ESTRO 36 Abstract Book

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S188 ESTRO 36
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plans needs to be built to achieve the full potential of
ART.
OC-0354 Dosimetric impact of anatomical changes in
photon and particle therapy for pancreatic cancer
K. Crama 1, A.C. Houweling1, J. Visser1, K. Fukata2, C.R.N.
Rasch1, T. Ohno2, A. Bel1, A. Van der Horst1
1 Academic Medical Center, Department of Radiation
Oncology, Amsterdam, The Netherlands
2 Gunma University, Gunma University Heavy Ion Medical
Center, Maebashi, Japan
Purpose or Objective
Radiotherapy of pancreatic tumors is toxic due to the high
dose to surrounding organs-at-risk (OARs). Irradiation with
charged particles is characterized by a sharp dose fall-off
around the target area. Compared to photon therapy,
OARs can be further spared while delivering a high dose to
the tumor. Treatment planning studies in pancreatic
cancer patients have shown this benefit of charged
particle therapy over photon therapy.
However, intra- and interfractional changes may greatly
affect the robustness of particle therapy. Past studies only
investigated differences in planned dose; studies
comparing the robustness of different modalities have not
been published yet.
We compared the dosimetric impact of interfractional
anatomical changes (i.e. body contour differences,
gastrointestinal gas volume changes and setup errors) in
photon, proton and carbon ion therapy for pancreatic
cancer patients. Intrafractional changes were not taken
into account in this study.
Material and Methods
Photon, proton and carbon ion treatment plans (36 Gy, 12
fractions) were created for 9 patients. For the particle
therapy plans, the relative radiobiological effectiveness
was taken into account. To simulate daily online setup
correction, the CBCTs were rigidly registered (only
translations) to the planning CT using fiducial markers.
Fraction dose calculation was then made possible by
deformable registering the planning CT to each of the 12
CBCTs. Gastrointestinal gas was delineated on each CBCT
and copied to the deformed CT, a relative density override
was applied for dose calculation (0.01).
Fraction doses were accumulated rigidly. To compare
planned and accumulated dose, for each radiotherapy
modality, dose volume histogram (DVH) parameters of the
planned and accumulated dose were determined for the
internal gross tumor volume (iGTV), internal clinical
target volume (iCTV) and OARS (duodenum, stomach,
kidneys, liver and spinal cord).
Results
Photon plans were highly robust against interfractional
anatomical changes. The difference between planned and
accumulated DVH parameters for the photon plans was
≤0.5% for the target and OARs. For proton therapy,
coverage of the iCTV was considerably reduced for the
accumulated compared to the planned dose: the mean
near-minimum dose (D98%) of the iCTV reduced from
98.1% to 90.3% [79.4%–95.3%](Figure). For carbon ion
therapy it was even worse; D98% was reduced with 10%,
from 98.6% to 88.6% [80.7%–92.7%]. The DVH parameters
of the OARs differed ≤3% between both particle
modalities. For all modalities the near-maximum dose
(D2%) did not differ.
Conclusion
Photon therapy is highly robust against interfractional
anatomical changes and setup errors in pancreatic cancer
patients. However, in particle therapy with either protons
or carbon ions, severe reductions in target dose coverage
were observed. Implementation of particle therapy for
pancreaticcancer patients should be done with great care
and interfractional anatomical changes must be accounted
for.
OC-0355 Which anatomical changes in Head&Neck
cancer lead to Repeat CT/planning?
S. Van Beek1, O. Hamming-Vrieze1, A. Al Mamgani1, A.
Navran1, J. Van de Kamer1, P. Remeijer1
1The Netherlands Cancer Institute, Department of
Radiation Oncology, Amsterdam, The Netherlands
Purpose or Objective