Impact of acquisition protocol variations on effective dose and risk for CT head examinations

Benhalim, MR

Abstract

PURPOSE: To propose a method for estimating the effective lifetime risk of radiation-induced cancer from different brain CT scan protocols in both paediatrics and adults and to develop a prospective method for estimating the number of cancer cases for patients undergoing CT brain scans when using different scanning parameters.

MATERIALS AND METHODS: A series of CT dosimetry experiments were conducted at the University of Salford. Both adult and paediatric ATOM phantoms were imaged using standard CT head protocols across a wide range of protocol variations. Dose measurements were made using metal oxide semiconductor field effect transducers (MOSFETs). Effective doses and lifetime attributable cancer risks were calculated from MOSFETs organ and tissue absorbed dose measurements in both phantoms. The method for lifetime cancer risk prospective estimation allowed the production of prospective risk data (Excel spreadsheet). This can predict attributable lifetime cancer risk, before undertaking a CT scan, and is of critical importance in order to predict more accurately the radiation risks from CT. Whilst such a tool has benefits for all patient ages, it would have particularly value for children - due to their increased radio-sensitivity. Using such a tool, dose optimisation can then occur on a more informed basis, by adjusting protocol parameters with a view to minimising organ and eye dose. Together with advanced image quality preserving techniques, like ACTM, the tool would allow for more informed clinical decisions to be taken to balance image quality and radiation dose on a patient by patient basis.

RESULTS: The brain, thyroid, thymus, lung, salivary glands, oral mucosa, extrathoracic region and bone marrow all receive more than 0.03 mSv during CT brain scanning. The range in effective dose across a range of CT brain protocols were 0.27 to 1.13 mSv and 0.34 to 1.55 mSv for adult sequential and helical protocols, respectively. For paediatric helical protocols, the effective dose ranged from 0.30 to 2.06 mSv. In addition to scan parameters, differences in risk are also attributed to patient age at the time of scanning and gender. As an example, the lifetime attributable cancer risk for a 30-year-old patient when undertaking either an adult sequential or helical CT brain scan were 8 females and 6 males per 106 and 16 females and 9 males per 106, respectively. By contrast, for the two paediatric helical CT brains protocols the effective risk (using 3 months to 3 years CT protocol) were 71 females and 36 males per 106 at birth and for the 3 years to 5 years protocol this decreases to 69 females and 35 males per 106 when aged 5. Using the risk data, a novel interactive spreadsheet has been developed and is reported within this thesis, which allows the proactive / prospective estimation of radiation risk for CT head examinations.

CONCLUSION: Data collected within this thesis has identified that the most dominant factors effecting absorbed dose are tube current, tube rotation time, detector configuration and helical pitch. When these factors increase the absorbed dose to body organs and tissues increases as well. The only effect of gantry angle is in decreasing the dose to the lens of the eye, this quantity does not factor into effective dose estimates due to the nature of the lens tissue. This thesis proposes a novel method to estimate effective lifetime risk of radiation-induced cancer from CT brain examinations in order to compare different brain CT protocols
(acquisition parameters). Absorbed dose measurements when considered in relation to the age
and gender of the patient can help provide estimations of effective risk, a potentially more useful indicator of the possible effects of radiation exposure from CT head examinations. This risk estimation method can be used to compare different CT brain protocol parameters immediately prior to imaging. Risk should be prospectively taken into account when planning a CT brain examination, especially for young ages and those undergoing serial imaging. Using the prospective risk data (spreadsheet) provided in this thesis can help to estimate the probability of cancer induction from specific CT brain protocols and can be considered by practitioners and manufacturers when developing CT examinations in the future.

Citation

Benhalim, M. (in press). Impact of acquisition protocol variations on effective dose and risk for CT head examinations. (Thesis). University of Salford

• Thesis Type: Thesis
• Acceptance Date: May 1, 2018
• Deposit Date: May 22, 2018
• Publicly Available Date: May 22, 2018