Susan Law

A plastic scintillation dosimeter for high dose rate brachytherapy.

In vivo dose verification in brachytherapy requires a small insertable dosimeter with a real-time readout capability. Fibre optic scintillation dosimeters, consisting of a plastic scintillator coupled to an optical fibre, are one of the most promising dosimeters for this application. We have developed two sizes of the BrachyFOD scintillation dosimeter which have external diameters of 2.2 mm and 1 mm and have determined their important dosimetric characteristics (depth dose relation, angular dependence, temperature dependence, energy dependence). We have shown that the background signal created by Cerenkov and fibre fluorescence does not significantly affect the performance in most clinical geometries using an (192)Ir source from an HDR brachytherapy unit. The dosimeter design enables readout at less than 0.5 s intervals. The BrachyFOD satisfies the need for a real-time in vivo brachytherapy dosimeter.

Cerenkov-free scintillation dosimetry in external beam radiotherapy with an air core light guide

Plastic scintillators have many advantages for dosimetry in external beam radiotherapy. The current method of transmitting the scintillation light to a remote detector is through a solid core optical fibre. When exposed in a high energy therapeutic radiotherapy beam this fibre is subject to an unwanted background signal from Cerenkov light which can exceed the scintillation signal at characteristic angles. We have constructed a plastic scintillation dosimeter that uses an air core light guide to transport the light from the scintillator to the light detector. We show that there is sufficient signal propagation in the air core light guide to allow the scintillator signal to be carried outside the primary beam of a radiotherapy linear accelerator and for a dosimeter to be constructed using a scintillator inserted into the end of the light guide. Studies of the background light generated in the air core light guide, as a function of the angle between the beam and the fibre axis, show that there is no characteristic Cerenkov peak generated in the air core. Depth dose measurements using the air core scintillation dosimeter with no correction for Cerenkov are compared to ionization chamber measurements for a 6 MV photon beam and a 9 MeV electron beam.

Plastic scintillation dosimetry: optimization of light collection efficiency

Practical contemporary radiotherapy dosimetry systems used for dose measurement and verification are ionization chambers (which typically have at least a 0.1 cm3 air cavity volume), thermoluminescent dosimeters (TLDs) and silicon diodes. However, during the last decade, there has been an increased interest in scintillation dosimetry using small water-equivalent plastic scintillators, due to their favourable characteristics when compared with other more commonly used detector systems. Although plastic scintillators have been shown to have many desirable dosimetric properties, as yet there is no successful commercial detector system of this type available for routine clinical use in radiation oncology. The objectives of this study are to identify the factors preventing this new technology from realizing its full potential in commercial applications. A definition of signal to noise ratio (S/N) will be proposed for this category of detectors. In doing so the S/N ratio for an early prototype design has been calculated and/or measured. Criteria to optimize the response and sensitivity of this category of detectors are presented.

Transmission of Cerenkov radiation in optical fibers

Cerenkov radiation is generated as an unwanted background when optical fibers carrying signals passthrough radiation fields. The angular dependence of the intensity of Cerenkov radiation transmitted in silica-core fibers was measured using 6 and 12 MeV electron beams from a Varian Clinac accelerator. These confirmed theoretical predictions that the angular variation of Cerenkov radiation transmitted along optical fibers depends only on the refractive index difference Δn between the core and the cladding, and that the peak intensity is proportional to the cube of the fiber core radius.

Optical fiber design and the trapping of Cerenkov radiation

Cerenkov radiation is generated in optical fibers immersed in radiation fields and can interfere with signal transmission. We develop a theory for predicting the intensity of Cerenkov radiation generated within the core of a multimode optical fiber by using a ray optic approach and use it to make predictions of the intensity of radiation transmitted down the fiber in propagating modes. The intensity transmitted down the fiber is found to be dominated by bound rays with a contribution from tunneling rays. It is confirmed that for relativistic particles the intensity of the radiation that is transmitted along the fiber is a function of the angle between the particle beam and the fiber axis. The angle of peak intensity is found to be a function of the fiber refractive index difference as well as the core refractive index, with larger refractive index differences shifting the peak significantly toward lower angles. The angular range of the distribution is also significantly increased in both directions by increasing the fiber refractive index difference. The intensity of the radiation is found to be proportional to the cube of the fiber core radius in addition to its dependence on refractive index difference. As the particle energy is reduced into the nonrelativistic range the entire distribution is shifted toward lower angles. Recommendations on minimizing the quantity of Cerenkov light transmitted in the fiber optic system in a radiation field are given.

Laser ultrasonic evaluation of human dental enamel during remineralization treatment

In this work a non-destructive laser ultrasonic technique is used to quantitatively evaluate the progressive change in the elastic response of human dental enamel during a remineralization treatment. The condition of the enamel was measured during two weeks treatment using laser generated and detected surface acoustic waves in sound and demineralized enamel. Analysis of the acoustic velocity dispersion confirms the efficacy, as well as illuminating the progress, of the treatment.

Čerenkov radiation in optical fibres

Theoretical predictions that the angular variation of Cerenkov radiation transmitted along optical fibres depends only on Deltan and that the peak intensity is proportional to the cube of the fibre core radius are experimentally verified.

Laser Ultrasonic Technique for Evaluating Human Dental Enamel

A non-destructive laser ultrasonic surface acoustic wave technique has been demonstrated to quantitatively evaluate the elastic response of human dental enamel. We demonstrate the system performance by measuring surface acoustic wave velocity in sound and demineralised enamel. In addition, progressive measurements were made to monitor the change in the enamel elasticity during a two week remineralisation process. The results are presented and they confirm the efficacy, as well as illuminating the progress, of the treatment.

An Air Core Scintillation Dosimeter for Megavoltage External Beam Dosimetry: Experimental Results

A plastic scintillation dosimeter, using an air core light guide to transport the scintillation signal, has been shown to be free of the unwanted Cerenkov background signal that usually results from exposure of fibre optics to megavoltage photon and electron radiation beams. We report the performance of the air core dosimeter in 6 MV and 18 MV photon beams and 6 MeV and 20 MeV electron beam. The percentage depth dose measurements for both photon beams agree with ionisation chamber measurements to within 1.6%, for depths up to 250 mm, except in the build up region where the positional uncertainty results in a slightly larger deviation. For a 6 MeV and 20 MeV electron beam, the percentage depth dose measurements agree with the ionisation chamber measurements to within 3.6% and 4.5% respectively.

Cerenkov radiation in optical fiber communication

Theoretical predictions regarding the angular variation of Cerenkov radiation transmitted along optical fibers are experimentally verified. The implications for transmission in a radiation environment are considered.