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S477
ESTRO 36
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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 (MRI
full
), and
for the planning MRI no rectal preparation was performed
(MRI
standard
). On both MRIs the tumor was delineated.
To evaluate tumor displacement, for both MRI
standard
and
MRI
full
, 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 MRI
full
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.
Poster: Physics track: Adaptive radiotherapy for inter-
fraction motion management
PO-0875 Dosimetric effects of anatomical changes in
proton therapy of head and neck (H&N) cancer
G. Miori
1,2
, L. WIdesott
1
, F. Fracchiolla
1
, S. Lorentini
1
, P.
Farace
1
, R. Righetto
1
, C. Algranati
1
, M. Schwarz
1,3
1
Trento Hospital, Protontherapy, Trento, Italy
2
University of Rome Tor Vergata, Postgraduate School of
Medical Physics, Rome, Italy
3
INFN, TIFPA, Trento, Italy
Purpose or Objective
Anatomical changes in H&N patients can affect dose
distributions especially in proton therapy. A retrospective
analysis of H&N patients undergoing repeat CTs and
treated at our Proton Therapy Center was done to
evaluate dose changes and to identify a dosimetric index
for the need of replanning. Furthermore, TCP analysis was
performed to evaluate the magnitude of changes with
radiobiological parameters. Finally, non-adapted and
adapted plans were compared.
Material and Methods
All H&N patients treated in our center between October
2014 and September 2016 with at least one repeat CT
(eCT) were considered. 21 patients were identified: 18
patients had at least one eCT (1 to 6 eCTs), but did not
need replanning, and 3 patients needed replanning at
some stage of the treatment. The original plan was
recalculated on each eCT. Differences were calculated for
each treatment fraction, considering a stepwedge
interpolation on fractions where the eCT was missing. D1
variations (ΔD1) for cord, brainstem, optic chiasm and
optic nerves, and Dmax differences (ΔDmax) for lenses
were considered. Target coverage analysis was based on
differences in CTV V95 (ΔV95). ΔV95 values were included
in
Non-replanned
(
controls
) if they came from non-
replanned patients or from replanned patient calculations
on CT preceding the replanning CT (rCT). On the contrary,
ΔV95 were included in
Replanned
(
cases
) if they came
from replanned patients on the rCT and the following CTs.
The choice was made to consider the trend in target
coverage after the point identified for replanning. A cut-
off ΔV95 for the need of replanning was identified by the
maximum Youden’s index on the ROC analysis between
control
and
cases.
Next, TCP differences with respect to
the planning TCP (ΔTCP) were calculated. ΔTCP values
were divided in
Non-replanned
and
Replanned
as for DV95
analysis. Finally, a comparison between adapted and non-
adapted plans for the 3 replanned patients was done. All
statistics were made by t-Student tests.