TECHNICAL PAPER Radiation dose measurements of an on-board imager X-ray unit using optically-stimulated luminescence dosimeters Leon Smith 1 • Mamoon Haque 2,3 • Johnny Morales 2 • Robin Hill 2,3 Received: 16 July 2014 / Accepted: 12 October 2015 / Published online: 19 October 2015 Ó Australasian College of Physical Scientists and Engineers in Medicine 2015 Abstract Cone beam computed tomography (CBCT) is now widely used to image radiotherapy patients prior to treatment for the purpose of accurate patient setup. However each CBCT image delivered to a patient increases the total radiation dose that they receive. The measurement of the dose delivered from the CBCT images is not readily per- formed in the clinic. In this study, we have used commer- cially available optically stimulated luminescence (OSLD) dosimeters to measure the dose delivered by the Varian OBI on a radiotherapy linear accelerator. Calibration of the OSLDs was achieved by using a therapeutic X-ray unit. The dose delivered by a head CBCT scan was found to be 3.2 ± 0.3 mGy which is similar in magnitude to the dose of a head computed tomography (CT) scan. The results of this study suggest that the radiation hazard associated with CBCT is of a similar nature to that of conventional CT scans. We have also demonstrated that the OSLDs are suitable for these low X-ray dose measurements. Keywords Optically stimulated luminescence Á Dosimetry Á Cone beam computed tomography Á Radiotherapy Á On board imaging Á Kilovoltage dosimetry Introduction On-board imaging (OBI) with an inbuilt kilovoltage X-ray imaging unit is a commonly-used feature on modern radiotherapy linear accelerators [1]. It enables the patient to be imaged immediately prior to treatment, to correct for errors in patient positioning. In addition to 2D planar imaging, modern OBI systems can also be utilized for cone-beam computed tomography (CBCT) imaging, in which multiple planar images are acquired as the kilo- voltage source is rotated around the patient. The images are reconstructed with a filtered backprojection algorithm in the same manner as conventional computed tomography, producing a 3D image of the patient. This procedure enables the tumour itself to be precisely located immedi- ately prior to treatment [2]. One possible concern with the use of OBI and CBCT is the uncertainty of the radiation dose delivered during scanning. As OBI involves additional radiation exposure to that given during treatment, the dose delivered to the patient will be increased. As the risk of adverse effects from diagnostic imaging becomes an increasing concern, it is desirable to quantify the radiation exposure patients receive during OBI and CBCT imaging procedures [3]. This is of considerable importance in determining the number of OBI or CBCT scans that can be safely per- formed, as more scans will enable a more conformal treatment with reduced potential for setup error, at the cost of increased radiation dose. The dosimetry of OBI units can be difficult with con- ventional radiotherapy dosimetry equipment, as they emit radiation in the kilovoltage energy range at much lower doses than are typically used for radiotherapy [4–7]. Most conventional radiotherapy dosimeters are sufficiently large to perturb kilovoltage X-ray beams, potentially leading to & Leon Smith lsmi7940@uni.sydney.edu.au 1 Sydney Medical School, The University of Sydney, Sydney, NSW, Australia 2 Department of Radiation Oncology, Chris O’Brien Lifehouse, Camperdown, NSW 2050, Australia 3 School of Physics, Institute of Medical Physics, The University of Sydney, Sydney, NSW, Australia 123 Australas Phys Eng Sci Med (2015) 38:665–669 DOI 10.1007/s13246-015-0386-x