Abstract
The main purpose of this work was to quantify patient organ doses from the two kilovoltage cone beam computed tomography (CBCT) systems currently available on medical linear accelerators, namely the X‐ray Volumetric Imager (XVI, Elekta Oncology Systems) and the On‐Board Imager (OBI, Varian Medical Systems). Organ dose measurements were performed using a fiber‐optic coupled (FOC) dosimetry system along with an adult male anthropomorphic phantom for three different clinically relevant scan sites: head, chest, and pelvis. The FOC dosimeter was previously characterized at diagnostic energies by Hyer et al. [Med Phys 2009;36(5):1711–16] and a total uncertainty of approximately 4% was found for in‐phantom dose measurements. All scans were performed using current manufacturer‐installed clinical protocols and appropriate bow‐tie filters. A comparison of image quality between these manufacturer‐installed protocols was also performed using a Catphan 440 image quality phantom. Results indicated that for the XVI, the dose to the lens of the eye (1.07 mGy) was highest in a head scan, thyroid dose (19.24 mGy) was highest in a chest scan, and gonad dose (29 mGy) was highest in a pelvis scan. For the OBI, brain dose (3.01 mGy) was highest in a head scan, breast dose (5.34 mGy) was highest in a chest scan, and gonad dose (34.61 mGy) was highest in a pelvis scan. Image quality measurements demonstrated that the OBI provided superior image quality for all protocols, with both better spatial resolution and low‐contrast detectability. The measured organ doses were also used to calculate a reference male effective dose to allow further comparison of the two machines and imaging protocols. The head, chest, and pelvis scans yielded effective doses of 0.04, 7.15, and 3.73 mSv for the XVI, and 0.12, 1.82, and 4.34 mSv for the OBI, respectively.PACS number: 87.57.uq
Highlights
Advances in radiation treatment delivery, such as intensity-modulated radiation therapy (IMRT), have made it possible to deliver large doses of radiation to a treatment volume with a high degree of conformity
Resolution was evaluated by the smallest resolution test pattern that could be visualized, and low-contrast detectability was evaluated by the smallest acrylic sphere that could be visualized in the image quality phantom
The tube current time product for all OBI protocols has been reduced substantially. All of these changes result in lower patient doses, with the dose to many organs being reduced by 50% or more from previously published works utilizing earlier versions of the manufacturer installed protocols.[13]. For the XVI system, there have been very few organ dose studies published in the literature.[4,11] these studies report the dose to only a limited number of organs, and they were performed before the release of software version 4.0, which included the introduction of new protocols and the use of bowtie filters
Summary
Advances in radiation treatment delivery, such as intensity-modulated radiation therapy (IMRT), have made it possible to deliver large doses of radiation to a treatment volume with a high degree of conformity. Previous studies with the XVI did not include the use of a bow-tie filter (and associated protocols to account for the filter), and a recent software upgrade to the OBI has reduced the total tube current time product used and offers a half rotation scan for head protocols. These technical changes to imaging parameters are intended to reduce the patient dose from cone-beam imaging, which is of increasing clinical significance with daily image acquisitions
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