Enchondroma

Most frequently on the small tubular bones of the hands, where it is the most common tumor.
Among the long tubular bones, enchondroma occurs more frequently in the proximal humerus and proximal and distal femur and tibia.
Rarely involves the pelvic bone, ribs, scapula, or sternum.
Enchondroma in the spine is very rare.
Almost never occurs in the craniofacial bones, that develop through intramembranous ossification.

Clinical Symptoms and Signs

Usually found on routine radiologic examination or as hot spots on the bone scan without specific symptoms.
The majority is asymptomatic.
In the hands and feet, enchondromas present as a palpable swelling with or without pain. In this location, they are frequently associated with pathological fracture and pain.
Chondroid tumors located in hands and feet, in subperiosteal location, and in the lining of joint capsules are almost certain to be clinically benign.
The behavior of a cartilage neoplasm is best predicted having in mind if the lesion is growing or not. Clinical, radiographic, and microscopic features should be interpreted in the light of this information.
Enchondromas usually do not grow after puberty.

Image Diagnosis

Radiographic Features

A sharply defined border, mild endosteal scalloping, and calcifications resembling a ring or an arc characterize the radiographic findings of enchondroma of the hand or foot.
Spotty calcifications are frequently seen.
Sometimes, calcifications may be dense (Fig. 16.1).
Cortical expansion or pathological fracture is a potential feature, mainly in tumors of the hands and feet.
Since enchondroma is a common type of benign tumor, solitary bone lesions with well-defined margins and scallopings of the endosteum in a bone of the hand should be considered an enchondroma until proven otherwise.
Enchondroma also shows typical radiological features on other sites. In long tubular bones, it shows a central or eccentric location with an osteolytic lesion and calcification in the medullary portion. Radiographic findings of enchondroma appearing in flat or irregular, nontubular bone may be hard to recognize (Fig. 16.2).

CT Features

Provides better evaluation of the cortical endosteal surface in medullary located lesions of long tubular bones.
It is a better technique for the study of lesions located in areas of difficult radiographic assessment, like the axial skeleton.

MRI Features

On MRI, the margin of the tumor is well defined by a low signal intensity on T1-weighted images and a high signal intensity on T2-weighted images.
Lobulated margins of the tumor are typical, and also low signal intensity is seen at the calcified zones.
The x-ray volume of the enchondroma overlaps the volume of the lesion as seen on MRI.

Image Differential Diagnosis

Low-Grade Chondrosarcoma

Differentiating between a low-grade chondrosarcoma and enchondroma can be challenging, and radiology is a useful adjunct in this process.
Radiologically, chondrosarcoma of the long bone exhibits expansion of bone and thickening of the cortex.
On x-ray and CT of tubular bones, extensive endosteal scalloping, cortical expansion, cortical thinning or thickening, cortical permeation, and ambiguous matrix calcification suggest chondrosarcoma.
Peripheral marrow edema around the lesion, subperiosteal edema, and mass in soft tissue, as seen in MRI, are findings that point toward low-grade chondrosarcoma.
MRI volume of the lesion bigger than x-ray volume is also suggestive of malignancy.
Epiphyseal location of the tumor also strongly suggests chondrosarcoma.

Pathology

Gross Features

Most enchondromas are curetted, so that it is very rare to examine an intact gross specimen.
Most enchondromas show confluent masses of bluish, semi-translucent hyaline cartilage with a distinct lobular arrangement.
The size of the lobule varies from millimeter to 1 cm. It is very rare to find one greater than 5 cm in size.
The periphery of the lesion is somewhat irregular, since individual lobules bulge into the surrounding marrow spaces.
Punctuate calcifications seen on radiographic imaging can be appreciated grossly.
Multiple enchondromas show a similar gross appearance to that of solitary tumors. However, in multiple enchondromas, normal marrow elements encircle the well-circumscribed nodules, which are often mistaken for an aggressive pattern of permeation.

Histological Features

Histological features are variable according to the location of the lesion. Accordingly, it is very important to consider the location of the lesion together with the radiological findings and clinical features.
Enchondromas are composed of mature hyaline cartilage arranged in lobules.
Cartilaginous lobules are surrounded by normal bone marrow (Fig. 16.3).
Enchondral ossification may also be seen.
Other lobules of enchondroma are frequently partially encased by mature lamellar bone.
In general, there is low cellularity and abundant extracellular matrix (Fig. 16.4). In enchondromas of the hands and feet, there may be a higher cellularity.
Usually, chondrocytes are located in lacunae, and the nucleus has a small round regular appearance. On a low-power view, the nucleus is hardly visible. Rarely, chondrocytes can be seen as clusters or sheets. Occasionally, chondrocytes can be seen as large cells or double-nucleated cells (Fig. 16.5) but lacking cytological atypia.
Calcification can be observed as fine purple-colored granular precipitate or a big mass (Fig. 16.6).
Prominent myxoid change or necrosis is hardly ever observed in enchondromas. However, they can be observed occasionally in the small bones of the hands and feet. When these histological features are present, even if the cellularity is low, the possibility of malignancy should be considered.
In case of small bones of the hands and feet, to make diagnosis of malignancy, there should be extreme hypercellularity, nuclei pleomorphism, and marked myxoid changes.
Calcifying or ossifying enchondroma is a variant that sometimes can be present at the metaphyseal region of the long bone. In this occasion, enchondroma can be seen as radiologically dense ringlike or flocculent agglomerated and heavily calcified and ossified radiopacities. Histologically, the lesion consists of mature hyaline cartilage and heavily calcified or ossified material with small scarce cartilage cells.

Pathologic Differential Diagnosis

Low-Grade Chondrosarcoma

Especially if arising in the metaphysis of the long bone in middle-aged to elderly patients.
Grossly, mucoid or myxoid changes are frequently observed in chondrosarcoma and only rarely in enchondromas.
Enchondroma does not show cortical erosion. Also, histologically, increased cellularity, marked myxoid change of the stroma, and cytologic features of malignancy are observed in chondrosarcoma.
Separation of cartilage lobules by fibrous bands is seen in low-grade chondrosarcoma.
In addition, the most important differential diagnostic feature is intramedullary invasion. In chondrosarcoma, the trabeculae of preexisting bone are embedded in the tumor tissue – entrapment of native trabeculae.
Radiographically, enchondromas of the small bones of the hands and feet show cortical thinning; however, microscopic permeation into the cortical tissue is not observed.
Enchondromas of the pelvis, spine, ribs, and sternum are very rare. Chondroid lesions occurring in these locations should be considered as aggressive cartilage lesions if they are not reactive or metaplastic in nature.