ESTRO 36 Abstract Book

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S448 ESTRO 36
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Purpose or Objective
Proton therapy, with or without breathing control
techniques, may be used to reduce the cardiopulmonary
burden in breast cancer radiotherapy. However, such
studies typically assume a constant RBE of 1.1 for protons.
This study aims to assess the impact of using a variable
RBE in breast proton radiotherapy and to evaluate the
sensitivity to respiratory motion when no breathing
control is applied.
Material and Methods
Tangential photon IMRT and 3-fields proton IMPT plans for
breast radiotherapy were generated for twelve patients,
both on a free-breathing (FB) CT and on a CT using breath-
hold-at-inhalation (BHI). 2 Gy(RBE) per fraction in 25
fractions were planned to the whole breast. The physical
proton dose was optimized assuming a constant RBE of 1.1.
Besides the constant RBE of 1.1, the variable RBE-model
by Wedenberg et al. (2013), assuming an α/β of 3.5 Gy for
the CTV/PTV and 3 Gy for the OARs, was used for plan
evaluation. Subsequently, the FB plans were recalculated
on the CT images of the two extreme phases (inhale and
exhale) to evaluate the sensitivity of a treatment delivery
without breathing control.
Results
All photon and constant RBE proton plans met the clinical
goals with similar target coverage. The target conformity
and homogeneity of the proton plans were superior to the
photon plans. The plan quality was generally independent
on whether the FB or BHI CT-scan was used. However, if
the heart was close to the target, the BHI plan lowered
the dose to the left anterior descending (LAD) artery in
most cases. Applying the variable RBE-model resulted in
an average of the mean RBE of 1.18 for the PTV and also
increased the heterogeneity. The predicted RBE values in
the OARs were also substantially higher than 1.1.
However, due to the low physical doses, this is expected
to have a minor impact. The dosimetric parameters for the
BHI plans are shown in Table 1.
The recalculation of the FB plans on the extreme phases
generally resulted in minor differences for the CTV
coverage and OAR doses for the proton plans. Small CTV
volumes may, however, receive a slightly lower dose for
the recalculated photon FB plans. The ranges of
dosimetric parameters for the FB plan for one patient are
shown in Table 2.
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 Boer1, A.J.A.J. Van de Schoot1, H. Westerveld1, M.
Smit1, M.R. Buist2, A. Bel1, C.R.N. Rasch1, L.J.A. Stalpers1
1Academic Medical Center, Radiation Oncology,
Amsterdam, The Netherlands
2Academic 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, V15Gy, V30Gy, V45Gy and Dmean 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.