1. Imaging Findings

From the provided X-ray, CT, and MRI images, the lesion is located on the posterior side of the knee joint (behind the tibia or near the posterior-lateral edge of the tibia) and is closely related to the bone cortex. The imaging characteristics show the following main findings:

• Plain X-ray: A clearly demarcated fat-density shadow can be seen on the posterior aspect of the knee joint, adjacent to the bone cortex. Local bone changes are visible, but there is no typical sign of a bony protrusion with continuous marrow space (differentiating from a typical osteochondroma).
• CT Scan: The lesion is mainly composed of fat density. Patchy or band-like calcifications/osseous shadows can be observed, “adhering” to the bone surface (cortex). Some portions of the lesion may contain fibrous, cartilaginous, or osseous components. There is no evident continuity between the tibial marrow and the bony components within the lesion.
• MRI Findings: On T1-weighted images, the lesion appears primarily as high signal intensity; on T2-weighted or fat-suppressed sequences, it also demonstrates high signal intensity, indicating a predominant fat component. Locally, irregular or patchy signals can be seen, suggesting cartilaginous or ossified components. It is closely attached to the bone cortex, with no obvious invasion of the bone marrow.

Overall evaluation: The above imaging findings are consistent with the characteristics of a fatty tumor on or adjacent to the bone surface (juxtacortical), with local cartilaginous or osseous components, suggesting a typical presentation of a parosteal lipoma (also referred to as “periosteal” lipoma).

2. Potential Diagnoses

1. Parosteal Lipoma: A rare benign lipoma closely attached to the bone surface, often containing osseous or chondroid components. On imaging, it is predominantly fatty and is closely related to the bone cortex but lacks continuity of the marrow space.
2. Osteochondroma: Commonly seen in children or adolescents, typically presenting with a bony protrusion that has continuity of the marrow space and cortex with the host bone. If only evaluated on plain films, one must carefully exclude osteochondroma. However, in this case, imaging does not demonstrate a typical marrow connection through a stalk.
3. Other Soft Tissue Tumors: If the tumor contains calcification or ossification, liposarcoma or chondrosarcoma could be considered. However, given the patient’s long-standing, slow-growing lesion and a lack of distinct malignant imaging features, the likelihood of malignancy is low.

3. Final Diagnosis

Considering the patient’s age, the long-standing painless or minimally painful swelling, and imaging findings of a predominantly fat-density lesion adhering to the bone surface without marrow continuity, the most likely diagnosis is Parosteal Lipoma.
A definitive diagnosis can be confirmed by pathological examination (surgical removal or biopsy) to clarify the histological type. However, parosteal lipoma is often diagnosed based on typical imaging findings. If there is no apparent functional impairment or complications, conservative observation or evaluation for surgical removal may be considered.

4. Treatment Plan and Rehabilitation

4.1 Treatment Strategies

• Conservative Follow-up: If the lesion remains stable in size with mild symptoms and no significant functional limitations, regular imaging follow-up can be performed to monitor tumor size and surrounding tissues.
• Surgical Resection: If there is local pain, restricted joint movement, or significant tumor growth affecting the patient’s quality of life, surgical removal may be considered. During surgery, care should be taken to protect important nerves and blood vessels, and to remove the tumor completely from the bone surface.

4.2 Rehabilitation and Exercise Prescription

Whether post-surgery or during conservative observation, patients should follow a personalized rehabilitation program based on joint function and overall health status. A gradual approach can help improve local muscle strength and joint mobility while minimizing adhesion and functional impairment. The following principles may be referenced:

1. Frequency: 3–5 times per week, adjusted according to pain and functional recovery.
2. Intensity: Begin with mild exercises, such as range-of-motion training and light strength exercises (e.g., straight leg raises, isometric quadriceps contractions). Gradually increase weights or resistance based on tolerance.
3. Time: 20–30 minutes per session; for those with lower fitness levels, sessions can be divided into shorter intervals in the morning, noon, and evening.
4. Type: Initially, focus on passive or assisted range-of-motion exercises and local muscle-strengthening. In later phases, incorporate closed-chain and low-impact lower limb exercises, like seated single-leg extensions or gentle squats. Avoid high-impact activities.
5. Progression: Once pain and swelling are well controlled, gradually increase load by adding resistance bands, light dumbbells, or elliptical training. Always follow the principle of “no significant pain or aggravated swelling.”

Special Note: For older patients, bone health and cardiovascular status should be considered. If osteoporosis or other chronic conditions are present, an individualized program under the guidance of a physician or rehabilitation therapist is advised. If abnormal swelling, acute pain, or functional impairment occurs, promptly seek a medical evaluation or adjust the rehabilitation plan.

5. Disclaimer

This report is provided as a reference based on the available imaging and clinical information and cannot replace in-person consultation or professional medical advice. If you have any concerns or notice worsening symptoms, please consult a specialist or visit a hospital as soon as possible.