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AIR CORE SCINTILLATION DOSIMETER

SUMMARYWe have shown that Cerenkov light can be reduced to a negligible level in scintillation dosimetry by using an air

core light guide to transport the scintillation light signal out of the radiation primary beam. This novel design has favourable characteristics for megavoltage photon and electron beam dosimetry, with high spatial resolution and

fast readout speed, critical for applications such as stereotactic radiosurgery and IMRT.

CONCLUSIONThe Air core scintillation dosimeter eliminates Cerenkov background light and is tissue equivalent. This feature, combined with its other characteristics of dose rate independence, high spatial resolution, energy independence and fast response, make the Air core scintillation dosimeter a promising new tool for complex dosimetry applications, such as IMRT and stereotactic radiosurgery.

Cerenkov-free Scintillation Dosimeter for use in

External Beam RadiotherapyDavid R. McKenzie1, Natalka Suchowerska1,2 , Jamil Lambert1,2, YongBai Yin1, Sue Law1, 1Royal Prince Alfred Hospital, Sydney, Australia

2University of Sydney, Sydney, Australia

REFERENCESLambert J, YinY., McKenzie D., Suchowerska N., Cerenkov free scintillation dosimetry in external beam radiotherapy with an air core light guide, Physics in Medicine and Biology(53) 3071-3080, 2008Elsey, J, McKenzie, DR, Lambert, J, Suchowerska, N, Law, S, Fleming, S Optimal Coupling of Light from a Cylindrical Scintillator into an Optical Fibre, Applied Optics, 46 397-404, 2007SH Law, N Suchowerska, DR McKenzie, SC Fleming, T Lin, The transmission of Čerenkov radiation in optical fibers, Optics Letters 32, (10), 1205-1207, 2007Law, SH, Fleming, SC, McKenzie, DR, Suchowerska, N, Transmission of Čerenkov radiation generated in the core of an optical fiber, Applied Optics 32[10] 2007

The Solution Our new scintillation dosimeter

design does not produce any Cerenkov signal in the air core. This

design has a small detection volume which is tissue equivalent,

it is energy and dose rate independent and provides real time

dose readings with high reproducibility and linearity.

Dosimeter light guide

PMT reading (nA)Signal to

Background ratioSignal Background

Air core (0.2 m length) 68.33 0.592 115.4

Air core (0.6 m length) 7.38 0.174 42.4

Air core (1.0 m length) 2.65 0.124 21.4

PMMA fibre (1 mm diameter) 706 182 3.9 0

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The Challenge

Current methods of transmitting the scintillation light to a remote detector use a solid core optical fibre. When exposed to radiation, these fibres generate an unwanted Cerenkov light as a background. The background can exceed the scintillation signal at characteristic angles.

SIGNAL:NOISE DEPTH DEPENDENCE

FIELD SIZE DEPENDENCE

CERENKOV SIGNAL – ANGULAR DEPENDENCE• The angular dependence of dosimeter performance was measured (Figure

1).• Dose readings were taken as a function of angle, integration time 0.5s• Air core dosimeter readings were within 2% (0.7%) for all angles• Solid core dosimeter exhibited a Cerenkov signal 2.5 times the magnitude

of the radiation induced light signal. • All scintillation dosimeters should be tested in this worst case situation to

validate performance.

• Table 1 shows the signal with and without a scintillator (signal and background respectively), when the scintillator axis is at 90 to the beam central axis

• Cerenkov generates 26% of the signal in PMMA fibre

• Signal is reduced by the air core, but the signal to background ratio is vastly improved.

Figure 1. Experimental design

PERFORMANCE

• Percentage depth dose for 6MV photon beam and 9MeV electron beam were measured in water using the air core dosimeter (Figure 3)

• For photons, measured dose agreed with 0.125cc SEMI-FLEX (PTW 31010) ionisation chamber readings to within 1.5% (0.3%)

• For electrons, measured dose agreed with Advanced Markus ionisation chamber readings to within 3% for depths 1.5 to 54 mm

Figure 3. Measured % depth dose

Figure 2.

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• The reading of the air core dosimeter with 1x5mm scintillator is shown as a function of field size in the range 1x1cm to 10x10 cm in a 6MV photon beam.

• The results agree with a 0.125cc SEMI-FLEX (PTW 31010) ionisation chamber

• Note that the diamond detector readings departs from both of the other detectors at 1x1cm fields.

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