Application of low-dose CT in image-guided radiotherapy based on CT-linac

Background: Image-guided radiation therapy (IGRT) involves online medical imaging systems. CT-guided imaging systems not only have high image quality but also allow for direct dose calculation for radiation treatment planning, making it an ideal image guidance method. However, the imaging dose from CT images may pose additional risks to patients. Therefore, the aim of this study was to evaluate the substitutability of using low-dose CT images for IGRT. Materials and methods: The CIRS-062 phantom, Catphan 604 phantom, CT dose index phantom were used to obtain the conversion curves of the CT values to the relative electron density (RED), image quality, and imaging dose of the low-dose CT, respectively. Catphan 604 phantom and clinical cases were used to analyze the feasibility of low-dose CT for dose calculation. Results: No significant differences were observed in the conversion curves of CT values to RED between low-dose and standard-dose CT scans. In terms of image quality, the geometric distortion of low-dose CT was 0.1 mm, which was the same as that of standard-dose CT. The spatial resolution of the images was 0.42-0.45 lp/mm, which was slightly better than that of standard-dose CT (0.40-0.44 lp/mm). The image uniformity was 98.4%- 99.0%, slightly worse than that of standard-dose CT (99.4%-99.5%), but still met the clinical requirements. The imaging dose was 2.96-4.62 mGy, significantly lower than that of standard-dose CT (9.71-26.68 mGy), especially for head CT protocol. In terms of dose calculation, the gamma passing rates were not lower than 98% for either the Catphan 604 phantom or the clinical cases. Conclusion: Low-dose CT can significantly reduce the imaging dose without a significant loss of image quality and can be directly used for dose calculation in radiation treatment planning, providing a safer and more effective image guidance method for IGRT.