Fetal and ovarian radiation dose in patients undergoing gamma knife radiosurgery

Abstract

BACKGROUND It is difficult to estimate the fetal or ovarian radiation dosage for female patients undergoing Gamma Knife radiosurgery. Our goals are to determine the fetal and ovarian radiation dose at various distances from a cranial isocenter, to provide a reference for practitioners to estimate the fetal dose with respect to gestational age, and to identify the components of pelvic extracranial radiation. METHODS An anthropomorphic phantom and ion chamber were used to measure the dose at 50, 60, and 70 cm from a cranial isocenter and at three points within the transverse plane for the supine position. Each measurement consisted of a 5-minute exposure. Additional measurements were taken for four collimator sizes, the prone position, off-axis, and with one-half of all collimator holes plugged. RESULTS The values of the fetal and ovarian dose rates ranged from 0.27 cGy/min to 0.05 cGy/min based on distance from the isocenter. The fetal and ovarian dose can be up to 8.1 cGy for a 30-minute Gamma Knife treatment. The dose fell off more rapidly than predicted by the inverse square law. There was no dependence of fetal dose rate on collimator size. No advantage to the prone position could be shown. Leakage and collimator scatter are the main components of extracranial dose 50 to 70 cm from the isocenter. CONCLUSIONS The fetal and ovarian dose is a function of treatment time and distance from the isocenter. We recommend pregnancy status assessment in women of reproductive age and treatment plan design using large volume shots in order to minimize treatment time.