Abstract:
A radiation dosimetry apparatus and method use a scintillating optical fiber array for detecting dose levels. The scintillating optical fiber detectors generate optical energy in response to a predetermined type of radiation, and are coupled to collection optical fibers that transmit the optical energy to a photo-detector for conversion to an electrical signal. The detectors may be embedded in one or more modular, water-equivalent phantom slabs. A repeatable connector couples the collection fibers to the photo-detector, maintaining the fiber ends in a predetermined spatial relationship. The detector fibers may be distributed as desired in a three-dimensional detection space, and may be oriented with their longitudinal axes at different orientations relative to a transmission axis of an incident radiation beam. A calibration method uses two measurements in two spectral windows, one with irradiation of the scintillator at a known dose and one with only irradiation of the collection fiber.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of dosimetry and, more particularly, to rapid, high-resolution dosimeters for advanced treatment technologies. 
       BACKGROUND OF THE INVENTION 
       [0002]    Treatment modalities such as Intensity Modulated Radiotherapy (IMRT), helical tomotherapy and radiosurgery are pushing current dosimeter technologies to their limits. Two major driving forces are involved: 1) complex, two-dimensional field patterns encountered in IMRT and tomotherapy require high resolution (i.e., small volume) dosimeters that can be stacked or arrayed to provide a rapid but precise two- or three-dimensional dose measurement, which in turn requires that the dosimeters be water-equivalent in order not to disturb the fluence in the measurement plane and 2) small fields used in all three treatment modalities, down to 1×1 cm 2  in IMRT and just a few millimeters in radiosurgery, give rise to volume effects such as spatial averaging in most dose detectors with detecting volumes larger than a few cubic millimeters. 
         [0003]    Radiographic films possess high spatial resolution and are used for two-dimensional dose measurements. They are, however, subject to drawbacks. The need to develop the films before reading makes their use for online assessment impossible. Moreover, the development process affects the film response. Radiographic films are also notorious for over-responding to low-energy photons and they are not water-equivalent. Finally, the precision of a radiographic film used for dose measurement in the clinic is often limited to ±5%. Radiochromic films, which do not require development and are closer to water-equivalence in the megavoltage energy range, can also be used to evaluate dose distributions. However, radiochromic films are temperature dependent and sensitive to ultraviolet light. Achieving better than 5% reproducibility in routine fashion with radiochromic film is also challenging. 
         [0004]    Detector arrays have been implemented in the clinic to achieve a faster and more precise dose reading than films. To date these arrays have been made of either semiconductor dose detectors or ion chambers. These arrays allow online evaluation of a dose pattern with the precision of a single dosimeter. The spacing between the detectors determines the resolution of an array: the closer the detectors are to each other, the more continuous the dose information will be. Because the materials used with current detector arrays are not water-equivalent (typically made of silicon or air), the use of such an array creates a perturbation in the particle fluence. Moreover, the non-water equivalence of these detectors prevents the use of three-dimensional arrays with closely packed detectors. The current detector arrays also suffer from other limitations depending on the type of detector in use. For semiconductor detectors, there may be significant angular dependence and poor reproducibility for low dose fractions. For ion chambers there may be some dose averaging because their detecting volume is usually larger than other dose detectors. Thermoluminescent detectors (TLD) have also been used in groups to measure dose at different location simultaneously, but the need to read each one individually limits frequent use of a large number of TLDs. 
         [0005]    All of the above-mentioned detectors allow, at most, measurements in two-dimensional planes. The only detectors that can be used for three-dimensional measurements are dosimetric gels. Dosimetric gels are either based on the behavior of ferrous ions or on the polymerization of a monomer. They can be produced using a large variety of chemical formulas and each has its own set of advantages and disadvantages. However, most gels share a delicate fabrication process and require a time-consuming development process that makes them unsuitable for online measurements. 
         [0006]    One of the key properties that is desired in the next-generation dosimetry systems is water-equivalence. Water-equivalence guarantees that the measurement instrument, when immersed in a water tank, does not perturb the beam fluence and allows stacking and arraying of multiple dosimeters in the treatment field for two- or three-dimensional dose measurements. In order to facilitate the quality assurance of complex treatment modalities, there exists a need for small, water-equivalent dosimeters. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with the present invention, a radiation dosimeter is provided for measuring a relative dose of a predetermined radiation type within a detection region by using a plurality of scintillating optical fibers. The scintillating optical fibers are located in a predetermined spatial arrangement in the detection region, and generate optical energy in response to irradiation with the predetermined radiation type. Optically coupled to the scintillation optical fibers are a plurality of collection optical fibers that receive the optical energy generated by the scintillation optical fibers. The collection optical fibers transmit the optical energy to a photo-detector that, in turn, generates electrical signals indicative of the optical energy received. 
