous dysplasia (FCD): FCD is a variant of fibrous dysplasia containing cartilaginous nodules and harbors GNAS mutations; FM is molecularly distinct (GNAS-negative) and should not be conflated with FCD.[2][6][7]
- Obsolete usage: Early reports occasionally applied the term "malignant" to this lesion; subsequent series demonstrated benign behavior, and the term malignant is no longer warranted.[3]
Epidemiology
- Frequency: Exceedingly rare; approximately 33 cases had been reported in the world literature as of 2017.[2]
- Age: Predominantly affects children, adolescents, and young adults. Reported age range spans from 3 months to 36 years, with a median of approximately 13 years.[2][8]
- Sex: Slight male predominance (male-to-female ratio approximately 1.75:1).[2]
- Anatomic distribution: Strong predilection for the metaphysis of long bones, particularly the tibia, humerus, femur, and fibula. The iliac-pubic bone and vertebrae are the most common non-long-bone sites. Involvement of the jaws has not been documented in the published literature; the entity is included in this atlas for completeness of the differential diagnosis of mixed lytic-calcified osseous lesions that may rarely involve the craniofacial skeleton.[2][3][8][9]
Clinical presentation
- Presentation: Pain and swelling are the most common symptoms, particularly when superficial bones are affected. Some patients are asymptomatic, with the lesion discovered incidentally.[2]
- Tempo: Growth can be rapid and the lesion may reach considerable size despite its benign nature. Pathologic fracture is a recognized presenting event.[2][4]
- Examination findings: A palpable, firm mass may be evident over superficial bones. In spinal cases, neurologic compromise has been reported when the tumor encircles the spinal cord.[9]
- Diagnostic pitfall: Because the characteristic cartilaginous component may be absent in small biopsies, initial biopsy frequently yields a nonspecific spindle cell lesion, leading to misdiagnosis as desmoplastic fibroma or low-grade spindle cell sarcoma. Repeat or excisional biopsy is often required to establish the diagnosis.[2][8]
Radiographic findings
Location and distribution
FM arises within the medullary cavity, most commonly in the metaphysis adjacent to the epiphyseal growth plate of long bones.[2][3] Flat bones (pelvis, vertebrae) and short tubular bones (metatarsals) are occasionally involved.[2][5] No cases involving the maxilla or mandible have been reported, but the imaging pattern could theoretically overlap with mixed lytic-calcified jaw lesions.
Periphery and shape
On conventional radiographs, FM typically appears as a poorly defined to moderately well-circumscribed radiolucent area.[2] A sclerotic rim may be present focally, reflecting the slow growth rate, but the margins are generally indistinct compared with classic benign lesions.[2]
Internal architecture
The characteristic internal feature is the presence of cartilaginous "popcorn-like" calcifications scattered within a predominantly lytic matrix.[2] These calcifications correspond histologically to the endochondral ossification occurring at the periphery of the growth-plate–like cartilaginous nodules. A sclerotic reaction within the lesion may also be seen, attributed to the low proliferative rate.[2]
Effects on adjacent structures
- Cortical involvement: Cortical destruction is common and represents a hallmark of the lesion's locally aggressive behavior. Cortical breakage with soft-tissue extension is frequently observed and may simulate a malignant process.[2]
- Medullary permeation: Diffuse infiltration of the medullary cavity by both the spindle cell and chondrogenic components is characteristic.[2]
- Effects on teeth: Not applicable in reported cases, as gnathic involvement has not been documented.
Growth pattern and extent
FM can attain large dimensions (reported range 4–17 cm).[2] Despite the aggressive radiographic appearance with cortical destruction and extraosseous extension, the biologic behavior remains that of a locally aggressive benign neoplasm.[2] Multifocal or polyostotic involvement has not been reported, distinguishing FM from polyostotic fibrous dysplasia.[2]
Recognized variants
Rare congenital cases have been described in infants, with a strikingly different clinical course including spontaneous regression or size reduction without intervention.[2][10] These cases remain controversial, as the histologic hallmark (growth-plate–like cartilage) was poorly documented.[2]
Other imaging findings
Medical CT
CT parallels conventional radiographic findings, with improved delineation of the lytic lesion, cartilaginous calcifications, and cortical destruction.[2] A peripheral sclerotic reaction is often more conspicuous on CT than on radiographs.[2] Strong contrast enhancement is characteristic and may raise suspicion for FM when combined with the calcification pattern.[2]
MRI
FM demonstrates low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, with avid, diffuse contrast enhancement after gadolinium administration.[2][11] MRI is superior for defining the intramedullary extent of the lesion and detecting soft-tissue invasion.[2]
Bone scintigraphy
Intense uptake of Tc-99m MDP is consistently observed, reflecting the metabolically active endochondral ossification within the tumor.[2]
Differential diagnosis
Given the extreme rarity of FM and its mixed lytic-calcified, locally aggressive radiographic pattern, the differential diagnosis centers on other fibrous, cartilaginous, and fibro-osseous bone lesions that share overlapping imaging and histologic features.
