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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
generate a library of plans for bladder treatments with a
combined target of the total bladder and the GTV.
Material and Methods
Two CT scans were acquired and registered (empty/full
bladder). The bladder CTVs and GTVs were delineated on
both CTs. An in-house developed script was developed to
calculate intermediate CTVs and GTVs based on the empty
and full bladder delineations. The script, which utilizes a
Robust Point Matching (RPM) algorithm (Osorio, 2012),
yields a deformation vector field that can transform the
target structure to the reference structure. The algorithm
can be tuned with the following parameters: stiffness,
density of points, number of iterations and the final
'temperature”.
To create intermediates, the deformation can be applied
partially, e.g., to create a structure in the middle of the
two input structures, a 50% deformation would be applied.
Dividing the maximum spacing required between
consecutive intermediate plans by the maximum distance
between reference and target structure, will give the
excitation percentages required to get to equidistant
intermediate structures. Bladder CTV and GTV need to be
handled by separate RPM processes because the required
parameters are very different due to large discrepancy of
deformation and size. The number of plans is set by the
maximum distance between full and empty CTVs.
Therefore first the intermediate structures for CTV are
created and then the same excitation percentages are
applied to GTV.
Results
Figure 1 shows an example of a generated library of plans
for CTVs and
GTVs.Toevaluate the results we create a
structure with 100% deformation, which should coincide
with the target structure. Using the default stiffness
parameter for 10 patients (1000 for CTV and 250 for GTV)
we found a success rate of 60%. By tuning the stiffness
parameter, intermediate structures were created
successfully for the remaining cases. On average it takes
3.50 and 2 minutes for the CTVs and GTVs to be created,
respectively.
Conclusion
We have developed a robust, quick and straightforward
method to generate a library of plans for a combined
bladder CTV and GTV using delineations of full and empty
bladder CTs. The method is able to generate plans at every
cm from full bladder.
EP-1666 Adaptive radiotherapy in prostate cancer:
when and why?
R. Muelas
1
, R. García
2
, L. Vidueira
2
, J. Bonaque
2
, A.
Conde
1
, C. Ferrer
1
1
Consorcio Hospitalario Provincial de Castellón,
Oncología Radioterápica, Castellón, Spain
2
Consorcio Hospitalario Provincial de Castellón,
Radiofísica y protección radiológica, Castellón, Spain