ESTRO 36 Abstract Book

486 / 1082

S471 ESTRO 36 2017
_______________________________________________________________________________________________
respiratory gating or deep inspiration breath hold (DIBH)
facilitate dose reduction to OAR.
Material and Methods
CT image sets from ten patients were analysed. Esophagus
and OAR were delineated on end expiration (EE) and end
inspiration (EI) phases of the 4DCT and on DIBH CT. 5 cm
long mock GTVs were delineated in the proximal (P),
medial (M) and distal (D) part of the esophagus. CTVs were
defined by expanding the GTVs according to our clinical
practice. CTV to PTV margin was 7 mm. Relative position
of OARs and target were evaluated with cumulative
distance volume histograms (DiVHs) [Wu et al. Med Phys
2009], calculated for the part of the OAR located in the
beam path. The most and least optimal phase for
treatment was selected by comparing the percent of the
OAR volume located within the distance intervals A (below
2.5 cm), B (2.5-5.0 cm) and C (5.0-7.5 cm) from the PTV.
The organ sparing achieved or lost, by changing treatment
from FB to a specific breathing phase, was estimated by
assuming that FB can be simulated with 50% EE and 50%
EI.
Results
Esophagus elongation during 4DCT was median 11mm
(range 2-20mm) and from EE to DIBH 23mm (10-42mm).
Lung volume increased 13.3% (6.9-24.9%) from EE to EI and
63.5% (34.1-120.8%) from EE to DIBH. Absolute volume of
lung in the beam path either increased or remained largely
constant upon inspiration in all patients. In seven P, four
M and four D targets, the absolute volume of lung located
within 5 cm of the PTV increased; however, increase in
the total lung volume still resulted in either a reduction or
a largely unchanged percent lung volume located within 5
cm of the PTV. Results extracted from DiVHs are presented
in Table 1.
DIBH was the optimal treatment phase for all P and M
targets and 8/10 D targets. For all targets EE was the least
optimal phase.
Heart displacement was ≤12mm on 4DCT and ≤26mm from
EE to DIBH. Relative heart volume DiVH’s are shown in
Figure 1 for 3 patients. The same respiration phase is
clearly not optimal for all patients, neither for M nor for
D targets. EE was most optimal for heart sparing in two M
and three D, EI in four M and three D and DIBH in four M
and four D targets. EE was least optimal in four M and two
D, EI in two M and three D and DIBH in four M and five D
targets.
Liver had median displacement of 12mm on 4DCT and
43mm from EE to DIBH. It was only in the beam path for D
targets. Even though volume in the beam path decreased
with median 1.1% (EI) and 2.6% (DIBH) compared to EE, EE
was still optimal in 2 and DIBH only optimal in 5 patients.
Conclusion
Lung sparing can be achieved in DIBH for proximal, medial
and most distal esophagus targets. For some medial and
distal targets heart sparing can be achieved. As the
optimal phase is not always DIBH, lung vs. heart sparing
must be prioritized. No general conclusions can be drawn
for liver. Further investigations are warranted.
PO-0873 Inter- and intra-fraction motion of the tumor
bed and organs at risk during IGRT for Wilms' tumor
F. Guerreiro1, E. Seravalli1, G. Jansses1,2, M. Heuvel-
Eibrink2, B. Raaymakers1
1UMC Utrecht, Department of Radiotherapy and Imaging
Division, Utrecht, The Netherlands
2Princess Máxima Center, Pediatric
Oncology/Hematology, Utrecht, The Netherlands
Purpose or Objective
Radiotherapy planning for Wilms' tumor (WT) is currently
done according to the SIOP-2001 protocol. The planning
target volume (PTV) is defined as the clinical target
volume (CTV) plus a margin of 10-mm while no planning
risk volume (PRV) margins are recommended. The aim of
this study is to assess inter- and intra-fraction motion of
the tumor bed and organs at risk (OARs) as well as patient
positioning uncertainty to estimate PTV and PRV margins
for flank irradiation in WT.
Material and Methods
Computed tomography (CT), 4D-CT and daily cone-beam
CTs (CBCTs), acquired during planning and treatment of
10 pediatric patients (mean 3.9 ± 2.1 years) were used.
OARs (kidney, liver and spleen) were delineated without
accounting for any motion in all image sets. OARs motion
was quantified in terms of absolute displacements of the
center of mass (CoM) in all orthogonal directions. Tumor
bed motion estimation was assessed using a quadratic sum
of the CoM displacements of 4 clips positioned at the
superior, lateral, medial, and inferior border of the tumor
during surgical resection. Intra-fraction motion was
estimated by calculating the CoM displacements between
the maximum inspiration and expiration phases of the 4D-
CT. For inter-fraction motion assessment, CoM
displacements were calculated using the planning-CT as
reference and daily pre-treatment CBCTs.
For intra-fraction patient positioning uncertainty,
translational and rotational bone off-sets between the