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S823
ESTRO 36
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Results
Results of point dose measurements, gamma-index
analysis and HDV-based comparisons are listed in table 1.
Absolute dose differences were all <1% with an average
value of 0.4±0.4%. Average differences of gamma passing
rate (%GP) with a low-dose threshold of 10% of the
maximum dose were 99.9±0.2% and 99.2±1.2% (2D global
3%/3mm and 2%/2mm criteria), 95.9±3.4% and 89.4±6.5%
(2D local 3%/3mm and 2%/2mm criteria), 99.5±0.9% and
97.0±2.9% (3D global 3%/3mm and 2%/2mm criteria),
96.9±2.8% and 89.2±6.0% (3D local 3%/3mm and 2%/2mm
criteria) respectively. Finally, DVH-based comparisons
between calculated and delivered fallback plans showed
differences of respectively -0.5±0.8% for the quality of
coverage (Q), -1.0±0.7% for the mean dose to target (MDT)
and 0.3±0.9% for the integral dose to organs at risks
(ID_OAR).
Conclusion
Fallback planning is an advanced RayStation feature that
uses a dose-mimicking function to automatically
replicate the DVH and the dose per voxel of a given plan,
but for an alternative treatment machine or technique.
Results presented here through a Helical Tomotherapy to
VMAT plan conversion show a good agreement between
planned and delivered dose for point dose
measurements, gamma-index analysis and DVH-based
comparisons, hence validating the dose-mimicking
algorithm used during the automatic optimization of the
fallback plans.
EP-1531 Collimator angle influence on dose coverage
for VMAT SRS treatment of four brain metastases
C. Ferrer
1
, C. Huertas
1
, A. Castaño
2
, A. Colmenar
2
, R.
Plaza
1
, R. Morera
2
, A. Serrada
2
1
Hospital universitaria La Paz, Radiofísica y
Radioprotección, Madrid, Spain
2
Hospital universitaria La Paz, Oncología Radioterápica,
Madrid, Spain
Purpose or Objective
To evaluate the collimator angle influence on the dose
coverage of 4 brain metastases treated with volumetric
modulated arc therapy (VMAT) stereotactic radiosurgery
(SRS).
Material and Methods
Three brain metastases were prescribed to 18Gy, and a
fourth one located in the cerebellar tonsil to 16Gy.
Treatment was planned with Elekta Monaco treatment
planning system (v. 5.00.00), and optimized using
biological and physical based cost functions for mono-
isocentric VMAT SRS treatment on an Elekta Synergy linear
accelerator equipped with a 160-leaf Agility MLC. Five non
coplanar partial arcs were used, plus a full clockwise-
counterclockwise arc with 0° couch rotation to modulate
only the fourth lesion with different prescription and away
from the other three. Planning target volume (PTV)
coverage and dose to organs at risk (OAR) have been
evaluated for three different collimator angle positions,
5°, 45° and 95°. Treatment constraints were the same for
the three plans, one treatment plan for each collimator
angle.
Results
The best plan in terms of target coverage and number of
monitor units was achieved with collimator angle set to
95°, with the 95% of the PTV volume receiving more than
95% of the prescription dose for the 4 lesions, with 35.8%
less total MU compared with the 5° collimator angle plan
(5176 MU versus 8061 MU). The target coverage for the 45°
collimator angle plan was lower than for the other two
plans. OAR maximal doses were similar for the brainstem,
optic nerves and eye lens, but maximum dose to the optic
chiasm was 42% and 49.1% lower for the 5° collimator
angle plan compared with the 95° and 45° angle plan
respectively.
Conclusion
The choice of collimator angle influences the target
coverage as well as the total MU and the doses to OAR.
The optimal choice of this angle in VMAT SRS treatments
improves the optimization outcome.
EP-1532 ITV optimization for SBRT lung treatment
planning accounting for respiratory dose blurring
C. Cases
1
, A. Latorre-Musoll
1
, P. Carrasco
1
, N. Jornet
1
, T.
Eudaldo
1
, A. Ruiz-Martínez
1
, M. Lizondo
1
, P. Delgado-
Tapia
1
, M. Ribas
1
1
Hospital de la Santa Creu i Sant Pau, Radiofisica i
Radioprotecció, Barcelona, Spain
Purpose or Objective
For SBRT lung treatments accurate 4D dose calculations,
accounting for heterogeneities and respiratory motion,
are crucial to determine an optimal ITV beyond a purely
geometric ITV. We propose a model to predict an optimal
ITV from a single figure computed from the Probability
Density Function (PDF) of the breathing waveform.
Material and Methods
We used the QUASAR Respiratory Motion Phantom (Modus
Medical Devices) with a cylindrical mobile wood insert as
lung substitute and an inner 30mm diameter sphere as
tumour substitute (GTV). We acquired 21 independent
scans (CT
z
) by axially shifting the mobile insert from z = –
10 to z = 10mm in 2mm steps. We generated 6 ITV: from
ITV
0mm
(static case, equal to the GTV of CT
0mm
) to ITV
10mm
(overlap of all GTV from CT
–10mm
to CT
10mm
). We planned a
SBRT treatment collimating to each ITV (from PLAN
0mm
to
PLAN
10mm
) in Varian Eclipse (AAA v13.5) using a 6MV non-
coplanar 3DCRT technique. Due to the
in silico
nature of
the study we added no extra margins to the ITV.
We considered 3 breathing patterns: sinusoidal (provided
by QUASAR software), free and trained (obtained by the
Varian RPM from real patients). We rescaled every