ESTRO 36 Abstract Book

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S105 ESTRO 36 2017
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optimization have been described in literature.
‘Minimax’ robust optimization is a relatively
straightforward implementation and is currently
incorporated in several treatment planning systems that
are commercially available. During minimax robust
optimization, dose-influence matrices are typically
calculated for the nominal scenario (without treatment
errors) and for a number of user-defined error scenarios,
and are subsequently used to optimize worst-case values.
The user can generally specify the number of included
error scenarios and the magnitude of the treatment errors
accounted for. In this way, one can account for errors in
patient setup and in particle range, and, in some
implementations, for anatomical changes.
The characteristics and practicalities of minimax robust
optimization in intensity-modulated proton therapy (IMPT)
for oropharyngeal cancer patients will be addressed in this
presentation:
1. Robustness recipes: Which robust optimization settings
(i.e. error scenarios) should be used for given population-
based distributions of setup and range errors
(systematic/random), in order to obtain adequate clinical
target volume (CTV) coverage in oropharyngeal cancer
patients? Available robustness recipes differ between
patients with unilateral or bilateral tumors and suggest
that setup errors and range errors can be accounted for
independently.
2. The price of robustness: What does robustness cost in
terms of dose to organs-at-risk (OARs)? An investigation on
the impact of the degree of robustness (i.e. magnitude of
the included error scenarios) on OAR doses and resulting
normal-tissue complication probabilities showed that
setup robustness had a substantially larger impact than
range robustness. This suggests that minimizing setup
errors should be given a higher priority than minimizing
range errors, in IMPT treatments for oropharyngeal cancer
patients.
3. Minimax robust optimization to account for anatomical
uncertainties. Anatomical robust optimization can
effectively deal with changes in nasal cavity filling,
providing substantially improved CTV and OAR doses
compared with the conventional margin-based approach.
Future investigations should reveal whether minimax
robust optimization can also be used to account for other
anatomical changes in oropharyngeal cancer patients.
SP-0211 Clinical implementation of coverage
probability planning in cervix cancer
J.C. Lindegaard1, A. Ramlov1, M. Assenholt1, M. Jensen 1,
C. Grønborg1, R. Nout2, L. Fokdal1, K. Tanderup1, M.
Alber3
1 Aarhus University Hospital, Department of Oncology,
Aarhus C, Denmark
2 Leiden University Medical Center, Department of
Radiation Oncology, Leiden, The Netherlands
3 Heidelberg University Hospital and Heidelberg Institute
for Radiation Oncology HIRO, Department of Radiation
Oncology, Heidelberg, Germany
Definitive radiotherapy in locally advanced cervical cancer
(LACC) often includes boosting of multiple pathological
pelvic nodes. The simultaneous integrated boost (SIB)
technique delivered by intensity modulated radiotherapy
(IMRT) or volumetric arc therapy (VMAT) is increasingly
being used as recent studies have shown excellent nodal
control with a boost of 55-60 Gy. However, nodal boosting
on top of elective whole pelvic radiotherapy at 45-50 Gy
invariably causes collateral higher dose to especially
bowel and pelvic bones, as metastatic regional nodes in
LACC are most often situated in the retroperitoneal
lymphatic space close to both bowel loops and the pelvic
wall. This dilemma may be even worse in situations where
para-aortic nodes are encountered and require
irradiation.
At present no consensus exists on the required margin for
nodal boosting by SIB, but margins of 5-10 mm from the
gross tumor volume of the node (GTV-N) to the nodal
planning target volume (PTV-N) have been
reported. Since the diameter of pathological nodes (GTV-
N) most often is about 10-20 mm, SIB dose planning using
a classical PTV concept of a dose plateau with full PTV-N
coverage will entail a relatively large volume being
treated to high doses compared to the actual GTV-N
volume. In addition, the robustness of SIB being embedded
in the 45-50 Gy irradiation of the whole pelvis is not fully
utilized.
Coverage probability treatment planning (CovP) has
previously been shown to provide robust dose escalation
for IMRT of prostate cancer with overlapping PTV and
rectum planning volume as well as superior patient
specific small bowel planning volume allowing for tighter
OAR margins with for instance para-aortic radiotherapy.
Reduction of the dose at the perimeter of the PTV-N could
therefore be considered by employing coverage
probability dose planning (CovP) for SIB in LACC. With
CovP local weights for each voxel are being used to create
a dose gradient at the edges of PTV-N according to the
presumed probability of finding the nodal target at this
coordinate in the treatment room. The shape of the fall-
off is based on assumptions about the position error of the
GTV-N. CovP has recently been implemented in the
prospective international multicentre EMBRACE II study
for SIB planning of nodal boosting in LACC
).
Clinical validation and implementation of CovP treatment
planning in LACC was performed at Aarhus University
Hospital in 2015 as a preparation for the Embrace II study.
Until then CovP had only been explored by use of
experimental treatment planning systems. A first step was
therefore to obtain a set of dose constraints based on a
number of CovP dummy runs performed in the research
dose planning software Hyperion. Based on assumptions
regarding the position of GTV-N over time, the dose
optimizer created a dose gradient around the CTV-N which
was allowed to lie partially inside PTV-N. From these
experimental CovP plans, dose constraints for use with the
clinical treatment planning system Eclipse were chosen
that captured the dose peak and dose gradient of the CovP
dose distribution for this particular setting of SIB boosting
in LACC: PTV-N D98 >90%, CTV-N D98 > 100% and a soft
constraint of CTV-N D50 > 101.5% of the prescribed dose.
The next step was then to analyze a number of previously
treated patients with LACC. In total 25 patients with 47
boosted nodes treated with SIB delivered by IMRT or VMAT
from 2012-2015 were investigated (Figure 1). Dose of
EBRT was 45 Gy/25 fx with a SIB of 55-57.5 Gy depending
on the expected dose from brachytherapy (BT). The
planning aim was to reach D98 > 57 GyEQD2. Nodes were
contoured on cone beam CT (CBCT) and the accumulated
dose in GTV-NCBCT and volume of body, pelvic bones and
bowel receiving >50 Gy (V50) were determined. Nearly all
nodes (89%) were visible on CBCT and showed considerable
regression . Total EBRT and BT D98 was >57 GyEQD2 in 98%
of the visible nodes. Compared to conventional planning,
CovP significantly reduced V50 of body, bones and bowel.
With CovP a new tool is available for nodal SIB in LACC
allowing for controlled underdosing at the edge of the
PTV. As this study is mainly based on pelvic nodes along
the major vessels it is still unclear how margin reduction
and CovP will perform for SIB of para-aortic nodes or nodes
in the groins. Nodes in the vicinity of organs which may be
displaced e.g. by the bladder or rectum may also need
monitoring in terms of delivered dose and eventually plan
adaptation during EBRT. However, CovP could be of
interest for nodal SIB in anal, rectal, vulvar, penile,
vaginal, prostate and bladder cancer. In EMBRACE II the
patients are treated with a reduced PTV-N margin (5 mm),
daily IGRT, IMRT/VMAT and CovP planning for SIB with
planning aims presented above. With an estimated accrual
of 800 patients, of which 50% will node positive disease, a