Abstract
There is a growing interest in the use of megavoltage cone‐beam computed tomography (MV CBCT) data for radiotherapy treatment planning. To calculate accurate dose distributions, knowledge of the electron density (ED) of the tissues being irradiated is required. In the case of MV CBCT, it is necessary to determine a calibration‐relating CT number to ED, utilizing the photon beam produced for MV CBCT. A number of different parameters can affect this calibration. This study was undertaken on the Siemens MV CBCT system, MVision, to evaluate the effect of the following parameters on the reconstructed CT pixel value to ED calibration: the number of monitor units (MUs) used (5, 8, 15 and 60 MUs), the image reconstruction filter (head and neck, and pelvis), reconstruction matrix size (256 by 256 and 512 by 512), and the addition of extra solid water surrounding the ED phantom. A Gammex electron density CT phantom containing EDs from 0.292 to 1.707 was imaged under each of these conditions. The linear relationship between MV CBCT pixel value and ED was demonstrated for all MU settings and over the range of EDs. Changes in MU number did not dramatically alter the MV CBCT ED calibration. The use of different reconstruction filters was found to affect the MV CBCT ED calibration, as was the addition of solid water surrounding the phantom. Dose distributions from treatment plans calculated with simulated image data from a 15 MU head and neck reconstruction filter MV CBCT image and a MV CBCT ED calibration curve from the image data parameters and a 15 MU pelvis reconstruction filter showed small and clinically insignificant differences. Thus, the use of a single MV CBCT ED calibration curve is unlikely to result in any clinical differences. However, to ensure minimal uncertainties in dose reporting, MV CBCT ED calibration measurements could be carried out using parameter‐specific calibration measurements.PACS number: 87.59.bd
Highlights
94 Hughes et al.: megavoltage cone-beam computed tomography (MV CBCT) electron densitythree-dimensional (3D) imaging at the time of treatment has increased, as has the possibility of using such images within the planning process.Cone-beam computed tomography (CBCT) is one possible mechanism and there are many recent developments of these systems utilizing both kilovoltage(1-5) and megavoltage (MV) radiation beams.[6,7,8,9] MV CBCT utilizes the therapy beam and involves the acquisition of a number of two-dimensional (2D) cone beam projection images from different angles around the patient which are used to reconstruct a 3D image dataset
For fan-beam CT systems, the addition of material outside the central volume does not result in the same impact, due to the small field size in the slice direction. This has been reported for a kV CBCT system.[27]. The results of our study suggest that a similar approach to that suggested for kV CBCT systems,(26,32) utilizing both site and parameter specific CT–electron density (ED) calibration curves generated under full scatter conditions, should be utilized for MV CBCT images used in treatment planning
The most accurate MV CBCT ED calibration curve will be that generated with the same acquisition and reconstruction parameters as the patient dataset considered and under similar scatter conditions. This investigation has highlighted several factors that affect the MV CBCT ED calibration curve determined for the Siemens megavoltage MVision system
Summary
Cone-beam computed tomography (CBCT) is one possible mechanism and there are many recent developments of these systems utilizing both kilovoltage (kV)(1-5) and megavoltage (MV) radiation beams.[6,7,8,9] MV CBCT utilizes the therapy beam and involves the acquisition of a number of two-dimensional (2D) cone beam projection images from different angles around the patient which are used to reconstruct a 3D image dataset This can be used to verify patient setup[10,11] and volume changes[12] over the course of the radiotherapy treatment, and to facilitate the optimization or adaption of treatment plans over the relatively long course of a treatment.[12,13] MV CBCT images can be used to replace or complement the kV CT planning data when there are high-density objects present, such as prosthesis or dental implants.[8]. Increased scatter from within the patient, the nonmonoenergetic nature of the beam, and the energy response of the electronic portal imaging device (EPID), all affect this linear relationship and need to be accounted for.[21]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call