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In Situ Verification of Radiation Therapy Dose Distributions From High-Energy X-Rays Using PET Imaging

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Q Zhang

Q Zhang1*, L Kai2 , X Wang2 , B Hua2 , L Chui2 , Q Wang2 , C Ma3 , (1) Wu Xi Yi Ren Tumor Hosiptal, Wuxi, Jiangsu, (2) ChangAn Hospital, Xian, Shaanxi, (3) Fox Chase Cancer Center, Philadelphia, PA


SU-D-BRF-2 Sunday 2:05PM - 3:00PM Room: Ballroom F

Purpose: To study the possibility of in situ verification of radiation therapy dose distributions using PET imaging based on the activity distribution of 11C and 15O produced via photonuclear reactions in patient irradiated by 45MV x-rays.

Methods: The method is based on the photonuclear reactions in the most elemental composition ¹²C and ¹⁶O in body tissues irradiated by bremsstrahlung photons with energies up to 45 MeV, resulting primarily in ¹¹C and ¹⁵O, which are positron-emitting nuclei. The induced positron activity distributions were obtained with a PET scanner in the same room of a LA45 accelerator (Top Grade Medical, Beijing, China). The experiments were performed with a brain phantom using realistic treatment plans. The phantom was scanned at 20min and 2-5min after irradiation for ¹¹C and ¹⁵, respectively. The interval between the two scans was 20 minutes. The activity distributions of ¹¹C and ¹⁵O within the irradiated volume can be separated from each other because the half-life is 20min and 2min for ¹¹C and ¹⁵O, respectively. Three x-ray energies were used including 10MV, 25MV and 45MV. The radiation dose ranged from 1.0Gy to 10.0Gy per treatment.

Results: It was confirmed that no activity was detected at 10 MV beam energy, which was far below the energy threshold for photonuclear reactions. At 25 MV x-ray activity distribution images were observed on PET, which needed much higher radiation dose in order to obtain good quality. For 45 MV photon beams, good quality activation images were obtained with 2-3Gy radiation dose, which is the typical daily dose for radiation therapy.

Conclusion: The activity distribution of ¹⁵O and ¹¹C could be used to derive the dose distribution of 45MV x-rays at the regular daily dose level. This method can potentially be used to verify in situ dose distributions of patients treated on the LA45 accelerator.

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