Rare Hip Tumor Masquerading as Infection Diagnosed Through Imaging and Pathology

A 56-year-old man who walked into a hospital with four days of severe right hip pain, weeks after a frightening episode of chills and high fever, has become the centerpiece of a case report that shines a light on one of orthopedic medicine’s most elusive diagnostic traps. His clinicians initially faced a confusing picture: a destructive-looking lesion eating into the acetabulum, the socket of the hip joint, combined with mildly elevated inflammatory markers and a recent febrile illness that seemed to point toward infection. The final answer, established only after surgery and microscopic examination, was something far rarer — a diffuse-type tenosynovial giant cell tumor of the hip with erosion of the acetabular bone, a diagnosis that required careful correlation of magnetic resonance imaging, computed tomography, surgical findings, and histopathology.

Tenosynovial giant cell tumor, or TGCT, is a neoplasm arising from the synovial lining of joints, bursae, or tendon sheaths. Under the 2020 World Health Organization Classification of Soft Tissue and Bone Tumours, the disease is divided by growth pattern into localized and diffuse types. The diffuse variant, historically known as pigmented villonodular synovitis, is a locally aggressive lesion marked by extensive synovial proliferation, nodular growth, and a strong tendency to recur. It most often strikes the knee, but also appears in the finger, ankle, and shoulder. Hip involvement is uncommon, and that rarity is precisely what makes it so dangerous diagnostically: when a deep, inaccessible joint begins to erode bone, the list of possible culprits is long and includes some far more alarming possibilities.

The patient’s story began one week before admission, when he experienced acute chills and high fever and received three days of empirical intravenous cefuroxime at a local hospital, with partial improvement. Then came the hip pain — acute, severe, worst at night, disturbing his sleep, and worsened by movement. Rest and repositioning did not help. On examination he was afebrile at 36.4°C, with marked tenderness over the right hip, pain provoked by internal rotation, adduction, and flexion, and mildly restricted range of motion. There was no redness, warmth, or swelling, no trauma, no weight loss, and no evening fevers. The distal neurovascular examination was unremarkable. In short, nothing about the physical presentation clearly separated infection from tumor.

Imaging told a more revealing story, though not a definitive one. Magnetic resonance imaging of both hips demonstrated abnormal intra-articular synovial tissue in the right hip, with mild bilateral joint effusion that was more pronounced on the right. The lesion, measuring approximately 27 by 35 by 25 millimeters, showed heterogeneous low-to-intermediate signal intensity on T1-weighted images, heterogeneous signal on T2-weighted images, and heterogeneous enhancement after contrast administration — a signal profile characteristic of TGCT, reflecting variable proportions of fibrous tissue, lipid-laden macrophages, fluid, and hemosiderin deposition. The adjacent anterior acetabular column appeared thinned, and mild swelling of the right obturator externus muscle was noted. Computed tomography then showed joint effusion, cystic low-density intra-articular lesions, and clear thinning and erosion of the anterior and inferomedial acetabular wall. Notably, the plain radiograph showed no definite abnormality, underscoring why advanced imaging is essential when plain films fail.

Laboratory testing deepened the ambiguity rather than resolving it. The white blood cell count was 6.46 × 10⁹/L with 68.4% neutrophils, C-reactive protein was elevated at 34.20 mg/L, the erythrocyte sedimentation rate was 25 mm/h, and procalcitonin measured 0.268 ng/mL — a pattern suggestive of low-grade inflammation but far from conclusive. Serum alkaline phosphatase and tumor markers were normal. Whole-body bone scintigraphy showed increased tracer uptake in both hips, sacroiliac joints, and shoulders, which clinicians judged most consistent with degenerative change. With a recent febrile illness, equivocal inflammatory markers, and a destructive-appearing acetabular lesion, the preoperative differential diagnosis spanned infectious or inflammatory synovitis, synovial chondromatosis, and other aggressive synovial or osseous lesions. Because neither imaging nor laboratory work could deliver a verdict, the team proceeded to surgical exploration after multidisciplinary discussion.

What surgeons found inside the joint settled the immediate uncertainty about the lesion’s nature while illustrating why preoperative diagnosis is so difficult. Intraoperative inspection revealed abundant yellowish joint fluid and diffuse proliferation of synovium-like tissue throughout the right hip, with irregular nodular lesions along the anteromedial and inferomedial acetabular wall. The team performed complete excision of the identified intra-articular lesions and involved synovial tissue, curetted the eroded inferomedial acetabular wall, treated it with alcohol ablation, and reconstructed the localized bone defect with an autologous iliac bone graft. Estimated blood loss was approximately 150 milliliters. Although the lesion contained nodular components measuring up to roughly 1.5 by 3.0 centimeters, it was classified as diffuse-type TGCT because of the widespread intra-articular synovial involvement, the acetabular erosion, and the absence of a solitary encapsulated mass.

