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(see
flow
chart).
We retrospectively applied this protocol to 88 historical
treatments (May 2011 - January 2015) performed within
our institute to evaluate its effect.
Results
Of 88 treatments in 86 patients, 36 were initially selected
for 3D CBCT in all fractions. From the remaining 52
treatments 25 would have been suitable for 3D CBCT after
comparing and combining 4D and 3D CBCT position
verification (see pie chart).
Conclusion
With use of the in-treatment decision protocol for 3D or
4D position verification, the number of patients having 3D
CBCT for position verification raised from 41% to 69%. This
is not only beneficial for patient comfort, it limits motion
related treatment degradation and it is also increases
treatment capacity. Therefore we propose to use the in-
treatment CBCT information and our decision protocol for
making optimal use of 3D CBCT.
EP-1635 Framework for the evaluation of interplay
effects between respiratory motion and dose
application
A. Von Münchow
1,2
, K. Straub
1
, J. Hofmaier
1,2
, P.
Freislederer
1
, M. Reiner
1
, C. Thieke
1
, M. Söhn
1
, M. Alber
3
,
R. Floca
4
, C. Belka
1
, K. Parodi
2
, F. Kamp
1
1
LMU Munich - Klinikum der Universität München,
Department of Radiation Oncology, Munich, Germany
2
LMU Munich - Faculty of Physics, Department of Medical
Physics, Munich, Germany
3
Heidelberg University Hospital, Department of
Radiation Oncology, Heidelberg, Germany
4
German Cancer Research Center, Software development
for Integrated Diagnostic and Therapy - Department of
Radiology, Heidelberg, Germany
Purpose or Objective
The interplay between respiratory motion of a tumor and
dose delivered by complex techniques like IMRT and VMAT
can potentially lead to undesirable and non-intuitive
deviations from the planned dose distribution. Small field
sizes and fluences used in these advanced therapy
techniques might amplify the dose deviations. We aim at
developing a 4D dose recalculation tool to simulate the
dose distribution for a moving target volume more
precisely. The impact of interplay effects can be
evaluated and compared for different treatment
techniques.
Material and Methods
We developed a workflow combining a Monte Carlo dose
calculation and a dose accumulation based on 4DCT
images and linac log files. Log data from the linac are
retrieved with
Delivery Parameters Log File Convertor for
Integrity™ R3.2
provided by Elekta. The time information
in these log files has a resolution of 0.04 s and is used to
divide the original treatment plan into small time intervals
correlated to the patient’s respiratory phases. All
resulting plan fragments (each corresponding to a certain
4DCT
phase)
are
then
recalculated
using
MCverify/Hyperion V2.4 (research version of Elekta
MONACO 3.2). As a final step the single doses are sorted
and combined to a total dose distribution. Different
respiratory cycles, e.g. changes in the breathing
frequency or pattern, and treatment methods, e.g.
stereotactic treatment or gating, can be simulated and
compared for different treatment techniques. The
handling and accumulation of the different dose fragments
are performed with AVID, a software framework for
radiation therapy data processing developed at
Deutsches
Krebsforschungszentrum
(DKFZ).
For
a
first
demonstration, implementation and verification, the
4DCT of a
Dynamic Thorax Phantom (CIRS)
is used. A 1D-
sinusoidal-rigid-motion with frequency 0.25 Hz and
amplitude 2 cm was set.
Results
A 3D-CRT and an IMRT plan were delivered to the
phantom. Fig. 1 shows the resulting DVHs for the GTV using
the 3D-CRT and the IMRT plan, respectively. In the plots,
two different starting phases are marked (“treatment
starting in inhale” and “treatment starting in exhale
phase”). The DVHs of the 3D-CRT plan remained
unaffected by the respiratory phase shift. Whereas for the
IMRT plan (optimized on a dose of 10Gy) the maximal dose
changed by 0.27 Gy (2.3%) from 11.95 Gy to 11.68 Gy after
the 50% phase shift.
Fig 1: Example for the occurrence of interplay effects:
IMRT plan segment on two CT Phases