Optical fibre-based sensors for improved radiation therapy
16 May 2013
Authors: Sinead O’Keeffe, Denis McCarthy, Elfed Lewis (Optical Fibre Sensors Research Centre, University of Limerick); Peter Woulfe (Department of Radiotherapy Physics, Galway Clinic); Mark Grattan and Alan Hounsell (Radiotherapy Physics, Northern Ireland Cancer Centre, Belfast City Hospital)
Research led by the University of Limerick – in collaboration with the Galway Clinic, Northern Ireland Cancer Centre and University of California, Los Angeles (UCLA) – is developing technology to ensure improved safety and more effective treatment for patients undergoing radiotherapy.
The sensor, based on optical fibre-sensing technology, provides real-time information on the radiation dose a patient receives and has many advantages over currently available technologies.
Radiotherapy is the use of ionising radiation for the treatment of cancer and it is involved in the treatment of over 40% of cancer patients [1,2]. Radiotherapy is delivered in the form of external beam radiotherapy, using linear accelerators (linacs), or brachytherapy, which involves the positioning of radioactive sources in, or near to, the tumour.
A common type of brachytherapy treatment involves the introduction of small radioactive ‘seeds’ directly into the prostate using long needle-like applicators. Radiotherapy destroys cells by damaging the genetic material of the tumour, making it impossible for these cells to continue to grow and divide.
However, it is inevitable that during treatment some surrounding healthy cells will also get damaged and so it is important that this damage to nearby healthy cells is minimised. The most commonly occurring clinical complications arising from radiation treatments of the pelvic region are a result of a high dose delivered to the portions of the rectum and bladder that are in close proximity to the tumour .
Knowledge of the radiation dose to critical structures is necessary to ensure that side effects can be minimised, while maintaining an adequate standard of treatment. These internal doses are calculated as part of the treatment planning process.
In-vivo dosimetry directly monitors the radiation dose delivered to a patient during radiation therapy. It allows comparison of prescribed and delivered doses and thus provides a level of radiotherapy quality assurance that supplements port films and computational double checks. A well-devised in-vivo dosimetry programme provides additional safeguards without significantly extending treatment delivery time.
Unlike other quality assurance (QA) methods, in-vivo dosimetry checks the dose delivered to the patient rather than the individual components prior to treatment. Current in-vivo measurements of treatment dose are limited to external parts of the patient, either as a means of confirming the radiation dose within the treatment field, or for measurement of doses to sensitive organs.
Traditional methods for estimating the dose delivered during treatment have involved the use of in-vivo detectors placed on the patient’s skin to monitor the actual dose delivered in comparison to that planned.
The most commonly used dosimetry methods for dose verification in radiation oncology include MOSFETs (metal oxide semiconductor field effect transistors), diodes, TLDs (thermo luminescent detectors) and scintillation detectors . MOSFET detectors operate by measuring the difference in voltage shift in the device before and after exposure to the ionising radiation. They are known to have an accumulated dose effect, which limits their lifetime, as well as a dependence on energy and temperature.
TLDs come in a number of different element types, including chip, rod or cube types. When subjected to ionising radiation, the electrons in the structure gain energy and are trapped in layers due to impurities in the crystal. After irradiation, the TLDs are heated and the electrons lose energy as they fall back to the valence band, with the energy given off as a photon in the visible part of the electromagnetic spectrum, and the amount of light detected by a PMT is directly proportional to the dose received by the TLD.
The main drawbacks of this type of detection system are that care must be taken in handling and identifying the individual TLDs, and that no real-time information is provided. Semiconductor diodes are commonly a p-type doped silicon detector and measure the movement of electrons and holes due to the radiation in the depletion layer.
While diodes can provide an instant readout, there are a number of known corrections that must be carefully applied to ensure an accurate reading is obtained. These corrections include temperature, angle of irradiation, dose rate, energy and cumulative radiation dose.
