ESTRO 36 Abstract Book

S768

ESTRO 36

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for small fields. Furthermore a difference was observed

between profiles performed in head-foot or foot-head

direction (Figure 2). Dose rate dependence was found to

be <0.3% while dose per pulse dependence showed an

increasing trend but still <0.6% for a maximum dpp of 0.2

cGy/pulse. At the nominal operating voltage of 300 V the

Razor Nanochamber exhibits a field size dependence of

the polarity correction > 2% between the 1x1cm

and the

40x40cm

field. The OF values were compared with diode

and scintillator measurements and show a good agreement

for fields >20x20 mm

. For smaller fields the volume effect

is huge and leads to strongly underestimated values.

Conclusion

Razor chamber is an interesting option for small field

measurements. Its use in orthogonal configuration raises

some stem effect issues evident when measuring inline

profiles. More measurements are required in order to fully

characterize this ion-chamber.

EP-1440 Monte Carlo determination of scintillator

quenching effect for small radiation fields

G. Valdes Santurio

, C. E. Andersen

Technical University of Denmark, Nutech, Roskilde,

Denmark

Purpose or Objective

Fiber-coupled organic plastic scintillator detectors are

excellent for measurement of the absorbed dose to water

in small MV photon fields. This is mostly because their

small active volume and their high degree of water

equivalence result in an almost negligible perturbation of

the radiation field. However, plastic scintillators are less

ideal when we consider the signal generation and the

signal detection. For the signal generation, it is known

that the light yield per absorbed dose for electrons below

100 keV produces less light than electrons with higher

energy which is the so-called ionization density

quenching. The objective of this work was to investigate

the potential implication of this quenching effect for

output factor measurements in small 6MV photon beams.

Monte Carlo modelling was used to compute changes in

light production for different field sizes using Birks

formula applied to electrons.

Material and Methods

The quenching effect can be predicted by the Birk´s

formalism which relates the amount of light per distance

travelled by a given particle to the ionization density from

that particle as expressed by the collision stopping power

of the medium (dE/dx). This formalism introduces the

quenching parameter (

), which describes that the light

produced by low energy electrons is not proportional to its

deposited energy. We implemented Birks formula in a

modified version of the application egs_chamber which is

part of the EGSnrc Monte Carlo system. The modified

application scored the light output over the absorbed dose

for each field size. This ratio will gives us how much the

quenching effect affects for that specific field size and

therefore, differences of the quenching effect when

changing the field size can be estimated. Moreover, this

ratio will give us how much the scintillator output factor

changes when the quenching effect is taken into account.

We computed light yields for square field sizes down to

0.6x0.6cm

with 10x10cm

as reference.

Results

The light output over the absorbed dose was calculated

for all field sizes. The uncertainty of all values was less

than 0.5%. Figure 1 shows the normalized scintillator light

output per dose for all field sizes and their respective

uncertainties (1 standard deviation). The straight line

represents the mean of all the obtained values and the

dashed lines represent 1% of deviation with respect to the

reference. As can be deducted from the figure, all the

ratios fell inside the range of +/- 1% of the deviation

respect the reference.

Conclusion

The study shows that for this specific value of

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

, R. Jones

, M. Fay

, D. Butler

, J. Lehmann

Calvary Mater Newcastle, Department of Radiation

Oncology, Newcastle- NSW, Australia

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)