         [0008]    In an exemplary embodiment, a phantom material, such as a water-equivalent material, is located within the detection space, and the scintillation optical fibers are embedded in the phantom material. The phantom material may take the form of a plurality of modular slabs, at least some of which have the scintillation optical fibers embedded within. Slabs with no scintillation optical fibers may also be used as desired for equilibrium material. 
         [0009]    The scintillation optical fibers are relatively short in length, and function as small local radiation detectors. These detectors may be distributed as desired within a three-dimensional detection space so as to gather data regarding dose distribution within the space. The scintillation optical fibers may be arranged parallel to each other or perpendicular, or in any other desired arrangement. In one embodiment, a longitudinal axis of the scintillation optical fibers is perpendicular to a transmission axis of a radiation beam being used while, in another, the longitudinal axis is parallel to the beam axis. Similarly, a plurality of the scintillating fibers may all reside in a common plane perpendicular to the beam axis, or there may be scintillating fibers located in multiple such planes. 
         [0010]    The photo-detector of the present invention has an imager that converts the optical energy transmitted by the collection optical fibers to an electric signal that may thereafter be converted to a dose. It may also include a set of optical filters that allow discrimination between the light produced through scintillation and light produced through other means. In an exemplary embodiment, a housing surrounds the photo-detector components and shields them from stray light and radiation. 
         [0011]    An optical connector may also be used for providing repeatable optical coupling between the collection optical fibers and the photo-detector. The photo-detector imager takes a two-dimensional image of the input optical signals. The connector may be such that it maintains the output ends of the collection optical fibers in a predetermined spatial relationship relative to one another, and connects to the photo-detector in such a way that the collection optical fibers have a precise location relative to the imager. In one embodiment, the connector maintains the collection optical fibers in an equally-spaced arrangement that minimizes optical crosstalk between the optical signals that are output from the fibers. Various arrangements of the collection optical fibers relative to the imager may be used, such that a correlation between the image taken by the photo-detector and the spatial distribution of the scintillating optical fiber detectors may be selected as desired by the user. 
         [0012]    Also provided with the present invention is a method of calibrating a radiation dosimeter that uses a scintillating optical fiber that generates optical energy in response to irradiation with a predetermined radiation type and that is coupled to a collection optical fiber. The dose measured by such a dosimeter may be characterized as being proportional to a linear combination of a first dosimeter gain coefficient times a magnitude of optical energy collected by the collection optical fiber in a first spectral window plus a second dosimeter gain coefficient times the optical energy collected by the collection optical fiber in a second spectral window. The first spectral window may then be selected as matching an emission spectrum of the scintillating optical fiber, and the second spectral window as being substantially outside of the first spectral window. 
         [0013]    In performing the calibration, a portion of the collection optical fiber is irradiated with the predetermined radiation type while irradiation of the scintillating optical fiber is avoided or minimized. During this irradiation, a first measurement is made of the magnitude of optical energy in the collection optical fiber for each of the first spectral window and the second spectral window. The scintillating optical fiber is then irradiated with a known dose of the predetermined radiation type and a second measurement is made of the magnitude of the optical energy in the collection optical fiber for each of the first spectral window and the second spectral window. From the first measurement, a non-scintillation optical energy ratio can be determined. This is the ratio between the magnitude of the optical energy in the first spectral window found during the first measurement and the magnitude of the optical energy in the second spectral window found during the first measurement. The first dosimeter gain coefficient may then be determined by dividing the known dose by a value equal to the magnitude of the optical energy measured in the first spectral window during the second measurement minus a multiplication product of the non-scintillation optical energy ratio and the magnitude of the optical energy measured in the second spectral window during the second measurement. 
         [0014]    The calibration method allows proper determination of a gain coefficient for the dosimeter despite the presence of parasitic radiation such as {hacek over (C)}erenkov light resulting from irradiation of the collection fiber with the predetermined radiation type. Such non-scintillation radiation is generated in the entire spectral range but with different yield in each spectral window. The method may also be applied to a radiation dosimeter having multiple pairs of scintillating optical fibers and accompanying collection optical fibers and, when the scintillating/collection fiber pairs are similar construction, calibration of one may be applied to the others. That is, the calibration values determined during calibration of one of the scintillating/collection optical fiber pairs may be applied to each of the other scintillating/collection optical fiber pairs of the dosimeter. Moreover, the calibration method may be conducted with only one predetermined dose measurement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic side view of a scintillating fiber dosimeter according to the present invention. 
