Does a breast cyst always need a needle biopsy radiology doc just suggest to do a follow up u/s and mamm in 6 months to make sure cyst has not grown?

No. A simple breast cyst or cyst with a single thin septation is very common and does not require biopsy or follow up. If it has a more suspicious feature (such a internal nodule, thick septation, etc.), then biopsy would be preferable to follow up. If a breast cyst is benign in appearance, it can still be therapeutically drained under ultrasound guidance to relieve symptoms.
Breast cyst. If it is completely a simple cyst without septation or nodularity then it does not need aspiration and following up with repeat sonography is fine.
No, it almost. never needs a needle biopsy, or even any follow up, if it is a simple cyst. Cysts can normally come and go, or get bigger or smaller. .
Breast cysts. Breast cysts are benign masses in the breast that soetimes cause pain and can cause concern because they feel like a solid mass. They can be easily managed by needle aspiration but if they are not causing pain or discomfort this is not neccesary. They can be followed by ultrasound if you prefer not to aspirate them.