ESTRO 36 Abstract Book

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S144 ESTRO 36 2017
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needle movements between MR-scan and treatment.
There was no relation between deviations in measured
dose and shifts of needles. E.g. patient 6 and patient 7
have similar shifts but very different accumulated dose
deviations. This illustrates how a small shift in a nearby
needle can lead to significant changes in the measured
dose, making it hard to use the accumulated dose for
treatment verification.
Conclusion
Accumulated dose and dose rate have been measured in
real-time for 22 treatments. We have used real-time in-
vivo dosimetry to determine the rela tive geometry
between needles and dosimeter with high precision. This
could potentially lead to real-time treatment verification
in BT.
OC-0280 Benefit of repeat CT in high-dose rate
brachytherapy as radical treatment for rectal cancer
R.P.J. Van den Ende1, E.C . Rijkmans1, E.M. Kerkhof1,
R.A. Nout1, M. Ketelaars1, M.S. Laman1, C.A.M .
Marijnen1, U.A. Van der Heide 1
1 Leiden Univers ity Medical Center, Department of
Radiation Oncology, Leiden, The Netherlands
Purpose or Objective
High-dose rate endorectal brachytherapy (HDR-BT) for
rectal cancer can be used to increase the dose to the
tumor while sparing surrounding organs due to a smaller
treated volume and the steep dose gradient.
Conventionally, one treatment plan is derived from a
planning CT with applicator in situ prior to the start of
treatment, which is then used for all further applications
(non-adaptive approach). An adaptive approach would be
to acquire a repeat CT scan at each application for
treatment planning. The purpose of this study was to
evaluate the difference in dose conformity and clinical
target volume (CTV) coverage between the non-adaptive
and the adaptive approach.
Material and Methods
Eleven patients included in a dose-escalation study were
included in this study. Patients received a radical
treatment consisting of 13x3 Gy external beam
radiotherapy (EBRT) followed by three weekly applications
HDR-BT of 5-8 Gy. A planning CT with applicator in situ
was acquired at application one and repeat CT scans with
applicator in situ were acquired at application two and
three. The CTV was defined as residual macroscopic tumor
or scarring after EBRT. The CTV, rectal wall without CTV,
mesorectum and anus were delineated by an expert
radiation oncologist and a resident radiation oncologist on
all repeat CT scans and consensus was reached. The
treatment plan of application one was projected on the
repeat CT scans to simulate the other applications.
Projected treatment plans were categorized as clinically
acceptable or unacceptable. Additionally, new treatment
plans were derived from the repeat CT scans by an
experienced treatment planner. A conformity index,
taking into account CTV coverage and dose to organs at
risk, was used to quantify conformity of both the
projected and the repeated treatment plans. Dose
distributions were scaled to a prescription dose of 7 Gy.
Using the Wilcoxon signed rank test, the conformity index
and cumulative CTV D98 of the projected and repeated
treatment plans were compared.
Results
Fourteen out of 22 projections were clinically
unacceptable. In 8 of those 14 projections, replanning was
of added value. In the remaining 6 unacceptable cases,
replanning was of limited value as first an intervention
would have been necessary to remove air and/or faeces.
The figure shows a repeat CT with an unacceptable
projection and corresponding replanning. The table
summarizes the conformity index and cumulative CTV D98
of the non-adaptive and the adaptive approach.
Parameters are presented both for all cases and for all
cases excluding those that needed an intervention. Repeat
CT-based adaptive HDR-BT resulted in a significantly
higher conformity.
Conclusion
Repeat CT-based adaptive HDR-BT resulted in a more
conformal treatment and should be standard practice in
radical treatment with HDR-BT in rectal cancer patients.
Poster Viewing : Session 6: Imaging
PV-0281 Lymph node MRI in regional breast
radiotherapy leads to smaller target volumes and lower
OAR dose
T. Van Heijst1, H.J.G.D. Van den Bongard1, N. Hoekstra1,
M.E.P. Philippens1, D. Eschbach1, J.J.W. Lagendijk1, B.
Van Asselen1
1UMC Utrecht, Radiotherapy, Utrecht, The Netherlands
Purpose or Objective
Elective axillary regional radiotherapy (RT) in breast
cancer patients is performed with RT-planning CT scans,
using delineation guidelines based on anatomical
boundaries. In contrast to CT, MRI can directly image
axillary lymph nodes (LNs) in RT position [van Heijst et al.
2016, BJR]. Our MRI linac (MRL) system is designed to be
able to treat those LNs precisely. LN-based target volumes