Subacute Radiation Effects

Definition - Subacute Radiation Effects occur after treatment has completed but before 6 months have elapsed.

Subacute radiation effects are an unusual group of pathologies that occur in several tissues and do not fit well into the 'early effects' due to loss of the proliferative compartment or the 'late effects' due to death of endothelial cells and stimulation of fibroblasts.

Examples include:

• Radiation pneumonitis, which is discussed in the Late Lung Reactions topic. It occurs 2 - 6 months following radiation treatment involving the lungs.
  • Radiation pneumonitis is thought to be due to stimulation of an inflammatory reaction within the alveoli of the lung, due to release of cytokines from type II pneumocytes, macrophages and endothelial cells
• Lhermitte's sign is due to a transient demyelination within the spinal cord and occurs 1 - 3 months after radiation exposure. It presents with pain radiation down the back from the neck to the sacrum.
  • Lhermitte's sign is due to loss of oligodendrocytes, the myelinating cells of the central nervous system. They have a longer life span than epithelial cells but still require replacement from a proliferative compartment. In this case, the loss of proliferative ability only becomes apparent after several months. Lhermitte's sign is usually reversible, unlike white matter necrosis which occurs due to a combination of oligodendrocyte loss coupled with endothelial cell death. See Late CNS reactions. Lhermitte's sign is also seen in patients with multiple sclerosis.

Occasionally late reactions may develop rapidly; examples could include osteoradionecrosis developing within 6 months of radiation delivery. The causes for this are most likely multifactorial and patient specific. Osteoradionecrosis of the jaw might be precipitated earlier if the patient has ongoing poor oral hygiene, for instance.

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