[Late radiation changes after small volume radiosurgery of the rat brain. Measuring local cerebral blood flow and...

The goal of this study was to investigate late effects following stereotactic single fraction and small volume irradiation on cerebral blood flow and histologic alterations in the rat brain parenchyma.

66 Copenhagen rats, separated into eleven groups of six animals each received single doses of 20, 30, 40, 50 and 100 Gy using a 15 MV linear accelerator. Six rats served as controls. Two cylindrical collimators of 2 mm and 3 mm aperture were used. The diameters of the spherical 80% isodose were 3.7 and 4.7 mm, respectively (Table 1). Irradiation was applied to a predefined area in the right frontal lobe. 19 months after irradiation local cerebral blood flow (LCBF) was measured by the autoradiographic method in one animal of each dose group between 20 and 50 Gy. 9 and 19 months after irradiation, half of the animals of each group were sacrificed for brain histology. All animals irradiated with 100 Gy were sacrificed 7 months after irradiation.

An increase of local cerebral blood flow was measured in brain structures within the 80% isodose in animals irradiated with 50 Gy (Figure 3) compared to the contralateral hemisphere. Measurements close to necrotic areas showed a strong decrease of local cerebral blood flow (Figure 1). A volume increase of the irradiated hemisphere was seen after 19 months (Figure 2). The histologic examination after 19 months showed necrotic areas in the 30-50 Gy groups (Figure 4b) but not in the 20 Gy groups (Figure 4c). The animals who received 100 Gy demonstrated brain necrosis within 9 months after irradiation (Figure 4a). At both points in time the groups irradiated with the 3-mm collimator showed more pronounced histomorphologic and functional changes compared to the groups irradiated with the 2-mm collimator.

Alterations of the local cerebral blood flow were measured as a late effect after single dose irradiation. The alterations of the local cerebral blood flow could be explained by the histomorphologic changes of the blood vessels. Using a semi-quantitative classification a dose, time and volume dependence for the endpoint radionecrosis was seen.