Pneumonitis is a general term that refers to inflammation of lung tissue. Technically, pneumonia is a type of pneumonitis because the infection causes inflammation, however pneumonitis is used when referring to other causes of lung inflammation. Pneumonitis can be caused by disease, infection, radiotherapy, allergies or irritation of the lung tissue by inhaled substances and SACT (National Cancer Institute [194]).

The primary function of the lungs is to provide gaseous interchange; therefore the lungs must be accessible to both the external and internal environments via inhalation and the circulation, respectively, leading to the evolutionary development of critical structural and physiological relationships between the air passages, respiratory parenchyma and vascular system (Notter et al. [215]). These relationships, together with the ease of access, make the lungs especially susceptible to a multitude of physical, chemical and biological stressors that appear to be able to disrupt the delicate functional balance of this system with relative ease. Depending on the source and severity, under many circumstances there is progression to a persistent, chronic pathology, which occurs through a complex cascade of processes beginning with the acute injury and followed by an associated innate inflammatory response, culminating in abnormal remodelling and tissue repair (Notter et al. [215]).

Pneumonitis is the lung tissue's reaction to cancer treatment; it is deemed dose‐limiting and thereby can compromise treatment and also lead to quality‐of‐life issues in survivors (Williams et al. [307]).

Radiation pneumonitis can occur within 6–12 weeks after thoracic radiotherapy and occurs in between 5% and 15% of patients (Oie et al. [219], Williams et al. [307]). The risk of pneumonitis is related to the volume of normal lung irradiated and the dose of radiotherapy delivered (Libshitz [146], McDonald et al. [174]). Therefore, it is most likely to occur with radical (potentially curative) treatments for lung and oesophageal cancer but can also occur following treatment for breast cancer and lymphoma (Williams et al. [307]). Symptoms usually arise within the first 90 days of radiotherapy treatment (acute effects of radiation) but can occur later than this during the development of lung fibrosis (McDonald et al. [174]). The chance of radiation pneumonitis is increased by the concomitant use of chemotherapy and is more likely to occur in patients with pre‐existing lung disease such as chronic obstructive pulmonary disease.

SACT, including bleomycin, busulfan, cyclophosphamide, carmustine, the taxanes and methotrexate, can cause inflammation in the alveoli, filling them with white blood cells and fluids which can lead to the development of pneumonitis (Limper [147], Matsuno [169], Vander et al. [297]). Targeted therapies such as mammalian target of rapamycin (mTOR) and EGFR inhibitors along with immunotherapies (CTLA‐4 or PD‐1/PD‐L1) are known to cause pneumonitis although the exact mechanism is unknown (Duran et al. [67], Zhang et al. [314]). Discontinuation of the drugs with prednisolone treatment in severe cases will resolve the pneumonitis (Duran et al. [67], Matsuno [169]).