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ESTRO 36
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PO-0877 Proton therapy of oesophageal cancer is more
robust against anatomical changes than photons
D.S. Møller
1
, M. Alber
2
, T.B. Nyeng
1
, M. Nordsmark
3
, L.
Hoffmann
1
1
Aarhus University Hospital, Department of Medical
Physics, Aarhus C, Denmark
2
Heidelberg University Hospital, Department of
Radiation Oncology, Heidelberg, Germany
3
Aarhus University Hospital, Department of Oncology,
Aarhus C, Denmark
Purpose or Objective
Anatomical changes such as changes in the mediastinum
and the diaphragm position are seen in oesophageal
cancer patients during the course of radiotherapy. Field
entrance through areas with a high risk of changes is often
unavoidable with intensity modulated photon
radiotherapy (IMRT) if target conformity and reduction of
dose to especially lungs and heart is pursued. Delivery of
proton therapy is highly sensitive to anatomical changes,
but using only one posterior field may avoid high risk
entrances. We investigate the sparing of normal tissue and
the potential gain in robustness towards anatomical
changes using intensity modulated proton therapy (IMPT)
instead of IMRT.
Material and Methods
Twenty-six consecutive patients with medial or lower
oesophageal or gastroesophageal junction(GEJ) cancer
treated with IMRT (5-8 fields) were retrospectively
planned with IMPT using one posterior beam. The
fractionation schedules were either 41.4 Gy/23fx (pre-
operative regime, 22 patients) or 50Gy/27fx (definitive
regime, 4 patients). To ensure dose coverage of the CTV
for photon plans, a PTV (5 mm AP, 5mm LR, 8 mm CC) was
used to account for uncertainties in planning and
delivery. For protons, three different strategies were
pursued. Robust optimization of the CTV (IMPT
CR
), robust
optimization of the CTV and full coverage of the PTV
(IMPT
PR
) and no robust optimization, but full coverage of
the PTV (IMPT
P
). Robust optimization was performed
accounting for 3mm isocenter shifts and 3% density
uncertainty.
IMRT and IMPT plans were compared in terms of dose to
lungs and heart. For all patients, an additional
surveillance CT-scan was obtained at fraction 10 and used
for recalculation of both IMRT and IMPT plans, analysing
the percentage of CTV receiving 95% of the prescribed
dose.
Results
Using IMPT instead of IMRT reduced the lung and heart
dose significantly regardless of the IMPT strategy (p<0.001
using a Wilcoxon signed rank test). The mean lung and
heart doses decreased from sample median = 8.7Gy
[1.6;16.3] and 17.1Gy [1.1;24.1] using IMRT to 2.2 Gy
[0.5;8.5] and 9.1 Gy [0;15.5], using IMPT
PR
.
Recalculation on the surveillance scans demonstrated that
7/26 (27%) IMRT plans showed CTV coverage < 99%. For
protons, IMPT
PR
plans were the most robust and only 4/26
(15%) decreased in coverage to below 99%. The CTV
coverage for all patients and plans are shown in Fig 1. The
most common anatomical changes are lateral target
deformations, enlargement of mediastinum and changes
in diaphragm position. The posterior proton plans are
sensitive to target deformations, while the multiple
photon fields are sensitive to all three types of changes
(see
Fig.
2).
Conclusion
Treating oesophageal cancer with protons has the
advantage of decreasing dose to organs at risk and at the
same time it improves the robustness towards common
anatomical changes. Frequent imaging is still needed to
identify patients with target deformations requiring
adaptive treatment planning.
PO-0878 Plan adaptation on the MR-Linac: first
dosimetric validation of a simple dose shift
R. Koopman
1
, A.J.A.J. Van de Schoot
1
, J. Kaas
1
, T. Perik
1
,
T.M. Janssen
1
, U.A. Van der Heide
1
, J.J. Sonke
1
1
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiation Oncology, Amsterdam, The
Netherlands
Purpose or Objective
Patient positioning on the MR-Linac (MRL; Elekta AB,
Stockholm) requires online plan adaptations to correct for
setup errors due to the fixed couch position. The aim of
our study was to validate the size and direction of such
plan adaptations (simple dose shifts) and evaluate
dosimetric differences for rectum cancer patients.