SU‐E‐J‐08: Dependence of Imaging Dose on Image Quality of Free‐Breathing 3DCBCT of Moving Tumors

Purpose: To evaluate the impact on free‐breathing CBCT (FBCBCT) image quality to properly visualize the motion range of moving tumors as a function of imaging dose. Methods: A multi‐purpose body phantom (QUASAR) with a cylindrical cedar wood (density = 0.330g/cc), and an embedded 3‐cm diameter Polystyrene sphere (density = 0.855g/cc) were used to simulate lung tumor motion. Varian Trilogy with OBI system was used to acquire CBCT images (high‐dose mode: 125kVp, 80mA, 25ms/frame & low‐dose mode: 110kVp, 20mA, 20ms/frame). As the FBCBCT projections were acquired, the sphere moved in accordance to 30 simulated sinusoidal patient breathing patterns using a programmable motion platform, which were given the parameters: inhalation‐to‐exhalation (I/E) ratio ranging from 1‐0.2131, amplitudes of 1 and 3 cm, and periods 2, 4, and 6 seconds. Following the acquisition of FBCBCT images, the ITV contrast, defined as = (target pixel values inside the sphere ‐ avg. pixel values in background)/(avg. pixel values in background), were calculated per image slice. Results: All parameters, I/E ratio, period, and amplitude did not seem to have much impact on the percentage change of the ITV contrast as a function of imaging dose. The percentage‐change for all coronal images with a reduced ITV contrast when going from high‐dose to low‐dose was ‐ 4.61 ± 3.04%, while the percentage‐change for all coronal images with an ncreased ITV contrast when going from high‐dose to low‐dose, was 8.19 ± 3.61%. The overall percentage‐change of all 30 coronal images was 5.21 ± 6.49%. Conclusions: We found that imaging dose did not have much impact on the visibility of the ITV volume, irrespective of the amplitude, I/E ratio, or period. Thus, it seems that low‐dose FBCBCT may be just as suitable for clinical use while sparing a significant imaging dose to the patients.