I. Radiological Findings

1. X-ray (Pelvic AP View): A relatively well-defined, lytic “geographic” lesion is observed in the left acetabulum (iliac bone). There is only slight expansion, with no obvious sclerotic rim, and no notable calcification or intraosseous formation.

2. CT (Coronal View): A large, low-density bone defect is seen in the left acetabular region. The lesion has a well-defined contour, locally thinned cortical bone, and mildly compressed adjacent soft tissue. There may be a slight protrusion of soft tissue component, but no obvious calcification.

3. MRI: On T1-weighted images, the lesion appears as medium to slightly low signal intensity in the left acetabulum. On T2-weighted images, parts of the lesion show low signal intensity, suggesting possible hemorrhage, hemosiderin, or fibrous content. A small amount of fluid-fluid levels is visible, suggesting changes similar to aneurysmal bone cyst (ABC) internally.

4. Post-contrast enhancement shows marked enhancement in the solid portion of the lesion, consistent with a lesion predominantly composed of mononuclear stromal cells mixed with vascularized or cystic areas. No significant evidence of distant metastasis is identified.

II. Differential Diagnosis

1. Giant Cell Tumor of Bone (GCTB):

• Commonly seen in adults with closed epiphyseal plates through middle age (approximately 20–50 years), which matches the patient’s age.
• Typically occurs near the epiphyseal ends of long bones but can also be found in the pelvis. The acetabulum is a less common site.
• Characteristic imaging appearances include well-defined osteolytic lesions adjacent to the articular surface, generally without calcification or intraosseous formation.

2. Aneurysmal Bone Cyst (ABC):

• Usually seen in individuals under 20 and in younger patients, although cases in middle-aged adults have been reported.
• Imaging often shows a septated, expansile osteolytic lesion; MRI can reveal pronounced fluid-fluid levels.
• If there is limited enhancing solid component, a pure ABC is more likely, but a large solid component lowers this possibility.

3. Metastatic Lesions, Osteosarcoma, or Other Malignant Bone Tumors:

• In older patients, bone metastases (e.g., from breast or lung cancer) should be considered. However, more extensive bone destruction or reactive sclerosis is often observed.
• Primary malignant bone tumors such as osteosarcoma may present with a soft tissue mass, widespread cortical destruction, and periosteal reaction.

Considering the imaging and clinical features in this case, a giant cell tumor of bone (GCTB) is the most likely.

III. Final Diagnosis

Based on the patient’s age (55), the presence of a lytic lesion in the left acetabulum, lesion morphology, and the findings on contrast-enhanced MRI, the most probable diagnosis is: Giant Cell Tumor of Bone (GCTB).

If uncertainties remain, a biopsy is recommended to confirm the pathological diagnosis and rule out other malignancies.

IV. Treatment Plan and Rehabilitation

Treatment Plan:

• Surgical Intervention: For resectable GCTB, the first choice is curettage with subsequent bone cement (PMMA) filling or bone grafting. If there is notable soft tissue extension or extensive local invasion, a more comprehensive surgical approach may be necessary following detailed evaluation.
• Medication: Denosumab can be effective in inhibiting RANKL and may be considered in certain cases, particularly if surgery is not feasible or in cases of postoperative recurrence.
• Radiotherapy: Generally not the first choice, but it may be considered for inoperable or recurrent lesions.

Rehabilitation and Exercise Prescription:

Postoperative rehabilitation should follow a gradual progression (FITT-VP principle) and be individualized:

1. Early Rehabilitation:
   • In the early postoperative period (about 1–2 weeks), protective weight-bearing is emphasized. Short-distance walking can begin with assistive devices such as braces or walkers.
   • Avoid overloading and high-impact activities to prevent fracture or loosening of implants.
   • Engage in range-of-motion exercises for the hip joint (passive or active) to maintain joint flexibility.
2. Mid-phase Training (approximately 2–6 weeks):
   • Gradually increase weight-bearing. Perform light strength training daily, such as straight-leg raises and isometric/ isotonic exercises for hip muscles.
   • If tolerated, introduce aquatic exercises (swimming or water walking) to reduce stress on the hip while building muscle strength.
3. Late-phase Training (6 weeks and beyond):
   • After bone healing or when repair is stable, moderate weight-bearing activities can be introduced, such as weight-bearing walks or aerobic workouts (e.g., elliptical or stationary cycling).
   • Ensure adequate nutrition and sufficient intake of calcium and vitamin D to aid bone recovery.

If the patient presents with fragile bones or other comorbidities, training intensity and pace should be adjusted under guidance from specialists and rehabilitation therapists. Regular monitoring of bone healing and soft tissue status is recommended to assess risks in a timely manner.