From the Guidelines

Radiation pneumonitis can develop both within and outside the radiation field.

The development of radiation pneumonitis is associated with dose-volume histogram (DVH) parameters, such as the mean lung dose (MLD) and the volume of lung receiving certain doses (e.g., V20) 1.
Patient factors, such as lung function, age, and sex, do not adequately select patients at high risk for radiation pneumonitis or fibrosis 1.
The use of specific chemotherapeutic drugs, such as platinum, etoposide, taxanes, and vinorelbine, and concurrent radiotherapy does not appear to increase the risk for radiation pneumonitis 1.

Radiation pneumonitis can occur within the radiation field, but the risk is also influenced by the dose to the contralateral lung and other organs at risk 1.
The contralateral lung V5 has been associated with the risk of radiation pneumonitis after lung-sparing surgery and adjuvant tomotherapy 1.
Limiting radiation doses to organs at risk is crucial to minimize the risk of radiation pneumonitis and other toxicities 1.

Hemithoracic radiation therapy after extrapleural pneumonectomy (EPP) has evolved to reduce the risk of radiation pneumonitis, with most centers now reporting rates of high-grade pneumonitis less than 10% 1.
The efficacy of EPP followed by hemithoracic radiation therapy has been promising, with in-field failure rates of approximately 15% to 35% 1.
However, the use of comprehensive radiation therapy after EPP is still a topic of debate, with some studies suggesting that it may not be necessary for all patients 1.

From the Research

Radiation pneumonitis does not only develop within the radiation field, as evidenced by cases of radiation-induced organizing pneumonitis occurring outside the direct radiation field 2.
Studies have shown that radiation pneumonitis can occur in the contralateral lung, outside of the high-dose area, and can be an immunologically mediated process resulting in a bilateral lymphocytic alveolitis 2, 3.
The development of radiation pneumonitis is influenced by both low-dose and high-dose lung volumes, and dosimetric risk factors include the volume of lung exposed to radiation 4, 5.

There are two separate mechanisms involved in radiation-induced lung damage: classical radiation pneumonitis, which is primarily due to radiation-induced local cytokine production confined to the field of irradiation, and sporadic radiation pneumonitis, which is an immunologically mediated process resulting in a bilateral lymphocytic alveolitis 3.
Radiation pneumonitis can be caused by direct cytotoxic effect, oxidative stress, and immune-mediated injury, and can lead to pulmonary fibrosis 6.

Radiation pneumonitis can have an unpredictable and sporadic onset, and can occur in only a minority of patients, with symptoms that are out of proportion to the volume of lung irradiated 3.
Patients with grade 5 pneumonitis often develop symptoms sooner than lower grade pneumonitis, and symptoms may not respond to steroid treatment or may return after steroid taper 4.