Tenosynovial Giant Cell Tumor of Diffuse Type Mimicking Bony Metastasis Detected on F-18 FDG PET/CT

Abstract

Tenosynovial giant cell tumor of diffuse type (TGCT-D) is a locally aggressive neoplasm that arises in the tendon sheath, bursa, or synovium. It typically involves the appendicular skeleton and rarely involves the axial skeleton. Because there are no specific findings of TGCT-D based on imaging studies or clinical symptoms, TGCT-D can be confused with other primary or metastatic bone tumors. We report findings of TGCT-D involving the T9 vertebra incidentally detected on F-18 FDG PET/CT in a patient with papillary thyroid cancer.

Introduction

Tenosynovial giant cell tumor of diffuse type (TGCT-D) is a locally aggressive neoplasm involving the synovial membrane of a joint [1]. Because there are no specific findings of TGCT-D based on imaging studies or clinical symptoms, it can be difficult to differentiate between TGCT-D and other benign inflammatory or neoplastic processes of the synovial lining [2]. TGCT-D mimics bony metastasis in patients with malignancy [3, 4] and typically involves the appendicular skeleton, while rarely involving the axial skeleton [5, 6]. We present findings of TGCT-D involving the T9 vertebra using fluorine-18 fluorodeoxyglucose (F-18 FDG) positron emission tomography/computed tomography (PET/CT) in a patient with papillary thyroid cancer.

Case Description

A 33-year-old female patient was diagnosed with papillary thyroid carcinoma and underwent total thyroidectomy followed by high-dose radioactive iodine therapy (6.7 GBq). After 4 years of treatment, the patient was referred for PET/CT as a routine follow-up. PET/CT revealed a focal hypermetabolic lesion (the maximum SUV = 7.1), and the CT images of PET/CT revealed a small osteolytic lesion with a sclerotic rim in the left facet of the T9 vertebra. MRI was performed 2 weeks after PET/CT and showed a mild gadolinium-enhancing lesion of 1.4 × 1.5 × 1.2 cm at the corresponding location (Fig. 1). Although the serum thyroglobulin level was negative and the patient had no symptoms, we could not exclude the possibility of bony metastasis to the T9 vertebra based on the findings of PET/CT and MRI. Therefore, she underwent surgical removal of the T9 lesion.

Fig. 1.

F-18 FDG PET/CT and MRI findings. Maximum intensity projection (MIP) (a) and transaxial fused image (b) of F-18 FDG PET/CT scan showed a hypermetabolic lesion with the maximum SUV of 7.1 in the left facet of the T9 vertebra (long arrows). The transaxial CT image (c) of the PET/CT scan showed a small osteolytic lesion (short arrow) and MRI (d) performed 2 weeks after the PET/CT scan revealed an enhancing lesion of 1.4 × 1.5 × 1.2 cm at the corresponding location of the T9 vertebra (arrowhead)

The gross specimen of the T9 lesion showed a yellowish osteolytic mass, and the histology revealed small histiocytoid cells, large epithelioid cells, osteoclast-like giant cells, and xanthoma cells. However, there was no evidence of malignant transformation (Fig. 2). Therefore, the tumor was diagnosed as TGCT-D. At a 9-month follow-up, there was no recurrence or metastasis.

Fig. 2.

Histologic findings. The tumor is composed of a small histiocytoid cells, large epithelioid cells, osteoclast-like giant cells, and xanthoma cells (original magnification × 100, H&E stain) (a). High-power view (b) shows the large epithelioid cells with scant cytoplasm often containing hemosiderin granules, multi-nucleated giant cells, and reniform mononuclear histiocytoid cells (original magnification ×400, H&E stain)

Discussion

TGCT-D was originally defined as a pigmented villonodular synovitis by Jeffe et al. in 1941 and was described as an inflammatory lesion involving the tendon sheath rather than as a malignant lesion [1]. According to the 2013 WHO classification of soft tissue tumors, TGCT can be subdivided into localized and diffuse forms [7]. TGCT-D is defined as a locally aggressive neoplasm and typically involves the appendicular skeleton, particularly the knee (80 %) and hip joint (15 %), and rarely involves the axial skeleton [5, 8]. The common symptoms of patients with diffuse-type TGCT include pain, limitation of motion, and minimal to mild joint swelling, heat, and tenderness [9]. Although wide excision is the treatment of choice for TGCT-D [1, 10], it has a higher recurrence rate (33 to 50 %) than that of the localized TGCT, and malignant behavior of TGCT-D has also been reported [2, 11].

However, TGCT-D can also be found incidentally in imaging studies of patients without any of these symptoms. TGCT-D manifests as a nonspecific well-defined soft-tissue mass with similar or slightly increased attenuation compared to that of adjacent muscle on CT images [5]. In MRI, TGCT-D shows low signal intensity with artifacts from hemosiderin deposits on T1- and T2-weighted images [2, 5]. The differential diagnosis based on imaging studies includes synovial sarcoma, rheumatoid arthritis, tuberculous arthritis, osteoarthritis, angiomas of the osseous origin, amyloidosis, fibrous dysplasia, multiple enchondromatosis, hemophilia, chronic indolent synovitis, and bony metastasis [3, 9]. Although there have been a few reports of TGCT-D located in the pelvic bones and knee joints detected on PET/CT [12, 13], there was no case of PET/CT involving the spine. The maximum SUV of TGCT-D has been reported to be variable, ranging from 4.4 to 25.0 [14, 15], so it would not be possible to differentiate between TGCT-D and other benign or malignant bone lesions by using SUV alone. Hence, the findings of CT or MRI should be correlated for better diagnosis.

TGCT-D is usually a large mass (often >5 cm) with synovial hypertrophy consisting of papillary, villous, nodular, or villonodular patterns [5]. Microscopically, TGCT-D comprises polymorphic cells, resulting from histiocytoid cells with abundant eosinophilic cytoplasm, large epithelioid cells, osteoclast-like giant cells, xanthoma cells, and inflammatory cells [2, 5]. The large epithelioid cells of TGCT-D often contain hemosiderin granules [2]. Previous studies have demonstrated the upregulation of the glucose transporter protein GLUT-1 in human macrophages [16, 17]. Also, a positive relation between the overexpression of GLUT-1 and F-18 FDG avidity was reported [18, 19]. Hoshi et al. suggested a close association of F-18 FDG avidity with strong hexokinase-2 activity in giant cells and spindle cells [20]. Based on those studies, we postulate that the increased F-18 FDG uptake of TGCT-D might be attributable to the overexpression of GLUT-1 and hexokinase-2 in macrophages and giant cells, respectively.

To the best of our knowledge, this is the first report of TGCT-D involving the spine incidentally detected on PET/CT. This report indicates that TGCT-D should be included in the differential diagnosis of the spinal mass in PET/CT, and correlation with conventional imaging modalities would be helpful.