Assessment of image quality and dose calculation accuracy on kV CBCT, MV CBCT, and MV CT images for urgent palliative radiotherapy treatments

Abstract

A clinical workflow was developed for urgent palliative radiotherapy treatments that integrates patient simulation, planning, quality assurance, and treatment in one 30-minute session. This has been successfully tested and implemented clinically on a linac with MV CBCT capabilities. To make this approach available to all clin-ics equipped with common imaging systems, dose calculation accuracy based on treatment sites was assessed for other imaging units. We evaluated the feasibility of palliative treatment planning using on-board imaging with respect to image quality and technical challenges. The purpose was to test multiple systems using their commercial setup, disregarding any additional in-house development. kV CT, kV CBCT, MV CBCT, and MV CT images of water and anthropomorphic phantoms were acquired on five different imaging units (Philips MX8000 CT Scanner, and Varian TrueBeam, Elekta VersaHD, Siemens Artiste, and Accuray Tomotherapy linacs). Image quality (noise, contrast, uniformity, spatial resolution) was evaluated and compared across all machines. Using individual image value to density calibrations, dose calculation accuracies for simple treatment plans were assessed for the same phantom images. Finally, image artifacts on clinical patient images were evaluated and compared among the machines. Image contrast to visualize bony anatomy was sufficient on all machines. Despite a high noise level and low contrast, MV CT images provided the most accurate treatment plans relative to kV CT-based planning. Spatial resolution was poorest for MV CBCT, but did not limit the visualization of small anatomical structures. A comparison of treatment plans showed that monitor units calculated based on a prescription point were within 5% difference relative to kV CT-based plans for all machines and all studied treatment sites (brain, neck, and pelvis). Local dose differences > 5% were found near the phantom edges. The gamma index for 3%/3 mm criteria was ≥ 95% in most cases. Best dose calculation results were obtained when the treatment isocenter was near the image isocenter for all machines. A large field of view and immediate image export to the treatment planning system were essential for a smooth workflow and were not provided on all devices. Based on this phantom study, image quality of the studied kV CBCT, MV CBCT, and MV CT on-board imaging devices was sufficient for treatment planning in all tested cases. Treatment plans provided dose calculation accuracies within an acceptable range for simple, urgently planned palliative treatments. However, dose calculation accuracy was compromised towards the edges of an image. Feasibility for clinical implementation should be assessed separately and may be complicated by machine specific features. Image artifacts in patient images and the effect on dose calculation accuracy should be assessed in a separate, machine-specific study.

Figure 1

Outline of the workflow for urgent radiotherapy treatments. Approximate times per steps are indicated above each section.

Figure 2

IVDC curves for kV CT and on‐board imaging systems. The VersaHD required two separate curves for rFOV and eFOV imaging protocols. The Artiste required three protocols, one for rFOV and two for eFOV.

Figure 3

Density profiles for the cylindrical and the pelvic water phantom. (left) CT image slice of the water phantoms. The red line indicates the path of the profile. (right) Density profiles for the acquired CT on each imaging system. The HU values are converted to density values using the machine‐specific IVDC. (*The air bubble in the cylindrical phantom was removed before image acquisition on the VersaHD.)

Figure 4

Dose difference maps. Percentage difference of simple dose plans based on kV CBCT, MV CBCT, and MV CT images compared to diagnostic kV CT‐based images. The left column bar shows the dose distribution within the CT image as percentage of the maximum dose. The columns to the right show the relative percentage differences where green is no difference, blue is underdosing, and red is overdosing on the linac's on‐board images.

Figure 5

CT images of the phantom used in Setup 3, 4, and 5.

Figure 6

Patient CT images of different treatment sites. The images were acquired using the indicated machines’ on‐board imaging system.