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therapy as an additional dose delivery check. The transit
dosimetry has the potential of testifying dose delivery, the
accuracy of MLC leaf positioning and the calculation of
dose to a patient or phantom.
Material and Methods
In total 42 patients with stereotactic plans were
evaluated. Delivery was carried out on a Varian TrueBeam
linac equipped with an aS1000 EPID. Continuous portal
imaging was performed at each treatment fraction during
the delivery of treatment for all beams. To validate the
method, we executed treatment plans on a commercial
respiratory motion phantom containing plastic spheres as
target. Phantom CT scans were made in different phases.
First phase were done by applying sinusoidal breathing
cycle in different motion amplitudes (-20, -10, 0, 10, 20
mm) in superior/inferior direction and second phase was
done by pre-defined breathing simulation with a short
pause after exhalation in oscillation mode. Three
techniques: 3D-CRT, IMRT and VMAT-SBRT were generated
and on board transit dose was collected by EPID during the
treatment. The daily obtained portal image were
compared with the reference image using the gamma
evaluation method with criterion 2% dose difference and
2 mm distance to agreement (DTA) criteria with a
threshold value of 5% of maximum value.
Results
The area gamma passing rate per arc in most of the plans
was higher than the acceptable limit but in some arcs it
had lower agreement, the lowest value was 3.7%. Besides
irradiating phantom in planned respiratory motion, we re-
irradiated the same plans due to displacement of the
target by stopping the movement or changing the
breathing speed. Gamma parameters such as maximum
gamma, average gamma, and percentage of the field area
with a gamma value>1.0 were analyzed. For all the VMAT
arcs in phantom measurements, the gamma evaluations
were within the tolerance limits (γmax = 3.5, γavg = 0.5
and γ% >1 = 2%) tough in some measurement 20 mm target
displacement was applied. For IMRT fields, measurements
were not in good agreement in different tumor motion.
3DCRT fields showed poorest gamma agreement in portal
dosimetry analysis.
Conclusion
This research increases the need of a tool for monitoring
inter-fraction errors by confirming the tumor position
within the treatment field over the course of therapy.
Using daily EPID images over the course of treatment could
potentially provide accurate verification of dose delivery
to heterogeneous anatomical regions in patients receiving
3D-CRT and IMRT radiation therapy treatments. However,
further studies are required to assess 3D IN VIVO dose
verification of VMAT techniques of various treatment
sites.
Electronic Poster: Physics track: Adaptive radiotherapy
for inter-fraction motion management
EP-1658 The effect of weight loss in head and neck
patients in the presence of a magnetic field
R. Chuter
1
, P. Whitehurst
1
, M. Van Herk
2
, A. McWilliam
2
1
The Christie NHS Foundation Trust, Christie Medical
Physics and Engineering CMPE, Manchester, United
Kingdom
2
University of Manchester, Manchester Academic Health
Science Centre MAHSC, Manchester, United Kingdom
Purpose or Objective
Head and neck patients tend to experience weight loss
during treatment in a predictable pattern loosing between
5-15% of their initial weight over the first two weeks.
Adaptive radiotherapy for these patients focuses on an
offline protocol where the patient is re-scanned and re-
planned two-to-three weeks through treatment. The MR-
Linac (Elekta, AB, Stockholm, Sweden) will provide
excellent soft tissue contrast which may be desirable for
this group of patients. However the electron return effect,
caused by the Lorentz force may potentially result in an
increased dose to superficial tissues, for example the
parotid glands. This effect can be controlled in plan
optimisation, however it is unknown whether the presence
of a magnetic field makes it necessary to adapt the plan
at an earlier stage or more frequently during treatment.
The purpose of this abstract is to assess the suitability of
the current off-line adaptive radiotherapy workflow for
head and neck patients in the presence of a magnetic
field.
Material and Methods
Ten patients treated with either 66Gy or 60Gy in 30
fractions, were selected from the clinical archive that had
shown significant weight loss during treatment which
required a repeat CT. Both the initial planning CT (pCT)
and the repeat CT (rCT) were fully contoured by an
oncologist specialising in head and neck cancer. Two plans
were optimised, at 0T and 1.5T using Monaco v5.09 (Elekta
AB Stockholm, Sweden) which met the departmental
constraints for Target and OAR doses. These plans were
copied to the rCT and re-calculated with a 1% statistical
uncertainty, allowing the segmentation and delivered MU
to remain constant. The magnitude of the change in dose
to the target and OARs due to weight loss was compared
between the 0T and 1.5T plans. The difference between
the dose distribution on the pCT was compared to the
distribution on the rCT and how this was affected by the
presence of the magnetic field.
Results
The percentage difference between 0T and 1.5T plans for
the targets showed statistical differences for the D95% for
PTV1, PTV2 and PTV3, D50% and mean dose for PTV2, and
mean dose and 2cc min for PTV3. The only statistical
difference for the OARs was the 2cc max dose for skin
which increased by 1.1% for 1.5T plans. However
differences between the 0T and 1.5T plans were on
average all within 2%, so were considered clinically
acceptable.