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S904
ESTRO 36
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organ (CTV) and global percentage gamma pass (%GP)
evaluation (criteria: 3%/3mm, 2%/2mm and 1%/1mm). For
each patient mean %GP, standard deviation (σ) and
angular coefficient of linear fit of %GP where
evaluated. In particular σ is used to monitor random set-
up and preparation errors while the angular coefficient is
used to monitor the target size variation and tumor
response during treatment course. The results were
correlated to treated pathologies.
Results
The phantom results showed a sensitivity equal to 100% in
detecting all simulated errors.
The obtained results are described in the table
ANOVA analysis pointed out that the significance of the
difference between %GP and pathologies exists only when
calculating mean %GP,
s,
and angular coefficient with the
1%/1mm CTV criterion, obtaining respectively p<0.006,
p<0.04, and p<0.04. Applying the other criteria, the
obtained results were p>0.05.
The results showed that gynecological patients, followed
by lung, head and neck, and rectum pathologies are the
most responsive patients.
Conclusion
Based on these results, we can state a general correlation
law between angular coefficient of %GP and treated
pathology to search a quantitative parameter to help
predicte adaptive radiotherapy. This methodology could
provide an important element toward informed decision-
making for adaptive radiotherapy.
EP-1663 Automated full-online replanning of SBRT
lymph node oligometastases for the MR-linac
D. Winkel
1
, P. Kroon
1
, J. Hes
1
, G. Bol
1
, B. Raaymakers
1
, I.
Jürgenliemk-Schulz
1
1
UMC Utrecht, Department of Radiotherapy, Utrecht, The
Netherlands
Purpose or Objective
Diagnostic imaging on the MR-linac most probably provides
better visibility of lymph nodes compared to CBCT on
conventional linacs. While commercially available plan
adaptation methods are feasible, full-online replanning is
the preferred method to reach good plan quality. The aim
of this study is to investigate the feasibility of fast online
replanning on the MR-linac to account for inter-fraction
motion for stereotactic body radiotherapy (SBRT) of lymph
node oligometastases.
Material and Methods
Patient imaging data and delineations from seven
advanced cervix cancer patients with a combined total of
33 lymph nodes in the abdominal and pelvic region were
included. A planning simulation study was performed on
these lymph nodes with a 7-field IMRT technique and a
prescription dose of 5x 7Gy to 95% of the PTV. Treatment
plans were automatically generated using the research
version of Monaco by Elekta AB (Stockholm, Sweden) with
the use of their research automation API and in-house
developed automated treatment planning software. A
CTV-PTV margin of 3mm in all directions was applied. To
decrease optimization time an additional margin of 5mm
around the PTV was applied and only the parts of the
OAR’s within this margin were considered as OAR during
the optimization (Figure 1). All plans were generated using
the MR-linac machine model and a 1.5T magnetic field in
superior-inferior patient direction. Dosimetric outcomes
were evaluated against clinical dose constraints and
optimization time was measured. When required, PTV
coverage (V
100%
>95%) was sacrificed to meet all OAR dose
constraints.
Figure 1: a) A schematic representation of the planning
strategy applied in this study to reduce optimization
times. b) Resulting dose distribution with isodoses
(cGy).
Results
For 30 (91%) of the 33 cases no clinical dose constraints
are violated in combination with sufficient PTV dose
coverage. In the other 3 (9%) cases PTV coverage is
reduced by 5.4 ± 3.0 % to meet all dose constraints of the
OAR. The average time required for optimization is 158 ±
95 s. The estimated dose delivery time, as reported by
Monaco, is 198 ± 32 s. This leads to a total average
optimization and delivery times of 357 ± 124 s, which fits
well within the proposed 30 minute time limit for
treatment on the MR-linac. Both the optimization and
delivery time are dependent on the volume of the PTV and
increases with increasing PTV. The average PTV is 6.4 ±
5.1 cc (range, 1.8 – 28.3 cc).
Conclusion
We have shown that automated full-online replanning for
the MR-linac to account for inter-fraction motion is
feasible for SBRT of lymph node oligometastases. With the
planning strategy as applied in this study we are able to
automatically generate treatment plans, suitable for
clinical use, within a timespan which is clinically
acceptable for treatment on the MR-linac.
EP-1664 Two-step verification of dose deformation in
presence of large inter-fraction changes during LACC RT
A. Gulyban
1
, M. Baiwir
1
, S. Nicolas
1
, M. Enescu
2
, V.P.
Nguyen
1
, M. Gooding
2
, T. Kadir
2
, J. Hermesse
1
, V. Baart
1
,
P.A. Coucke
1
, F. Lakosi
3
1
Liege University hospital, Department of Radiation
Oncology, Liege, Belgium
2
Mirada Medical Ltd., Department of Research, Oxford,
United Kingdom
3
University of Kaposvar, Health Science Center,
Kaposvar, Hungary
Purpose or Objective
Dose accumulation is one of the most challenging parts of
modern radiotherapy, especially in the presence of large
inter-fraction motion. Determining actual dose to a given
organ during external treatment of locally advanced
cervical cancer (LACC) is one of the most prominent
examples. In our current investigation we aimed to
evaluate the residual dose deformation errors during the
summation of dose for clinical target volume (CTV),
bladder and rectum.