Radiation to the neck area can bruise the thyroid. When that happens, thyroid levels can fluctuate and eventually end up either high or low. This results in primarily shortness of breath along with other issues such as appetite, constipation, overall weakness, lethargy.
A simple blood test for thyroid stimulation hormone or TSH along with thyroid levels of T3&4 will either rule out this as a major cause of shortness of breath or pinpoint this.
My thyroid went low/high/ about two months after being done with radiation. Felt like anemia or a mild heart attack. It then settled out as overactive or high thyroid. A thyroid uptake scan showed one enlarged or bruised wing of the thyroid. Options were to ablate the thyroid and live on artificial thyroid medication for life, or take a thyroid reduction medication and see if the thyroid heals itself.
Over many years the reduction medication worked and became a lower dose in small steps. After twenty four years, my thyroid levels got to the point where medication is no longer necessary.
The reason I went that route rather than the ablation road, which is far more common by the way, is because a patient can live without the reduction medication but not without the artificial thyroid medication, in case of natural or global disaster.
So, shortness of breath after radiation? Rule out the thyroid.