the MD Anderson Cancer Center Orlando, Florida. The results demonstrate that PMMA optical fibres exhibit a quantifiable response to low doses (up to 12Gy) of ...
Low dose plastic optical fibre radiation dosimeter for clinical dosimetry applications S. O’Keeffe1, E. Lewis1, Optical Fibre Sensors Research Centre, University of Limerick, Limerick, Ireland
A. Santhanam2,3, A. Winningham3, J.P. Rolland2,4 2 CREOL, University of Central Florida, Orlando 3 MD Anderson Cancer Center Orlando, Florida 4 The Institute of Optics, University of Rochester, New York
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Abstract — PMMA optical fibres offer the opportunity to monitor ionising radiation doses online and in real-time. The work presented here investigates the use of these fibres in clinical dosimetry applications. Tests to determine the sensitivity of the PMMA optical fibres at low radiation doses were carried out at the MD Anderson Cancer Center Orlando, Florida. The results demonstrate that PMMA optical fibres exhibit a quantifiable response to low doses (up to 12Gy) of radiation, similar to that received during radiation treatment.
I.
INTRODUCTION
The use of ionising radiation as a method of treating cancerous tumours dates back to 1895, almost immediately after the discovery of X-rays by Wilhelm Roentgen, and was further established after the discovery of radioactivity by Bacquerel and pollodium and radium by Marie Curie [1]. External beam radiotherapy (EBRT) uses ionisong radiation external to the body to target malignant tumours in the patient. It destroys cells in a precisely controlled manner in the area being treated, referred to as the “target tissue”, by damaging the genetic material of the tumour, making it impossible for these cells to continue to grow and divide. Low energy photon based EBRT (120–300kV, produced by X-ray tubes) is used for treating superficial lesions, while high energy photons (6-25MV, produced by linear accelerators), which penetrate deeper, are used to treat deep-seated tumours in a patient’s body [2]. Although the technique dates back to the early twentieth century, an accurate delivery system wasn’t available and so it was not until the development, in the 1950s, of high energy 60 Co machines, which could penetrate deeper in to the body, that it regained popularity in oncology [3]. Much research has been done in the ensuing decades in the area and today it is the most common forms of radiation therapy, due in large to the improved radiographic and data-processing capabilities, in particular. the development of 3-D conformal radiotherapy, which allows the radiation beam to be set to the same size and shape as the target tumour. Intensity-modulated radiation therapy (IMRT) is the latest advancement in radiotherapy technology, allowing for the high precision delivery of ionising radiation to a tumour, specifically to an area inside a tumour. The method uses 3D computer tomography that manipulates computer controlled linear accelerators to distribute precise radiation doses to the affected area by modulating and adjusting the intensity of the radiation beam [2].
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Radiation dosimetry is used to ensure that accurate radiation doses are delivered during radiotherapy and to ensure the safety of the patient and the hospital personnel. Clinical dosimetry is used in a number of different aspects of the radiation therapy dose verification. It can be divided in to two main sections: 1) Basic beam verification, which is used to characterise the radiation beam and 2) Patient monitoring, which involves inphantom and in-vivo patient monitoring [2]. In-vivo patient monitoring is used to verify the dose of ionising radiation delivered to the patient during radiotherapy treatment. The dosimeter is placed either on the skin or in a body cavity, depending on the tumour location. Thermoluminescent detectors (TLD) are the most commonly used dosimeters for invivo patient monitoring, however it does not provide an immediate response and so real-time information is not available. This project investigates the use of Poly(methyl methacrylate) (PMMA) based plastic optical fibres for on-line dosimetry, with applications in both beam verification and invivo patient monitoring. Optical fibres offer many advantages for monitoring gamma radiation doses, such as their immunity to electromagnetic interference. Of particular advantage is their ability to remotely monitor radiation in real-time. The sensor can be placed several hundred metres from the control electronics, which means that they can be employed in harsh environments, such as a radiation chamber, and monitored online, in real-time, from a control room. The work presented include the initial results in the characterisation of PMMA optical fibres on exposure to low doses of X-ray radiation emitted from the Novalis radiotherapy facility. II.
RADIATION DEGRADATION OF PMMA OPTICAL FIBRES
The effect of ionising radiation on Poly(methyl methacrylate) (PMMA) has been studied in depth by Yoshida and Ichikawa [4]. The effects can generally be divided into two main sections: 1) main-chain scission (degradation) and 2) crosslinking. In many polymers both processes take place in parallel, however in certain cases, in polymers known as degrading polymers, such as PMMA, the scission dominates the crosslinking. The side-chain is initially affected by ionising radiation and the radical thus formed is a precursor for the main chain scission. When PMMA is irradiated with ionising
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radiation, such as gamma radiation, a free radical is generated
− COOC H 2 . on the ester side-chain,
The Beer-Lambert Law is used to determine the radiationinduced attenuation (RIA). The radiation-induced attenuation is given by equation 6 and can be further explained through the diagram in figure 1.
