Case Reports

Giant Solitary Synovial Chondromatosis Mimicking Chondrosarcoma: Report of a Rare Histologic Presentation and Literature Review

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SCM is a benign disorder emerging from the synovium as a result of proliferative changes in the synovial membrane of the joints, tendon sheaths, or bursae, leading to the formation of numerous cartilaginous nodules, usually a few millimeters in diameter.8 In a rare presentation of the disease, the nodules may coalesce to form a large mass, or a single cartilaginous nodule may enlarge to form a mass. Edeiken and colleagues7 named this previously unrecognized SCM feature as GSSCM when there was a major single mass larger than 1 cm in diameter. There have been other SCM cases with multiple giant masses.9,10 In the English-language literature, we found 15 GSSCM cases, which include the first reported, by Edeiken and colleagues7 (Table). However, earlier SCM cases would be reclassified GSSCM according to their definition.11

The present case brings the total to 16. Nine of the 16 patients were male. Mean age at presentation was 41 years (range, 10-80 years). The knee was the most common GSSCM site (6 cases), followed by the temporomandibular and hip joints (3 each). Regarding gross pathology, 10 lesions were solid, and 6 (including the present one) were formed by conglomeration of the chondromatosis nodules. Lesions varied in size (16-200 mm), and 2 were primarily extra-articular (foot). One common issue with most of the cases was the initial diagnosis of chondrosarcoma. The exact surgical technique used was described for 6 cases (cases 11-16); the technique was marginal excision. In no case was recurrence 14 to 60 months after surgery reported.

This chondroproliferative process is potentially a diagnostic challenge, as distinguishing it from a chondrosarcoma, a more common lesion, could be difficult based on clinical and imaging findings, and, as is true for other chondral lesions, even histologic differentiation of the conditions might not be conclusive.12,13 Confusion in diagnosis was almost universal in this series of patients.

One important differentiating feature of benign and malignant skeletal lesions is the time course of the disease. Malignant tumors are expected to demonstrate rapid enlargement and local or systemic spread. Unfortunately, often SCS cannot be distinguished by this characteristic, as grade I or II chondrosarcoma is usually a slow-growing tumor and does not metastasize early.14 Although lack of recurrence is assuring, recurrence is not necessarily a sign of malignancy, as a considerable percentage of benign chondromatosis lesions recur.8

Radiologic differentiation between SCM and SCS is another challenge. Although bone erosion caused by a lesion not originating from bone is usually considered a sign of malignancy, GSSCM was reported as causing bone erosion in 5 of the 16 cases in our literature review.7,15 Our patient did not experience any bone erosion. However, lack of bone erosion is not a reliable criterion for excluding SCS, and bone erosion was noted in only 3 of the 9 SCS cases in the series reported by Bertoni and colleagues.6 Moreover, tumor size and propagation of tumor to surrounding tissue could be surprising in GSSCM. Large size (up to 20 cm) and extra-articular spread of a lesion originating in a joint are common findings.6,16 Our case was an obvious extension of a hip GSSCM to the iliopsoas and obturator externus bursa, which is the most common pattern of extracapsular spread of hip SCM.17 An interesting feature of the present case, however, was the relatively superficial location of the mass immediately under the fascia.

Calcified matrix is key in diagnosing a chondral lesion on imaging studies, but, in some cases, SCM does not demonstrate any radiographically detectable calcification at time of diagnosis.18 However, all the GSSCM cases reported to date had obvious calcified matrix.

The hypercellularity, cellular atypia, binucleated cells, and pleomorphism in the histologic examination of the present case are not features of malignancy in SCM.8 On the contrary, several other characteristics, including qualitative differences in the arrangement of chondrocytes (sheets rather than clusters), myxoid matrix, hypercellularity with crowding and spindling of the nuclei at the periphery, necrosis, and, most important, permeation of the trabecular bone with the filling up of marrow spaces, have been assumed to be indicative of malignancy.8 Furthermore, Davis and colleagues8 found no mitotic activity in the histopathologic investigation of 53 SCM cases. Even in 3 cases that developed malignant transformation to SCS, mitosis was not found in the initial biopsy specimens before transformation. This was compatible with the common opinion that SCM is not a neoplastic, but a metaplastic, process. Histopathologic data were available for only 8 of the previous 15 GSSCM cases. There were no reports of mitosis, and necrosis was found in only 1 case.16 In our patient’s case, however, the first biopsy did show remarkable mitotic activity. This was not the case for the second biopsy, when mature chondrocytes associated with marked calcification and ossification were prominent features (Figures 6A, 6B). We presume that, within a limited period during earlier stages of tissue maturation in SCM, mitotic activity might be a possible finding. Of note, none of the other aforementioned histologic criteria for malignancy was seen in the first or second biopsy in the present case (Figures 3, 6C).

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