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ESTRO 36 2017
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Agility
®
collimator system included 160 MLC, minimum
leaf width was 5 mm. MLC effective speed was 6.5 cm/sec
and leaf travel was 15cm over the central axis. VMAT plans
were generated on Monaco 5.1
®
(Crawley, Elekta)
treatment planning system with Monte Carlo algorithm. All
calculation parameters were grid spacing 0.3
cm, minimum segment width 1.0 cm, Max. 180 of control
Points Per Arc, Fluence smoothing medium, Statistical
Uncertainty 1% per plan, increment of gantry 30° and dose
to medium.
The VMAT-TBI tecnique consisted of three isocentres and
three dual overlapping arcs from top of head to the
bottom of pelvis region. The prescribed dose was 90% of
target volume receiving dose of 12Gy. Mean dose to lung
and kidney were restricted less than 10Gy and maximum
dose to lens were restricted less than 6Gy. The plans were
verified using 2D array IBA Matrixx
®
and CC13 ion chamber.
The comparison between calculation and measurement
were made by γ-index (3%-3mm) analysis and absolute
dose measurement at the isocentre.
Results
An average total delivery time was determined 923±34
seconds and an average monitor unit (MU)s was
determined 2614±231MUs for dual arc VMAT technique.
When we evaluated organ at risk(OAR)s, mean dose to
lungs was 9.7±0.2Gy, mean dose to kidneys was
8.8±0.3Gy, maximum dose to lens was 5.5±0.3Gy and
maximum point dose was 14.6±0.3Gy, HI of PTV was
1.13±0.2, mean dose to PTV was 12.6±0.15Gy and mean γ-
index (%3-3mm) pass rate was %97.1±1.9. Absolute doses
were measured by CC13 ion chamber and we determined
%2.0±0.6 dose difference between measurement and
treatment planning system's (TPS) calculation at the
isocentre.
Conclusion
The results show that dose coverage of target and OAR’s
doses are feasible for TBI using VMAT tecnique on the
coach. A benefit could be demonstrated with regard to
dose distribution and homogeneity and dose-reduction to
organs at risk. Additionally,we determined highly precise
dose delivery by patient QA and point dose measurement
at the isocentre. Based on the dose distributions we have
decided to plan TBI in our clinic with dual arc VMAT
technique on the treatment coach.
PO-0911 Can the therapeutic benefits of microbeam
radiation therapy be achieved using a clinical linac?
N. Suchowerska
1
, V. Peng
1
, L. Rogers
1
, E. Claridge-
Mackonis
1
, D.R. McKenzie
2
1
Chris O'Brien Lifehouse, Radiation Oncology,
Camperdown- Sydney, Australia
2
University of Sydney, School of Physics, Sydney,
Australia
Purpose or Objective
The increasing availability of high definition multileaf
collimators (HDMLCs) with 2.5mm leaves provides an
opportunity for ‘grid’ therapy to more closely approach
the clinical outcomes of Microbeam Radiation Therapy
(MRT). However, periodic spatial modulation of the dose
in the target volume runs counter to current clinical
practice. To optimize the modulation, a better
understanding of cell dose responses to such treatments is
needed. The aim of this study is to determine if some of
the therapeutic benefits of MRT can be achieved using a
clinical linac with HDMLCs and if so, to develop a
predictive model to optimize the benefits of such
treatments.
Material and Methods
Varian Novalis Tx
TM
HD120-MLCs were used to generate
grid patterns of 2.5mm and 5.0mm spacing, which were
dosimetrically characterized using Gafchromic
TM
EBT3 film
[Figure 1]. Clonogenic survival of normal (HUVEC) and
cancer (lung NCI-H460, breast HCC-1954, melanoma
MM576) cell lines were compared in vitro for the same
average dose, following irradiation with periodically
modulated and open 6MV photon fields.
Results
Survival of normal cells in a 2.5mm striped field was the
same as for an open field, but the survival of the cancer
cells was significantly lower. However for cancer cell lines
in the 5.0mm modulated fields, the response compared to
an open field was no longer statistically significant. A
mathematical model was developed to incorporate the
dose gradients of the spatial modulation into the standard
linear quadratic model. Our new extended bystander LQ
model assumes spatial gradients drive the diffusion of
soluble factors that influence survival through bystander
effects. The model successfully predicts the experimental
results that show an increased therapeutic benefit.
Conclusion
We have confirmed that HDMLCs can create spatially
modulated fields that increase the therapeutic advantage
between normal and cancer cells. Our results challenge
conventional radiotherapy practice and propose that
additional gain can be realized by prescribing spatially
modulated treatments to harness the bystander effect.
PO-0912 Short- and long term stability of the isocenter
of a three-source Co60 MR guided radiotherapy device
D. Hoffmans
1
, M.A. Palacios
1
, J.P. Cuijpers
1
1
VU University Medical Center, Radiotherapy,
Amsterdam, The Netherlands
Purpose or Objective
Recently a 0.35T Co
60
MRIdian system (Viewray Inc.,
Cleveland) is implemented at our institution. In a similar
way as for other image guided radiotherapy techniques,
the coincidence of the radiation therapy (RT) and imaging
isocenter is of major importance. The purpose of this
study is to present a method for daily QA of MR-RT
isocenter coincidence and to assess its short- and long
term stability using daily film-based isocenter QA.
Material and Methods
Two pieces of radiochromic film (GafChromic EBT3) are
taped to square inlays on the top and the side of a cubic
water-filled phantom. The phantom is aligned to the MRI
isocenter using MR guided setup relying on three internal
cylindrical markers.
A treatment plan, consisting of an AP and a lateral square
field is delivered. The direction of the lateral field is
altered daily between 90
o
and 270
o
in order to monitor all
3 treatment heads (Head 1 and 2 for the lateral fields
respectively, head 3 for the AP field). The films are
digitized and the positions of the square fields with
respect to the phantom are determined. These data
provide a daily measurement of the coincidence of the RT-
and MR- isocenters in 3 dimensions. The AP field provides