Description
Radiation therapy treatment planning for patients with dynamic tumors, such as lung cancer, is often performed using four-dimensional computed tomography (4DCT) due to the presence of breathing respiratory motion. All potential locations of the tumor during its motion create the internal target volume (ITV), and 4DCT is used to define it. Since the method of treating the ITV is designed during the treatment planning stage, the correct positioning of the patient and the localization of the ITV, on the day of treatment, is critical in ensuring an effective treatment. In practice, most patients have breathing patterns where its amplitude, period, and inhalation-to-exhalation (I/E) ratio vary throughout time hence the breathing pattern during the 4DCT could differ from the breathing pattern on the day of treatment. That increases the uncertainty in the defined ITV which leads to possible inaccuracies in patient positioning and radiation beam setup. Currently, in-room cone-beam computed tomography (CBCT) is used immediately before treatment, due to the patient's inconsistent breathing, to define and localize the ITV. If any differences exist between the 4DCT and CBCT images, the treatment plan is updated. Unfortunately, CBCT provides images of mediocre quality when scanning dynamic targets, images which include dynamic artifacts. That flaw leads to further uncertainty within the defined ITV. Four-dimensional (4D) CBCT has been created and has been shown to be as effective as 4DCT, but is not in practice due to the needed increase in CT projections which will lead to an increase in the patient imaging dose if current high-dose settings are unchanged. Most clinics set the CBCT scans to its highest-dose settings (125kVp, 80mA, 25ms/frame) due to it providing the best image-quality when compared to images using lower-dose settings. In this work, low-dose CBCT parameters (110kVp, 20mA, 20ms/frame) were used to acquire images of a 3cm diameter sphere moving in accordance to 30 simulated sinusoidal breathing patterns. The low-dose CBCT images were quantitatively/qualitatively compared with the high-dose CBCT images to see if using low-dose CBCT parameters would have a significant impact on the defined ITV. In the quantitative analysis, the greatest percent reduction in ITV-contrast when going from high-dose to low-dose was found to be 10.1052% for a breathing pattern with I/E ratio of 0.2131. The overall percent change of the ITV-contrast, going from high-dose to low-dose images, was 5.2047±6.490%. In the qualitative analysis, when using the true ITV as reference with respect to the defined ITV in the low-dose and high-dose CBCT images, the ITV underestimation was found to vary by 5.2237%-11.573% for 1cm amplitude and 9.2285%-34.776% for 3cm amplitude. The percent difference between the defined ITV of the high-dose and low-dose CBCT images was 0.5220±0.5398%. Due to the insignificant difference between the defined ITV of the high-dose and low-dose images, acquiring more projections using low-dose CBCT parameters to apply 4DCBCT scans is clinically viable.
