Childhood radiation for treatment of a variety of illnesses can interfere with breast development.
Since your current state is the consequence of a medical illness, correction of this condition should be classifed as breast "reconstruction".
Your insurance plan should cover at least some of the associated cost(s) for reconstruction of your breast(s).
There are many options available to you - the very same options as for breast cancer reconstruction. These include either a breast IMPLANT or a tissue FLAP.
Since you do not have a large amount of breast tissue and the skin is likely tight, stretching of the area will be required, using a type of expander.
All tissue expanders involve placement of a prosthesis (some are temporary, others are permanent) that are filled with SALINE. The reason for this is to enable the implant to be filled gradually and for the volume to be modified to suit your aesthetic goals (i.e. A cup, B cup, C cup, etc.).
Once the breast has been expanded to the desired size, the implant or expander is often removed and a permanent implant is placed. This can be either saline or SILICONE. Silicone is a fixed volume and is not usually placed at the intial procedure, as gradual stretchng is safest.
The other option for flap reconstruction can involve transplanting tissue from the tummy area (DIEP flap, SIEA flap, TRAM flap), the inner thighs (TUG flap), back, or buttocks. Many women choose this option if they have areas of excess skin and fat (such as after pregnancy).
The nipple and areola may also require reconstruction - this is often done as a secondary outpatient (day surgery) procedure.
Be sure to see a Board-Certified Plastic Surgeon who has a great deal of experience in breast reconstruction for their expert opinion. Your procedure may indeed be more complicated than most, given the previous radiation.
As insurance may cover this procedure, select a doctor who takes your insurance to decrease the overall costs to you.
Best of luck!