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ESTRO 36 2017
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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.
Material and Methods
Eleven LACC patients were included in this study treated
between 06/2015 and 06/2016. Before each of the 25
treatment session, online corrected CBCT acquisition was
performed (XVI 5.0, Elekta Ltd., Crawley, UK). Using the
daily CBCTs the CTV, bladder and rectum were delineated
(actual position), and the actual dose volume histogram
(DVH_actual) was calculated using the reference dose
matrix (rigidly transferred). For a topological co-
registration a constraint-based deformation using Radial
Basis Function with Robust Point Matching (RBF-RPM) was
performed between the current and the reference
position of each given organ using Mirada RTx (1.6.3,
Mirada Medical ltd, Oxford, UK). Hausdorff-distance
distributions (HDDs) from the reference volume towards
the initial and deformed positions were assessed and the
accuracy of the RBF-RPM deformation was evaluated.
Further two DVHs were generated by deforming the dose
matrix (transferred previously to the CBCT) in combination
with the actual contour deformed (DVH_deformed) or with
the reference delineation (DVH_reference). Differences
between the relative DVHs were assessed in two steps: 1)
the residual error of the deformation (DVH_actual vs.
DVH_deformed) and 2) the volumetric mismatch sourced
from the constraint-based RBF-RPM approximation
(DVH_deformed vs. DVH_reference). Volume-specific
confidence intervals were determined for the separated
and combined steps.
Results
A total of 621 DVHs were generated. The HDDs (Figure 1,
from reference) were reduced from the initial 30.5 mm
(standard deviation, SD = 16.6) to a reasonably good 10.4
mm (SD = 6.4) confirming a good performance of the
constraint-based RBF-RPM (Figure 2, bladder). The initial
deformations were responsible for maximum of 3.8%/6.9%
and 5.7% errors for CTV, bladder and rectum respectively,
reaching a total combined maximum discrepancy of
4.6/7.2/6.2%. For CTV deviations are observed between
40-55 Gy, while fore bladder and rectum after 25 Gy errors
can be seen. The interquartile errors remained within +/-
5% deviations for the entire dose range.