Imaging of rare appendicular non-acral soft-tissue chondromas in adults with histopathologic correlation

Introduction

Soft-tissue chondroma (STC) is a rare benign soft-tissue tumor, composed of hyaline cartilage, that arises primarily in extra-skeletal locations (1,2). Radiologically and pathologically, STCs may resemble other soft tissue chondroid masses such as periosteal chondroma, synovial chondromatosis, soft-tissue chondrosarcoma, and metaplastic chondroid lesions (such as myositis ossificans and chondrolipoma) (2). However, conventional STCs are readily recognized by their small size (often less than 3 cm) and peri-articular acral location, typically in the hands and feet (1,3–6). They are rarely found in other parts of the body (7–10). STCs are usually treated with conservative local excision sparing the patient unnecessary radical surgery and decreasing morbidity (11,12), with local recurrences reported in 10–18% of cases (4–6).

Occasionally, STCs may arise in more proximal non-acral locations in the appendicular skeleton, accompanied by non-classic imaging features and a relatively larger size compared to their acral counterparts. Radiologically, these tumors are problematic posing diagnostic concerns for more aggressive tumors that warrant aggressive radical surgery, and accordingly they are often labeled as indeterminate lesions (11–14). Similarly, distinguishing benign from malignant cartilaginous tumors at extra-osseous sites may present a diagnostic challenge to the pathologist because of the frequent occurrence of atypical cellular features in STCs (15). Therefore, accurate diagnosis requires close radiological–pathological correlation.

Herein we present a series of six primary soft-tissue chondroid tumors that fulfilled the criteria for non-acral STC, and explicate the potential of diagnostic imaging in the identification and characterization of these lesions with emphasis on their morphologic MRI features.

Material and Methods

Results

MRI findings

All six cases had MRI studies available with 5/6 cases also having contrast-enhanced T1W images with or without fat suppression. All tumors had well defined margins with a more or less lobulated contour (Figs. 2–4). They were completely separable from adjacent osteo-articular structures and showed no surrounding soft-tissue edema except for one case where peri-lesional fat stranding was noted (Fig. 3a and b). The non-calcified matrices of all tumors exhibited: (i) a homogenous texture with no detected areas of breaking down; (ii) isointense signal intensity on T1W images; and (iii) predominantly high-signal intensity on water-sensitive sequences, compared to adjacent skeletal muscles, with linear low-signal bands of the capsules and septae (Figs. 2–4). Remarkable peripheral enhancement, with or without septal enhancement, was identified following intravenous gadolinium infusion in 4/5 cases with available post-contrast MRI series. The supra-patellar lesion showed characteristic segmental peripheral and patchy enhancement of its non-ossified tumor due to heavy ossification (Fig. 1c). OPEN IN VIEWER

Fig. 2.

A mass in the Hoffa’s fat pad of left knee from case 2. MRI; sagittal (a) T1W image and (b) FS PD-weighted image the lesion exhibits isointense signal on (a) and hyperintensity on (b). (c) Post-contrast FS T1W axial image displaying marginal enhancement (white arrows). (d) Microscopic examination of the needle core biopsy reveals hyaline cartilage featuring bland chondrocytes in lacunae, consistent with a well-differentiated cartilaginous neoplasm (H&E stain, ×20 magnification).

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Fig. 3.

An intra-articular mass of left knee from case 3. Coronal MRI (a) T1W and (b) FS PD images show a well-defined soft-tissue mass filling the supero-lateral synovial recess under the ilio-tibial tract (white arrowheads in (a) and (b)). The mass exhibits homogenous isointensity to skeletal muscles on (a) and hyperintensity on (b). Note the adjacent peri-lesional fat stranding on T1W image and hyperintensity on PD images inferring adjacent edema small (small black arrows in (a) and (b)). Post-contrast FS T1W images in (c) coronal and (d) axial planes showing marginal enhancement (small white arrows in (c) and (d)). Of note, no reactive cortical changes are observed.

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Fig. 4.

A suprasternal mass from case 6. (a) Non-contrast MRI axial T1W and (b) coronal FS T2W, showing the mass superior to the right sternoclavicular joint (white stars in (b) and (c)), stretching and splaying the distal sternomastoid insertion (small white arrows in (a) and (b)) and bulges into the overlying subcutaneous tissue. The mass emits isointense low signal on (a) and predominantly hyperintense on (b). (c) Post-contrast coronal FS T1W image, following IV gadolinium administrations, the mass exhibited remarkable marginal enhancement (white arrowheads). (d) Gross resection specimen shows a circumscribed mass adherent to the outer surface of the sternoclavicular joint without infiltrating into the bone or adjacent structures (yellow arrow indicates the bone). (e) Microscopically, the lesion is composed of chondromyxoid matrix with foci of hyaline cartilage, consistent with a well-differentiated cartilaginous neoplasm (H&E stain, ×20 magnification).

Discussion

STCs are uncommon slow-growing tumors of the soft tissues characterized by their small size (mean < 3 cm in diameter) (6) and predilection to appendicular acral locations (4,5), with the hands being more common than feet (5,6,16). Thus, it is understandable that such tumors often pose a diagnostic challenge to the radiologist and pathologist alike when they are encountered as large masses in unusual proximal sites. Indeed, the relatively larger tumor size in the current series (range = 2.6–10.2 cm, mean = 5.6 cm) contributed to the diagnostic dilemma. Similar to previous reports (12,17,18), our cases showed a predilection for the lower limb, specifically the knee region. One tumor was in close proximity to the sterno-clavicular joint but completely separable from the joint cavity and the clavicular bone. The only two earlier reports of clavicle-related chondromas described in the English literature were periosteal variants at the lateral and medial ends of the clavicle, respectively (19,20). Our cases were in the third to sixth decades of life (age range = 26–58 years, mean age = 40.5 years) with no sex predilection, in agreement with previous data (4,6).

