Radiation Exposure of Extracranial Organs at Risk during Stereotactic Linac Radiosurgery

Stereotactic radiosurgery of intracranial tumors and vascular malformations can be performed by either a linear accelerator (Linac) or gamma knife. The aim of this first study on patients was to determine the radiation exposure of organs at risk and assess the risk for late effects including secondary tumors and/or hereditary disorders after stereotactic Linac radiosurgery. Thermoluminescent dosimetry (TLD) measurements were done on 21 consecutively admitted patients with various intracranial lesions scheduled for Linac radiosurgery. The TLD chips were placed on the eyelid, thyroid, breast and the regions of the ovary or testes. The mean doses in organs at risk were 276 ± 200 mGy (eye lens), 155 ± 83 mGy (thyroid), 47 ± 22 mGy (breast), 20 ± 12 mGy (ovary), and 9 ± 3 mGy (testes). The doses decreased significantly with a larger distance from the isocenter. The absorbed doses to the extracranial organs at risk in patients undergoing Linac radiosurgery were very low, ranging from 0.025% (testes) to 0.76% (eye lens) of the mean maximum target dose (36 Gy). Nevertheless, while the majority of radiosurgery patients have benign tumors or arteriovenous malformations and their life expectancy is long, all doses should be kept as low as reasonably achievable. This could be accomplished by the use of modern irradiation techniques including conformal beams with micro–multileaf collimator and avoiding beams directed to the trunk.