OPTIMAL FIBRE SENSOR TECHNOLOGY
The use of optical fibres in sensing technologies for a wide range of applications, particularly in areas such as structural health monitoring and environmental monitoring, has been the topic of much research in recent years.
The most significant feature of an optical fibre sensor is that the information is transmitted using optical signals as opposed to electrical signals. Consequently, optical fibres are immune to electrical and electromagnetic interferences, which can be a problem for many electronic and electrochemical based sensors. The ability to remotely monitor radiation is also an advantage of optical fibres.
The sensor can be placed several hundred metres from the control electronics, which means that they can be employed in harsh environments, such as in high-radiation-level areas in the vicinity of a nuclear reactor. Optical fibre sensors can also be multiplexed so that a single controller can monitor a number of sensors.
Research at the Optical Fibre Sensors Research Centre at the University of Limerick, in collaboration with the Galway Clinic, Northern Ireland Cancer Centre at Belfast City Hospital and UCLA are developing an advanced radiation dosimetry system based on optical fibre sensor technology.
The optical fibre dosimeter is constructed by coating the end of an exposed PMMA-based (polymethyl methacrylate-based) plastic optical fibre with a specific radiation sensitive scintillating phosphor material, terbium-doped gadolinium oxysulfide (Gd2O2S:Tb). The scintillating tip of the sensor fluoresces on immediate exposure to ionising radiation. The resultant emitted fluorescent light penetrates the PMMA optical fibre and propagates along the fibre to a distal scientific-grade spectrometer, where the intensity of the peak wavelength of the fluorescent light is measured.
The measured optical intensity directly relates to the amount of radiation a patient receives and by monitoring the optical intensity in real-time, the total radiation dose can also be determined [4,5].
Optical fibre sensors offer numerous advantages over conventional dosimeters, such as TLDs and diodes. The small dimensions of the optical fibre dosimeter make them suitable for minimally invasive in-vivo applications. This would allow the radiation dosimeter to be placed either directly into or in close proximity to the tumour, or in the case of a brachytherapy implant alongside the seeds, or radioactive sources, in a manner which has not been previously achieved to provide real-time dosimetric information e.g. in close proximity to the implants in the tumour itself or critical tissues requiring monitoring.
Optical fibre sensors offer a cost-effective radiotherapy dosimetry system that can be multiplexed so that a single controller can monitor a number of sensors. A further potential advantage of these sensors is that optical fibres generally comprise only silica (glass) or plastic as their constituent material and therefore are uniquely, and ideally, suited for use in the magnetic resonance imaging (MRI) environment as they are non magnetic, do not cause interference on the image and are themselves immune to the intense magnetic field and radio frequency pulses present in the MRI environment.
This would be of advantage for possible new techniques being developed, where the radiation therapy system features a combination of radiotherapy delivery and simultaneous MRI.
Dr O’Keeffe is chair of a pan-European COST Action TD1001 aimed at developing fibre-optic sensor systems for reliable use in safety and security relevant applications in society.
Dr O’Keeffe was recently awarded the Institute of Electrical and Electronics Engineers’ Sensors Council Early Career (GOLD) Award.
 Murray, LJ and Robinson, MH. ‘Radiotherapy: technical aspects.’ Medicine, 39(12): 698-704, December 2011.
 Podgorsak, EB. Radiation Oncology Physics: A Handbook for Teachers and Students (Vienna: International Atomic Energy Agency, 2005).
 Lambert, J et al. ‘In vivo dosimeters for HDR brachytherapy: a comparison of a diamond detector, MOSFET, TLD, and scintillation detector.’ Med. Phys. 34(5) May 2007, pp. 1759-1765.
 McCarthy, D. et al. ‘Optical Fibre X-Ray Radiation Dosimeter Sensor for Low Dose Applications.’ IEEE Sensors Conference, October 2011, Limerick.
 O’Keeffe, S. et al. ‘Radiotherapy dosimetry based on plastic optical fibre sensors.’ Fifth European Workshop on Optical Fibre Sensors, May 2013.