Side Effects of Lung Radiation

The side effects of radiation to the lung region are related to how large the radiation field (the larger, the more side effects) how much normal lung is in the way (the more normal lung, the more scar tissue or fibrosis) what other structures are in the way (if the tumor is near the esophagus, then a sore throat or swallowing problems would be more likely) and whether the patients is also getting chemotherapy (which increase the frequency and severity of side effects.)

Radiation can definitely inflame/damage normal lung tissue in the way (see pictures of radiation pneumonitis here, here, here and here, radiation chest wall inflammation here and radiation fibrosis here).

There are limits to the safe doses of radiation (go here, here, here, here and here) which basically recommend keeping the total lung volume that gets 20Gy (V20) below 25% or the V10 below 40% and the mean lung dose under 20Gy.

See the NCCN guidelines here).

New techniques are being used to evaluate the functional parts of the lung (go here).

Typical side effects are listed in the consent form below from the recent RTOG lung trial
Short term there is little risk from radiating the heart (though the dose should be limited as noted here) but long term there is an increased risk of coronary artery disease (see here.)

Consent form for RTOG 0324 Risks from Radiation Therapy
Radiation therapy to the chest
Very Likely
Difficulty, pain, or burning sensation when swallowing, which is temporary; the use of chemotherapy withradiation may increase this risk. You should avoid acidic or spicy foods and alcoholic beverages.
Fatigue (tiredness) for no apparent reason, which is temporary
The skin in the treatment area may become reddened and/or dry, and chest hair may not grow back.
Decrease in blood counts while undergoing treatment, which could lead to an increased risk of infection, weakness, and/or in bleeding and bruising easily Cough
Some difficulty breathing, due to lung damage, as described below

Less Likely, But Serious
Irritation of the lining around the heart, which can cause chest pain, shortness of breath, and irregular or rapid heart beat; rarely, this can require surgery to correct.
Irritation and/or damage to the muscle of the heart; rarely, this can cause a heart attack, heart failure, and/or death.
Irritation and/or damage to the spinal cord (the major nerve within the spine), which can lead to weakness, tingling or numbness of the lower body and legs; very rarely, this can lead to inability to move or control the lower half of the body.
Narrowing of the esophagus (tube to the stomach)

Chest radiotherapy can cause changes in normal lungs. These changes can be as unimportant as small amounts of "scarring" seen on x-rays that does not cause symptoms. Sometimes chest radiotherapy can cause lung damage that leads to symptoms such as shortness of breath, cough, or fever. Rarely, these symptoms can be severe or life threatening. The combined use of chemotherapy and radiotherapy, as in this study, may increase the risk of developing symptoms due to lung damage.