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S980
ESTRO 36 2017
_______________________________________________________________________________________________
K. Koiwai
1
, A. Shinoda
1
, T. Ozawa
1
, H. Matsushita
1
, A.
Fukazawa
1
, K. Sakai
1
, M. Kadoya
1
1
Shinshu University School of Med., Radiology,
Matsumoto, Japan
Purpose or Objective
In radiotherapy for head and neck cancers, postural
reproducibility is sometimes deteriorated due to the non-
rigidity of the head and neck. Image-guided corrections
cannot completely compensate for the deterioration. We
evaluated whether aligning the chest with a treatment
couch just before immobilisation improved the postural
reproducibility in radiotherapy for head and neck cancers.
Material and Methods
Fifty patients with head and neck cancers who underwent
radiotherapy from February 2015 to January 2016 were
selected for this study. Twenty five patients were treated
with the “aligning-the-chest” method, and others not.
When we acquired planning CT images of the patients with
the aligning-the-chest method, we drew marks on the skin
of their anterior chest in line with the fixed marks on the
both sides of the couch of the CT scanner which were
geometrically the same as on the treatment couch. On the
set-up for the treatment, the marks on their chest were
aligned with the fixed marks on the both sides of the
treatment couch just before immobilisation. For all
patients, 2 oblique X-ray images (image 1 and image 2)
were obtained on the first session of the treatment. We
generated digital reconstructed radiographies (DRRs)
representing an ideal posture from the planning CT images
with the maximal similarity to the corresponding X-ray
images with 6 degree-of-freedom registration. On the both
X-ray images and DRRs, we measured the mandibuler-
spinal angle that was defined as the angle formed by the
line projected parallel to the base of the mandible and the
line projected parallel to the alignment of the lower
cervical spine. As an index for postural reproducibility, the
difference of the mandibuler-spinal angle between the X-
ray image and the DRR (DMSA) was calculated in each
patient. The raw and absolute DMSA were analysed.
Results
In the mean value of the raw DMSA, there was no
difference between the patients with the aligning-the-
chest method and without the method (image 1 and 2, -
0.5° vs. 1.7° and 1.2° vs. 3.0°, p = 0.1713 and p = 0.1072,
respectively). The mean value of the absolute DMSA in the
patients with the aligning-the-chest method was
significantly smaller than that in the patients without the
method (image 1 and 2, 1.3° vs. 4.7° and 2.0° vs. 5.7°, p
= 0.0004 and p < 0.0001, respectively).
Conclusion
Aligning the chest with a treatment couch before
immobilisation improved postural reproducibility in
radiotherapy for head and neck cancers.
EP-1812 A study on the patient positioning accuracy for
stereotactic radiotherapy of brain lesions
O. Pashkovskaya
1
, E. Polovnikov
1
1
NSRICP, Oncology and radiotherapy, Novosibirsk,
Russian Federation
Purpose or Objective
The purpose of this study is an investigation of the patient
positioning accuracy (inter-fraction and intra-fraction
shifts) for stereotactic radiation treatment of the brain
lesions using invasive Lexsell Coordinate Frame G®, non-
invasive vacuum-activated head frame system HeadFix®
and individual thermoplastic masks, and comparison
position errors of these systems.
Material and Methods
A total of 532 patients, which were treated for benign and
malignant brain tumors in our center, represented study
population. The invasive Lexsell Coordinate Frame G®
(LFG), vacuum-activated head frame system HeadFix®
(HF) and individual thermoplastic masks (TM) were used to
immobilize patients during pre-radiation preparation and
radiation treatment. Dose distributions were typically
planned with a volumetric modulated arc therapy
technique and noncomplanar arcs if needed for optimal
dose distributions. Positioning accuracy was estimated
using linac-mounted cone beam computer tomography
(CBCT) scan system. The target intra-fraction offsets were
determined by comparison target positions before and
after the irradiation procedures with planning positions.
To determine inter-fraction shifts data from daily CBCT
scans were used. Statistical samples of target offsets in six
coordinates were collected and statistical analysis was
carried out.
Results
For 386 patients TMs were used, for 31 LFG and for others
115 HF. Analysis of inter-fraction shifts, based on
scaterplots, showed no significant correlations and
selected
systematic
errors
in
patient’s
positioning. Quantitative estimations of mean values and
standard deviations also demonstrated acceptable
accuracy. It was concluded that it’s necessary to do daily
CBCT control, if shifts at least in one direction are greater
or close to 5 mm during first four procedures. This is due
to patient’s specific characteristic and TM doesn’t provide
sufficient immobilization. Mask remaking does not always
solve the problem. As the result of intra-fraction offset
estimations, the position errors for LFG and HF were less
than 0.3 mm in coordinates and less than 0.3 degrees in
rotation.
Conclusion
Study of inter-fraction shifts showed that TMs provide
acceptable setup reproducibility, as consistent with PTV.
With respect to intra-fraction shifts, due to the slight
differences of the offset error results, the choice between
LFG and HF should be determined by the individual patient
characteristics and does not affect on the accuracy of the
delivered
dose.
EP-1813 The investigation of the immobilization
devices and localization methods for brain cancer in P-
SRS.
R.D. Lin
1
, J. Gao
1
, P. Chen
1
1
Chang Gung medical hospital, Proton and radiation
therapy center, Taoyuan, Taiwan
Purpose or Objective
There are two aims of this study: one is to assess the
accuracy of the Base of Skull (BoS) immobilization cast and
BRW headring for proton stereotactic radiosurgery (P-
SRS). The other one is to investigate the accuracy of
imaging-guided localization method by using bony
structure and fiducial markers as different landmarks.
Material and Methods
To assess the accuracy of the immobilization devices, we
immobilized the CIRS Radiosurgery Head Phantom with
hidden target by BoS cast and headring, respectively. Then
we implemented the imaging guidance by using bony
structure. Compare to the localization method, the CIRS
Radiosurgery Head Phantom with hidden target was
immobilized by headring followed by implementation of
the imaging guided process using bony structure and
fiducial markers, respectively. The overall couch shifts
were performed by the radiation therapists, the deviation
with respect to the proton isocenter was measured by
using the hidden target with paired t-test and 3D vector
.
Results
For BoS cast and headring, the deviations (mm) were
0.54±0.68 and 0.48±0.28 (p
=
0.412) in right-left (RL),
0.22±0.28 and -0.11±0.34 (p
=
0.186) in superior-inferior
(SI), -0.30±0.11 and -0.15±0.22 (p
=
0.167) in anterior-
posterior (AP) directions, and 0.85±0.42 and 0.60±0.32 (p
=
0.017) in 3D vector, respectively. For the imaging-
guided localization method, the deviations
(mm) of bony