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ESTRO 36 2017
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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
practice without any guidance. Responses were evaluated
by the EAG and used to inform on best practice and
identify centres where additional support was required.
Results are given here for PTV margins used for metastatic
disease. PTV margins are particularly important when
treating multiple mets as they can increase the volume of
normal brain irradiated and the commissioning criteria
requires the total treated volume to be below 20cc, so the
choice of PTV margins can impact patient eligibility for
treatment.
Results
All 20 centres responded to the questionnaire with one
centre excluded as they were in process of changing
equipment. Responses are summarized in Table 1.
The most common platform used was the Elekta
Gammaknife system. A variety of linacs were used, the
majority of those used for the commissioning were
specialised units (e.g. Novalis, Truebeam STx) or had been
adapted (e.g. fixed cones) for SRS treatments.
Centres used a 0, 1 or 2mm margin for brain mets. All
Gammaknife centres used a 0mm margin, but other
platforms varied depending on the centre as seen in Figure
1. Only four centres used a 2mm expansion for treating
brain mets, three of these were non specialised linacs.
Conclusion
There is significant variation in the equipment used to
treat SRS nationally. A 0mm PTV expansion was the most
common for SRS treatment regardless of platform.
Gammaknife centres were consistent with their PTV
margins, which is based on historical practice but other
platforms varied depending on the centre. No system has
an end to end accuracy of 0mm, however many centres
are choosing to use, which may lead to under-coverage of
the target.
Following feedback, centres using non-specialised
equipment are planning to acquire either stereotactic
linacs or upgrades such as the Apex head, with some
frameless users acquiring Exactrac systems to reduce
uncertainty in patient positioning. These will facilitate
margin reduction at centres using 2mm PTV expansions, in
line with the ≤1mm recommended by the EAG.
PO-0914 Helium Beam Radiography System based on
pixelized semiconductor detectors
T. Gehrke
1
, G. Arico
1
, S. Berke
2
, J. Jakubek
3
, M.
Martisikova
4
1
Heidelberg University Hospital, Radiation Oncology and
Radiation Therapy, Heidelberg, Germany
2
German Cancer Research Center, Medical Physics in
Radiation Therapy, Heidelberg, Germany