Peripheral doses in children undergoing Gamma Knife radiosurgery and second cancer risk

Gamma Knife radiosurgery stereotactically delivers a high single dose of external radiation to a small well-defined intracranial lesions. Due to a large amount of dose delivered in a single fraction (10–30 Gy), dose outside the treatment volume is an important issue. The aim of the study was to measure the out-of-field doses during the Leksell Gamma Knife Model C radiosurgery for children. The children population was chosen due to their higher susceptibility to radiation. Also, due to smaller size of their bodies, the larger doses are expected to all out-of-field tissues and organs than for adults for the same irradiation conditions. The purpose was to identify doses delivered to the eye lens, thyroid glands, breasts, sternum, upper abdomen and gonads from the doses measured with dosimeters placed on the some positions on the surface of the patients. There were 5 patients between 4 and 14 years old. The relation of the organ doses and doses on the surface was determined by measurements using the anthropomorphic phantom (CIRS, ten-year old child). For dosimetry measurements two types of thermoluminescent dosimeters were used, LiF:Mg,Ti and LiF:Mg,Cu,P and radiophotoluminescent glass dosimeters type GD-352M. The agreement between three types of dosimeters was on average within 4%. Doses on the patients have shown large dissipation for the same positions above specified organs. This is due to different influential parameters (position and volume of the target, height and weight of patient, irradiation dose). For example, doses to the eyes and gonads varied from 0.7% to 2.55% and from 0.05% to 0.22% of the mean target dose, respectively. Results of organ dose measurements in the phantom and on the surface of the phantom show differences between 5 and 25% for different organs. Lifetime attributable risks of cancer incidence associated with a chosen Gamma Knife radiation treatment were calculated for selected radiosensitive organs.

► TLD and RPL are suitable for out-of-field dose measurements in radiotherapy.
► Doses to the patient's eyes varied from 0.7% to 2.55% of the mean target dose.
► Doses to the patient's gonads varied from 0.05% to 0.22% of the mean target dose.
► Surface doses give possibility of dose estimation in the underlying organs.
► Knowledge of the peripheral doses enables estimation of the second cancer risk.