458 / 1096
S443
ESTRO 36
_______________________________________________________________________________________________
Germany
2
German Cancer Consortium DKTK, Partner Site Freiburg,
Freiburg, Germany
3
Medical Center University of Freiburg - Faculty of
Medicine - University of Freiburg, Department of
Radiation Oncology, Freiburg, Germany
4
Medical Center University of Freiburg - Faculty of
Medicine - University of Freiburg, Department of
Pathology, Freiburg, Germany
5
Medical Center University of Freiburg - Faculty of
Medicine - University of Freiburg, Department of Nuclear
Medicine, Freiburg, Germany
6
University of North Carolina, Department of Radiation
Oncology, North Carolina, USA
7
Karolinska Institutet - Stockholm University,
Department of Medical Radiation Physics, Stockholm,
Sweden
Purpose or Objective
The goal of this work is to show the technical feasibility
and to evaluate the normal tissue complication probability
(NTCP) and the tumor control probability (TCP) of the
intensity modulated radiation therapy (IMRT) dose
painting technique using
68
Ga-HBED-CC PSMA-PET/CT in
patients with primary prostate cancer (PCa).
Material and Methods
We studied 10 RT plans of PCa patients having PSMA-
PET/CT scans prior to radical prostatectomy. One contour
was semi automatically generated for each patient on the
basis of the 30% of SUVmax within the prostate (GTV-PET).
For each patient, two IMRT plans were generated: PLAN
77
,
which consisted of whole-prostate radiation therapy to 77
Gy in 2.2 Gy per fraction; PLAN
95
, which consisted of
whole-prostate RT to 77 Gy in 2.2 Gy per fraction, and a
simultaneous integrated boost to the GTV-PET to 95 Gy in
2.71 Gy per fraction. The feasibility of these plans was
judged by their ability to reach prescription doses while
adhering to the FLAME trial protocol. Comparisons of TCPs
based on co-registered histology after prostatectomy
(TCP-histo) and normal tissue complication probabilities
(NTCP) for rectum and bladder were carried out between
the plans.
Results
Prescription doses were reached for all patients plans
while adhering to dose constraints. The mean doses on
GTV-histo for [Plan
77
and Plan
95
] were 75.8±0.3 Gy and
96.9±1 Gy, respectively. In addition, TCP-histo values for
Plan
77
and Plan
95
were 70±7 %, and 95.7±2 %, respectively.
PLAN
95
had significantly higher TCP-histo (p<0.0001)
values than PLAN
77
. There were no significant differences
in rectal (p=0.563) and bladder (p=0.3) NTCPs between
the 2 plans.
Conclusion
IMRT dose painting for primary PCa using
68
Ga-HBED-CC
PSMA-PET/CT was technically feasible. A dose escalation
on GTV-PET resulted in significantly higher TCPs without
higher NTCPs.
PO-0825 Multi-scenario sampling in robust proton
therapy treatment planning
E. Sterpin
1
, A. Barragan
2
, K. Souris
2
, J. Lee
2
1
KU Leuven, Department of Oncology, Leuven, Belgium
2
Université catholique de Louvain, Molecular imaging-
radiotherapy and oncology, Brussels, Belgium
Purpose or Objective
Beam specific PTVs (BSPTV) or robust optimizers are
superior to conventional PTVs for ensuring robustness of
proton therapy treatments. In these planning strategies,
realizations ('scenarios”) of a few types of uncertainties
are simulated: errors in patient setup, CT HU conversion
to stopping powers, and, more recently, breathing
motion. However, baseline shifts of mobile targets should
also be taken into account, which complicates the
sampling of the space of possible scenarios. We compare
here several sampling strategies. We will also show that
current robust optimizers sample scenarios in a
statistically inconsistent way.
Material and Methods
Sampling must optimize the trade-off between clinical
optimality and robustness. Both were assessed by
computing the volume of the BSPTV and a confidence
interval (CI), respectively. The latter is defined as the
percentage of all possible ranges and beam positions that
the BSPTV encompasses. The findings can then be applied
later to robust optimizers.
We have designed a simulation phantom to model
uncertainties in lung tumors (Figure 1). Standard
deviations of the Gaussian distributions for (systematic)
setup errors, baseline shifts, and CT conversion errors
were 5 mm, 5 mm, and 2%, respectively. The errors were
sampled following three different methods:
1.
M1 (conventional approach): sampling of setup
errors and baseline shifts within conventional
lateral PTV margin for systematic errors
(encompassing 90% of possible beam positions).
The distal and proximal margins encompass 98%
of possible proton ranges scaled by a flat CT
conversion error (±3.3% to include 90% of
possible CT conversion errors).
2.
M2: same as M1 with random sampling of the CT
conversion error.
3.
M3: all errors are simulated within an iso-
likelihood hypersurface including 90% of all
possible scenarios.
A fixed breathing-induced motion amplitude of 1 cm has
been considered for every scenario.
Results
BSPTVs equaled 430, 420 and 564% of the CTV volume for
the three methods, respectively (see figure 2 that
illustrates the range margins). M1 does not ensure
statistical consistency because of the flat CT conversion
error, which overemphasizes unlikely scenarios (large
geometrical AND large CT conversion errors) and makes
non-trivial the computation of the CI. M3 guarantees at
least 90% CI, but with a 34% increase of the irradiated
volume. The latter is due to the non-prioritization of