ESTRO 36 Abstract Book

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S445 ESTRO 36 2017
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house multicriterial optimizer to generate input
parameters for automated plan generation in Multiplan,
including patient-specific parameters to maximally
control integral dose. Plan comparisons were made for 15
patients. Both for automatic and manual planning, the
goal was to deliver a single fraction of 12 Gy, with planning
priorities PTV V100% ≥ 98%, Brainstem Dmax < 12.5 Gy,
while at the same time keeping the integral dose as small
as possible. For un-biased plan quality comparisons,
AUTOplans were generated such that the resulting CK
treatment time was similar to that for the corresponding
MANplan.
Results
AUTOplans were comparable to manual MANplans in terms
of PTV coverage (AUTO: 99.4 ± 0.5 %, MAN = 99.1 ± 0.5 %,
p=0.1) and treatment time (AUTO = 39.5 ± 4.7 min, MAN =
38.9 ± 5.9 min, p=0.3). On average, the brainstem D2%,
D1cc and Dmean were very similar, i.e. 9.5 vs. 9.6, 8.6 vs.
8.5, and 2.0 vs. 2.2 Gy for the AUTO- and MANplans,
respectively (p>0.2). Patient volumes receiving more than
1, 2, 4, and 6 Gy were highly reduced in the AUTOplans
for the majority of patients, as visible in figure 1 (upper),
with average reductions of 26.0% (SD= 15.4%, p < 0.001 ),
14.7% (SD=10.5%, p < 0.001), 9.8% (SD= 10.3%, p = 0.002 ),
and 6.3% (SD=10.4%, p = 0.010). Conformality was also
better in the AUTOplans, and spiky dose leakage away
from the target was less frequent and severe, as visible in
figure 2. The D2% in ring structures at 1, 2, and 3 cm
distance from the PTV were 3.6, 1.9, and 1.3 Gy in
AUTOplans vs. 4.7, 2.4, and 1.6 Gy in the MANplans (p<
0.001). For almost all patients, ring structures’ D2% were
lowest in the AUTOplan (see figure 1, lower).
Conclusion
With automated Cyberknife planning, highly patient-
specific parameters for optimal plan generation in
Multiplan are automatically established, resulting in
substantial reductions in integral dose in treatment of
benign vestibular schwannoma tumors, without degrading
PTV dose delivery, increasing OAR doses, or enlarging
treatment time.
PO-0835 PTV margin for pelvic lymph nodes in IGRT
guided prostate radiotherapy
H.R. Jensen1, C.R. Hansen1,2, S.N. Agergaard 1, E.L.
Lorenzen1,2 , L. Johnsen1, S. Hansen2,3, L. Dysager3, C.
Brink1,2
1Odense University Hospital, Laboratory of Radiation
Physics, Odense, Denmark
2University of Southern Denmark, Faculty of Health
Sciences, Odense, Denmark
3Odense University Hospital, Department of Oncology,
Odense, Denmark
Purpose or Objective
In recent years irradiation of the pelvic lymph nodes for
high risk prostate cancer has received strong interest, as
a potential way to increase locale control probability.
However the prostate and the pelvic lymph nodes move
independently of each other. The purpose of this study is
to calculate the additional PTV margin needed for
covering the pelvic lymph node region, when performing a
registration and setup on the prostate with implanted gold
fiducials.
Material and Methods
All 40 prostate patients treated at the same accelerator in
2015 were included in the study. The majority of the
patients had stage T3 disease. All patients had three gold
fiducials implanted into the prostate 2-3 weeks before CT
simulation, which were used in the daily online IGRT. A
total of 1284 cone beam CT scans were analyzed. An
automatic gold seed algorithm (used as a surrogate for the
prostate) and bone algorithm covering the upper pelvic
and lower spine area (used as a surrogate for the lymph
nodes) were performed. The deviation between the two
registrations was calculated and the population based
random and systematic setup error was calculated. To
estimate the PTV margin needed the Van Herk margin
formula was used M = 2.5 * Σsystematic + 0.7 * σrandom
Results
The setup margin needed for the lymph node region of this
patient cohort is 2.1, 6.9 and 6.6 mm for the LR, CC, AP
directions, respectively (see table). This margin does not
incl. any other uncertainties. The minimum deviation
between prostate seed and pelvic bone match is shown as
a cumulative histogram in the figure for the individual
directions. More than 15.4% of the fractions have a
deviation of more than 5 mm, and 5% of the fractions have
a larger deviation than 7mm. The largest deviations are
seen in the CC and AP direction, and a small deviation in
the LR direction. The systematic and random errors are
shown in the table.