What does it mean if having a swollen sac inside the bones of my left ankle as it was seen in the mri pictures.any advices..could it be bone cancer?

Not cancer: Based on your description, a sac known as a "bursa" that cushions the bones of your ankle, may be inflammed, which is a common injury and may be caused by increased exercise, poorly fitting shoes, or basically a type of low grade, longer term irritation. This bursitis can be treated with a steroid injection directly into the area, or by oral anti-inflammatories. Bone cancer is unusual in your age.

Inflammation,probabl: Sounds like you are talking about the joint space in the anlle which is covered with a membrane which can swell with inflammation or trauma. Not likely to be a bone cancer as they are quite rare.