SP-0029 EXPLOITING HYPOXIA DEPENDENT DNA REPAIR DEFECTS

Abstract

model for MC CBCT reconstruction. The accuracy of the MC CBCT based correction strategy was first quantified using a 4D phantom. For each fraction, a first MC CBCT is acquired to align the target to the planned position, a second scan to verify this correction and two additional scans are acquired concurrent with VMAT delivery to monitor intra-fraction motion. The first scan of each fraction was reconstructed as a 4D MC CBCT to assess differences between the prior motion model and actual patient motion. Subsequent scans were 3D MC CBCT if the residual target motion amplitude ≤ 8 mm and otherwise regular 4D CBCT were used. Results: Phantom experiments demonstrated alignment accuracy better then 1 mm/1° compared to 4D CBCT and a residual amplitude on 4D MC CBCT ≤ 2 mm. Six patients have been evaluated so far using MC CBCT protocol since it was clinically implemented in October 2011 on one of our treatment machines. The mean (SD) tumour amplitude derived from the 4D planning CT was 2 (2) mm (LR), 5 (4) mm (CC) and 5 (3) mm (AP). A total of 25 fractions were analyzed. In one patient, the residual tumour amplitude exceeded 8mm; so we reverted to a 4DCBCT guidance protocol. The residual tumour amplitude derived from the first scan of each fraction using 4D MCCBCT scans for the remaining patients was 1.4 (0.6) mm (LR), 1.6 (0.7) mm (CC) and 1.7 (0.5) mm (AP), showing validaty of the motion compensation (figure 1).