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average dose, following irradiation with periodically
modulated and open 6MV photon fields.
Results
Survival of normal cells in a 2.5mm striped field was the
same as for an open field, but the survival of the cancer
cells was significantly lower. However for cancer cell lines
in the 5.0mm modulated fields, the response compared to
an open field was no longer statistically significant. A
mathematical model was developed to incorporate the
dose gradients of the spatial modulation into the standard
linear quadratic model. Our new extended bystander LQ
model assumes spatial gradients drive the diffusion of
soluble factors that influence survival through bystander
effects. The model successfully predicts the experimental
results that show an increased therapeutic benefit.
Conclusion
We have confirmed that HDMLCs can create spatially
modulated fields that increase the therapeutic advantage
between normal and cancer cells. Our results challenge
conventional radiotherapy practice and propose that
additional gain can be realized by prescribing spatially
modulated treatments to harness the bystander effect.
PO-0912 Short- and long term stability of the isocenter
of a three-source Co60 MR guided radiotherapy device
D. Hoffmans
1
, M.A. Palacios
1
, J.P. Cuijpers
1
1
VU University Medical Center, Radiotherapy,
Amsterdam, The Netherlands
Purpose or Objective
Recently a 0.35T Co
60
MRIdian system (Viewray Inc.,
Cleveland) is implemented at our institution. In a similar
way as for other image guided radiotherapy techniques,
the coincidence of the radiation therapy (RT) and imaging
isocenter is of major importance. The purpose of this
study is to present a method for daily QA of MR-RT
isocenter coincidence and to assess its short- and long
term stability using daily film-based isocenter QA.
Material and Methods
Two pieces of radiochromic film (GafChromic EBT3) are
taped to square inlays on the top and the side of a cubic
water-filled phantom. The phantom is aligned to the MRI
isocenter using MR guided setup relying on three internal
cylindrical markers.
A treatment plan, consisting of an AP and a lateral square
field is delivered. The direction of the lateral field is
altered daily between 90
o
and 270
o
in order to monitor all
3 treatment heads (Head 1 and 2 for the lateral fields
respectively, head 3 for the AP field). The films are
digitized and the positions of the square fields with
respect to the phantom are determined. These data
provide a daily measurement of the coincidence of the RT-
and MR- isocenters in 3 dimensions. The AP field provides
information about the alignment along the lateral (X) and
the longitudinal (Y) axis. The lateral fields provide the
shift in the vertical (Z), and the Y directions. Data were
collected daily over a period of 4 months. A linear
regression is performed in order to determine any trends
in time. Furthermore, the correlation between the two
daily values for the shift along the Y-axis is assessed.
Results
The average 3D vector of the daily shift is found to be
0.8mm (P95 = 1.3 mm). The average misalignments as
determined by the individual heads are shown in Table 1.
The systematic shift in 3D is zero in X and Y direction and
0.2 mm in the Z direction, which is caused by th e vertical
shift measured with head 1. No time trend in the shift is
observed in any direction as the regression coefficients
were not statistically significant different from zero:
p=0.39, 0.64 and 0.50 for the X, Y and Z axes respectively.
The pearsman correlation coefficient between the Y-shift
determined using the two perpendicular fields was very
weak and found to be 0.24.
Conclusion
A method is developed for daily assessment of the
coincidence of the MV- and MR-isocenter for an integrated
MR-RT unit. The alignment of the MR- to the RT-isocenter
is found to be stable during a time period of 4 months. A
small systematic shift in vertical direction was found, a
star shot measurement confirmed that this was caused by
a slight misalignment of Head 1. This misalignment can be
compensated by realignment of the MLC leaf positions.
The weak correlation in the pair of Y-measurements
suggests that the daily misalignment is dominated by
random measurement inaccuracies such as the placing
markers on the film and rotational setup misalignments of
the phantom.
PO-0913 A national review of equipment, techniques
and PTV margins used for SRS
R. Patel
1
, J. Lee
1
, C. Walker
2
, D.J. Eaton
1
1
Mount Vernon Hospital, Radiotherapy RTTQA,
Northwood Middlesex, United Kingdom
2
Northern Centre for Cancer Care, Radiotherapy,
Newcastle, United Kingdom
Purpose or Objective
As part of a national commissioning programme,
treatment providers were required to complete a SRS
quality assurance review in order to benchmark current
practice. The process was designed to ensure providers
were able to deliver a service in line with parameters
developed by a multidisciplinary expert advisory group
(EAG).
The long term aim of this programme was to progress a
system of standardisation and quality improvement of
service by promoting consistency and the development of
services over time. The short term goals were to highlight
any significant variation in practice in order to identify
centres that may require further support or mentoring in
order to meet nationally agreed parameters.
Material and Methods
A questionnaire was circulated to 20 centres to establish
the equipment, treatment techniques and PTV expansions
used to deliver SRS. Centres reported on their current