Table I.

Features of acute and chronic radiation toxicity

Parameter	Acute radiation toxicity	Chronic radiation toxicity
Effects	Cells with high proliferation rate (stem cells, Paneth cells, enteroendocrine cells, etc.)	Mostly fibroblasts
Pathogenesis	Inhibition of stem cell proliferation and differentiation. Intensification of apoptosis and intestinal epithelial stem cell impairment	Fibroblast stimulation to proliferation and accumulation in the submucosa. Submucosal fibrosis is related to increase of collagen synthesis. Increase in secretion of hepatocyte growth factor (HGF), transforming growth factor β (TGF-β1), and cytokines. Mesenteric thrombosis and vasculitis leads to intestinal ischaemia
Time since start of radiotherapy to onset of symptoms	2–4 weeks	Months – years
Symptoms and complications	Nausea, vomiting, diarrhoea, water and electrolyte disturbances, mucositis, dysbiosis	Ileus, fistulas, bleeding, ulceration, perforations
Treatment	Conservative	Mainly surgical
Disease course	Acute – resolves spontaneously after radiotherapy	Chronic, irreversible. Natural mechanisms are not able to repair the damage