MEDICAL IMAGING—RADIATION ONCOLOGY—ORIGINAL ARTICLE Audit of radiation dose delivered in time-resolved four-dimensional computed tomography in a radiotherapy department Patricia Hubbard, 1 Jason Callahan 2,3 , Jim Cramb, 4 Ray Budd 4 and Tomas Kron 3,4 1 Department of Radiation Therapy, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia 2 Department of Molecular Imaging, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia 3 Department of Medical Imaging and Radiation Science, Monash University, Melbourne, Victoria, Australia 4 Department of Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia P Hubbard BSc; J Callahan BAppSc; J Cramb MSc; R Budd PhD; T Kron PhD. Correspondence Professor Tomas Kron, Department of Physical Sciences, Peter MacCallum Cancer Centre, St Andrews Place, East Melbourne, Vic. 3002, Australia. Email: tomas.kron@petermac.org Conflict of interest: None. Submitted 14 April 2014; accepted 1 January 2015. doi:10.1111/1754-9485.12284 Abstract Introduction: To review the dose delivered to patients in time-resolved com- puted tomography (4D CT) used for radiotherapy treatment planning. Methods: 4D CT is used at Peter MacCallum Cancer Centre since July 2007 for radiotherapy treatment planning using a Philips Brilliance Wide Bore CT scanner (16 slice, helical 4D CT acquisition). All scans are performed at 140 kVp and reconstructed in 10 datasets for different phases of the breath- ing cycle. Dose records were analysed retrospectively for 387 patients who underwent 4D CT procedures between 2007 and 2013. Results: A total of 444 4D CT scans were acquired with the majority of them (342) being for lung cancer radiotherapy. Volume CT dose index (CTDIvol) as recorded over this period was fairly constant at approximately 20 mGy for adults. The CTDI for 4D CT for lung cancers of 19.6 ± 9.3 mGy (n = 168, mean ± 1SD) was found to be 63% higher than CTDIs for conventional CT scans for lung patients that were acquired in the same period (CTDIvol 12 ± 4 mGy, sample of n = 25). CTDI and dose length product (DLP) increased with increasing field of view; however, no significant difference between DLPs for different indications (breast, kidney, liver and lung) could be found. Breathing parameters such as breathing rate or pattern did not affect dose. Conclusion: 4D CT scans can be acquired for radiotherapy treatment planning with a dose less than twice the one required for conventional CT scanning. Key words: imaging for treatment planning; motion management; radiation dose. Introduction One of the most important recent technical advances for radiation therapy has been the ability to character- ise motion of targets and critical structures in radio- therapy planning and adjust the treatment approach to suit the motion observed. Although motion of some type is inherent in many parts of the human anatomy, breathing motion is typically the one which is largest. As breathing motion is periodic and usually regular it is also possible to take motion into account for patient treatment. 1–3 ‘Motion management’ in radiotherapy has many differ- ent aspects but at the heart of it all is a computed tomography (CT) dataset that can depict motion. Four- dimensional (4D) CT relies on acquisition of enough X-ray projections of the patient in each phase of the breathing cycle to reconstruct three-dimensional (3D) image datasets in each part of the breathing cycle. 4–6 Most commonly an external surrogate marker for breath- ing motion is used to indicate which phase of the breath- ing cycle a patient is in and associate the appropriate CT projections to it. External markers can be flexible belts or infrared markers placed on the patient’s chest wall or Journal of Medical Imaging and Radiation Oncology •• (2015) ••–•• © 2015 The Royal Australian and New Zealand College of Radiologists 1