ESTRO 36 Abstract Book

200 / 1082

S187 ESTRO 36 2017
_______________________________________________________________________________________________
Results
Photon plans were highly robust against interfractional
anatomical changes. The difference between planned and
accumulated DVH parameters for the photon plans was
≤0.5% for the target and OARs. For proton therapy,
coverage of the iCTV was considerably reduced for the
accumulated compared to the planned dose: the mean
near-minimum dose (D98%) of the iCTV reduced from
98.1% to 90.3% [79.4%–95.3%](Figure). For carbon ion
therapy it was even worse; D98% was reduced with 10%,
from 98.6% to 88.6% [80.7%–92.7%]. The DVH parameters
of the OARs differed ≤3% between both particle
modalities. For all modalities the near-maximum dose
(D2%) did not differ.
Conclusion
Photon therapy is highly robust against interfractional
anatomical changes and setup errors in pancreatic cancer
patients. However, in particle therapy with either protons
or carbon ions, severe reductions in target dose coverage
were observed. Implementation of particle therapy for
pancreaticcancer patients should be done with great care
and interfractional anatomical changes must be accounted
for.
OC-0355 Which anatomical changes in Head&Neck
cancer lead to Repeat CT/planning?
S. Van Beek1, O. Hamming-Vrieze1, A. Al Mamgani1, A.
Navran 1, J. Van de Kamer1, P. Remeijer1
1 The Netherlands Cancer Institute, Department of
Radiation Oncology, Amsterdam, The Netherlands
Purpose or Objective
During a course of radiotherapy for head and neck (H&N)
cancer, non-rigid anatomical changes can occur. For
example, changes in volume of the target, changes in neck
diameter (contour) due to edema or weight loss, shifts of
hyoid or thyroid bone or other localized soft tissue
deformations. These anatomical changes cannot be
corrected for by a couch shift, but they can be observed
on daily Cone Beam CT (CBCT) and are scored digitally by
RTTs according to a traffic light protocol (TLP)(green: no
action, orange: evaluation of dose consequences before
the next fraction, red: immediate evaluation of dose
consequences). Orange and red scores can lead to a new
radiation plan, either on the original planning CT scans (O-
pCT) with local adjustment of target volumes or on a new
pCT scans (N-pCT) with complete re-delineation. In this
work, we evaluated how often re-planning was done for
non-rigid anatomical changes and which anatomical
changes lead to which new plan actions during the 7 weeks
of treatment.
Material and Methods
A consecutive series of H&N cancer patients (416) treated
from January 2015 until September 2016 were
retrospectively selected using the digital log of CBCT scans
(10862 H&N logs). These digital logs were analysed for the
number of new treatment plans on an O-pCT or a N-pCT.
Reasons for re-planning were categorized into: target
volume increase, target volume decrease, contour
decrease, contour increase and shift of target volume. To
evaluate the timing of re-planning, the week in which
delivery of the new plan started was scored as well.
Results
In 9% (37/416) of the H&N patients included in this
analyses, the treatment plan was adapted due to
anatomical changes detected during radiation treatment
on CBCT. Re-planning on a N-pCT with complete re-
delineation was done 22 times. In fifteen cases a new plan
was created after adjustment of contours on the O-pCT.
For 4 patients, two actions were taken, first a new plan
on the O-pCT and secondly (further in the treatment) a
new plan on a N-pCT. Figure 1 shows the anatomical
changes observed at the time of re-planning, as well as
the time of occurrence during treatment. In the early
weeks of treatment, the most observed reason for re-
planning was a target volume increase, both on a N-pCT as
well as on the O-pCT. In the last part of treatment, re-
planning on a N-pCT was mainly done because of contour
decrease, while re-planning on the O-pCT was chosen in
the event of local shifts of target volume. The majority of
adaptive treatment plans were made in the second, third
and fourth week of treatment for relatively 10, 9 and 10