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

Synovial chondromatosis (SCM) is a relatively rare benign lesion of the synovium.¹ Its pathogenesis has been thought to be a chondral metaplasia of the subintimal layer of the intra- or extra-articular synovium.² However, evidence supporting a neoplastic cause of the disease is emerging.³ When intra-articular, any joint can be affected, though large joints are more prone to the disease; the knee, hip, and elbow are the most common locations.⁴ The synovial layer of tendons or bursae can be the origin of extra-articular SCM.⁵

Synovial chondrosarcoma (SCS), an even rarer pathology, can be caused by malignant transformation of SCM or can appear de novo on a synovial background.⁶ Histologic differentiation from SCM might be difficult because of the high incidence of hypercellularity, cellular atypia, and binucleated cells.⁶ Some features, such as presence of a very large mass or erosion of the surrounding bones, have been indicated as possible signs of malignancy.³ An unusual presentation of SCM, giant solitary synovial chondromatosis (GSSCM), can be hard to distinguish from SCS because of the large volume and possible aggressive radiologic findings.⁷ Some histologic features, such as presence of necrosis and mitotic cells, have been suggested as distinctive criteria for malignancy.⁸

In this article, we present a case of benign GSSCM with a histologic feature that has not been considered typical for benign SCM. The patient provided written informed consent for print and electronic publication of this case report.

Case Report

An 18-year-old woman presented with a large mass over the right hip. The mass had been growing slowly for 2 years. One year before presentation, a radiograph showed a large hip mass with fluffy calcification (Figure 1), and magnetic resonance imaging (MRI) showed a large nonhomogeneous mass anterior to the hip capsule and extending into the hip joint back to the posterior part of the joint (Figures 2A, 2B). Open incisional biopsy was performed in a local hospital at the time, and the histologic analysis revealed presence of atypical binucleated cells and pleomorphism, in addition to some mitotic activity (0 to 1 per high-power field) (Figure 3). These findings suggested malignancy. The patient declined surgery up until the time she presented to our hospital, 1 year later.

Clinical examination findings on admission to our hospital were striking. The patient had a large mass in the groin region. It was fairly tender and firm to palpation, immobile, and close to the skin. Hip motion was mildly painful but obviously restricted.

The mass was restaged. New radiographs and MRI did not show any significant changes since the previous year, computed tomography (CT) did not show any bone erosion (Figure 4), and chest radiograph, CT, and whole-body bone scan did not demonstrate any signs of metastasis.

Given the clinical presentation and previous histopathologic findings, a diagnosis of GSSCM with possible malignant transformation was made. The patient was scheduled for surgery. During surgery, the tumor was exposed through the Smith-Petersen approach. The mass was extruding under the fascia between the femoral neurovascular bundle medially and iliopsoas muscle laterally. There was no adhesion of the surrounding structures, including the femoral neurovascular bundle, to the mass. The muscle was sitting on the anterolateral surface of the mass, which was considered located in the iliopsoas bursa but extending to the joint. In the vertical plane, the mass extended down to the subtrochanteric area. The entire solid extra-articular mass was excised en bloc, and hip capsulotomy was performed inferior to the area of emergence of the mass. The joint was occupied by a single solid cartilaginous mass molding around the femoral neck, filling the piriformis fossa and propagating to the posterior joint space. Obtaining enough exposure to the back of the joint required surgical hip dislocation. The visualized acetabular fossa revealed chondral fragments, which were excised. Bone erosion or significant osteoarthritis was not detected in any part of the joint. A nearly total synovectomy was performed, leaving the ascending retinacular vessels intact. Meticulous technique was used to avoid contaminating the extra-articular tissues. The wound was closed in the routine way after hip relocation.

The 16×9.5×9-cm mass (Figure 5A) had a conglomerated internal structure (Figure 5B). Multiple specimens from the intra- and extra-articular portions of the mass were sent for histopathologic analysis, which revealed clusters of mature chondrocytes arranged in a lobular pattern and separated by thin fibrous bands. Areas of calcification and ossification were appreciated as well (Figures 6A-6C). No necrosis, mitosis, or bone permeation was detected. These findings were compatible with typical SCM. Given these pathologic findings and the lack of clinical deterioration over the previous year, a diagnosis of GSSCM with extension along the iliopsoas and obturator externus bursae was made. The already-performed marginal excision was deemed sufficient treatment. At most recent follow-up, 38 months after surgery, the patient was pain-free and had good hip range of motion and no indication of recurrence.