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We can conclude that the patients with HS mask have
larger displacements than the H mask patients, but further
investigations
should be
done.
All errors are below 3mm. This result agrees with the
literature for H&N displacements.
In summary, HexaPOD couch in combination with daily
CBCT can considerably improve the accuracy of patient
positioning during VMAT treatment for H&N treatments.
EP-1649 Comparison of two thermoplastic
immobilization shells for frameless stereotactic
radiotherapy
I. Gagne
1
, S. Zavgorodni
1
, A. Alexander
2
, I. Vallieres
2
1
BC Cancer Agency - Vancouver Island, Medical Physics,
Victoria, Canada
2
BC Cancer Agency - Vancouver Island, Radiation
Oncology, Victoria, Canada
Purpose or Objective
The primary goal of this study was to compare the inter-
fraction 6DOF corrections applied between patients
immobilized with an open-face shell system and patients
immobilized with a reinforced closed-shell system. The
intra-fraction motion was also compared between these
two groups of patients.
Material and Methods
Sixty patients have been treated with frameless
radiotherapy on a Varian TrueBeam STx linear accelerator
equipped with a six-degree of freedom (6DOF) couch. All
patients had a planning CT scan with an immobilization
system that comprised of a CIVCO head cup, a CIVCO
customizable pillow and a thermoplastic shell. An open-
faced shell from CIVCO was used on the first 15 patients
in anticipation of the ALIGN RT optical tracking system
installation. A closed-faced shell from AUBO with extra
reinforcement was employed on subsequent patients due
to delays in the approval of ALIGN RT in Canada and larger
than expected setup and intra-fraction motions with the
open-faced immobilization system. Two pre-treatment
CBCTs were acquired; the first to correct using 6DOF bone
anatomy matching the initial inter-fraction setup error,
the second to correct using 4DOF the residual setup error
following the 6DOF couch moves. A post CBCT was
acquired to determine the intra-fraction motion using
6DOF bone anatomy matching.
Results
Datasets from 12 patients with an open-faced
immobilization shell and 29 patients with a reinforced
closed-face immobilization shell were obtained for a total
of 2451 CBCT scans. Table 1 summarizes the population
average of the patient average and largest 6DOF
corrections (vrt, lng, lat, pitch, roll, rtn) from CBCT1 for
both the open-face and closed-face shells. Also included
is the population average of the average and largest intra-
fraction motion (vrt, lng,lat,pitch, roll, rtn) recorded from
CBCT3 for both the open-face and closed-face shells. A
student t-test for uneven sample and variance was applied
to determine which parameters had statistically
significant differences at the p=0.01 level. No statistically
significant differences were found between the two
patient populations when patient 6DOF correction
averages, however, statistically significant differences
were found between the two patient populations when the
patient largest rotational corrections were used in the
test. Statistically significant differences between the two
patient populations were also noted for the patient largest
6DOF pitch and roll intra-fraction motion as well as the
patient average 6DOF pitch intra-fraction motion.
Conclusion
Moving to a reinforced closed-face immobilization shell
from an open-faced immobilization system has
significantly reduced the magnitude of the rotational
corrections as well as significantly reduced the magnitude
of the pitch and roll motion errors.
EP-1650 Setup uncertainty in head and neck assessed
by a 1.5T MR-sim with thermoplastic mask
immobilization
Y.H. Zhou
1
, W.W.K. Fung
2
, J. Yuan
1
, O.L. Wong
1
, G.
Chiu
2
, K.F. Cheng
2
, K.Y. Cheung
1
, S.K. Yu
1
1
Hong Kong Sanatorium & Hospital, Medical Physics &
Research Department, Happy Valley, Hong Kong SAR
China
2
Hong Kong Sanatorium & Hospital, Department of
Radiotherapy, Happy Valley, Hong Kong SAR China
Purpose or Objective
This pilot study aims to prospectively assess the setup
uncertainty in head and neck (HN) using thermoplastic
mask immobilization on a dedicated 1.5T MR-sim for
radiotherapy based on a cohort of healthy volunteers.
Material and Methods
11 healthy volunteers received a total of 142 scans (each
scanned 4-40 times) on a 1.5T MR-sim in a treatment
position immobilized with customized 5-point
thermoplastic mask with shoulder fixation to simulate HN-
RT fractions. Volunteers were carefully aligned and
positioned by RT therapists using a 3D external laser
system. The imaging protocol consisted of a T1w SPACE
sequence (TR/TE = 420/7.2ms, FOV = 470mm, 256 coronal
slices, isotropic voxel size of 1.05mm, acquisition time
~5min, geometric distortion correction ON). Six Degree-
of-Freedom rigid body registration based on normalized
mutual information (sampling factor = 5%) were used to
pair-wisely register images with respect to the first session
for each subject. Systematic and random errors of
translation and rotation in positional setup were
calculated. One sample t-test and box-plot were used to
evaluate positional variation of translation and rotation
with a significance level of 0.05.
Results
The overall positional setup repeatability results were
presented in Table I. The group mean translation was
<1mm and mean rotation was <1º, respectively. The
systematic error in LR, AP and SI translation was 0.48mm,
0.23mm and 0.53mm, respectively. The systematic error
in roll, pitch and yaw rotation was 0.07°, 0.002° and
0.27°, respectively. The random error in corresponding
direction was 1.83mm, 0.62mm and 1.88mm for
translation, and 0.36°, 0.01° and 1.09° for rotation,
respectively. 20 (~14.0%), 0 (~0%), and 25 (~18.6%) out of
all 142 scan sessions had displacements >1mm in the LR,
AP, and SI directions, respectively. Displacements >2 mm
were seen only in the SI direction in one session (~0.7%).
One-sample t-test showed significance of group mean
error in AP translation (
p<
0.001) and all rotations (
p
= 0.02
in roll and
p
<0.001 in pitch and yaw). The box-plot of