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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
to optimize the dose sensitivity and the Signal-to-Noise
Ratio. In particular, geometrical distortion, image
homogeneity, artifacts, image texture, dose accuracy and
resolution, limit of detectability (LOD) and quantification
(LOQ), Fe
3+
spatial diffusion (Fricke-gels) and dose rate
dependence were evaluated. Finally, a pre-treatment
dosimetry of a SBRT plan was acquired and a relative
planar profiles comparison with a standard dosimeter
(Gafchromic EBT2) was performed. Ad hoc Matlab codes
were developed for images analysis.
Results
The chemical composition, MRI acquisition and
reconstruction parameters were optimized for each gel
system. No image correction maps were needed, since
geometrical distortion, artifacts and inhomogeneity were
always negligible, and no dependence on photon beam
dose rate was observed. 3D spatial resolution (voxel
dimension) was 1x1x3mm
3
. Dose accuracy was under 4% in
the range 18-25Gy, but worst for lower doses. Dose
resolution was about 1Gy, while LOD was less than 0.5Gy.
Differences between gel systems and Gafchromic profiles’
FWHMs were in the range 0,5mm – 5,5mm, mean dose
deviations in flat region were always around 2%, while
penumbra differences were about 2mm. Negligible
diffusion and time effects were observed up to 3 hours
from irradiation for all gel systems.
Conclusion
This study showed that both Fricke/Polymer- gel
dosimeter could be a suitable tool to perform pre-
treatment QA, with particular focus on SBRT and SRS
treatments, thanks to their optimal spatial resolution,
their practicability and their capability to perform 3D
dosimetry. Further studies are ongoing to standardize a
protocol to perform 3D pre-treatment dosimetry.
EP-1443 Measurement of 3D dose distributions from
an MR Linac with gel dosimetry
Y. Roed
1,2
, L. Pinsky
1
, G. Ibbott
2
1
University of Houston, Physics, Houston, USA
2
The University of Texas MD Anderson Cancer Center,
Radiation Physics, Houston, USA
Purpose or Objective
To demonstrate the potential value of polymer gels to
measure 3D dose distributions delivered with an MR-image
guided radiotherapy delivery machine.