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Conclusion
The use of the variable RBE-model results in substantially
higher predicted doses to the CTV compared to the
constant 1.1, due to the low α/β associated with breast
cancer. Substantially higher RBE values are also predicted
for the OARs. This decreases the potential benefit with
protons, but could probably be neglected in cases where
the physical doses are low. However, if e.g. the LAD is
close to the target this could lead to substantially higher
predicted doses. The variable RBE could therefore be of
importance in certain cases when employing a NTCP model
based comparison between proton and photon plans.
PO-0833 Reducing small bowel dose for cervical cancer
using IMPT and target tailoring in treatment planning
P. De Boer
1
, A.J.A.J. Van de Schoot
1
, H. Westerveld
1
, M.
Smit
1
, M.R. Buist
2
, A. Bel
1
, C.R.N. Rasch
1
, L.J.A. Stalpers
1
1
Academic Medical Center, Radiation Oncology,
Amsterdam, The Netherlands
2
Academic Medical Center, Gynaecology and Obstetrics,
Amsterdam, The Netherlands
Purpose or Objective
Current radiotherapy standards for cervical cancer
patients lead to irradiation of large bowel volumes and
bladder during external beam radiotherapy (EBRT). Highly
conformal techniques such as IMRT, arc-rotation therapy
and image guided adaptive radiotherapy (IGART) have
resulted in considerable reduction in volume to organs at
risk (OARs), but there remains room for further
improvement. We previously showed that cervical invasion
into the uterine corpus assessed by MRI correlates well to
pathological invasion [1]. In the present study we wish to
investigate the potential clinical benefit from target
tailoring by excluding the tumor free proximal part of the
uterus during IGART. Furthermore, we compare this
benefit with the advantage of an improved dose
conformity by intensity-modulated proton therapy (IMPT).
Material and Methods
Diagnostic MRIs and planning-CTs from eleven patients
with locally advanced cervical cancer were used; all
previously had photon radiotherapy and a substantial (>4
cm) tumor-free part of the proximal uterus as visualized
by MRI. IGART and robustly optimized IMPT plans were
generated for both conventional target volumes (including
the entire uterus), and MRI-based target tailoring
(excluding the non-invaded proximal part of the uterus),
which yielded four treatment plans per patient. For each
plan, V
15Gy
, V
30Gy
, V
45Gy
and D
mean
for bladder, sigmoid,
rectum and bowel bag were compared. The clinical
benefit of either and both approaches were estimated by
calculating the normal tissue complication probability
(NTCP) for at least grade II acute small bowel toxicity.
Results
Both IMPT or target tailoring by excluding the proximal
uterus resulted in significant reductions of V
15Gy
, V
30Gy
,
V
45Gy
and D
mean
for bladder and small bowel. Compared to
conventional volumes, target tailoring by excluding the
non-invaded uterus resulted in an average reduction of the
primary ITV and PTV of 37% and 8%, respectively. IMPT
would have reduced the estimated NTCP for small bowel
toxicity (≥grade 2) from 25% to 18%, and would be
additionally reduced to 9% when IMPT were combined with
MRI-based target tailoring. Major NTCP reductions of >10%
were predicted in four patients (36%) by IMPT, and in six
patients (55%) when IMPT were combined with MRI-based
target tailoring. Patients benefitted most (NTCP reduction
>10%) from one of the investigated approaches if the V
45Gy
for bowel cavity was >275 cm
3
during standard IGART
alone; a similar reduction in NTCP from the combined
approached would have been obtained in patients with a
V
45Gy
for bowel cavity >200 cm
3
.
Conclusion
In patients with cervical cancer, both 1) proton therapy
and 2) target tailoring by excluding the radiologically
uninvolved part of the uterine corpus led to a significant
dose reduction to surrounding OARs, which separately
would already yield a clinically important decrease in
small bowel toxicity, which is cumulative if both
approaches would be combined.
Reference
[1] de Boer P, Bleeker MCG, Spijkerboer AM, et al. Eur J
Radiol Open. 2015;2:111–7.
PO-0834 Automated planning to reduce integral dose
in robotic radiosurgery for benign tumors
L. Rossi
1
, A. Méndez Romero
1
, M. Milder
1
, E. De Klerck
1
,
S. Breedveld
1
, B. Heijmen
1
1
Erasmus Medical Center, Radiation Oncology,
Rotterdam, The Netherlands
Purpose or Objective
Highly conformal dose distributions and minimizing
integral dose are essential in radiosurgery of benign
vestibular schwannoma (VS) tumors to avoid long term
side effects. This includes avoidance of secondary tumor
induction in these long surviving patients. High delivery
accuracy can be obtained with the robotic CyberKnife (CK,
Accuray Inc, Sunnyvale, USA) due to real time image-
guided tracking, allowing small PTV margins. However,
optimal plan quality may be hampered by the current
trial-and-error planning approach, as it strongly depends
on the planner’s experience and available planning time.
We have developed a system for fully automated
CyberKnife treatment planning. In this study, we have
used this system to automatically generate plans for
vestibular schwannoma patients (AUTOplan) and we have
compared them with plans that were manually generated
in clinical routine (MANplan), both with the IRIS
collimator.
Material and Methods
Both MANplans and AUTOplans were genereated with the
Multiplan TPS (Accuray Inc). For AUTOplanning, a fully
automatic pre-optimization was performed with our in-