ESTRO 36 Abstract Book

922 / 1096

S906 ESTRO 36
_______________________________________________________________________________________________
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 evaluate 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. Muelas1, R. García2, L. Vidueira2, J. Bonaque 2, A.
Conde1, C. Ferrer1
1Consorcio Hospitalario Provincial de Castellón,
Oncología Radioterápica, Castellón, Spain
2Consorcio Hospitalario Provincial de Castellón,
Radiofísica y protección radiológica, Castellón, Spain
Purpose or Objective
To evaluate if planned doses for prostate and rectum are
equal to the doses which are actually delivered and to
determine adaptation points for the accumulated dose.
Material and Methods
Twenty four patients with intermediate and high-risk
prostate cancer who were going to be treated with image
guided radiotherapy were enrolled. A plan-CT (pCT) and
nine treatment kilovoltage conebeam-CT (kvCBCT) scans
were acquired prospectively during the first three weeks
of a prostate lGRT treatment (a total of 240 CTs). A rectal
emptying preparation and a full bladder protocol were
used. For each patient, a deformable image registration
(DIR) from the pCT to each of the nine kvCBCT was
performed with RayStation treatment planning system. All
registers were revised and recontoured by a Radiation
Oncologist, establishing regions of interest (ROIs) for a
second DIR with control of such ROIs. For every patient, a
hypofractionated VMAT schedule (15 x 3.82 Gy) was
planned and correlated with their kvCBCT images, being
able to determine the accumulated and total doses that
would have been actually delivered. Since the pCT day, a
nutritional evaluation control with anthropometric and
biochemical parameters was performed for each of the 24
patients.
Results
A significant difference between planned and delivered
D98 CTV 57 (p=0.026) and D2 CTV 57 (p=0.005) was
observed; however, the average D98 CTV 57 delivered was
higher than the prescription dose. Despite not having
observed a significant difference in V36.5 of the planned
and delivered to the rectum, the delivered doses to 50%
of the rectum exceeded the planned constraints in 37.5%
of the patients. A significant rectum volume variation was
observed during the first week of treatment. An
accumulated delivered dose to 50% of rectum > 1194 cGy
in fraction five was a significant predictor for exceeding