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ESTRO 36 2017
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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.
Conclusion
This work shows that the dosimetric effect of weight loss
does not cause any clinically significant changes in the
presence of a magnetic field, as the difference between
pCT and rCT for 0T and 1.5T are similar. Therefore,
current off-line strategies for adaptive planning for head
and neck patients are valid for use on the MR-Linac.
EP-1659 Quantitative triggering of plan adaptation:
monitoring plan quality by recalculation on CBCT scans
R. Canters
1
, M. Wendling
1
, M. Kusters
1
, R. Monshouwer
1
1
Radboud University Medical Center, Radiation oncology,
Nijmegen, The Netherlands
Purpose or Objective
Since the introduction of 3D imaging on the linac,
anatomical changes observed on CBCT scans regularly lead
to plan adaptation. However, adaptation is often triggered
by qualitatively assessing anatomical changes between
CBCT and planning CT. This regularly leads to unnecessary
replanning, disrupting the regular workflow in the clinic.
In this study, we created an automated evaluation tool,
that recalculates the treatment plan on recorded CBCT
scans to indicate if a replanning may be necessary. The
aim of this work is to assess its potential for regular
clinical use.
Material and Methods
The recalculation tool imports planning CT a nd CBCT
scan, after which the treatment plan is transferred to the
CBCT scan. Subsequently, the plan is recalculated on the
CBCT using Pinnacle, and DVH’s are compared (Figure 1).
The CT-CBCT match is derived from the CBCT match at the
linac. Since Hounsfield units (HU) of the CBCT are not
calibrated, a CT to CBCT HU conversion table was created