           [0016]      FIG. 2  is a schematic side view of a scintillating fiber dosimeter similar to that of  FIG. 1 , but for which a single slab is used for a phantom component. 
           [0017]      FIG. 3A  is a schematic view of a layout of scintillating fiber detectors embedded in a water-equivalent slab, where the fiber detectors are parallel and aligned at a mid-line of the slab. 
           [0018]      FIG. 3B  is a schematic view of an alternative layout of scintillating fiber detectors embedded in a water-equivalent slab, where the fiber detectors are distributed in an “X” pattern. 
           [0019]      FIG. 3C  is a cross sectional view of a water-equivalent slab according to the present invention having scintillating fiber detectors fully embedded inside. 
           [0020]      FIG. 4  is a schematic view of an embodiment of the invention in which the scintillating fiber detectors are embedded in a vertical direction in a water-equivalent slab. 
           [0021]      FIG. 5A  is a detailed schematic view of a scintillating fiber detector that may be used with the present invention. 
           [0022]      FIG. 5B  is a detailed schematic view of a scintillating fiber detector similar to that of  FIG. 5A , but for which the detector is coated with a reflective material. 
           [0023]      FIG. 6A  is a schematic view of a photo-detector component that may be used with the present invention. 
           [0024]      FIG. 6B  is a schematic view of a photo-detector component similar to that of  FIG. 6A , and which includes a set of optical filters and mirrors. 
           [0025]      FIG. 7  is a cross sectional view of a connector system that may be used with a scintillating fiber dosimeter array according to the present invention. 
           [0026]      FIG. 8A  is a graphical depiction of a relative radiation dose measurement taken using a scintillating fiber dosimeter array according to the present invention, for which the dose distribution of a radiotherapy beam is in a lateral direction. 
           [0027]      FIG. 8B  is a graphical depiction of a relative radiation dose measurement taken using a scintillating fiber dosimeter array for which a dose distribution of a radiotherapy beam is in an axial direction. 
           [0028]      FIG. 9A  is a schematic depiction of a first measurement step for a calibration procedure according to the present invention. 
           [0029]      FIG. 9B  is a schematic depiction of a second measurement step for a calibration procedure according to the present invention. 
           [0030]      FIG. 10  is a graphical depiction of a relative radiation dose measurement similar to that shown in  FIG. 8A , but which made use of a more accurate calibration method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The present invention provides a dosimeter that makes use of scintillating optical fiber detectors. In an exemplary embodiment of the invention, miniature plastic scintillating fiber detectors are used that have a unique combination of properties. Their water equivalence is maintained over a broad energy range (e.g., 0.2 to 25 MeV). Furthermore, they provide a highly sensitive medium, which enables small sensitive volumes (e.g., less than 2 mm 3 ) and high resolution, linearity to dose, dose rate independence, energy-independent response, and real-time readout. 
         [0032]    Shown in  FIG. 1  is a scintillating fiber dosimeter array according to the present invention. The dosimeter is located on a radiotherapy treatment table  13 , a phantom component  10  being composed of a series of water-equivalent modular slabs. Slabs  12   a ,  12   b  and  12   c  have scintillating fiber detectors embedded in them, while slabs  11   a  and  11   b  do not. While five slabs are shown in the embodiment of  FIG. 1 , those skilled in the art will recognize that it is possible to have more or fewer slabs depending on the particular application. The slabs may be made of a common plastic material with properties similar to water, such as polystyrene, acrylic or Lucite, or may be some other special chemical compound designed to be similar to water or human tissue. With the scintillating optical fibers being embedded in a water-equivalent phantom, beam perturbation is minimized. 
         [0033]    During operation, the slabs are irradiated by a radiation beam  14 . The impact of the radiation on the scintillating fiber detectors embedded in the slabs  12   a ,  12   b  and  12   c  results in the generation of scintillation light in the detector fibers in an amount proportional to the radiation dose detected. The scintillation light is transported by optical fiber cable  15  to photo-detector component  17 , to which it is connected by a connector system  16  that provides reliable, reproducible coupling. 