RIA(dB) = −
(1)
This side chain radical may be generated in a number of ways (Ichikawa 1995);
⎧⎪ P (λ , t ) ⎫⎪ 10 log⎨ T 0 ⎬ L0 ⎪⎩ PT (λ ) ⎪⎭
(6)
where PT(λ,t) is the measured optical power at the end of the irradiated fibre and PT0(λ) is the optical power of the reference fibre, which is not irradiated.
by direct action of ionizing radiation:
− COOCH 3 + γ → −COOC H 2 + H
(2)
by proton transfer of the side-chain cation:
− COOCH 3 + γ → −COOCH 3.+ + e − − COOCH 3.+ → −COOC H 2 + H +
(3)
or by hydrogen abstraction: Figure 1. Diagram showing how radiation-induced attenuation measurements are obtained from the transmission spectra.
− COOCH 3 + H → −COOC H 2 + H 2
III.
(4)
EXPERIMENTAL PROCEDURE
A. Experimental Set-up The side-chain radical remains stable at temperatures below 200K. Above 210K, the side-chain radical converts to a scission type radical due to the detachment of the side chain. This is followed by the β-scission of the main-chain radical and is shown in equations 5 and 6 (Yoshida and Ichikawa 1995). A. Radiation-Induced Attenuation of PMMA Optical Fibres The Beer-Lambert Law can be used to determine the radiation dose absorbed. The absorption co-efficient, α, is given by equation 5.
α =−
⎧ P (λ ) ⎫ 1 log⎨ 0 ⎬ L0 ⎩ P (λ ) ⎭
(5)
Where L0 is the irradiated length of fibre, P(λ)/P0(λ)is the ratio of the spectral radiant power of light transmitted through the irradiated dosimeter to that of the light transmitted in the absence of the dosimeter.
Figure 2 detailed the experimental set-up used for this investigation. It consists of a tungsten halogen white light source that was used to illuminate the fibre, with a linear variable filter to allow only wavelengths between 500nm and 700nm to be transmitted and a variable attenuator was used to ensure the spectrometer can be operated without going in to saturation. An Ocean Optics S2000 Dual-Channel Spectrometer was used to spectrally resolve the optical signal from the PMMA fibre. The test fibre was connected to the spectrometer using a separate 25m optical fibre, allowing for remote monitoring of the dosimetry measurement in real-time. An internal ADC1000-USB analogue to digital converter interfaced the spectrometer with a notebook PC via the USB port. As the spectrometer captured and detected the light, the resulting spectrum was displayed in real-time on the notebook PC using a LabVIEW V.I. [5]. The spectrometer detector consisted of a high sensitivity 2048-element linear silicon charged coupled device (CCD) array. Each of the 2048 detector elements represents a specific wavelength between 200nm and 850nm. The integration time set within the LabVIEW V.I. was used to control how long the CCD pixels accumulate light before completion, in a similar way to the shutter speed on a camera.
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An integration time of 1 second was used throughout the experiment.
allowing for optimisation of the test system. The fibre under test can be seen in figure 3, with figure 4 showing a close up of the fibre covered with the water-based membrane. IV.
Figure 2: Diagram of experimental set-up
B. Irradiation Procedure Testing of the PMMA fibres was conducted at the MD Anderson Cancer Center Orlando, Florida, using the Novalis Radiotherapy Facility. The Novalis system, emits X-ray radiation, using 6 MV photon energy. The system allows for a maximum field size of 100mm x 100mm, which was used for these initial experiments.
RESULTS AND DISCUSSION
The transmission spectra were monitored online and were recorded every two seconds for analysis of attenuation changes as the radiation dose was increased. The resulting spectra at various doses are presented in figure 5 and although, when viewing the overall spectra, it is not clear that there are any discernible changes in attenuation, when specific wavelengths are identified a more noticeable trend is apparent. The inset of figure 5 shows an amplified view of the first peak in the transmission spectra, around 565nm. From this graph, small changes in the spectra can be observed as the radiation dose increases. The intensity of the optical signal decreases as the dose increases, indicating that there is a measureable radiationinduced attenuation.
Novalis X-Ray System
PMMA Optical Fibre Figure 5. Transmission spectra for varying radiation doses, indicating a decreasing intensity as the dose increases (inset).
Figure 3. Fibre under test at Novalis facility Water-based membrane
Applying the radiation-induced attenuation (RIA) calculation, given in equation 6, to specific wavelengths of the transmission spectra there is an evident increase in attenuation. The results of the RIA calculations are shown in figure 6 for two peak wavelengths, 565nm and 594nm. There is an immediate and distinct increase in the RIA as the PMMA optical fibre is exposed to the x-ray radiation. The initial results indicate a sensitivity of the fibres to radiation of 0.001dBm1 /Gy. However further repeatability studies are planned to establish this precisely.
PMMA Optical Fibre Figure 4. Close up of fibre under test, with membrane cover.
A 5m length of PMMA optical fibre was coiled to fit within this field area, to allow for maximum irradiation along the fibre length. The PMMA based plastic optical fibres used were supplied by Fibre Data and consisted of a 1mm PMMA core, flourinated PMMA cladding and polyethylene jacket, (ncore ~ 1.49, attenuation ~ 0.15 dB/m), terminated with SMA connectors. The fibre was covered with a 1.5cm thick waterbased membrane used to simulate a patients’ skin tissue,
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V. Radiation-Induced Attenuation 0.018 565nm 594nm
RIA (dB/m)
0.013
0.008
0.003
-0.002 0
2
4
6
8
10
12
14
Accumulative Dose (Gy)
Figure 6. Radiation Induced Attenuation for two wavelengths, 565nm and 594nm.
The RIA calculation was also performed at a number of other wavelengths, and while they also exhibit a similar trend, the signal-to-noise ratio was too low to obtain sufficient information from the resulting plots. Characterisation of PMMA optical fibres at higher radiation doses have indicated that radiation sensitivity increases with decreasing wavelength in the visible region and therefore by increasing the optical source power at wavelengths between 500–550nm, monitoring at those wavelengths will lead to improved radiation sensitivity [6]. A.
Post-irradiation Recovery of the Fibre
Figure 7 shows the radiation-induced attenuation in the plastic optical fibre immediately following irradiation of the target. It shows that the attenuation continues for approximately one minute after the radiation was switched off, following which it levels off. Previous work has shown that following exposure to high radiation doses PMMA fibres exhibit a recovery, due to the spontaneous reforming of the polymer bonds damaged during irradiation [6]. Further analysis, over a longer time period, is necessary to determine if, following exposure to low radiation doses, similar post-irradiation recovery occurs. Recovery of POF at 565nm 0.022
RIA (dB/m)
0.02 0.018 0.016 0.014 0.012 0.01 0
20
40
60
80
100
Time after irradiation (seconds)
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CONCLUSIONS
An invesitgation in to the effect of low doses of ionising radiation on PMMA based plastic optical fibres have shown an immediate degradation of the transmission characteristics of the fibre. This results in a quantifiable radiation-induced attenuation of the optical signal transmitted through the fibre. By monitoring this optical signal at specific wavelengths (565nm and 594nm) it is possible to determine the dose received in the fibre, even at low radiation doses (up to 12 Gy). The results show that although there is some noise affecting the results, there is a distinct increasing trend in the RIA signal with increasing radiation. Further signal processing can be applied to improve the resulting RIA and the resolution of the sensor at these low doses. Future studies will determine the repeatability of the fibre measurements and the sensitivity of the fibre at a number of wavelengths. Initial post-irradiation measurements have indicated that there is a short-term increase (~ 1 min) in the polymer degradation before recovery begins. However it is necessary that these measurements are performed over a much longer time period to determine the exact recovery. The postirradiation recovery of the PMMA optical fibre is a significant feature of the optical fibre dosimeter that must be determined and future work will investigate this. The online monitoring of the fibre after irradiation will determine if the fibre can be reused or if the PMMA is permanently damaged. ACKNOWLEDGMENT The authors would like to thank the staff of MD Anderson Cancer Center Orlando, Florida for their help and patience during the course of these experiments. This research was funded under the James and Esther King Biomedical Research Program and the Florida I4-Corridor and also by the European Commission under the 7th Framework Programme through the ‘Marie Curie Re-integration’ action of the ‘Peoples’ Programme, (PERG04-2008-239207). REFERENCES [1] Rockwell, S., Experiemental Readiotherapy: A Brief History. Radiation Research, 1998. 150(5): p. S157-S169. [2] Chen, Z., F. d'Errico, and R. Nath, Principles and requirements of external beam dosimetry. Radiation Measurements, 2007. 41: p. S2 - S21. [3] Denmeade, S.R. and J.T. Isaacs, Timeline: A history of prostate cancer treatment. Nature Reviews Cancer, 2002. 2: p. 389-396. [4] Yoshida, H. and T. Ichikawa, Temperature effect on the radiation-degradation of poly(methyl methacrylate). Radiation Physics and Chemistry, 1995. 46(4-6, Part 1): p. 921. [5] National Instruments. http://www.ni.com. [cited April 2008]. [6] O'Keeffe, S., et al., Real-time gamma dosimetry using PMMA optical fibres for applications in the sterilization industry. Measurement Science and Technology, 2007. 18: p. 3171-3176.
Figure 7. Post-irradiation recovery of the fibre at 565nm.
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