Soft tissue chondroma (ECR 2015 Case of the Day)

A 79-year-old man presented with pain and swelling at the plantar side of his left foot for several months. There was no history of acute trauma. His past medical history was unremarkable.

Radiography and magnetic resonance imaging (MRI), including post-gadolinium images, were performed.
Radiographs show a distinct round lesion with ring and arc-like calcifications in the soft tissue at the plantar side of the left foot. There is also a consolidated fracture of the diaphysis of the fourth metatarsal.
MRI demonstrates a well-defined, lobulated lesion with a maximum diameter of 5 cm, without a clear connection to the metatarsals. The lesion is heterogeneous and has mainly a high signal intensity on both T1-weighted and T2-weighted images. After administration of gadolinium contrast material, peripheral rim enhancement is seen. There are intralesional areas of low signal intensity on all sequences, compatible with calcifications. No significant perilesional oedema is depicted on fat-suppressed proton-density weighted images.

Background
Soft tissue chondromas are benign soft tissue tumours occurring in extraosseous and extra-synovial locations. [1] They constitute 1.5% of benign soft tissue tumours. [2] The most commonly accepted theory of tissue origin is synovial. [3] Soft tissue chondromas are predominantly composed of hyaline cartilage but can also contain osseous, fibrous and/or myxoid stroma. [1]

Clinical Perspective
The majority of patients are middle-aged, with an age range of 1-85 years. [2] There is a slight male predominance. [2] Most lesions arise in the hands and feet. [2-5] Patients usually present with a slowly growing soft tissue mass, occasionally with pain or tenderness. [2]

Imaging Perspective
Radiographically, soft tissue chondromas are well demarcated, lobulated lesions with central and peripheral calcifications, often curvilinear in nature. [2-5] Calcifications are radiographically visible in 33-70%. [2] Although they do, by definition, not originate from intraarticular synovium or periosteum, [1] remodelling, erosion, or sclerosis of nearby bone can occur. [2, 5] The MRI appearance is variable and probably related to the degree of calcified matrix. [3] The main role of MRI is to localize and determine the extent of the lesion. In our case, location and the presence of chondroid-like calcifications suggest soft tissue chondroma; synovial sarcoma, heterotopic ossification, and calcific myonecrosis are less probable. Synovial sarcoma may calcify in 30% of cases and is usually seen in patients of 30-40 years of age. The MRI enhancement pattern of synovial sarcoma is more pronounced than of soft tissue chondroma. In 50% of heterotopic calcifications due to myositis ossificans, there is a trauma history. Calcific myonecrosis shows typical plaque-like calcifications along the orientation of the muscle fibres. MRI showed no direct connection with bone, ruling out osteochondromatous lesions. [6] An extraskeletal chondrosarcoma, however, could not be ruled out by imaging alone. Therefore, biopsy was performed, showing cartilage and some bone, without signs of malignancy, confirming the diagnosis of a soft tissue chondroma. Notably, the presence of an old healed fracture of the fourth metatarsal may have been a coincidental finding. Yet, it has also been suggested that trauma may induce soft tissue chondroma formation. [7]

Outcome
Local excision was performed, which is usually curative. However, 15-25 % of soft tissue chondromas may locally recur. [2] Chondrosarcoma transformation has not been described. [1]

Take Home Message
A soft tissue chondroma is a rare benign soft tissue lesion, most frequently arising at the hands or feet. Radiography typically demonstrates a well-demarcated, lobulated soft tissue lesion with chondroid-like calcifications. MRI appearance of the lesion can be variable, but there is no osseous connection.