ESTRO 36 Abstract Book

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S472 ESTRO 36 2017
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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. Kleijnen1, M. Intven1, B. Van Asselen1, A.M.
Couwenberg1, J.J.W. Lagendijk1, 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
GTV position. Purpose of this study is to investigate the
impact of a varying rectum filling on tumor position and
quantify potential tumor shifts.
Material and Methods
For the analysis, nine patients were included who were
scanned twice on MRI in supine position. First on a 1.5 T
MRI for diagnostic purposes and next on a 3T MRI for
treatment planning. For the diagnostic MRI, the rectum
was filled using an ultra sound transducer gel (MRIfull), and
for the planning MRI no rectal preparation was performed
(MRIstandard). On both MRIs the tumor was delineated.
To evaluate tumor displacement, for both MRIstandard and
MRIfull, three distances in cranial-caudal (CC) direction
were determined between the bony anatomy; i.e. the
sacrum promontory and the tumor cranial border, the
tumor caudal border and the center of mass (COM), (figure
1, C & D). For each distance measure, displacements were
then determined by taking the difference in distance
between both MRI scans.
Results
In all patients a shift in tumor COM in CC direction was
observed, ranging between 6.9 and 28.3 mm. Mean tumor
displacements between MRI standard and MRIfull were found to
be 16.7 mm, 16.5 mm and 17.7 mm for the cranial and
caudal tumor border and the COM, respectively (figure 1
C & D). Displacements were all found to be significantly
different from zero (p<0.002 for all distance measures).
Displacement was larger for tumors situated higher up in
the rectum (figure 2).
Conclusion
In all patients, tumor position changes considerably under
influence of rectal filling. The found mean displacements
are larger than the typical PTV-margins for rectal GTV
(Brierley et. al 2011). The higher situated rectal tumors
show the largest displacements under influence of rectal
filling. To avoid geometrical miss of the tumor, rectal
volume preparation prior to boost radiotherapy or
adaptive RT with online tumor visualization using MRI
(Lagendijk et al. 2008) seems beneficial. Especially for
tumors located high in the rectum.