TITLE:
A Sensitivity Dosimetry Study of the Setup Uncertainties during Machine Commissioning and Annual QA
AUTHORS:
Vi Nhan Nguyen, Brian Wang, Chengyu Shi
KEYWORDS:
Linac Commissioning, Annual QA, Gamma Analysis, TG 142, Flattening Filter and Flattening Filter Free Beams
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.5 No.4,
November
21,
2016
ABSTRACT: This
study is to investigate three common potential setup uncertainties during Linac
commissioning and annual QA and to evaluate how these uncertainties propagate
into the quality of beam profiles and patient dosimetry using gamma analysis. Three
uncertainty scenarios were purposely introduced for gantry position tilted from
0˚ - 3˚ (scenario 1), isocenter position misaligned from 0 - 6
mm (scenario 2) and SAD changed from 99.5 - 103 cm (scenario 3). A 60 × 60 × 60
cm3 water phantom cube was created to replicate a 3D
water tank in VarianEclipse (V.11) treatment planning system (Varian Medical
Systems, Palo Alto, CA). For each scenario, beam data profiles (crossline and
diagonal) and PDD curves were calculated at different field sizes and depths
for three energies: 6 MV, 6 MV-FFF and 10 MV-FFF. Gamma analysis method was
used to compare a total of 263 profiles to baseline using a 1%/1mm parameter
with 90% gamma passing rate criteria. For scenario 1, a ≥90% gamma passing rate
and ≤1% dose difference were seen on both crossline and diagonal profiles, and
PDD curves for gantry tilted up to 2˚. For 3˚ degree tilt, the
gamma passing rate decreased to ≤90% at depth of ≥20 cm for 6MV/6MV-FFF and
depth of ≥12 cm for 10MV-FFF. For scenario 2, a ≤90% gamma passing rate and ≥1%
dose difference were seen at depths from dmax to 20 cm
for all energies. For depths ≥20 cm, mostly ≥90% gamma passing rate and ≤1%
dose difference were seen. For scenario 3, a ≥90% gamma passing rate and ≤1%
dose difference were seen on ≤4 mm isocenter misalignments for all energies. In
summary, gamma analysis of the beam profiles is a very sensitive test for SAD
deviation scenarios and can reveal issues of sub millimeter setup uncertainty. However,
it is not as sensitive for isocenter misalignment scenarios. The test is also
more sensitive for FFF beams than flattening filter beams.