It would be great to have more details about your symptoms and the results of an exam. But it sounds like the “cyst” you’re describing is likely due to one or more of these common conditions:

• Retrocalcaneal bursitis. This is an inflammation of a sac that lies between the Achilles tendon and the back of the heel bone (called the calcaneous).
• Achilles tendonitis. Inflammation of the Achilles tendon can cause swelling in just one area of the Achilles tendon..
• Haglund’s deformity (also called “Pump Bump”). This is a piece of normal bone that sticks out at the back of the heel. And that may irritate the Achilles tendon, the retrocalcaneal bursa — or both. Footwear with a stiff back (including womens’ pumps or mens’ dress shoes) may contribute to the development of this condition. But a high arch and genetics can as well.
• A skin callous or blister. This may be due to pressure or rubbing of the back of the heel on your shoe.

While each of these conditions may cause pain, the swelling may persist during recovery — even after the pain has subsided.

There are other possible causes. So I recommend that you see your doctor for evaluation.

In the meantime, if your “cyst” isn’t painful or getting larger, you may not require specific treatment. I would not recommend surgical removal based only on what you have described. In fact, this problem may cause you no trouble in the future. And depending on the cause, you may be able to prevent future symptoms by wearing comfortable shoes with a soft back and shoe inserts (such as heel pads).