Chronic damage after radiation therapy: challenge to radiation biology.

After local irradiation of the heart or the recto-sigmoid of rats, chronic radiation damage develops after several months, leading to well-defined clinical syndromes of fatal pericarditis and myocardial necrosis or fatal ileus. In both organs, the LD-50 is below 20 Gy, and a pronounced split dose recovery has been measured with fractionated irradiation. In both organs, the primary radiation damage appears to be to the capillaries, which then leads to secondary parenchymal atrophy. Damage to structured vessels and fibrosis was only seen in areas of necrosis, which in the gut might be precipitated by secondary trauma to the atrophic mucosa.