Basic imaging features
On standard radiographs, central chondrosarcoma appears as a lytic lesion (lytic lesion). At low grades (low-grade), the margins of the lesion usually remain well-defined, whereas with increasing aggressiveness (higher-grade), the contours become ill-defined, accompanied by cortical destruction and soft-tissue extension (soft-tissue extension).
Due to the slow growth of the tumor, reactive changes occur within the bone. Parallel to endosteal erosion (scalloping), periosteal bone formation takes place, ultimately leading to characteristic bone expansion with a variable cortical thickness.
Typical chondroid calcification occurs in 75% of cases and is described on radiographs using the following patterns:
- Punctate (punctate);
- Stippled (stippled);
- Ring-and-arc (ring-and-arc);
- Popcorn (popcorn).
Computed tomography (CT) is used to detect occult calcified matrix that is not visible on standard radiographs.
Magnetic resonance imaging (MRI)
Cartilage tissue contains a high amount of water, which is why on fluid-sensitive sequences, chondrosarcoma demonstrates a pronounced hyperintense signal with a lobulated (multilobular) structure. Islands of calcification appear as areas of low signal intensity across all pulse sequences.
Tumors of this group are typically poorly vascularized; therefore, upon administration of a contrast agent (gadolinium), enhancement is limited and occurs predominantly peripherally or along internal septa.
Important MRI sign: If a penetrating lytic area with low or intermediate signal intensity appears on T2-WI adjacent to the typical bright lobulated cartilage tissue, this indicates tumor dedifferentiation. Such a transformation process into a more aggressive sarcoma is observed in one-third of cases. It is precisely these zones with altered MRI signal that serve as the primary target for image-guided biopsy.
Differential diagnosis with enchondroma
Distinguishing grade 1 chondrosarcoma (atypical cartilaginous tumor) from a benign enchondroma is a challenging task. Most advanced imaging modalities have limited utility here:
- Scintigraphy: Unreliable, as up to 30% of conventional enchondromas take up the radiotracer due to ossification processes.
- MRI (including diffusion-weighted): Does not provide clear criteria to differentiate between benign and borderline/malignant processes.
- 18F-FDG PET: Not routinely recommended, although low SUVmax values favor a benign etiology.
The primary criteria remain anatomical characteristics:
- Size: Chondrosarcomas are on average larger (about 8 cm versus 5 cm for enchondromas).
- Depth of erosion: Involvement exceeding 2/3 of the cortical thickness is detected in 67% of chondrosarcomas and only 11% of enchondromas.
Secondary and rare variants
Secondary peripheral chondrosarcoma develops against the background of malignant transformation of an osteochondroma (exostosis). Suspicion should arise when an exostosis resumes growth and pain appears in a patient with closed growth plates. The imaging marker of malignancy is a cartilage cap (cartilage cap) thickness greater than 2 cm. It must be measured strictly perpendicular to the bone; otherwise, the result will be overestimated. An inflamed bursa overlying the exostosis can cause diagnostic error, as it also appears bright on T2-WI, mimicking thick cartilage.
Clear cell chondrosarcoma (about 2% of cases) predominantly affects the subchondral regions (femoral and humeral heads) in men aged 30–50 years. On radiographs, it appears as a well-circumscribed lytic defect without typical calcification, which is why it is frequently mistaken for chondroblastoma. However, on MRI, clear cell chondrosarcoma lacks pronounced bone marrow edema.
Mesenchymal chondrosarcoma occurs in younger patients and frequently affects the jaws, vertebrae, and ribs. It is characterized by high aggressiveness, a tendency for early local recurrence, and late metastases (sometimes after more than 20 years), which requires lifelong imaging monitoring.
To remember the types of chondroid calcification, imagine an evening at the cinema: popcorn (popcorn) all around, punctate (punctate) and stippled (stippled) footprints on the floor, and a ring-and-arc (ring-and-arc) logo on the screen.
Class and exam questions
Why can a tumor appear on radiographs as bone expansion?
This is the result of prolonged and relatively slow growth: while the tumor destroys the cortical layer from the inside (endosteal resorption), the periosteum on the outside manages to form new bone, resulting in visible expansion.
Are the bones of the hands and feet affected by chondrosarcoma?
Yes, but this occurs extremely rarely. The small tubular bones of the hands and feet are a typical location for benign enchondromas.
How to distinguish juxtacortical chondrosarcoma from peripheral chondrosarcoma?
Juxtacortical (surface) chondrosarcoma is located on the bone surface (usually the proximal humerus or distal femur) but lacks continuous anatomical connection with the medullary cavity, unlike peripheral chondrosarcoma (malignant exostosis).
What else to study on this topic
- WHO morphological classification: criteria for atypical cartilaginous tumor (ACT)
- Parosteal and periosteal osteosarcoma: differential diagnosis with cartilaginous tumors
- Imaging findings of osteochondroma complications: bursitis and pseudoaneurysms
- Image-guided biopsy tactics for multilobular bone lesions
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