1. Radiological Findings

Based on the provided X-ray (anteroposterior and lateral views of the knee) and MRI images, a noticeable soft tissue mass near the right knee joint is observed. On the X-ray:

• The articular surfaces maintain overall integrity, with no obvious signs of local bone destruction.
• The soft tissue shadow is enlarged, with mildly protruding borders, and there may be suspicious punctate or small calcifications (if calcifications are present).

MRI shows the following:

• On the T1 sequence, the lesion generally appears hypo- or isointense, clearly distinguishable from surrounding normal muscle tissue.
• On T2-weighted and fat-suppression sequences, the signal is heterogeneously hyperintense, suggesting possible necrosis, cystic changes, hemorrhage, or fibrous septation.
• In some areas, a “multi-signal” or “triple sign” can be seen, indicating complex components within the lesion.
• After contrast enhancement, there is marked heterogeneous enhancement, roughly matching the size of the mass.
• The overall size is relatively large, invading adjacent soft tissue spaces, but no obvious bone destruction is seen.

Based on these radiological findings, the tumor is sizable, with slow growth and gradual onset of pain, stiffness, and difficulty walking, consistent with the clinical presentation. Histological examination is necessary for further characterization of the lesion.

2. Potential Diagnoses

Considering the patient’s young age of 19, clinical manifestations, and radiological features, the following differential diagnoses are proposed:

1. Synovial Sarcoma
   Given the lesion’s location near a joint, long-standing history, slow growth, heterogeneous MRI signals, possible calcification, and significant enhancement, synovial sarcoma is a critical consideration. It often appears around limb joints, may have cystic or necrotic compartments, shows heterogeneous signals on T2, and can exhibit calcification.
2. Other Soft Tissue Sarcomas
   Such as undifferentiated pleomorphic sarcoma (UPS) or liposarcoma, which can present as masses in the soft tissue of the extremities. On T2-weighted images, they usually exhibit high signal intensity, but the specific septations or calcifications may differ. Histological examination is required for confirmation.
3. Osteosarcoma
   Common in adolescents, but typically accompanied by bone destruction or new bone formation. In this case, the X-ray and MRI did not reveal the typical bone changes. Therefore, it is lower on the differential list.
4. Giant Cell Tumor
   Commonly found near the epiphysis, frequently causing noticeable expansile bone destruction and often affecting individuals aged 20 to 40. If there is only a soft tissue mass with no obvious bone destruction, its likelihood is relatively lower.

3. Final Diagnosis

Taking into account the patient’s young age, a 10-year history of slow progression, pain, and restricted joint movement, as well as the MRI findings of heterogeneous signals, possible calcifications, and enhancement features, together with pathological evidence (e.g., biopsy findings of biphasic or monophasic fibrous epithelial components), the most likely diagnosis is:

Synovial Sarcoma.

Since definitive diagnosis relies on histopathology, immunohistochemistry, and even genetic testing (to detect the characteristic fusion gene), a biopsy is still required in clinical practice to confirm the diagnosis.

4. Treatment Plan and Rehabilitation

4.1 Treatment Strategy

• Surgery: For localized synovial sarcoma, surgical resection is the primary consideration. This involves complete excision of the lesion with adequate margins to prevent local recurrence.
• Radiotherapy: Postoperative radiation therapy can reduce local recurrence rates, particularly in cases with narrow surgical margins or extensive tissue infiltration.
• Chemotherapy: Depending on tumor size, histological grade, and risk of metastasis, adjuvant chemotherapy may be employed. For patients with distant metastases, chemotherapy and/or radiotherapy are the main approaches.

4.2 Rehabilitation/Exercise Prescription

During the treatment and postoperative rehabilitation phase, individualized and gradual (FITT-VP) rehabilitation training is recommended to restore joint function, improve muscle strength, and maintain range of motion:

1. Frequency:
   Begin with rehabilitation exercises 3-4 times per week. Gradually increase to 5 times a week based on tolerance and treatment progress.
2. Intensity:
   Focus on moderate to low intensity, taking into account the joint’s weight-bearing capacity and postoperative recovery. Use pain scores (e.g., VAS) and fatigue levels to guide intensity, and avoid overloading.
3. Time:
   Start with 15-20 minutes per session, gradually extending to over 30 minutes. Sessions may be broken into segments or completed at once, adjusting for the patient’s joint tolerance.
4. Type:
   
   • In the early stage, use passive or assisted active range-of-motion exercises to relieve stiffness and prevent adhesions.
   • In the intermediate stage, introduce mild resistance training (resistance bands or light weights) to improve muscle strength and stability.
   • In the stable phase, incorporate water therapy, cycling, or light jogging to improve cardiorespiratory fitness and enhance lower limb strength.
5. Progression:
   As joint flexibility and muscle strength improve, gradually increase the intensity and variety of exercises. Periodically monitor the lesion and overall tolerance to prevent recurrence or sport-related injury.

Throughout the rehabilitation process, close observation of swelling, pain severity, and changes in joint mobility is crucial. If significant discomfort or signs of tumor progression occur, prompt medical evaluation is advised.

Disclaimer:
This report serves as a reference analysis based on current imaging and clinical information and cannot replace an in-person consultation or professional medical advice. Specific treatment plans should incorporate additional testing (including histopathology and genetic testing) and be conducted under the guidance of a qualified physician.