The combined use of 2D scout and 3D axial CT images to accurately determine the catheter tips for high‐dose‐rate brachytherapy plans

Abstract Purpose To develop a method combining CT scout images with axial images to improve the localization accuracy of catheter tips in high‐dose‐rate (HDR) brachytherapy treatments. Materials and Methods CT scout images were utilized along with conventionally reconstructed axial images to aid the localization of catheter tips used during HDR treatment planning. A method was developed to take advantage of the finer image resolution of the scout images to more precisely identify the tip coordinates. The accuracies of this method were compared with the conventional method based on the axial CT images alone, for various slice thicknesses, in a computed tomography dose index (CTDI) head phantom. A clinical case which involved multiple interstitial catheters was also selected for the evaluation of this method. Locations of the catheter tips were reconstructed with the conventional CT‐based method and this newly developed method, respectively. Location coordinates obtained via both methods were quantitatively compared. Results Combination of the scout and axial CT images improved the accuracy of identification and reconstruction of catheter tips along the longitudinal direction (i.e., head‐to‐foot direction, more or less parallel to the catheter tracks), compared to relying on the axial CT images alone. The degree of improvement was dependent on CT slice thickness. For the clinical patient case, the coordinate differences of the reconstructed catheter tips were 2.6 mm ± 0.9 mm in the head‐to‐foot direction, 0.4 mm ± 0.2 mm in the left‐to‐right direction, and 0.6 mm ± 0.2 mm in the anterior‐to‐posterior direction, respectively. Conclusion Combining CT scout and axial images demonstrates the ability to provide a more accurate identification and reconstruction of the interstitial catheter tips for HDR brachytherapy treatment, especially in the longitudinal direction. The method developed in this work has the potential to be implemented clinically together with automatic segmentation in modern brachytherapy treatment planning systems, in order to improve the reconstruction accuracy of HDR catheters.

of the catheters, especially their tips, which can often be difficult to identify on images, directly affects the determination of dwell positions of HDR source in the plan, which subsequently influences the accuracy of dose delivery and thus may potentially lead to negative impacts on the treatment outcomes. 3 Currently, identification of the implanted catheters in the scope of patient anatomy primarily relies on the MSCT images. 4 However, partial volume effects, which are caused by the finite thickness of CT slices and the finite size of the target, 5 are a common issue in MSCT images. This partial volume effect may compromise the reconstruction accuracy of the HDR catheters from the CT images, especially for the catheter tips, if the reconstruction relies on the axial CT images alone. Moreover, the magnitude of the effect can be highly dependent on the type of reconstruction algorithm and kernel used by the commercial CT vendor. 6,7 Conversely, as a result of a completely different processing scheme, CT scout images have a much higher image resolution in the longitudinal direction, and thus are not affected by the partial volume effect. Previous works, such as that of Yue et. al., 8 have demonstrated that by combining orthogonal scout CT images along with the axial images, the radioactive seeds used in low-dose-rate (LDR) brachytherapy can be clearly distinguished and the partial source artifact can be reduced in CT-based brachytherapy planning.
The purpose of this work is to further that method into the identification and reconstruction of catheters/needles in HDR brachytherapy treatment and to evaluate whether scout images help to improve the accuracy of the localization especially that of the catheter tips or ends. The accuracies of catheter positions obtained from the developed method were evaluated with a phantom study, as well as with a clinical case. Since both needles and catheters are frequently used in HDR interstitial brachytherapy, both terms will be interchangeably used for the same purpose of description and illustration in the following sections.

2.A | Phantom study
In order to best replicate the clinical scenario, a 17-gauge 25-cm stainless hollow steel (Alpha Omega Services, Bellflower, CA) needle was inserted into tissue-equivalent bolus and imaged in a CTDI head phantom (Sun Nuclear, Melbourne, FL) with a GE LightSpeed 16 CT simulator (GE Healthcare, Chicago, IL). A photograph demonstrating the setup of the needles in a CTDI head phantom is shown in Fig. 1.

2.C | Clinical case study
The images of one clinical patient (age 33, female, primary cancer of the cervix) were selected for this study. This patient received brachytherapy boost HDR treatment (30 Gy in 5 fractions) with a Syed interstitial applicator (tandem, cylinder and 16 needles). The treatment was planned based on axial CT images. As part of routine treatment planning, the patient underwent orthogonal CT scout scans and an MSCT scan for pretreatment simulation after needle insertion. The scout scans included an AP scout acquired with 140 kVp and 10 mAs, and a lateral scout (90 degree) with 140 kVp and 40 mAs. The parameters of the CT scan protocol were 1.25 mm slice thickness, 140 kVp, 470 mAs, and 50 cm of FOV.

2.D | Quantification of the needle positions
The coordinates of a point determined from the scout images and its coordinates determined from the corresponding MSCT images are associated, and the association is briefly described as follows. 8 On the scout images, in the plane perpendicular to longitudinal direction (head to foot), there is a magnification factor. Assuming the X axis is defined as the horizontal direction (left to right), Y axis is defined as the vertical direction (anterior to posterior), and Z axis is defined as the longitudinal direction (head to foot), the coordinates of a point on the scout image (X ap, scout , Y lat, scout , Z scout ) are related to its coordinates on the corresponding CT image (X CT , Y CT , Z CT ) as SAD is the distance from CT source to the isocenter of the scan-   (2)] provides a way to calculate the projection for this arbitrary angle scout. Actually, [Eq. (1)] represents special cases when this θ equals 90, corresponding to AP scout and θ equals 0, corresponding to lateral scout. X θ,scout ¼ ½Y CT cosðθÞ À X CT sinðθÞ SAD SAD À Y CT sinðθÞ À X CT cosðθÞ Z θ,scout ¼ Z CT (2) In conclusion, with this proof-of-concept study, we have demon- Study design, data acquisition, revision of manuscript.