ESTRO 36 Abstract Book

S759

ESTRO 36 2017

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For the majority of ion chambers in SW phantom CFs were

up to 1.01, except of the IBA CC01 were a correction of 4%

is needed mainly due to perturbation of the high density

central electrode. Regarding ABS phantom larger

corrections are needed up to 1.05 for IBA CC01 and CC13

and up to 1.02 for PTW 31010 and Exradin A1SL, attributed

to the different orientation of the detectors in GK PFX

stereotactic space.

Conclusion

An experimental procedure is proposed for the

determination of CFs for the GK PFX radiosurgery unit and

CFs were determined for a set of ion chambers allowing

for accurate dosimetric measurements.

Acknowledgement: This work was financially supported by

the State Scholarships Foundation of Greece through the

program “Research Projects for Excellence IKY/SIEMENS”.

EP-1439 Small field dosimetry: preliminary

characterization of a nano-chamber with a focus on

stem effect

A. Stravato

, G. Reggiori

, P. Mancosu

, F. Lobefalo

, L.

Paganini

, F. Zucconi

, V. Palumbo

, S. Tomatis

, M.

Scorsetti

Istituto Clinico Humanitas, Department of radiotherapy

and radiosurgery, Rozzano Milan, Italy

Purpose or Objective

Micro and nano-chambers cannot be as small as solid state

detectors but present some advantages in terms of energy

independence and absolute dose measurement that make

them fundamental for small field dosimetry in the SBRT

scenario. A preliminary characterization of a nano-

chamber prototype (Razor Nanochamber, IBA) was carried

out with a particular focus on stem effect. Response under

10 MV FFF beams was observed too.

Material and Methods

The study included characterization of leakage, dose rate

and dose per pulse dependence, measurement of small

beam profiles, and depth dose curves. Profiles were

acquired both in orthogonal (i.e chamber axis orthogonal

to beam axis) and parallel (i.e chamber axis parallel to

beam axis) configuration. Ten repeated inline profile

measurements were performed in head-foot and foot-

head direction to better quantify the stem effect. Ion

collection efficiency and polarity effects were measured.

The values of P

ion

were verified with

1/V

versus

1/Q

curves

(Jaffé plots). The 6 MV and 10 MV FFF photon beams of a

Varian EDGE were used. Output factors for small fields

were compared with Razor Diode (IBA) and FOD scintillator

values.

Results

The 2mm diameter guarantees a very high spatial

resolution comparable to some commercially available

diodes, with penumbra values 0.5-0.8mm larger than

those measured with a Razor Diode for the same fields

(Figure 1). When used with the chamber axis

perpendicular to the beam axis a strong stem (and cable)

effect was observed leading to asymmetric inline profiles

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