Untitled

Abstract

Cone beam computed tomography (CBCT) images obtained from linac-based kV imagers are typically used for image-guided radiotherapy, in particular to perform three-dimensional image matching. CBCT image sets can also be used for adaptive radiotherapy where the treatment plan is modified on the basis of periodic imaging throughout the treatment course. CBCT images provide both anatomical information and Hounsfield unit (HU) values, which are required for dose calculations. This study evaluates treatment plans based on CBCT datasets calibrated using the Catphan 504 phantom to investigate the feasibility of using CBCT for adaptive replanning. The CBCT images were acquired from a Varian On-Board Imager system. Conventional planning CT (PCT) images obtained from a Philips Brilliance Big Bore CT scanner were used as reference images. The HU-density calibration curves of CBCT were obtained using a Catphan 504 phantom and a CIRS density phantom and compared with the clinical PCT calibration curve (obtained using the CIRS density phantom). Treatment plans created using the different calibration curves were compared. Identical targets were delineated on CBCT and PCT images on four different-sized phantoms and planar dose maps were generated. The dose-volume histograms of PCT-and CBCT-based plans were compared and evaluated by gamma analysis. To extend the study to a typical clinical situation, two prostate cases were included. The dose distribution comparison between PCT-and CBCT-based plans for patients yielded similar results to those obtained using phantoms. The study also analyzed the effect of phantom dimensions on HU values and its impact on dose calculations. The isodose distributions computed based on PCT and CBCT using the Catphan calibration curve agree to within ± 1% compared to that based on CBCT using the density phantom calibration curve. However, for phantoms of larger diameter, there is a pronounced discrepancy in the 50% and 60% isodose lines, with the dose difference being about ± 3%. For phantoms whose thickness is less than the cone beam scan length (16 cm) and for phantoms whose diameter is less than that of the calibration phantom, the variation in HU values is high. The effect of a change in radial diameter has a larger impact on dose calculations. This study shows that the CIRS density phantom is not suitable for CBCT calibration and that individual calibration curves obtained using phantoms of appropriate dimensions should be used for planning individual treatment sites.

An evaluation of techniques for dose calculation on cone beam computed tomography

“…3 Due to their acquisition arrangement, the three-dimensional (3D) CBCT images generated by OBIs are of inferior quality compared to the planning CTs (pCTs), generated by diagnostic CT scanners used to plan a patient's radiotherapy treatment. 4 However, there have been a number of studies investigating the accuracy of treatment planning dose calculations performed using CBCT images and potential methods to improve observed limitations in their calculation accuracy. [5][6][7][8][9][10][11] Dose calculations derived from anatomy outlined on CBCT have been used to determine more accurate estimates of the physical dose that patients actually receive during prostate radiotherapy treatments.…”

Applications of linac-mounted kilovoltage Cone-beam Computed Tomography in modern radiation therapy: A review

“…The results were compared with the conventional PCT as a reference. Although studies that investigated CBCT for treatment planning displayed good agreement with PCT of within 1–3% for phantoms and within 2% for prostate patients using deformable image registration, the level of accuracy breaks down with changes in patient/phantom size [44,45]. Thus, the results cannot be assumed to be the same for all volumes of scanned object and hence there is no uniformly accepted solution so far.…”

“…Thus, the results cannot be assumed to be the same for all volumes of scanned object and hence there is no uniformly accepted solution so far. This indicates the need for reliable HU calibration curves appropriate for a particular treatment site for planning in order to use CBCT potentially for ART [44]. This can be achieved by using calibration phantoms that closely match the size of the treatment site to be scanned (e.g.…”

“…Figure 6A shows concentric rings in a uniform water phantom, which impairs the diagnostic quality of the image by creating a dark smudge at the center of the image. Although the causes of these ring artifacts have been determined and methods to suppress them were developed [44–49], enhanced reconstruction methods will be required in the future to further reduce these artifacts. Figure 6B shows the corresponding reconstructed slice of a water phantom acquired from a 16-slice fan beam CT scanner (Philips Medical Systems, Cleveland, Ohio, USA) under similar scanning conditions for comparison purposes.…”

Systematic Parameterization, Storage, and Representation of Volumetric DICOM Data

“…2DPR can also be used for efficient image distribution between clinical departments to ensure that images can be viewed by physicians in other disciplines with the optimal settings. Approaches for parameterizing the representation of medical data have also been applied to radiotherapy [3], security [4], and other extensions [5]. …”