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S193
ESTRO 36
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Material and Methods
Seven patients were studied, treated with MRI-guided PDR
BT (two times 24 x 0.75 Gy, given in two applications BT1
and BT2). DIR was performed using the Feature-Based
Deformable Registration (FBDR) tool, connected to a
research version of Oncentra®Brachy (Elekta
Brachytherapy, Veenendaal, the Netherlands). The
delineated rectums were converted to 3D surface meshes,
and a mapping was established to propagate elements on
the surface of rectum
BT1
to the surface of rectum
BT2
. The
transformation vectors were used to deform the BT1 dose
distribution. Next, the BT1 and BT2 doses were summed
voxel-by-voxel. To investigate the dose warping
uncertainty a physically realistic model (PRM) describing
rectal deformation was used. In this model the central
axes of rectum
BT1
and rectum
BT2
were constructed. The
axes were assumed to be fixed in length. For both
rectum
BT1
and rectum
BT2
, orthogonal planes were
reconstructed at 5 evenly spaced positions on the axis
(Fig. A). 100 points were evenly distributed over the
intersection curve of each plane with the rectal wall. It is
assumed that the most dorsal point of the rectum is fixed
and also that the rectal wall only stretches
perpendicularly to the central axis. For point pairs on
rectum
BT1
and rectum
BT2
that were at the same location
according to the PRM, the dose for BT1 and BT2 was added
(D
PRM
) and compared as a 'ground truth” to the DIR
accumulated dose (D
DIR
) in the BT2 point. For BT, the high
dose regions in the OAR are most relevant and points
within the 2 cm
3
volume receiving the highest dose should
be correctly identified. We therefore evaluated the
percentage of points where D
PRM
and D
DIR
were both >D
2cm3
.
Results
Over all patients, D
DIR
varied between 1.1-44.4Gy
EQD2
and
D
PRM
varied between 1.1-40.1Gy
EQD2
(α/β=3Gy for late OAR
toxicity, T
1/2
=1.5 hours). For point pairs, the absolute
difference between D
DIR
and D
PRM
was 0-8.3Gy
EQD2
(Fig. B).
The 2 cm
3
volumes receiving the highest dose according to
the two models have an overlap of 66% (Fig. C).
Conclusion
With the rectal model it is feasible to quantify dose
warping uncertainties, which could be as high as 8.3
Gy
EQD2
. Most points (>66%) in high dose regions were
correctly identified as part of D
2cm3
.
OC-0361 Commissioning of applicator-guided SBRT
with HDR Brachytherapy for Advanced Cervical Cancer
S. Aldelaijan
1
, S. Wadi-Ramahi
1
, A. Nobah
1
, N.
Jastaniyah
2
1
King Faisal Specialist Hospital and Research Center,
Biomedical Physics, RIyadh, Saudi Arabia
2
King Faisal Specialist Hospital and Research Center,
Radiation Oncology, RIyadh, Saudi Arabia
Purpose or Objective
There is emerging evidence that dose escalation to the
“GEC ESTRO defined” high-risk clinical target volume
leads to improved clinical outcome in patients with
cervical cancer. For those with large residual disease or
with unfavorable topography of parametrial spread,
achieving such high doses is limited by the dose to organs
at risk. Options include a parametrial boost by EBRT which
lack precision and lead to prolongation of overall
treatment time or the addition of interstitial needles
which require a specialized brachytherapy (BT) program.
The option of combining brachytherapy with SBRT, using
the applicator as a guide, is being explored at our
institution. The purpose of this work is to show how this
idea can be successfully implemented using an EBT3
Gafchromic film-based dosimetry system. The effect of
positional inaccuracies on overall dosimetric outcome is
studied as well.
Material and Methods
A cube phantom was constructed to snuggly accommodate
an intrauterine tandem (IU), Fig1a. Pieces of EBT3 film
were taped on both sides of the IU to capture the dose
distribution. The phantom was CT-scanned and the
physician contoured a CTV mimicking large residual
parametrial disease, Fig1b. The plan was such that the
7Gy isodose adequately covers the near-distance CTV. The
BT plan was used as input for the SBRT plan and the 7Gy
to 2.0Gy dose gradient were used to create dose shells,
each having its own dose objective and constraint. Three
VMAT arcs were used to achieve the goal of D
98%
> 95% to
the entire CTV. Later, HDR BT treatment was delivered
using microSelectron v2 and the SBRT was delivered using
TrueBeam®. Positioning accuracy of the phantom was
done using CBCT imaging with the applicator for image
registration. Films were scanned with 10000XL EPSON
scanner at 127 dpi and dosimetry was done using the green
channel and an in-house MATLAB routine. Intentional