S812
ESTRO 36 2017
_______________________________________________________________________________________________
Purpose or Objective
To demonstrate with end-to-end tests the ability of
RayStation v5.02 (RaySearch Laboratories AB, Stockholm,
Sweden) fallback planning module (RFP) to perform an
accurate Helical Tomotherapy (HT) to volumetric
modulated arc therapy (VMAT) plan conversion by
validating the dose-mimicking algorithm used during the
automatic optimization of the fallback plans.
Material and Methods
Thirty patient plans of various treatment sites previously
treated with HT were switched to 6 MV dual-arc VMAT
plans using RFP and default dose-mimicking algorithm
parameters. For the purpose of this study no further
optimizations were performed and delivery quality
assurance (DQA) were designed for each fallback plan.
DQA were delivered on a TrueBeam linear accelerator
(Varian Medical Systems, Palo Alto, CA) and
planar/absolute dose measurements were acquired using
the ArcCHECK diode array (Sun Nuclear Corporation,
Melbourne, FL) with an insert containing an Exradin A1SL
ionization chamber (Standard Imaging, Middleton, WI). 3D
dose distributions in the patient geometry were
reconstructed within 3DVH software (Sun Nuclear
Corporation, Melbourne, FL) by using ArcCHECK Planned
Dose Perturbation (ACPDP). Agreement between planned
and delivered dose was eventually evaluated with global
and local 2D/3D gamma-index analysis (3%/3mm and
2%/2mm criteria) and DHV-based comparisons were
performed using the following dosimetric parameters:
quality of coverage (Q=D98%/Dref), mean dose to target
(MDT=Dmean/Dref) and integral dose to organs at risks
(ID_OAR=∑·Di·Vi).
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