  Collimator angle is usually rotated when planning volumetric modulated arc
therapy (VMAT) due to the leakage of radiation between multi-leaf collimator
(MLC) leaves. We studied the effect of the collimator angles on the results of
dosimetric verification of the VMAT plans for head and neck patients. We
studied VMAT plans for 10 head and neck patients. We made 2 sets of VMAT plans
for each patient. Each set was composed of 10 plans with collimator angles of
0, 5, 10, 15, 20, 25, 30, 35, 40, 45 degrees. Plans in the first set were
optimized individually and plans in the second set shared the 30 degree
collimator angle optimization. Two sets of plans were verified using the
2-dimensional ion chamber array MatriXX (IBA Dosimetry, Germany). The
comparison between the calculation and measurements were made by the
$\gamma$-index analysis. The $\gamma$-index (2\%/2 mm) and (3\%/3 mm) passing
rates had negative correlations with the collimator angle. Maximum difference
between $\gamma$-index (3\%/3 mm) passing rates of different collimator angles
for each patient ranged from 1.46\% to 5.60\% with an average of 3.67\%. There
were significant differences (maximum 5.6\%) in the passing rates of different
collimator angles. The results suggested that the accuracy of the delivered
dose depends on the collimator angle. These findings are informative when
choosing a collimator angle in VMAT plans.
