Paediatric organ doses from CT-simulation in brain tumour GK radiosurgery treatment – phantom study

For the preparation of Gamma Knife (GK) treatments of brain tumours, a head CT examination is necessary when using a new CT-based convolution dose calculation algorithm included in Leksell Gamma Plan software (version 10.1.1). The purpose of this study was to measure the organ doses in paediatric anthropomorphic phantoms during a head CT scan and compare them with the measured GK treatment doses. CT doses were measured with radiophotoluminescence (RPL) dosimeters and compared with results of MC simulations. Measured doses for organs within the CT scan volume were comparable for both phantoms. The highest organ CT dose was evaluated for eyes: 27.6 0.2 mGy and 27.3 0.6 mGy for 5 and 10 year old phantom respectively. Doses for organs not included in the scan volume decreased with distance from the scan volume and were on average 3.5 times higher for the 5 year old phantom compared with the 10 year old phantom. Comparison of measured and MC dose values showed good agreement. For the 10 year old phantom, CT doses were 22%, 6%, 5% and 3% of the GK dose for thyroid, lungs, eyes and breasts respectively. Enhancement of the total organ doses for the whole GK treatment due to CT scanning is not negligible but nevertheless, the use of the new convolution algorithm seems reasonable