The abscopal effect is the ability of radiotherapy to cause effects beyond the radiation field. Fatigue during radiotherapy is the most commonly observed effect. The distant bystander effect is one type of abscopal effect.
There are three proposed mechanisms for the abscopal effect:
- Radiotherapy may lead to release (or loss) of various cytokines and hormones, which exert an endocrine effect on other parts of the body.
- Radiotherapy may induce an immune response, sensitising the immune system to self antigens. These immune cells may then mount an autoimmune response against tissues elsewhere in the body.
- The pseudo-abscopal effect is seen in lymphomas, and is thought to be due to cells entering the field during treatment from other parts of the body.
For the development of radiation pneumonitis outside the radiotherapy field, the former two mechanisms are both implicated.
- Damage to endothelial cells within the lung may lead to the release of cytokines, stimulating fibrosis and pneumonitis in other areas of the lung.
- Damage to pneumocytes may lead to their antigens being exposed to the immune system, with the possibility of developing an autoimmune response. This response would then attack other pneumocytes outside the radiotherapy field.
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