ESTRO 36 Abstract Book

90 / 1096

S77 ESTRO 36
_______________________________________________________________________________________________
One beam (1 GyE) was applied to a CIRS head phantom and
monitored with the PGI slit camera. To investigate the
influence of the spot dose, the same beam with 5 GyE was
also delivered and measured. Global and local (5 cm in
diameter) range shifts were introduced and the PGI
profiles (prompt-γ counts over depth) with and without
shifts were compared. Sum profiles containing prompt-γ
counts over the entire fraction were used for the
comparison of DS and PBS. Moreover, PGI profiles
measured in PBS were analyzed spot-wise and will also be
compared with simulated profiles for absolute range
determination.
Results
A good agreement between introduced and measured
global shifts was found in the sum profile evaluation for
both modalities, PBS and DS (Table 1). Relative
differences were below 2, 7 and 12 % for the 10, 7 and 4
mm shifts, respectively. Local shifts are not detectable
using sum profiles. For the applied local shifts, a spot-wise
comparison of PGI profiles in PBS allows the detection and
localization of global and local shifts (Figure 1). For
interpretation, neighboured spots should be clustered, as
shifts detected for single spots are less reliable due to low
statistics. Higher doses (5 vs. 1 GyE) allow the detection
of smaller shifts as shown in Figure 1 for the 4 mm local
shift.
Table 1: Measured global shifts between sum profiles in DS
and PBS with 1 and 5 GyE.
Figure 1: Spot-wise analysis of the determined range
shifts: The points represent PBS spots of one energy layer,
the size corresponds to the dose per spot, the color to the
detected shift between two PGI profiles. Spots influenced
by the local shifts (black ring) are highlighted with a black
edge. Global and local shifts with 1 and 5 GyE were
measured.
Conclusion
The systematic sensitivity study revealed the capability of
the PGI slit camera to detect range shifts under clinical
conditions. In both treatment modalities, global range
shifts can be detected. Additionally, in PBS a spot-wise
comparison allows also the determination of
interfractional local range shifts. Moreover, a still ongoing
evaluation of PBS measured and simulated spot-wise
profiles for absolute range verification will be presented.
OC-0154 Proton therapy patient selection for
oropharyngeal cancer patients: the impact of treatment
accuracy
M. Hoogeman1, S. Breedveld1, M. De Jong2, E.
Astreinidou2, L. Tans1, F. Keskin-Cambay 1, R. Bijman1, S.
Krol2, S. Van de Water1, T. Arts1
1Erasmus MC Cancer Institute, Radiation Oncology,
Rotterdam, The Netherlands
2Leids University Medical Center, Radiation Oncology,
Leiden, The Netherlands
Purpose or Objective
Comparative treatment planning including Normal Tissue
Complication Probability (NTCP) evaluation has been
proposed to select patients for proton therapy. NTCP,
however, does not only depend on the type of radiation
used, but also on the size of the safety margins or degree
of robustness needed to account for treatment-related
uncertainties. In this study, for the first time to our
knowledge, the impact of margins and robustness settings
to the selection of oropharyngeal cancer patients is
investigated using fully automated comparative treatment
planning.
Material and Methods
CT and contour data of 78 consecutive oropharyngeal
patients were imported in our in-house developed system
for automated treatment planning for Intensity-Modulated
photon (IMRT) and proton radiotherapy (IMPT). Treatment
plans were generated fully automatically for a
simultaneously integrated boost scheme prescribing 70
GyRBE to the primary tumor and pathological lymph nodes
and 54.25 GyRBE to the elective nodal areas in 35 fractions.
IMRT treatment plans were generated with a 0, 3, or 5mm
margin. IMPT 'minimax” robust optimized treatment plans
were generated for five different setup and range
robustness settings. Five validated NTCP models (see Fig.
1) proposed for IMPT patient selection were used in this
study. Following Dutch consensus guidelines, patients
were selected for IMPT if IMPT reduced NTCP by 10% or 5%
for a grade II or a grade III complication, respectively.
Results
In total 624 treatment plans were generated automatically
and approved by the authors. Figure 1 shows that the
percentage of patients selected for IMPT decreases with
increasing robustness setting for a given margin and also
decreases with decreasing margin for a given robustness
setting. In contrast to the size of the margin, the degree
of robustness has little impact on patient selection for
tube feeding dependence, which is the only grade III
complication. For the other complications the impact of
the degree of robustness setting is noticeably higher. For
patient-rated sticky saliva, nearly no patient is selected
for IMPT if robustness is included. If we consider high-
precision IMRT using a 3mm margin and high-precision
IMPT using a robustness setting of 3mm for setup and 3%
for range errors, most patients are selected for proton
therapy based on problems swallowing solid food (51.3%),
followed by tube feeding dependence (37.2%) and
decreased parotid flow (29.5%). Patient-rated sticky saliva
and patient-rated xerostomia contributed only with 1.3%
and 7.7% respectively.