http://www.emedicine.com/radio/topic590.htm
I had a cough for a while as well, it was due to radiation Pneumonitis; most radiation oncologist will deny that this is the cause of your cough.... and I think incidents of this are much higher than what is reported. Basically, the damage to your lungs from the radiation, causes the body to "overreact" with wbc's causing inflammation and a persistent cough.
It does get better with time...
Radiation Pneumonitis
Clinical Details: Classic radiation pneumonitis has 3 main phases.
Early phase (first month): This represents a latent period of pneumonitis. During this phase, loss of both type I and type II pneumonocytes occurs. Type II pneumonocytes produce surfactant, and decreased amounts result in transudation of serum proteins into the alveoli. This leads to edema of the intersitial spaces.
Intermediate phase (1-6 months): This is characterized by dose-dependent leakage of proteins into the alveolar space, thickening of the alveolar septa, and development of clinical symptoms. Common clinical symptoms include nonproductive cough, low-grade fever, tachycardia, and dyspnea.
Late phase (6 months and later): This is characterized by a loss of capillaries and increased collagen deposition. This results in restrictive changes within the lung characterized by reductions in vital capacity, lung volumes, diffusing capacity of lung for carbon monoxide (DLCO), and total lung capacity.