439 / 1096
S424
ESTRO 36
_______________________________________________________________________________________________
recalculation of delivered dose to the planning CT.
PO-0796 Optimisation of plan robustness to sinus filling
in a magnetic field.
A. Pollitt
1
, R. ChuteR
1
, P. Whitehurst
1
, R. MacKay
1
, M.
Van Herk
2
, A. McWilliam
2
1
Christie Hospital NHS, Radiotherapy, Manchester,
United Kingdom
2
University of Manchester, Division of Molecular and
Clinical Cancer Science, Manchester, United Kingdom
Purpose or Objective
The MR Linac (Elekta AB, Stockholm, Sweden) will provide
on-treatment MR imaging allowing for excellent soft tissue
imaging. Such a machine will become an integral part of
the drive towards daily online adaptive radiotherapy.
However, the presence of the magnetic field results in the
Lorentz force and will cause an increase or decrease in
dose to superficial tissues (Raaijmakers et al. 2007). This
is particularly pertinent for sinus cancers, of which 60%
are squamous cell carcinoma’s and primarily on the
surface layer of the nasal cavity. Recent studies (Bol et al
2015, Uilkema et al. 2015) have been performed to
determine the effect of the Lorentz force on low density
cavities in the body. This abstract aims to investigate the
effect of the magnetic field on plan quality and
optimisation for varying sinus filling and emptying states.
Material and Methods
Ten patients with PTV’s overlapping the sinus cavity were
selected from the clinical archive. For each patient four
plans were optimised at 60Gy in 30 fractions, 2 with no B-
field and 2 with the 1.5T B-field present. For each, 1 plan
assumed full sinuses with the volume overridden to 1gcm
-
3
and the other assumed empty sinuses with the volume
overridden to 0gcm
-3
. All plans were created using Monaco
(v5.19.07, Elekta AB Stockholm, Sweden) and met the
departmental constraints for Target and OAR doses. To
investigate the effect of a change in sinus filling, plans
were recalculated on their opposite filling state, i.e plan
optimised on a full sinus was recalculated on an empty
sinus. The difference in dose between the two plans for
target coverage and OARs was calculated. This comparison
will determine the magnitude of the effects from sinus
filling in each scenario. Investigating the range of dose
differences will provide information on how to optimise
these plans to minimise the effect of the Lorentz force.
Results
The change in dose to the Target for the different filling
and magnetic field combinations can be seen in Figure 1.
Several of the dose differences for plans optimized on an
empty cavity, for both with and without B-field show a
shift of the mean of the distribution which is greater than
2% (considered potentially clinically significant). i.e. mean
Dose = 2.36%; V
50%
= 2.26%; V
5%
= 3.12%; V
2%
= 3.21%. An
OAR which also saw a difference greater than 2% was the
Brainstem PRV 1cc max = 2.16 %.
Conclusion
This abstract shows greater dosimetric differences due to
sinus filling in a 1.5T magnetic field for plans optimised
with an empty cavity. Without a B-field plans optimised
on full and empty cavities show similar results.
The dose to the PTV is also less conformal optimising on
an empty cavity due to hotspots caused by the ERE close
to the surface shown by a higher effect for the max dose
to 2% of the Target. The results indicate that optimising
with a full sinus cavity makes the plan more robust to the
Lorentz force and therefore to changes in filling.
PO-0797 Studies on optical fiber dosimeters for in-vivo
dosimetry in HDR brachytherapy
L. Moutinho
1
, H. Freitas
1
, J. Melo
1
, J.F.C.A. Veloso
1
, P.J.
Rachinhas
2
, P.C.P.S. Simões
2
, J.A.M. Santos
3
, A. Pereira
3
,
J. Silva
3
, S. Pinto
3
1
University of Aveiro, Physics Department, Aveiro,
Portugal
2
CHUC, Radiology, Coimbra, Portugal
3
IPO-Porto, Radiotherapy, Porto, Portugal
Purpose or Objective
Dose verification and quality assurance in radiotherapy
should be assessed in order to provide the best treatment
possible and minimize risks for patient. Notwithstanding,
due technical constraints in certain treatments there’s no
such tools capable to perform real-time dose
measurement. An ideal dosimeter for prostate
brachytherapy should provide real-time and in-vivo dose
measurement, present high sensitivity and no
dependencies on energy, dose and dose rate and
temperature. Also should be detectable in the anatomic
volume to check its position, easy to use and calibrate and
not expensive/disposable use of its implantable part.
Material and Methods
We developed a fiber optic dosimeter suitable for real-
time dose monitoring in breast and prostate
brachytherapy, thus opening the possibility for real-time
dose correction.
The dosimeter comprehends a sensitive optical fiber probe
of 1mm or 0.5 mm diameter where a 5 mm length
scintillating optical fiber is coupled. The clear optical
fiber providing scintillation light guidance into the
photodetectors.