Ifmbe proceedings 2503 - a tissue-equivalent radioluminescent fiberoptic probe for in-vivo dosimetry based on mn-doped lithium tetraborate

A tissue-equivalent radioluminescent fiberoptic probe for in-vivo dosimetry based
on Mn-doped lithium tetraborate
M. Santiago1,2, M. Prokic3, P. Molina1,2, J. Marcazzó1,2 and E. Caselli1,4 1 Instituto de Física Arroyo Seco, Universidad Nacional del Centro de la Provincia de Buenos Aires, Pinto 399, 7000 Tandil, Argentina 2 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Rivadavia 1917, 1033 Buenos Aires, Argentina 3 Institute of Nuclear Sciences, Vinca, P.O. Box 522, 11000 Belgrade, Serbia 4 Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CICPBA), calle 526 entre 10 y 11, 1900 La Plata, Argentina Abstract— The fiberoptic (FO) dosimetry concept, which re-
any external high-voltage bias, c) its rugged design makes is lies on the assessment of dose by measuring the intensity of the
suitable for the routine tasks carried out by radiotherapy light emitted by a tiny sample of a radioluminescent (RL)
technicians, d) since the reading is obtained during irradia- compound coupled to an optical fiber cable, is gaining impor-
tion, the FO technique allows for in-vivo dosimetry, etc. tance as a promising dosimetry technique for in-vivo do-
Many materials have been tested as FO scintillators: rare- simetry. In this work the design, construction and test of a
tissue-equivalent RL probe is described. The intensity of the
earth oxides [2,3], Cu1+-doped quartz [4], plastic scintilla- RL emission of Mn-doped Lithium Tetraborate samples dur-
tors [5], scintillating fibers [6], Ce3+-doped SiO2 optical ing irradiation is used as an estimate of the relative dose. The
fibres [7], Tb3+-doped fluorides [8], NaI(Tl), CsI(Tl) and influence of spurious luminescence is discussed and a removing
ZnS(Ag) [9], carbon-doped Al2O3 [10,11,12], etc. Although method based on simple optical filtering is implemented. Fi-
all of them show promising performances, Al2O3:C is one of nally, the response of the probe in the context of a typical
the most investigated compounds in the context of this ap- depth-dose experiment in a 60Co radiotherapy facility is ana-
plication probably due to its commercial availability (by lyzed and compared to the response of a standard ionizing
Landauer, Inc.), besides its outstanding properties as RL chamber.
phosphor. As to the optical fiber component, both plastic, Keywords— radioluminescence, radiotherapy, fiberoptic do-
polymethyl methacrylate (PMMA) [11,13] and fused silica simetry, tissue-equivalent detector.
[9, 12, 14] fibers have been used. Although PMMA fibers have higher attenuation, they are cheaper, have lower bend-ing radius and are more tissue-equivalent than silica fibers [15,16], what is important from the point of view of their application in dosimetry practice. Besides, PMMA fibers The development of new radiation-based treatments for show lower spurious intrinsic luminescence than silica fi- cancer claims for dosimetry systems accomplishing more and more demanding characteristics. Among them, the The intrinsic RL of the scintillator at the sensitive end of possibility of performing in-vivo dosimetry has shown to be the optical fiber is not the only source of light during the an increasing necessity in the different radiotherapy con- irradiation of the FO probe. Spurious light produced in the texts [1]. Among the different approaches developed so far fiber by Cherenkov effect and the intrinsic luminescence of to achieve this goal, the recently established fiberoptic do- the fiber also reaches the detector. The Cherenkov radiation simetry method offers interesting potentialities. This tech- is emitted whenever charged particles pass through dielec- nique is based on the use of an efficient, either organic or tric matter with a velocity beyond the velocity of light in the inorganic, scintillating material, which is placed in the point medium (fiber). The intensity of Cherenkov light increases where dose or dose-rate is to be assessed. During irradia- inverse to the third power of the wavelength. Consequently, tion, part of the energy absorbed by the scintillator is re- in the visible spectrum the blue color dominates. On the emitted as light of characteristic wavelength. This light is other hand, the intrinsic luminescence of the optical fiber collected by means of an optical fiber to which the scintilla- depends on the characteristics of the kind of fiber being tor is glued or mechanically coupled, and taken outside the used. Several methods have been proposed in the literature irradiation room up to a suitable high-gain detector. Gener- to get rid of the spurious luminescence, also dubbed stem ally the scintillation yield is proportional to the dose-rate, effect: simple optical filtering, subtraction of the back- what makes the system suitable for dosimetry. Overall, the ground signal, and time gating. In the first case long-pass FO technique shows interesting characteristics: a) the small filters remove the short wavelength components of the light size of the detector permits accurate dose measurements in reaching the detector, which are supposed to be more af- regions of high dose gradients, b) the system does not use fected by the Cherenkov emission. In the second case, the O. Dössel and W.C. Schlegel (Eds.): WC 2009, I FMBE Proceedings 25/III, pp. 367–370, 2009. www.springerlink.com M. Santiago et al.
signal of a blank optical fiber having no detector at its end is Solid sintered LTB phosphors are relatively complex ma- used to estimate the contribution of the stem effect. In the terials with characteristics strongly influenced by the prepa- third case, which is only useful for measurements at linear ration method, the phases of the Li2O-B2O3 system present, accelerators (LINAC), the spurious contribution of the stem the basic chemical used, the sintering temperature, and the effect is avoided by measuring the RL signal from the de- proper choice of the chemical form of activators and co- tector between the LINAC pulses, when the stem effect activators, etc. [18]. luminescence is negligible [5]. In-lab RL spectra have been obtained at room- In the field of radiotherapy dosimetry, the possibility of temperature by means of an Acton Research VM-504 0.39 relying on tissue-equivalent detectors, say, detectors having m monochromator featuring an Electron Tubes P25PC-02 an energy-dependent response similar to water, has been photon counting head as detector. This detector has a sensi- always welcome. For this reason several researchers have tivity window that goes from 250 up to 650 nm. The meas- used plastic scintillators when developing FO dosimetry urements have been made with a resolution of approxi- systems [14]. However, many inorganic materials showing mately 5 nm. In order to perform the spectral measurement also tissue-equivalent response to photons should not be the pellets were placed onto the entrance slit and irradiated dismissed. Among them, lithium tetraborate (LTB), which from their back side by means of a 3.7 u 108 Bq ophthalmic has been extensively investigated as thermoluminescent 90Sr beta-source located 1 cm away from the sample. dosimeter for its nearly tissue-equivalent effective atomic In order to build the FO probe, a 1mm3 piece has been number (Zeff = 7.3) clearly constitutes an interesting alterna- cut from a LTB:Mn pellet and glued to the end of a PMMA tive for RL dosimetry [18]. Although the RL of LTB doped fiber (1mm dia core, 2mm outer jacket). The sensitive end with different activators, such as Cu, Eu, Mn and Ag, has has been coated with three layers of opaque, water resistant been reported [19,20], its application to FO dosimetry has been never explored. In-situ measurements with the FO probe have been made In this article first results on the performance of a FO at room temperature in a Theratron 80 60Co radiotherapy probe featuring a doped LTB scintillator are reported. The facility rendering 0.3 Gy/min at 5 mm water depth (80 cm most suitable dopant has been chosen by taking into account SSD, source-surface distance). The emission of the FO the corresponding RL spectra and the spectrum of the stem probe has been measured by means of a Hamamatsu H9319 effect. As a test evaluating the response of the system under photon counting head having sensitivity between 300 and tissue-equivalent conditions, a percent depth dose (PDD) 850 nm. A long-pass colored-glass filter Schott OG530 has curve in a water phantom has been recorded and compared been placed between the end of the fiber and the detector in to that obtained with a standard ionization chamber. order to cut off the spurious contribution of the stem effect emission. To obtain the PDD with the FO probe a 40u40u40 cm3 water phantom has been employed. During the meas- II. MATERIALS AND METHODS urements the SSD and the field size have been set to 80 cm and 10u10 cm2 respectively. PDD readings have been made Lithium tetraborate samples doped with Mn, Cu and starting from the water surface up to 34 mm water depth. Cu,Ag,P have been used in this work. They have been de- Measurements performed in the water phantom with the FO veloped by M.P at the Institute of Nuclear Sciences, Bel- probe have been checked with a 0.6 cm3 PTW Farmer grade. These compounds have been prepared by a sintering chamber model 30013 and a PTW UNIDOS E electrometer. technique reported in Ref. [18,21,22], which renders pellets In-situ spectral measurements have been made at room (4.5 mm dia and 0.95 mm thickness) made by cold-pressed temperature with a resolution of approximately 10 nm em- polycrystalline powder having grain sizes between 75 and ploying an Acton Research SP-2155 0.150 m monochroma- 200 microns, which were sintered at 880ºC for Mn acti- tor equipped with a Hamamatsu H9319 photon counting vated, and at 850ºC for Cu and Cu,Ag,P activated LTB phosphors. The sintered LTB:Mn pellets are semitranspar-ent, and pale blue-colored for LTB:Cu and LTB:Cu,Ag,P. The activator concentrations for LTB:Mn was 0.1 wt%, for III. RESULTS AND DISCUSSIONS LTB:Cu was 0.03 wt% and for LTB Cu,Ag,P samples were 0.03 wt%, 0.03 wt% and 0.8 wt% respectively. As described In Fig. 1 the RL spectra of LTB:Cu, LTB:Cu,Ag,P and in the references cited before, the optimal stoichiometric LTB:Mn under beta irradiation are shown. The RL spec- ratio of the reagents has been carefully chosen in order to trum of Cu:LTB shows a single peak at 380 nm, what eliminate the effect of moisture on the prepared lithium matches very well the RL spectrum of LTB:Cu single crys- borate phosphor. tals reported in Ref. [19]. On the other hand, the spectrum IFMBE Proceedings Vol. 25 A Tissue-Equivalent Radioluminescent Fiberoptic Probe for In-Vivo Dosimetry Based on Mn-Doped Lithium Tetraborate of Cu,Ag,P:LTB presents a peak at approximately 435 nm. = 80 cm, field 10u10 cm2) while the angle spanned by the Similar results have been obtained by Can et al. [23] when fiber axis and the beam axis was set to 90 and 45º. In prin- they irradiate Cu,Ag,P:LTB pellets with X-rays at room ciple, it is expected that approximately at 45º the contribu- temperature. The RL emission of LTB:Mn shows a broad tion of the Cherenkov emission reaches its maximum value peak stretching between 300 and 500 nm and a minor, [14]. The results of the spectral measurement are shown in longer-wavelength peak centered at 590 nm, what resembles Fig. 1. For our setup the contribution of the stem effect the spectrum obtained by Holovey et al. for LTB:Mn single spans a wide wavelength range, which goes from 375 up to crystals under X-ray irradiation [24]. By taking into account about 600 nm with maximum at 400 nm. The strong de- the previous results, we have chosen LTB:Mn as the best pendence of the intensity on the angle confirms that the candidate to build a FO probe, since it shows intense RL stem effect is made up mainly of Cherenkov emission. By beyond the short-wavelength region where the stem effect is taking into account this information and the RL spectrum of usually important. LTB:Mn shown in Fig. 1, a OG530 Schott long-pass filter has been used in the rest of the measurements between the FO probe and the detector head in order to suppress by simple optical filtering the stem effect contribution. The typical response of the Mn:LTB probe under 60Co ir- radiation as function of time is illustrated in Fig. 2. The emission curve shows an abrupt rise as soon as irradiation is switched-on and a decaying behavior when irradiation is turned off. This so-called afterglow is usually linked to the presence of defects, which trap and release free charge car- riers at room temperature during irradiation [11]. Fig. 1 In-lab spectra corresponding to the RL emission of the different doped LTB samples and to the spurious luminescence (stem effect) pro- Fig. 2 Radioluminescent curve of the Mn:LTB FO probe obtained at 5mm duced in a blank fiber under 60Co irradiation at different angles between the water-depth in 60Co (dark counts have been previously subtracted). beam and fiber axes In Fig. 3 the PDD obtained by using the Mn:LTB probe When a FO probe is planned to be employed for in-situ is compared to that obtained in identical conditions with the measurements, it is important to determine the actual influ- ionizing chamber. As a relative estimate of the dose ab- ence of the stem effect for the particular setup being used. sorbed by the FO probe, the integral under the RL curve Indeed, if the simple optical-filtering technique will be used between switch-on and switch-off instants has been used. to remove the spurious luminescence produced in the fiber, As can be seen from the figure both curves reach their it is necessary to learn about the spectrum of the stem effect. maxima at 5 mm water depth, as expected [25]. Besides, For this reason, we irradiated in the 60Co radiotherapy facil- they match each other fairly well within the whole depth ity a blank PMMA fiber, say, a fiber having no sample at its range of interest (5-35 mm). Within the build-up region end, and recorded the resulting spectrum. The end of the differences between the response of the FO probe and the fiber was placed at 5 mm depth in the water-phantom (SSD chamber are observed mainly due to the different effective IFMBE Proceedings Vol. 25 M. Santiago et al.
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