ESTRO 36 Abstract Book

S402

ESTRO 36 2017

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screen has been replaced by a water equivalent build-up

material [1]; a dual detector combining a standard EPID

and an array dosimeter [2]; and an EPID comprising a

plastic scintillator fibre array (PSFA) in place of the

metal/phosphor screen [3]. Our performance

specifications were to achieve imaging performance

equivalent to standard EPIDs, and a dose response

equivalent to standard clinical dosimeters. Quantitative

metrics such as detective quantum efficiency (DQE) for

imaging and field size response for dosimetry were used in

both experimental and Monte Carlo (MC) studies. There

are three arms to this project that shall be described; i)

MC simulations to characterise and design scintillators, ii)

Prototype construction and experimental evaluation, iii)

clinical implementation.

Results

All prototype detectors exhibited near equivalent dose

response with ionisation chambers in both non-transit and

transit geometries (± 2%), including 2D clinical dosimetry

of IMRT fields. The X-ray quantum efficiency of the direct

and PSFA detectors is approximately 9% compared to 2%

for the standard EPID and dual detector. The imaging

performance of the standard EPID and dual detector

remains superior to the other prototypes because of the

greater efficiency of optical photons detected per

incident X-ray and better spatial resolution. MC

simulations demonstrate potential improvements in

imaging with the PSFA. A model for clinical

implementation has been developed that exploits the

water equivalence of the detectors. A water equivalent

EPID provides more direct and robust verification than can

be achieved with current EPID dosimetry. A water

equivalent EPID that retains imaging capability is better

suited than current EPIDs for modern radiotherapy.

Conclusion

This work demonstrates the feasibility and advantages of

alternative EPID designs that better meet the needs of

modern radiotherapy.

PO-0768 Electron Paramagnetic Resonance signal from

a new solid polymer material aimed for 3D dosimetry

M.R. Bernal-Zamorano

, N.H. Sanders

, L. Lindvold

, C.E.

Andersen

DTU, Nutech, Roskilde, Denmark

Purpose or Objective

We have developed a water-equivalent solid polymer

dosimeter material aimed for 3D dosimetry in

radiotherapy beams. The material responds to ionizing

radiation by changes in its optical absorbance and by

generation of fluorescence centers. The latter signal is of

particular interest as the fluorescence centers facilitate

detailed mapping the 3D dose distribution us ing laser

stimulation. However, in addition to the optical si gnals

we also expect that the material could have an electron

paramagnetic resonance (EPR) dose response related to

the production of stable free radicals. To test this

hypothesis, point detector experiments were therefore

performed where the material was casted into 5 mm

diameter pellets identical in size to the alanine dosimeters

that we routinely use for reference EPR dosimetry in our

laboratory. The pellets of the new material and alanine

were irradiated in

Co beams and EPR signals were

recorded afterwards.

Material and Methods

The dosimeter is based in pararosaniline leuco dye, which

is chemically transformed into its dye-form by the effect

of radiation. The leuco dye is dissolved in a poly(ethylene

glycol) diacrylate matrix (PEGDA-575 g/mol) that contains

diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO)

used for photocuring. We cured the material in a mold

with a 395 nm LED for a few minutes. We made 4

cylindrical pellets of 4.75 mm diameter and 2.78 mm

thickness (same size than alanine dosimeters used in this

work).

Pellets of the new material and alanine were irradiated in

Co gamma source with a dose rate of about 8 Gy min

-1

They were given doses of 5, 10, 20, 30, 50, 75 and 100 Gy.

The EPR signal for both dosimeters was obtained by a

Bruker EMX-micro spectrometer by inserting the pellets

into the resonator in a quartz tube. Absorbance and

fluorescence signals of the pellets of our material were

measured with a Shimazdu UV-2700 spectrophotometer

and an Ocean Optics QE6500 spectrometer respectively.

Fluorescence was excited with a diode laser.

Results

A clear EPR signal was obtained for our material, and this

signal increased with dose. The peak-to-peak amplitude of

the EPR spectra are shown in the figures.

Although alanine and PEGDA have similar characteristics

in terms of its water equivalence (similar effective atomic

number, mass density and electronic density), their EPR

signal is very different.

Conclusion

We have obtained an EPR signal for our solid polymer

dosimeter. The EPR signal increases linearly with dose for

the medical dose range, but it saturates for higher doses.

Although it is not comparable to the EPR dosimetry using

alanine, this signal could be a source of improved

understanding of the underlying dosimetric characteristics

of this material and it may be a supporting feature to the

optical signals from the dosimeter. We further foresee

interesting applications in particle therapy beams since

the signal production in solid-state dosimeters are

generally dependent on the ionization density.

PO-0769 A microDiamond for determination of

absorbed dose around high-dose-rate 192Ir

brachytherapy sources

V. Kaveckyte

, A. Malusek

, H. Benmaklouf

, G. Alm

Carlsson

, A. Carlsson Tedgren

Linköping University, Radiation Physics IMH, Linköping,

Sweden

Karolinska University Hospital, Radiation physics,

Stockholm, Sweden

Purpose or Objective

Experimental dosimetry of high-dose-rate (HDR)

192

brachytherapy (BT) sources is complicated due to steep