         [0034]    The phantom material and the scintillating fiber detectors may be arranged in any desired configuration. Since each of the detectors is small, and may act as a point detector, detection points may be selected as desired within a three-dimensional detection space. The embodiment shown in  FIG. 1 , for example, has three slabs within which detectors are embedded, as well as slabs on the top and on the bottom of the phantom in which there are no embedded detectors. These top and bottom slabs therefore serve only to provide equilibrium material for the phantom, while the detectors provide detection points within each of three different vertical planes. It may also be desired to use the detectors in an application that has no phantom. For example, the scintillating fiber detectors may be used during a radiotherapy treatment of a patient. 
         [0035]      FIG. 2  shows another example of the invention in which only a single slab  12  is used which has embedded scintillating fiber detectors. Two slabs  11   a ,  11   b , above and below the slab  12 , have no detectors, and serve only as equilibrium material. While  FIGS. 1 and 2  show two possible configurations of the present invention, those skilled in the art will recognize that numerous other combinations of scintillating fiber detectors, with or without different combinations of equilibrium material, may be used without departing from the intended scope of the invention. Moreover, the slabs shown in the figures are modular, allowing them to be mixed and matched as desired to form a phantom component of a particular configuration. However, other arrangements of phantom material may also be used. 
         [0036]    Different layouts of scintillating fibers may be used within a slab.  FIG. 3A  depicts one possible configuration, in which small, equal-sized pieces of the scintillating fibers are arranged in a parallel configuration, equally spaced along a midline  18  of the slab  23 . The scintillating fiber segments are coupled to non-scintillating collection fibers  19 , which extend from the side of the slab  23 . As indicated in the figure, the scintillating fiber segments  28  occupy only a small space relative to the overall fiber combination, and the detection that they provide is therefore only along the midline  18 . The lengths of combined scintillating and collection fibers may be embedded in grooves machined in the slab which, for example, may be made of water-equivalent plastic. The grooves may be of the same depth as the fiber diameter, so that the fibers are completely contained within the grooves. For example, one-millimeter diameter fibers may be embedded in grooves having a depth of one millimeter. The grooves are machined to provide a tight fit to the optical fibers, and to leave as small of an air gap as possible. As shown in the configuration of  FIG. 3A , the grooves are equally spaced and extend from a first edge of the slab to the mid-line  18 . The portions of the collection optical fibers that protrude from the slab  23  together form optical cable  15  that is connected to the photo-detector component  17  (as shown in  FIGS. 1 and 2 ). 
         [0037]      FIG. 3B  shows another possible configuration for fibers embedded in a detector slab  23 . In this example, the detector fibers are arranged in the shape of the letter “X,” which allows them to provide more spatial information. As in the arrangement of  FIG. 3A , the scintillating fiber segments of  FIG. 3B  are located only at the very end of the fibers shown in the figure, e.g., in the last one millimeter. In this configuration, half of the detectors are located at the ends of the collection fibers of fiber group  22 , which are embedded in one side of the slab  23 , one quarter of the detectors are located at the ends of the collection fibers of fiber group  20 , which are embedded in a direction perpendicular to the group  22 , and the remaining one quarter of the detectors are located at the ends of collection fibers of fiber group  21 , which extend from the side of the slab  23  opposite that of group  20 . Thus, the scintillating fiber detectors, being very short, together form an “X” shape across the slab  23 , and thereby represent a variety of detection points spanning a two-dimensional space. 
         [0038]    For a slab within which fibers are embedded in grooves made in the slab, it may be desirable to fix a thinner slab of water-equivalent material with shallow grooves atop the slab holding the fiber detectors.  FIG. 3C  is a schematic view showing a cross section of two such slabs  23 ,  24  that are fixed together, for example, by glue to form what is essentially a single slab. This configuration allows the fiber detectors to be tightly surrounded by equilibrium material, with little or no air space in between. It may also serve to keep a light-tight enclosure around the scintillating fiber detectors, so that no external light can reach them. 
         [0039]    Another embodiment of the invention is shown in  FIG. 4 . In this embodiment, the fiber detectors are oriented in a vertical configuration, such that the terminal ends of the fibers face toward the radiation source. That is, rather than the fibers being oriented in a direction perpendicular to a transmission direction of the radiation source, they are parallel thereto. This configuration allows the scintillating fiber detector to have a smaller cross section relative to the beam direction and therefore better spatial resolution. As in the other orientations, the scintillating fiber segments are relatively short, and are coupled to collection fibers  25  that protrude from the bottom of the slab and together form the optical cable  15  that is connected to coupler  16 . The scintillating fiber segments may each be located as desired within the three-dimensional space of the phantom material  10 , thus allowing a user to tailor the detection arrangement to a specific application. 