The cartilaginous nature of the current cases was suspected based on the high signal intensity of the matrix and low signal intensity of the intervening septa on T2W MRI, followed by peripheral and septal vascular enhancement on post-contrast series. All STCs in our series were well-defined and lobulated lacking invasion into adjacent tissues and host tissue reaction except for one case that showed mild peri-lesional soft-tissue edema on MR. Although the presence of surrounding soft-tissue edema may suggest a more aggressive neoplastic lesion (21–23), the edema in our case may be explained by the mass effect exerted by the tumor on the fat surrounding the superior-lateral recess of the knee causing venous backpressure. All cases lacked adjacent bone remodeling on radiography and CT. Other radiological clues that pointed to the benign nature of the lesions in the current series were signal homogeneity on T1W images and absence of tissue heterogeneity on T2W images apart from that caused by calcified matrix. This is comparable to the results of previous reports (21–23) that addressed the use of different MR morphologic and signal criteria to differentiate benign and malignant soft-tissue tumors, especially those in the extremities. Post-contrast MR studies in our series revealed a characteristic peripheral and septal pattern of enhancement in all lesions, also in agreement with previous studies (16,24–26). This pattern is uncommon in malignant cartilaginous tumors which usually show intense and homogenous patterns of enhancement (16,22–24).

Due to their lobular growth pattern, cartilaginous tumors exhibit a characteristic arciform pattern of calcification, recapitulating the final stages of endochondral ossification (27). In up to 70% of STCs, various patterns of calcifications have been described on radiographic and/or CT studies, including arc and ring patterns, calcium flecks, and granular and diffuse patterns (26,28). Our cases also showed a spectrum of mineralization from a central calcium fleck to diffuse dense ossification. These areas of calcification showed loss of signal on all MR pulse sequences due to paucity of free precision water molecules. Detection of these calcification patterns on ancillary radiographic and CT imaging is invaluable in recognizing cartilaginous lesions within soft tissues and predicting their histology (29).

Histologically, STCs are composed of lobules of mature hyaline cartilage separated by fibrous septa and surrounded by a fibrous pseudo-capsule (1,24,27), a picture that parallels what is seen on MRI and explains the peripheral/septal enhancement post contrast. However, this pattern is not specific for benign chondromas (24,30) and one needs to exclude cytological atypia and tumor permeation beyond the capsule. Although all of our cases were proven to be of chondroid nature histologically, none was firmly diagnosed as chondroma at the outset. The main limiting factor was the scanty material available for pathological assessment, making it very difficult to establish a definite diagnosis without radiological guidance. Indeed, our suprasternal tumor was originally diagnosed as myxoid chondrosarcoma on core-needle biopsy based on the large tumor size, unusual location, and extensive myxoid matrix. Re-evaluation of the pathology slides, in view of the radiological features, revealed lack of significant cytological atypia, necrosis, mitosis, and permeation into adjacent tissues and the final diagnosis of STC was rendered in consensus.

From imaging perspectives, STCs should be differentiated from other appendicular soft-tissue masses, especially those with calcifications, such as myositis ossificans, depositional entities; like tumoral calcinosis, gout, and calcium pyrophosphate deposition disease (CPPD); and benign neoplasms as synovial chondromatosis. Further malignant entities such as synovial sarcoma, pleomorphic sarcoma (aka malignant fibrous histiocytoma [MFH]), extraskeletal chondrosarcoma, and/or osteosarcoma have to be considered while interpreting extraskeletal soft-tissue masses.

Myositis ossificans is commonly characterized by its zonal pattern on radiography and CT with recall of traumatic precedents (5). Depositional diseases tend to have peri-articular amorphous or bizarre-shaped calcifications with relatively poorly defined margins and an attenuation value of about 160 HU (31). Synovial chondromatosis usually exhibits multiple calcific nodules, with classic ring and arc patterns, related to a large joint and/or bursa (5,6).

Synovial sarcoma prevails in younger age group, occurs in peri-articular locations and 30% of the tumors show calcification, which is more dense and peripheral (32). On the other side, pleomorphic sarcoma is more common in older age groups with lesser calcification tendency (33). Extra-skeletal chondrosarcomas are larger masses with discontinuous cartilaginous calcific patterns. It lacks both lobular growth pattern and vascular septal pattern of enhancement seen in STCs (34). Juxtacortical osteosarcomas are related to bone surface and usually exhibits a cloudy pattern of amorphous calcification mirroring osteogenesis in the tumor without enchondral ossification (35).

We acknowledge that our study is limited by the small number of cases included, but this is inevitable due to the rarity of these non-acral STCs. Another limitation is the lack of assessment of quantitative enhancement parameters due to the retrospective nature of the study, although the validity and clinical applicability of these parameters are still controversial (36).

In conclusion, non-acral STCs are benign cartilaginous tumors that may pose a diagnostic challenge radiologically and pathologically. Collaborative imaging and pathologic workup is needed for better characterization of non-aggression of these lesions, to avoid diagnostic pitfalls and unnecessary radical resections.