SU-E-T-291: Study of Reduction Technique for the Secondary Dose Due to CBCT in Image Guided Radiotherapy

Abstract

Purpose: To reduce the secondary dose by shielding the critical structures and to estimate the effect of the shielding for the secondary dose during the cone beam computed tomography (CBCT) scan in radiotherapy. Methods: The reduction in secondary doses during the CBCT was measured using glass dosimeter as a function of shielding material thickness. Lead and tin alloy with variable thickness was used as a shielding material which has a form of sheet. The CBCT scans were performed using pelvis mode (125kV, 80mA, 13ms) and two glass dosimeters in the phantom were placed 40 cm from the beam isocenter, 5cm from the central axis and 10cm above the couch. 30 scans of CBCT were assumed for image guided radiotherapy (IGRT) treatment. Results: The secondary dose of 3.62 cGy per 30 scans, which was measured without shielding material, was reduced to 0.065, 0.050, 0.030, 0.015 cGy per 30 scans due to 1, 3, 5, 7 mm thick shielding materials, respectively. The Result shows that about 98% of secondary dose can be shielded with 1 mm thick of lead and tin alloy sheet. Although the secondary doses were significantly reduced with the 1mm thick of lead and tin alloy, the reduction rate was decreased as the thickness is increased showing that only 1% of additional secondary dose reduction can be achieved with 5 mm thick of lead and tin alloy sheet. Conclusion: This study suggests that secondary dose due to the CBCT is not negligible compared to secondary dose due to radiation treatment and should be reduced if there is a critical structure or side effect is expected. Our results shows that only few mm thick lead tin alloy can effectively reduce the secondary dose.