921 / 1096
S905
ESTRO 36
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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.
Conclusion
Using a two-step clinical verification of the dose
deformation confirms the feasibility to perform accurate
dose accumulation for CTV, bladder and rectum during
LACC RT. These values are within the range of
uncertainties originated from dose calculation, residual
positioning errors or anatomical changes, confirming the
reasonable clinical usage.
EP-1665 Library of plans approach for bladder cancer
radiotherapy including a simultaneous integrated boost
S. Nakhaee
1
, L. Hartgring
1
, M. Van der Burgt
1
, F. Pos
1
, P.
Remeijer
1
1
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiotherpay, Amsterdam, The Netherlands
Purpose or Objective
With image guided radiotherapy the positioning of
patient can be corrected accurately by a table shift after
a registration procedure. However, for large
deformations of the target area, for example due to
inter-fractional changes in bladder filling, table shift
might not fully compensate the variation. Compared to
full bladder treatments, the need for accuracy in dose
delivery is even more profound for bladder patients
receiving simultaneously increased dose to the gross
tumor volume (GTV). A daily plan selection from a library
of plans is a strategy to tackle this challenge. With this
approach, a number of radiation treatment plans are
made for a set of anticipated shapes and positions of the
target prior to treatment. At every fraction the most
suitable plan can then be selected. The purpose of this
study was to develop an interpolation method to