Estimated Risk For Radiosensitive Organs In Stereotactic Radiotherapy And In Various Radiosurgery Treatment Modalities

Keywords: gamma knife, Risk factors, Radiation Therapy, radiation injury, radiosurgery

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       Radiation delivery correlates with risk to different organs.
     The purpose of this work is to compare peripheral doses absorbed during stereotactic treatment of a brain lesion, delivered using different devices. Data were used to estimate the risk of stochastic effects.
     Six treatment plans were created using an anthropomorphic Rando® phantom, and delivered with a 6 MV LINAC, a CyberKnife, a TomoTherapy® and a Leksell Gamma Knife. Peripheral doses, dose to the OARs and the target coverage were compared. To create CyberKnife and TomoTherapy treatment plans, a CT scan of the first ten slices of the phantom was acquired. To create LINAC and Gamma Knife treatment plans, the scan was repeated by applying to the head of the phantom a localization system. The same target volume, at the level of the cavernous sinus, was drawn on each CT scan. 5 Gy were prescribed to the target. The prescription isodose was different for each technique (70% for LINAC and CyberKnife, 95% for TomoTherapy and 50% for Gamma Knife).
     Mean doses ranged from of 4.1 mGy (Gamma Knife) to 62.8 mGy (LINAC with cones) in the thyroid, from 2.3 mGy (TomoTherapy) to 30 mGy (pre-shielding CyberKnife) in the sternum, from 1.7 mGy (TomoTherapy) to 20 mGy (pre-shielding CyberKnife) in the upper part of the lungs, from 0.98 (Gamma Knife) to 15 mGy (pre-shielding CyberKnife) in the lower part of the lungs, and between 0.3 mGy (Gamma Knife) and 10 mGy (pre-shielding CyberKnife) in the gonads.
     This is a retrospective study.
     The peripheral dose absorbed in the sites of interest with a 5-Gy fraction is low then the risk of induction of secondary cancers is very small in considered organs.
     The higher risk is to develop a tumour (fatal and non fatal) within the thyroid after the treatment with the LINAC, which is estimated around 0.08%. Dose absorption in the gonads involves a risk of induction of somatic and hereditary effects that are completely negligible.


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