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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. Guerreiro
1
, E. Seravalli
1
, G. Jansses
1,2
, M. Heuvel-
Eibrink
2
, B. Raaymakers
1
1
UMC Utrecht, Department of Radiotherapy and Imaging
Division, Utrecht, The Netherlands
2
Princess 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
planning-CT and post-treatment CBCTs were recorded.
Inter-fraction patient positioning uncertainty was null as
online patient position correction was always performed.
Margins were determined by combining systematic (Σ) and
random (σ) errors. Van Herk (2.5∑ + 1.7σ) and McKenzie
(1.3∑ ± 0.5σ) analytic solutions were used for PTV and PRV
margin expansions, respectively.
Results
Tumor bed and OARs mean CoM displacements were less
than 3-mm for all directions for both inter- and intra-
fraction motion. Largest displacements were seen in the
cranio-caudal (CC) direction (Figure 1). Inter-fraction
motion was larger than intra-fraction motion (Figure 1).
Mean intra-fraction patient positioning uncertainty was
considered negligible (translation <1-mm; rotation <1
o
). Σ
and σ errors differed less than 2.5-mm for organ motion
and 0.5-mm for patient positioning uncertainty. The
calculated PTV and PRV margins (Table 1) were up to a
maximum of 6/5-mm in the CC direction, respectively.
Conclusion
Imaging data collected before and during radiotherapy
demonstrated limited motion of the tumor bed and OARs
and reduced patient positioning uncertainty. By combining
4D-CT and daily CBCTs information, PTV margins can be
reduced to 6-mm in the CC direction compared with the
existing protocol. The use of PRV margins for OARs
protection is also advised. In addition, margins should be
applied anisotropically and individualized for each
patient.
PO-0874 The impact of rectal filing on rectal tumor
position
J.J.E. Kleijnen
1
, M. Intven
1
, B. Van Asselen
1
, A.M.
Couwenberg
1
, J.J.W. Lagendijk
1
, B.W. Raaymakers
1
1
UMC Utrecht, Radiotherapy department, Utrecht, The
Netherlands
Purpose or Objective
In 15% of rectal cancer patients, a pathological complete
response (pCR) is observed after neo-adjuvant
chemoradiotherapy. To increase this pCR rate, many
studies are being performed, in which the GTV dose is
escalated. To avoid an increase in toxicity and potential
surgical complications, PTV margins must be minimized
and geometrical miss has to be avoided. However, rectal
filling can change from day-to-day as can be observed in
daily practice (see figure 1, A & B), which might alter the