Based on the provided ankle MRI images, a spindle-shaped abnormal soft tissue shadow is observed posterior to the right ankle in the Kager’s triangle region. The signal of this soft tissue is approximately isointense to the surrounding normal muscle tissue on both T1 and T2 weighted images. However, due to the significantly high signal intensity of the surrounding adipose tissue, this mass is more readily identifiable on T1 weighted images.

The muscle belly courses slightly medially, with a clear boundary from the normal soleus muscle, suggesting that it may independently originate from the posterior tendon sheath or the posterior surface of the tibia, and possess an independent attachment near the calcaneus or Achilles tendon. There are no obvious signs of bone destruction, no evident bone marrow edema, or ligament rupture in the surrounding tissues. Mild edema or signs of inflammation can be seen within the soft tissue.

1. Accessory Soleus Muscle: Based on the imaging features, the patient’s age, and ballet training history, an abnormal muscle mass attached to the calcaneus or Achilles tendon is most likely an accessory soleus muscle. It can lead to posterior ankle pain and impingement syndrome, especially apparent during weight-bearing or dance movements.
2. Other Posterior Ankle Soft Tissue Lesions (such as soft tissue tumors or synovial cysts): For example, giant cell tumors, lipomas, or rare synovial cysts. However, these lesions typically present signal characteristics on T1 and T2 weighted images that differ from muscle (e.g., lipomas typically show high signal on T1 images).
3. Paratenonitis or Tenosynovial Lesions: Primarily related to Achilles tendon or soleus tendon inflammation. However, the anatomical structure and signal seen on imaging are consistent with an independent muscle, making this less likely.

Given that the patient is a classical ballet dancer, experiencing repeated overextension and load on the posterior ankle, and that the MRI shows a soft tissue signal consistent with muscle tissue, the most likely diagnosis is an accessory soleus muscle.

Combining the patient’s clinical symptoms (persistent right ankle pain and a background in dance) with the MRI findings, the most likely diagnosis is:

Accessory Soleus Muscle Causing Posterior Ankle Impingement Syndrome

If symptoms persist and are accompanied by joint dysfunction, further confirmation can be obtained via physical examination, histopathological evaluation, or additional diagnostic methods.

Treatment strategies can be tailored according to the severity of the patient’s symptoms and functional needs:

1. Conservative Treatment:
   • Temporarily reduce or avoid activities that cause excessive compression in the posterior ankle (e.g., standing on tiptoe or maximal plantar flexion).
   • Use an ankle brace or heel lift to lessen impact on the posterior ankle region.
   • Short-term use of oral anti-inflammatory and analgesic medications (e.g., NSAIDs) for pain relief.
   • Physical therapy and rehabilitation: Local treatments such as heat therapy, ultrasound, or low-frequency electrical stimulation can help reduce muscle tension and inflammation.
2. Surgical Treatment:
   • If significant posterior ankle impingement symptoms or notable muscle hypertrophy impacts dance performance or quality of life, surgical removal of the accessory soleus muscle may be considered to relieve symptoms.

Exercise Prescription & Rehabilitation Highlights (Using the FITT-VP Principle):

• Frequency: At least 3–4 rehabilitation sessions per week.
• Intensity: Start within a pain-free range at mild intensity, gradually progressing to moderate intensity. If the pain scale exceeds 4 out of 10, reduce intensity accordingly.
• Time: Each session should last 20–30 minutes, including warm-up and stretching.
• Type:
  • Low-impact activities such as swimming or cycling can be used for cardiovascular and foundational lower limb strength training.
  • Ankle stability and small muscle group strengthening exercises, including balance board training and resistance band exercises.
• Volume & Progression: As pain decreases and ankle stability improves, gradually increase tiptoe maneuvers, half squats, and jumping exercises in terms of frequency and sets. Adhere to a pain-free or low-pain principle throughout.

Note: Since the patient is still in a growth and development stage, care should be taken to avoid overtraining that might cause soft tissue or growth plate injuries. Under the guidance of a specialist physician or athletic trainer, gradually increase the load to prevent recurrence and long-term complications.