AIR CORE SCINTILLATION DOSIMETER SUMMARY We 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. CONCLUSION The 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 Radiotherapy David R. McKenzie 1, Natalka Suchowerska 1,2, Jamil Lambert 1,2, YongBai Yin 1, Sue Law 1, 1 Royal Prince Alfred Hospital, Sydney, Australia 2 University of Sydney, Sydney, Australia REFERENCES Lambert 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) , 2008 Elsey, 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, , 2007 SH Law, N Suchowerska, DR McKenzie, SC Fleming, T Lin, The transmission of Čerenkov radiation in optical fibers, Optics Letters 32, (10), , 2007 Law, 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 ratio SignalBackground Air core (0.2 m length) Air core (0.6 m length) Air core (1.0 m length) PMMA fibre (1 mm diameter) 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. Cerenkov background 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. Solid core