Room: ePoster Forums
Purpose: To test the efficacy of Anatomy-Based Shift (ABS) by use of the conformity index (CI) of target between target and prescribed dose cloud.
Methods: Cone beam CT (CBCT) sets of ten head and neck (H&N) patients were imported and contoured in the RayStation treatment planning system (TPS: RaySearch Laboratories AB, Sweden, V. 18.104.22.168) using deformable image registration to the planning CT. The isocenters for dose calculation have been 3-D (translational) shifted with 1 mm increments up to 3 mm for each coordinate from the original setup isocenter, which resulted in total 342 shifted positions for each CBCT. After this procedure, PTV and OAR dose were calculated with initial plan. From the outputs from this procedure, CI was determined from all shifted positions for each CBCT sets. Among these, each CBCT sets with the best CI with higher 95% of prescribed target dose (D95) than the original CBCT positions, while not compromising the OAR doses were selected. These selected CBCTs images were shifted again allowing 6-D (six degrees of freedom) shifts, and then compared target and OAR doses to 4-D shift.
Results: Total 73 CBCTs of 10 patients were used to calculate CIâ€™s of different isocenters. Eleven CBCTs showed higher D95 values (average: 0.87%, Ïƒ=1.08%). However, only 5 of them (from 2 patients) higher than 0.5% (average: 1.77%, Ïƒ=1.03%). These CBCTs were calculated again after manual 6D shift. Average PTV D95 improvement with 6-D shift than that of 3-D was 7.45% (Ïƒ=12.27%).
Conclusion: The ABS provided improved dose coverage for H&N cases. 6-D shift enhances even more for some cases.