776 / 1082
S760
ESTRO 36 2017
_______________________________________________________________________________________________
The study shows that for this specific value of
kB
studied
and all the limitations of the model, the quenching effect
will not significantly affect the scintillator output factor
measurements in small 6 MV photon fields, and the
quenching correction factor will be therefore close to the
unity (u=1%). The impact of the stem signal (i.e. Cerenkov
and fluorescence light produced in the optical fiber cable
during irradiations) therefore remains the main
influencing factor on such measurements.
EP-1441 Repurposing of a small clinical x-ray source
for radiobiology irradiations
M. Barry
1
, R. Jones
1
, M. Fay
1
, D. Butler
2
, J. Lehmann
1
1
Calvary Mater Newcastle, Department of Radiation
Oncology, Newcastle- NSW, Australia
2
Australian Radiation Protection and Nuclear Safety
Agency, Radiation Therapy Section, Yallambie, Australia
Purpose or Objective
Around the clock availability of irradiation capability is
desirable for creative design of radiobiology experiments.
Clinical radiation systems are often only available after
hours. Dedicated commercial cell irradiation systems are
bulky and expensive. They might not be feasible due to
financial or administrative constraints.
This work reports on the repurposing of a retired clinical
intraoperative kV irradiation system (Intrabeam™) for cell
irradiations.
Material and Methods
The Intrabeam system is designed to deliver spherical dose
distributions to surgical cavities. The in the adaption for
cell irradiations, one of the supplied applicators has been
modified to be fitted with custom collimators aiming to
deliver a homogenous field across the cell dish. Several
collimator designs have been tested and measured used
using radiochromic EBT3 film. Additionally, measurements
without a collimator were done in comparison and to
support
Monte
Carlo
simulations.
Film calibrations were performed with national standard
beams covering the energy range of the device.
The BEAMnrc code and the 'NRC swept BEAM” source
model have been used to characterize the dose
distributions and to aid collimator development.
Results
Using the film measurements, the parameters of the
Monte Carlo source model (swept angle and beam radius)
were tuned to produce the final model. Very good
agreement between measured and simulated dose profiles
for the open source at 5, 7.5 and 10 cm distance from the
tip was observed. (Figure 1)
Figure 1: Profiles of the dose distributions 10 cm from the
open tip of the source (no collimator) as measured with
EBT3 film and simulated using the BEAMnrc Monte Carlo
code.
Measurements with steel and aluminum collimator designs
identified desirable characteristics for a suitable
collimator: a long extension beyond the tip of the source
and a diameter beyond the projected field size.
Based on measurements and simulations, a cell culture
plate irradiation rig has been designed and built, allowing
for radiobiology experiments with different cell dishes and
incorporating film measurements to verify dose delivery.
(Figure 2)
Figure 2: Cell irradiation rig with Intrabeam, collimator
and 96 well cell culture plate.
Conclusion
The repurposed x-ray system will allow for flexible
irradiation of cell cultures for radiobiology experiments.
Future plans include extension to small laboratory animal
irradiations, as the unique design with the source of
radiation being at the tip of an extended metal tube
allows for high dose rates to small fields when in close
proximity to the target.
EP-1442 Fricke and Polymer gel dosimeters for
radiotherapy pre-treatment 3D dosimetry
G.M. Liosi
1
, L. Trombetta
2
, P. Salmoiraghi
2
, M. Mariani
1
,
F. Locatelli
2
, E. Bombardieri
2
1
Politecnico di Milano, Energy- Nuclear Engineering
Division, Milano, Italy
2
Cliniche Humanitas, Gavazzeni, Bergamo, Italy
Purpose or Objective
Pre-treatment dosimetry represents a fundamental step
for the verification of radiation therapy outcome, and, in
particular, an accurate and precise measurement of the
3D dose distribution with high spatial resolution has
become of paramount importance. Aim of this work was
the study and characterization of two gel-dosimetry
systems (Fricke- and Polymer-gels), suitable for
volumetric patient-specific 3D dosimetry.
Fricke-gel dosimeters are based on the dose dependent
oxidation of Fe
2+
ferrous ions –dispersed into a tissue
equivalent gel matrix– into Fe
3+
ferric ions. Thus, the Fe
3+
concentration is linearly related to the absorbed dose. The
MRI acquisition of gels through T1-weighted images
permits measurement of Fe
3+
concentration, obtaining at
the same time the absorbed dose mapping within the
irradiated volume. On the other hand, as regard Polymer-
gel dosimeters, a dose-dependent polymerization occurs,
hence dose assessment and spatial information can be
obtained by means of T2-weighted MRI analysis.
Material and Methods
A preliminary optimization of the chemical composition
for both Fricke/Polymer- gel dosimeters was performed.
Afterwards, the calibration method, MRI (1.5T) acquisition
and reconstruction parameters were set for each system