Abstract Purpose: To propose and establish a practical institutional imaging dose guideline by estimating organ and effective doses from a standard and large pelvis CBCT protocol. Methods: CBCT imaging was performed using the on-board imaging system of the TrueBeam linear accelerator. CTDIvol and dose length product (DLP) values for standard pelvis and large pelvis protocols were recorded to estimate organ doses to the rectum, bladder and bowel using published conversion coefficients. The effective dose was calculated using region-specific factors recommended by the International Commission on Radiological Protection (ICRP Publication 103). Results: CTDIvol for the standard and large pelvis protocols were 15.98mGy and 36.7mGy, with corresponding DLP values of 345.3mGy·cm and 794.6mGy·cm. For a 25-fraction treatment, cumulative organ doses for the standard pelvis protocol were 0.48Gy(rectum), 0.44Gy(bladder), and 0.52Gy(bowel), while the large pelvis protocol resulted in 1.10Gy, 1.01Gy, and 1.20Gy, respectively. Although CBCT dose seem negligible, repeated imaging does increase cumulative exposure. Additional CBCTs were performed in approximately 40% throughout the treatment course due to anatomical variation, increasing the cumulative organ doses to approximately 0.7 Gy for the standard pelvis protocol and 1.67 Gy for the large pelvis protocol. This additional dose becomes relevant when the therapeutic OAR doses approach established constraints. Moreover, the cumulative effective dose was 164.02mSv for the standard pelvis protocol and 377.43mSv for the large pelvis protocol. Based on the BEIR VII Phase 2 Report, the cumulative effective doses of 164.02mSv and 377.43mSv correspond to excess lifetime cancer risks of approximately 0.82% and 1.89%, respectively. Conclusion: CBCT is essential for accurate IGRT but contributes additional dose. Although limiting one CBCT per fraction is considered the gold standard, additional imaging should be clinically justified and imaging protocols should be carefully selected. CBCT imaging frequency should adhere to the ALARA principle and be incorporated into imaging guidelines.
Koh et al. (Thu,) studied this question.