         [0040]    The configurations shown in  FIGS. 3A-3C  and  4  show only a small number of detectors shown to provide for better clarity. However, those skilled in the art will understand that many more detectors may be used to provide good resolution within the detection space. 
         [0041]      FIG. 5A  is detailed schematic view of a scintillating fiber as it might be embedded in the slabs of a phantom component of the present invention. A piece of scintillating fiber  26  is coupled to a light carrying optical collection fiber  27 . The scintillating fiber  26  has a core  28  that produces scintillation if stimulated by radiation, and a non-scintillating cladding  29  that improves the guidance of scintillation light produced in the core  28 . The collection fiber  27  also possesses a core  30  and cladding  31  but does not produce scintillation. A black jacket  32  surrounds the portion of the collection fiber  27  that is external to the water-equivalent slab to shield it from ambient light. A coupling  33  between the scintillating fiber  26  and the light carrying optical fiber  27  is made with a bonding agent that will minimize the differences in refractive index between the scintillating fiber  26  and the light carrying optical fiber  27 . A lens, a fiber optic taper or other device may also be inserted in the coupling region to increase coupling efficiency. 
         [0042]    The embodiment shown in  FIG. 5B  is similar to that of  FIG. 5A , but a reflective coating is added to the scintillating fiber. The reflective coating surrounds the piece of scintillating fiber, except for the portion that is coupled to the non-scintillating fiber  27 . The use of this reflective coating tends to increase the collection of a usable amount of light by the scintillating fibers in that it minimizes loss from the fiber by reflecting inward scintillation light generated therein. The reflective coating may also act as a shield by preventing external light from entering the fiber segment. In this way, the entry of ambient light into the fiber is minimized. 
         [0043]    A photo-detector system that may be used with the scintillating fiber detector of the present invention is shown in  FIG. 6A . An external casing  35  anchors the photo-detector components, and shields them from ambient light. An imager  36  converts the light coming from the input fiber optic cables to a quantitative electronic signal in the form of an image. The imager can be a CCD camera, a CMOS sensor, or any other device capable of producing a reproducible image either in monochrome or color with the level of light produced by the scintillating fiber detectors. Also located within the external casing are an objective lens  37 , an intermediate space  38  and a connector system  39 , which connects the non-scintillating optical fiber to the photo-detector system. The outer casing may be made of thick metal to act as a radiation shield that will protect the imager  36  from stray radiation. In case where the thickness is not sufficient to completely protect the imager, a post-processing algorithm may be used to remove transient noise produced by scatter radiation. 
         [0044]    A variation of the photo-detector system of  FIG. 6A  is shown in  FIG. 6B . In this embodiment, a set  42  of optical filters and mirrors is added to the intermediate space  38  to allow for filtration or separation of undesired light from another source such as Cerenkov radiation. Such an undesired light source can contribute to the noise of the system, and it is desirable to remove it. Filtration can be achieved by different techniques including, but not limited to, the use of band-pass filters to isolate the spectral component of the scintillation emission spectrum that overlaps only minimally with that of the undesired light source. One other possible way of filtering the undesired light source is by measuring the signal that contains both the scintillation light and the overlapping undesired light with different color filters, thereby applying spectral decoupling. 
         [0045]    The objective lens  37  of the photo-detector system collects the light from the optical fiber coupled to the connector system  39  and projects an image onto the photosensitive surface of the imager  36 . This allows for simultaneous reading of the output of all the scintillation detectors coupled to the connector system. For maximal collection efficiency, this objective lens should have a small F-number. The collection fibers are coupled to the connector system in a known arrangement, such that a correlation is made between the signal detected from each fiber and the corresponding scintillating fiber detector within the detection space. Thus, the image projected onto the imager is indicative of the distribution of radiation for the detectors that are coupled to the connector system  39 . Those skilled in the art will recognize that the specific correlation between the image of the photo-detector system and the spatial location of the detectors is completely customizable, and may be selected as desired by the user. 