| Entity | Why it is considered | Best discriminator |
|---|---|---|
| Fibrocartilaginous dysplasia (FCD) | Lytic lesion with ground-glass opacity and cartilaginous calcifications in a young patient | FCD shows woven bone without osteoblastic rimming, less cellular stroma, GNAS mutation positivity, and lacks cortical destruction with soft-tissue extension[2][7][12] |
| Low-grade central osteosarcoma | Metaphyseal lytic or sclerotic lesion with cortical disruption in a young patient | MDM2/CDK4 amplification present in osteosarcoma but absent in FM; osteosarcoma lacks epiphyseal growth-plate–like cartilaginous nodules[1][2][13] |
| Desmoplastic fibroma of bone | Purely lytic, well-marginated expansile lesion with bland spindle cell histology on biopsy | Desmoplastic fibroma lacks any cartilaginous component; FM biopsy may mimic it when cartilage is absent from the sample[2][8] |
| Mesenchymal chondrosarcoma | Lytic lesion with chondroid calcifications and biphasic histology in a young patient | Mesenchymal chondrosarcoma contains high-grade small round cells (not low-grade spindle cells) and lacks the organized growth-plate–like cartilage architecture of FM[2][14] |
The highest-risk diagnostic pitfall is misidentification of FM as a malignant spindle cell neoplasm (low-grade osteosarcoma or dedifferentiated chondrosarcoma) on small biopsy, where the pathognomonic cartilaginous nodules may be absent.[2][8] When initial biopsy yields only a bland spindle cell proliferation in a young patient with a lytic-calcified lesion, repeat biopsy or excisional biopsy should be pursued, and molecular testing for GNAS, MDM2, and IDH1/2 can help exclude the principal mimics.[1][2]
Histopathology
Morphologic hallmarks
FM is defined by three interrelated components:[2]
- Spindle cell proliferation: Long, slender cells arranged in sweeping bundles or intersecting fascicles with mild nuclear atypia and hyperchromasia. Mitotic activity is absent or negligible. The spindle cells permeate medullary fat, encircling adipocytes, and may mimic a low-grade spindle cell sarcoma.[2]
- Epiphyseal growth-plate–like cartilaginous nodules: Discrete islands of benign-appearing hyaline cartilage with chondrocytes arranged in long parallel columns, recapitulating the normal physis or the cap of an osteochondroma. This is the morphologic hallmark.[2]
- Endochondral ossification: Bone trabeculae rimmed by osteoblasts form at the periphery of the cartilaginous nodules, reproducing the sequence of embryonal endochondral bone formation.[2][15]
Scattered benign multinucleated giant cells may occasionally be present.[2] The spindle cell component predominates in most cases, though the ratio of spindle cells to cartilage varies.[2]
Immunohistochemistry
Smooth muscle actin is negative in the spindle cells, arguing against myofibroblastic differentiation.[2] Negative staining for SATB2, CDK4, and MDM2 helps exclude low-grade central osteosarcoma.[8]
Molecular findings
- GNAS mutations: Absent, distinguishing FM from fibrous dysplasia and its fibrocartilaginous variant (FCD).[1][2][7]
- MDM2 amplification: Absent, excluding low-grade central osteosarcoma.[1][2]
- IDH1/2 mutations: Absent, excluding dedifferentiated chondrosarcoma.[1][2]
These negative molecular findings collectively support FM as a distinct entity.[1][2]
Radiologic-pathologic correlation
The popcorn-like calcifications seen on imaging correspond to the foci of endochondral ossification at the margins of the cartilaginous nodules. The lytic component reflects the spindle cell stroma, while cortical destruction and soft-tissue extension correlate with the diffuse permeative growth pattern observed histologically.[2]
Management and prognosis
Workup and referral
Any lytic bone lesion with cartilaginous calcifications and cortical destruction in a child or young adult warrants referral to a musculoskeletal oncology center. Biopsy is essential but may be nondiagnostic on the first attempt; if only a bland spindle cell lesion is obtained, repeat or excisional biopsy should be considered before definitive treatment.[2][8]
Definitive treatment
En-bloc excision with tumor-free margins is the treatment of choice.[2][16] Intralesional excision (curettage) carries a high rate of local recurrence and should be avoided when feasible.[3] In selected cases where wide resection is not anatomically possible, intralesional resection supplemented by adjuvant measures (phenol, ethanol cauterization) has been reported with short-term disease-free outcomes, though long-term data are limited.[5]
Recurrence
Local recurrence is the principal risk and is strongly associated with incomplete surgical margins.[2][3] In the largest combined series, recurrence followed intralesional excision but was prevented by complete resection.[2]
Metastasis and survival
No metastases or tumor-related deaths have been documented across all published cases.[2][3] The prognosis is favorable when adequate surgical margins are achieved.
Surveillance
Given the propensity for local recurrence, periodic clinical and imaging follow-up of the surgical site is appropriate, particularly in the first several years after resection. No standardized surveillance protocol exists for this rare entity; follow-up intervals should be guided by the adequacy of surgical margins and institutional practice for locally aggressive bone tumors.
Special considerations
Congenital and infantile cases
Rare cases presenting at birth or in early infancy have demonstrated a distinctly different natural history, including spontaneous regression or size reduction without surgical intervention.[2][10] However, the histologic documentation in these cases is limited, and whether they represent true FM or a related but distinct process remains debated.[2] These observations should not alter the standard recommendation for surgical excision in older children and young adults.
Biopsy sampling error
The cartilaginous component—essential for diagnosis—may be entirely absent from small biopsy specimens.