Program Information
Evaluation of a Hybrid (optical and KV) Pre-Treatment Setup Protocol
S Oh*, L Padilla , S Kim , J Palta , M Rosu , Virginia Commonwealth University, Richmond, VA
Presentations
MO-RPM-GePD-J(A)-1 (Monday, July 31, 2017) 3:45 PM - 4:15 PM Room: Joint Imaging-Therapy ePoster Lounge - A
Purpose: Evaluate the following multistep hybrid (optical and 2D/3D kV) SBRT setup protocol, with correction of all shifts without minimal action levels: (1) tattoo setup (S0), (2) AlignRT surface-based alignment (S1), (3) 2D kV residual bony alignment (S2), (4) 3D kV residual tumor alignment (S3).
Methods: 80 imaging sessions from 17 patients [lung(10), liver(4), spleen(1), spine(1), pelvis(1)] were audited. AlignRT alignment was considered for consistent and more accurate overall body alignment (eg. placement of arms on cushions) and more straightforward correction for rotations. The following inter-fraction quantities were evaluated, along vertical, longitudinal and lateral axes for the entire population, and for the ABC/non-ABC subgroups: mean shifts (M), systematic errors (Σ) and random errors (σ). The required margin in the absence of the alignment protocol was also calculated using the conservative formula by van Herk. These analyses were performed for the S0→S1 (initial setup shifts), S1→S2 (residual shifts after 2D kV), S2→S3 (residual shifts after 3D kV), S0→S3 (overall shift per protocol).
Results: Population metrics (Vert,Long,Lat): S0→S1: M(2.9,1.3,2.0)mm, Σ(7.0,5.7,6.4)mm, σ(5.8,6.3,4.3)mm; S1→S2: M(-0.7,-1.0,0.5)mm, Σ(2.0,2.6,2.6)mm, σ(3.3,4.3,3.9)mm; S2→S3: M(-0.7,0.0,0.2)mm, Σ(1.9,2.9,2.0)mm, σ(2.1,2.8,2.9)mm; S0→S3: M(1.4,0.4,3.2)mm, Σ(7.9,5.9,7.9)mm, σ(2.7,2.9,4.5)mm. Larger shifts were noted in the ABC subgroup, owing to free-breathing tattoo setup, with the reference dataset acquired with ABC in place. The corresponding setup margins in the absence of the setup protocol would be (21.8, 16.8, 22.9)mm.
Conclusion: As expected, the protocol obviates the need for large PTV setup margins. The process is long enough that there is an increased likelihood that intra-fraction setup uncertainties are blended into the residual shifts. The initial AlignRT setup corrects substantially the tattoo positioning shifts; margins up to 10 mm would be required to account for the residual uncertainties after AlignRT setup, if additional imaging doses and/or the uncertainties associated with the execution of 2D and/or 3D kV are to be minimized.
Contact Email: