1. Imaging Findings

Based on the provided chest X-ray and left lower limb MRI, the following key features are noted:

• Chest X-ray: The overall lung fields appear clear, with no evident signs of acute chest pathology. The mediastinal silhouette is roughly midline, and there are no remarkable abnormalities in the cardiac silhouette or bilateral hilar regions. In terms of bony structures, no obvious new lesions or bony destruction are observed.
• Left Lower Limb MRI: A sizable mass-like lesion is observed in the thigh region (or near the knee joint), exhibiting a distinct bony outgrowth in continuity with the medullary and cortical bone, which is typical of an osteochondroma (exostosis). There is compression or mild displacement of the surrounding soft tissue. The lesion shows heterogeneous signal characteristics, with both cartilaginous and osseous components. In certain layers, the cartilaginous cap appears relatively thickened.
• Given the patient’s complaint of “growth and pain,” close attention should be paid to the thickness of the cartilaginous cap and any signs of invasion into adjacent soft tissue.

2. Potential Diagnoses

Based on the imaging findings and the patient’s history of Hereditary Multiple Exostoses (HME), the following diagnoses are worth considering:

1. Multiple Osteochondromas (Exostoses) with Simple Enlargement:
   Explanation: The patient has a known diagnosis of HME. If the lesion is only an enlargement of an existing osteochondroma without definitive signs of malignancy, it could still cause local compression or pain.
2. Malignant Transformation of Osteochondroma into Chondrosarcoma:
   Explanation: In patients with HME, rapid growth and newly emerging pain should raise suspicion for malignant transformation to chondrosarcoma, which occurs in fewer than 5% of osteochondroma cases. For solitary osteochondroma, the rate of malignant transformation is about 1%. On MRI, significant thickening of the cartilaginous cap (usually >1.5 cm) warrants concern for possible malignancy.

3. Final Diagnosis

Considering the patient’s young age, familial genetic background (HME), recent rapid growth of the lesion, and worsening pain, along with imaging findings of an osteochondroma-like lesion and possible thickening of the cartilaginous cap, the most likely diagnosis is:

• Hereditary Multiple Exostoses (HME) with Suspicious Malignant Transformation (Possibility of Chondrosarcoma)

A definitive diagnosis should be confirmed by pathological biopsy or complete surgical resection followed by pathological examination.

4. Treatment Plan and Rehabilitation

4.1 Treatment Strategy

• Surgical Treatment: In cases with significant symptoms (pain, functional limitation) or suspected malignant transformation, surgical resection or curettage plus reconstruction is typically advised. If malignancy is confirmed, wide resection followed by additional therapy (e.g., radiotherapy or chemotherapy) should be considered, depending on pathological grading.
• Medication: If significant pain or localized inflammatory reactions occur before or after surgery, a short course of analgesics or anti-inflammatory drugs may be used. For cases without suspected malignancy but recurring symptoms, symptomatic pain relief and anti-inflammatory treatment can be considered.
• Observation and Follow-up: When uncertainty remains, if the cartilaginous cap thickness is borderline or the lesion does not significantly affect function, periodic imaging follow-up (e.g., MRI every six months or annually) can be considered to monitor changes in size and cartilaginous cap thickness.

4.2 Rehabilitation / Exercise Prescription

1. Early Stage (Postoperative or Severe Symptom Phase):
   
   • Focus on protecting the affected limb and alleviating pain; use crutches or other assistive devices to reduce weight-bearing.
   • Perform gentle joint range-of-motion exercises (passive or assisted active) to prevent contractures.
   • Training frequency: 1–2 times per day, 10–15 minutes each session, low intensity, avoiding marked pain.
2. Intermediate Stage (Stabilized Lesion or After Suture Removal / Healing):
   
   • Gradually increase joint and muscle strength training, such as resistance exercises using elastic bands or light weights. Maintain proper form to avoid excessive joint twisting.
   • Engage in water-based activities (swimming, walking in water) to decrease joint load.
   • Exercise frequency: 3–4 times per week, 20–30 minutes per session, moderate intensity, aiming for mild fatigue without significant pain.
3. Late Stage (Functional Recovery and Strengthening):
   
   • Enhance strength and functional training, integrating it with daily activities or sports, gradually resuming running, jumping, etc.
   • Once adequate strength and flexibility are achieved, higher-level skill training may be introduced, but monitor closely for any recurrence of pain or joint swelling.
   • Exercise frequency: 3–5 times per week, 30–45 minutes each session, moderate-to-high intensity, adjusting individually as needed.

Throughout the rehabilitation process, observe the FITT-VP principles (Frequency, Intensity, Time, Type, Progression, and Volume). Adjust exercises based on the patient’s bone health status and postoperative recovery.

If there is bone fragility or suspected malignancy, exercise caution with strength training to avoid heavy loads or high-impact activities.

Disclaimer: This report is based on the current clinical and imaging information for reference purposes and cannot substitute for in-person consultation or expert medical advice. Specific treatment recommendations must be integrated with the actual clinical situation and determined by a specialist.