Histopathologic examination provided the gold-standard confirmation. Microscopy showed proliferative synovium-like tissue with focal cyst wall-like structures lined by flattened to cuboidal cells, a stroma containing proliferative fibrous tissue, histiocytes, prominent congested vessels, diffuse chronic inflammatory cells, scattered neutrophils, and focal coagulative necrosis. Scattered and clustered foamy histiocytes and variable numbers of osteoclast-like multinucleated giant cells were present, along with hemosiderin granules within the cytoplasm of mononuclear cells and histiocytes — the pigment that gave the old disease its name. Immunohistochemistry showed strong CD68 positivity in the multinucleated giant cells and a subset of mononuclear cells, focal S-100 expression, and a Ki-67 labeling index of approximately 5% to 10%, compatible with low proliferative activity. Focal coagulative necrosis was interpreted as a secondary degenerative or ischemic change, and no cytologic atypia, atypical mitotic activity, or sarcomatous overgrowth was identified. Joint-fluid cultures were negative for bacterial and fungal growth, though the authors caution that prior cefuroxime exposure may have reduced culture sensitivity, so infection could never be excluded on microbiologic grounds alone.

The molecular backdrop of this tumor is one of the more fascinating chapters in modern soft-tissue oncology. TGCT is associated with overexpression of colony-stimulating factor 1, or CSF1, which recruits macrophages to the lesion through what researchers call the landscape effect — a minority population of neoplastic cells orchestrating a massive influx of reactive, non-neoplastic immune cells that constitute most of the tumor’s bulk. This mechanism also underpins CSF1/CSF1R-targeted therapies, which are reserved for selected patients with residual, recurrent, or incompletely resectable disease. In this case, however, gross excision and synovectomy were achieved without postoperative radiotherapy or systemic antitumor therapy, with management transitioning instead to clinical and imaging surveillance.

The recovery course was largely favorable, punctuated by one unrelated complication. The patient experienced marked pain relief immediately after surgery, but on postoperative day three he developed fever, mild cough, and fatigue. Chest CT revealed bilateral patchy opacities, pulmonary infection was diagnosed, and intravenous cefuroxime with supportive care normalized his temperature and resolved his systemic symptoms. Follow-up imaging told a reassuring story: CT on the first postoperative day showed satisfactory filling of the acetabular defect by the bone graft, and at two months the graft maintained its position with osseous incorporation and healing. At the two-week mark the incision was well healed and hip motion had improved beyond preoperative levels. By six months, the patient reported sustained pain improvement and had returned to independent daily activities and work, although mild limitation of right hip motion persisted.

The case, reported in Cancer Reports, carries an educational message that resonates well beyond a single patient. Because the hip joint is deeply situated and its capsule has limited capacity to accommodate proliferative synovium, osseous erosion is a recognized feature of hip D-TGCT — yet a destructive-appearing acetabular lesion inevitably raises concern for septic arthritis, inflammatory synovitis, synovial chondromatosis, or a more aggressive neoplasm. The authors emphasize that MRI remains the preferred modality for mapping the extent of synovial disease, while CT more clearly delineates the osseous erosion relevant to operative planning, making the two techniques complementary rather than interchangeable. Ultimately, imaging alone could not reliably distinguish this tumor from infectious or aggressive etiologies; definitive diagnosis depended on integrating the clinical course, intraoperative observations, and characteristic histopathologic features. The authors frame the report primarily as an educational account of diagnostic evaluation and multidisciplinary management rather than evidence of durable local control, noting that the follow-up period remains short and that recurrence is a recognized concern given the complex anatomy of the hip. Longer surveillance will be needed to assess recurrence, joint function, and treatment durability. The broader lesson stands: diffuse-type tenosynovial giant cell tumor deserves a place on the differential diagnosis of any unexplained erosive intra-articular hip lesion, even when the clinical context screams infection.

Subject of Research: Diffuse-type tenosynovial giant cell tumor of the hip with acetabular bone involvement diagnosed by radiologic-pathologic correlation.

Article Title: Diffuse‐Type Tenosynovial Giant Cell Tumor of the Hip With Acetabular Bone Involvement: A Case Report With Radiologic‐Pathologic Correlation

Article References: Quanbing, W., Huanhuan, X., Xing, Z., Tao, Y., Jun, Z., Lei, Z., Kai, L., Lei, L., Feng, A., & Wei, Y. (2026). Diffuse‐Type Tenosynovial Giant Cell Tumor of the Hip With Acetabular Bone Involvement: A Case Report With Radiologic‐Pathologic Correlation. Cancer Reports, 9(9), Article e70696. https://doi.org/10.1002/cnr2.70696

Image Credits: AI Generated

DOI: 10.1002/cnr2.70696

Keywords: tenosynovial giant cell tumor, diffuse-type TGCT, hip, acetabular erosion, MRI, CT, synovectomy, histopathology, CSF1, case report, orthopedic oncology, diagnostic imaging