Radiation dose to the fetus during CyberKnife radiosurgery for a brain tumor in pregnancy

PurposePregnancy during radiosurgery is extremely rare in clinical practice. We report fetal dose results during CyberKnife radiosurgery for a brain tumor in pregnancy.Methods and materialsA 26 year old pregnant woman with a rapidly growing deep-seated grade-III glioma was treated during the third trimester of gestation using CyberKnife. Ultrasound imaging was used to determine the position of the embryo prior to treatment. A dose of 1400 cGy was prescribed aiming to control tumor growth until delivery of the child. Prior to radiosurgery, the treatment was simulated on an anthropomorphic phantom. Radiation dose to the embryo was measured using a Farmer chamber and EBT3 films.ResultsFetal doses of 4.4 cGy and 4.1 cGy were measured for the embryo's head and legs, lying at 56 cm and 72 cm from the isocenter, respectively, using the Farmer chamber situated at 8.5 cm depth beneath the phantom surface. Dose results of 4.4 cGy, 3.5 cGy and 2.0 cGy were measured with the films situated at depths of 6.5 cm, 9.5 cm and 14.5 cm, respectively. An average dose of 4.2 cGy to the fetus was derived from the above values. A corresponding dose of 3.2 cGy was also calculated based on results obtained using EBT3 films situated upon the patient skin.ConclusionsThe measured fetal doses are below the threshold of 10 cGy for congenital malformations, mental and growth retardation effects. The radiogenic cancer risk to the live-born embryo was estimated less than 0.3% over the normal incidence. The treatment was administered successfully, allowing the patient to deliver a healthy child.     

Radiation Hormesis and Cancer

Despite the long history of radiation hormesis and the public health concerns with low-level exposures to ionizing radiation, there has been surprisingly little formal evaluation of whether hormetic effects are displayed with respect to radiation exposure and cancer incidence (i.e., reduced cancer risk at low radiation doses compared to controls, enhanced cancer risk at higher doses) until relatively recently. This paper reviews data relevant to the question of radiation hormesis and cancer with particular emphasis on experimental studies in animal models exposed to low levels of ionizing radiation. Data exist that provide evidence both consistent with and/or supportive of radiation hormesis. Other biomedical research provides potentially important mechanistic insight: low dose exposures have the capacity to activate immune function to prevent the occurrence of tumor development and metastasis; low doses of radiation have been shown to reduce mutagenic responses and induce endogenous antioxidant responses. These findings are consistent with epidemiological data suggesting an inverse relationship between background radiation and cancer incidence and with occupational epidemiological investigations in which low-dose exposure groups display markedly lower standardized mortality rates than the referent or control group.