Assessment of imaging radiation dose from tomotherapy systems

Background: Tomotherapy systems employ image-guided radiotherapy (IGRT) using either megavoltage (MVCT) or kilovoltage (kVCT) computed tomography for daily patient positioning verification. However, these imaging procedures can deliver a non-negligible radiation dose to the patient. According to the American Association of Physicists in Medicine (AAPM) Task Group 180, this dose should be incorporated into the prescribed treatment dose if it exceeds 5%. Objectives: This study aimed to assess IGRT imaging doses and determine whether the cumulative dose exceeded 5% of the prescribed treatment dose. Materials and methods: Imaging doses from IGRT were evaluated under clinical scan protocols across four tomotherapy machines at three centers, including two MVCT systems and two kVCT systems. Doses were measured as Weighted Multiple Slice Average Dose (MSADw) using a CTDI phantom. Cumulative imaging dose was calculated by multiplying MSADw by the number of imaging sessions, then compared to the prescribed dose for each patient. Results: The mean MSADw values for MVCT ranged from 0.85 to 0.86 cGy per scan, while for kVCT it ranged from 0.19 to 2.65 cGy per scan. Over the treatment course, cumulative imaging doses accounted for 0.34-0.44% of the prescribed dose for MVCT and 0.28-0.87% for kVCT. Notable inter-center variations were observed, with differences of up to 51.16% in breast cancer cases and 35.71% in prostate cancer cases. Conclusion: Cumulative imaging doses from both MVCT and kVCT across all centers remained below the 5% threshold, indicating no need to adjust the prescribed dose in clinical practice. Nevertheless, further studies are required to identify the causes of inter-center variations and to develop optimized imaging protocols aimed at minimizing patient dose while maintaining adequate image quality.