571 / 1096
S556
ESTRO 36
_______________________________________________________________________________________________
approach did so due to poor PTV coverage (n=5) or
unacceptable lung dose (n=2). Of the 18 patients who had
an acceptable plan generated (regardless of planning
approach), one was achieved with the 8-fraction regime,
with the remaining needing the 12-fraction regime. OAR
PRV max doses were 2-3.5% over the OAR dose for the
conventional and hybrid approaches and 6% for the SABR
approach, highlighting the need for IGRT.
Conclusion
SABR was feasible for approximately half of the locally-
advanced NSCLC patients we assessed and for almost all
of these cases only a 12-fraction scheme was feasible. If
the alternative to SABR is no treatment at all,
compromises to tumour coverage or OAR tolerances may
be acceptable, increasing feasibility. This data will inform
a phase I study testing the safety of SABR for locally
advanced NSCLC.
Poster: RTT track: Image guided radiotherapy and
verification protocols
PO-1009 Evaluation of setup margins using cone-beam
CT for prostate and pelvic nodes irradiation
A. Van Nunen
1
, T. Budiharto
1
, B. De Vocht
1
, D. Schuring
1
1
Catharina Ziekenhuis, Radiotherapie, Eindhoven, The
Netherlands
Purpose or Objective
In 2014 radiotherapy for prostate and pelvic nodes was
introduced in the Catharina hospital. For this tumour site,
CBCT is used for position verification. Due to variation in
prostate position in relation to lymph nodes, large setup
margins are required to deliver the correct target dose to
both volumes. A CTV-PTV margin of 1 cm is used for both
prostate and lymph nodes. The aim of this study was to
evaluate the required setup margins using different
correction and registration strategies.
Material and Methods
CBCT-scans of 20 patients were included in this study. 220
scans were analysed retrospectively. Patients were
treated with an offline SAL correction protocol with an
initial action level of 10 mm and a maximum number of 3
measurements. When large day-to-day variations were
observed, an online correction protocol was performed.
All CBCT-scans were registered automatically using a grey
value, seed or bone match algorithm of the XVI software
(Elekta, Crawley, UK). For these automatic matches either
a clipbox containing bony structures and the entire PTV, a
mask consisting of the prostate or a mask consisting of
lymph nodes CTV was used (figure 1). Registration of the
lymph node area was performed to determine the
correlation between bony anatomy and the position of the
pelvic lymph nodes. For all these registrations all
translations, rotations and table corrections were
collected. From these results the random and systematic
setup errors were determined. The required setup
margins were then calculated using the margin recipe M =
2.5Σ+0.7σ (Σ: systematic error, σ: random error).
Results
There was a large correlation between bony structures and
lymph nodes in all directions (correlation coefficient >
0.82). Correlation between bony structures and the
prostate position was large in lateral direction and small
in longitudinal and vertical direction due to large variation
in rectal filling. This resulted in larger margins in this
direction. The
required setup margins are summarised in
Table 1. In this margin calculation, we did not account for
rotations, intrafraction variation, delineation, treatment
and match uncertainties.
When applying online position verification on the prostate,
setup margins for lymph nodes must be 0.19 cm (lateral),
0.73 cm (longitudinal) and 0.57 cm (vertical). When
applying online setup correction on bony structures, setup
margins for the prostate must be 0.25 cm, 0.73 cm and
0.73 cm respectively. Offline setup correction on prostate
resulted in the largest margins for both volumes.
Conclusion
The required setup margin depends on the applied
correction strategy. When applying position verification
on bony structures, larger margins are required for CTV
prostate. When applying position verification on prostate,
larger margins are required for CTV lymph nodes. When
applying these margins clinically, additional margins are
needed to account for rotations, intrafraction variation,
delineation, treatment and match uncertainties.
PO-1010 Investigation of reproducibility of bolus
position based on kV CBCT imaging
E. Dabrowska
1,2
, P. Czuchraniuk
3
, M. Gruda
1
, P.
Kukolowicz
1
, A. Zawadzka
1
1
Maria Sklodowska-Curie Memorial Cancer Center and
Institute of Oncology, Department of Medical Physics,
Warsaw, Poland
2
Faculty of Physics University of Warsaw, Department of
Biomedical Physics, Warsaw, Poland
3
Maria Sklodowska-Curie Memorial Cancer Center and