Patient characteristics
This retrospective study was reviewed and approved by HKSH Medical Group Research Committee (RC-2021-24). Our institution has been implementing ClearRT for clinical use since it was installed on a Radixact system (v3.0.0.10) at the end of June 2021. A total of 23 patients treated between June and August 2021 were retrospectively enrolled in this study. Each patient underwent at least one MVCT scan and one kVCT scan during the treatment course. Only one single scan per fraction (MVCT or kVCT) was performed for ethical reasons. Among them, tumors in the head (6), thorax (9) and pelvis (8) regions were treated. No patient had metal implants or prostheses.
Workflow performance metrics
The kVCT and MVCT were compared by examining scanning time, nominal dose length product (DLP), registration time and translational (IEC X, Y and Z) and rotational (IEC Roll only) corrections in patient positioning. MVCT and kVCT data were extracted from two consecutive fractions in order to minimize the inter-fractional anatomic change, and the same radiation therapist performed the registration on both fractions. Although an iterative MVCT reconstruction algorithm was introduced at the launch of the Radixact system, the Standard reconstruction algorithm was adopted in this study for a general comparison. Key parameters of the most used kVCT and MVCT imaging protocols in our institution are summarized in Table 1. Unlike the data in Velten et al. (2022) that mainly focused on Fine and Coarse mode, our institution preferred Normal mode for the clinical workflow. Wilcoxon signed-rank tests were performed to determine whether significant differences existed between the two cohorts.
We also conducted a survey. Six experienced radiation therapists were invited to score the performance of kVCT and MVCT (where a score of 1 represented the worst performance, and 5 indicated the best performance) regarding signal-to-noise ratio (SNR), the frequency and extent of artifacts including beam hardening, motion, aliasing, ring, scatter artifacts (Schulze et al., 2011), tissue differentiation and overall experience of the image registration.
Table 1
Key parameters of the most used kVCT and MVCT imaging protocols in our institution.
ClearRT helical kVCT | FOV (mm) | Slice Thickness (mm) | CTDIvol* (cGy) |
Anatomy | Body Size | mA | kV | Mode# |
Head | Small | 80 | 100 | Normal | 440 | 2.4 | 0.8 |
Thorax | Medium | 125 | 120 | Normal | 440 | 3.6 | 0.9 |
Pelvis | Medium | 125 | 140 | Normal | 440 | 3.6 | 1.3 |
MVCT | | | |
Nominal energy | Pitch# | | | |
3.5 MV | Normal | 400 | 2 | 1.3 |
* Measurements were conducted by the vendor in a 32 cm diameter Polymethyl methacrylate (PMMA) CTDI phantom representative of an adult body and a 16 cm diameter PMMA CTDI phantom representative of a head, neck region, or pediatric patient. |
# The nominal pitch is 0.75 and 2 for kVCT and MVCT with Normal mode, respectively. |
Image performance characterization
The CTP515 low-contrast module in a Catphan® 504 phantom (The Phantom Laboratory, Greenwich, NY), which provided nominal target contrast levels of 0.3%, 0.5% and 1.0%, was scanned to check the visualization of low-contrast targets. The discernible circle with the smallest diameter was recorded.
Image uniformity and noise were evaluated by scanning the uniform region of a 30 cm diameter cylindrical TomoTherapy Phantom HE (‘Cheese Phantom’, Sun Nuclear, Melbourne, FL). Image uniformity is defined as the maximum CT number difference (in Hounsfield units) between the peripheral and central ROIs (20 cm2). Noise is expressed as the pixels’ standard deviation (SD) in a large central ROI (400 cm2).
Geometrical distortion was evaluated by measuring the distances between the fiducial markers inside the Cheese Phantom. The kV to MV alignment accuracy was assessed by comparing the IEC X, Y and Z offsets of kVCT and MVCT after manually aligning a dedicated fiducial with the laser center during image registration.
In addition, spatial resolution was evaluated by scanning a high contrast resolution plug inside the Cheese Phantom, which has seven sets of five holes with a diameter ranging from 2 to 0.8 mm. The diameter of the smallest pinholes visible in the image, with each hole distinguishable from neighboring holes, represented spatial resolution.
To benchmark and compare the imaging dose, the multiple scan average dose (MSAD) was measured for each imaging protocol using a calibrated A1SL ion chamber (Standard Imaging, Middleton, WI) placed at the center of the Cheese Phantom with a scan range covering the whole phantom.
Another phantom with different density plugs (CIRS, Norfolk, VA) was scanned to create the CT number versus electron density calibration curves and evaluate the SNR and the contrast-to-noise ratio (CNR) for all protocols. SNR is defined as the mean divided by the SD of the pixels in a small ROI (2 cm2), and CNR is calculated by dividing the pixel difference between soft-tissue and water ROIs with the SD of water ROI.
All the phantoms mentioned above and examples of their kVCT images are shown in Fig. 1.
CT number stability and its dosimetric impact
The CT number stability was monitored by scanning the CIRS phantom weekly with different imaging protocols for over two months. The routine quality assurance (QA) of CT number calibration (water and air only) was also performed weekly using the Cheese Phantom as recommended by the vendor. In order to quantify the dosimetric impact of the CT number variation, three clinical plans with treatments sites in the head & neck, thorax and pelvis regions were recalculated using the upper and lower range of CT calibration curves collected over time. Dose metrics of these plans were recorded and compared after that.
Dosimetric evaluation of planning with kVCT
For further exploring the possibility of dose assessment and plan adaptation using kVCT, as illustrated in Fig. 2, six clinical plans were created based on images of a RANDO phantom (Alderson Research Laboratories, Stanford, CT) scanned with a CT-simulator (Siemens Somatom Confidence). Treatments sites were located in the brain, nasopharynx, lower neck, lung, T-spine, and pelvis. The prescription of all plans was 60 Gy to 95% of the planning target volume (PTV) in 30 fractions, and plan criteria followed institutional dosimetric protocols. Especially, PTVs were contoured in regions where kVCT artifacts might occur for the lower neck, T-spine, and brain plans. The same RANDO phantom was scanned on the treatment position using different kVCT imaging protocols (Table 1). A dose recalculation and dose metrics comparison were carried out using the original plans as a reference after these kVCT images were imported into the treatment planning system.