         [0046]    Mechanical reproducibility is an important feature of an absolute dosimeter array. Shown in  FIG. 7  is a schematic view of a connector system that may be used with the present invention. The connector attaches to the photo-detector system and is configured so as to accommodate a plurality of non-scintillating collection fibers that transport light from the fiber detectors. The connector shown in  FIG. 7  has a plate with a row of grooves  40  within which the collection fibers reside. The grooves are spaced at an equal pitch (e.g., two millimeters) along the length of the connector, and may thereby serve to fix numerous fibers in an equally-spaced configuration. The grooves may be sized so that each provides just enough space for one of the collection fibers. Thus, if a flat surface is placed atop the plate portion containing the grooves, the fibers are snugly contained within the groove space, with very little air space surrounding them. This helps to minimize noise from being introduced due to stray ambient light. It also provides for the ability to stack several plates together to form a connector with multiple rows of collection fibers. The plate having the grooves  40  in  FIG. 7  also has a second set of grooves  41  on the opposite side. The set of grooves  41  can accommodate another row of collection fibers in the same manner as the set of grooves  40 . As this plate is stacked on another plate with a flat top surface, the fibers are snugly contained within the grooves  41 . 
         [0047]    Those skilled in the art will understand that the connector may have multiple plates with different number of grooves as may be desired by the user. Moreover, for clarity the drawing shows a relatively small number of grooves per row, and each row may have many more grooves. For example, in one embodiment, a connector may hold sixty fibers per row. In addition, several of the connectors can be mounted to the external casing of the photo-detector system thus allowing a very large number of optical fibers to be read simultaneously by the imager. By virtue of the predictable groove configuration, the connector enables repeatable positioning of the fibers relative to the imager across multiple connector insertion and extraction cycles. It is also desirable that all of the fibers being imaged simultaneously have their ends (from which optical energy is emitted) in a common plane. This allows for proper imaging of the optical outputs onto the image. Also shown in  FIG. 7  are mounting holes  45 , by which the connector may be mounted to the photo-detector system. These mounting holes aid in repeatable positioning of the connector relative to the photo-detector. 
         [0048]      FIGS. 8A and 8B  are graphical depictions of dosimetry measurements obtained with a scintillating fiber dosimeter array such as that shown in  FIG. 2 , using a slab containing scintillating fiber detectors  12  in a configuration like that depicted in  FIG. 3A . For this specific application, twenty-nine detectors were embedded in the slab  12 .  FIG. 8A  shows the results of measurement of three different “in-plane” beam profiles. The respective field sizes (i.e., the collimated beam sizes at a distance of 100 cm from the source) for the three results shown in the figure are 4×4, 10×10 and 20×20, all in square centimeters. The corresponding results in  FIG. 8A  (having a position axis in millimeter units) are based on a standard calibration using a conventional calibration method, and suffer from some inaccuracies in the tail (out-of-field) of the distributions. 
         [0049]      FIG. 8B  shows a depth-dose curve acquired with the same apparatus as is used for the results shown in  FIG. 8A . In each of  FIG. 8A  and  FIG. 8B , the symbols making up the points on the curves represent measurements performed with the present invention, while the lines represent measurements taken with a single ion chamber scanned across a volume of water. Thus, as can be seen, the scintillating fiber detector array provided a correct relative output factor (within 0.3%) for each of the different field sizes. The standard deviation on a series of ten consecutive measurements was less than 1% within the field. The maximum relative difference between the scintillation detector and ionization chamber was equal to 0.9% in-field for the profiles ( FIG. 8A ) and 1.6% for the depth dose ( FIG. 8B ). 
         [0050]    Also provided with the present invention is a calibration method for the dosimeter described herein. One of the issues facing the fiber dosimeter is the presence of ionizing radiation called {hacek over (C)}erenkov radiation, which is a parasitic optical signal caused by the interaction of high energy electrons with the light-carrying optical fiber that transmits the optical signal from the scintillating fiber. The {hacek over (C)}erenkov radiation represents optical noise with a wavelength range that overlaps with that of the optical signal from the scintillating fiber. Thus, an accurate measurement requires that the effect of the {hacek over (C)}erenkov radiation be accounted for in determining the dose from the scintillation light. 
         [0051]    The dose measured using a scintillating fiber dosimeter may be characterized as a linear combination of light measured in two orthogonal spectral windows. In practice, light in two different spectral windows may be measured from a light signal in a collection fiber by using a detector capable of discriminating the different wavelength ranges or, for example, by using a photodiode with an appropriate set of filters. In the following example, two spectral ranges are considered, referred to herein as the “blue range” and the “green range.” If the variable “B” is used to designate the magnitude of the blue light and “G” used to represent the magnitude of the green light, and “a” and “b” used to represent the respective calibration coefficients for these two values, the dose, “D”, may be represented as: 
         [0000]        D=aB+bG   (1) 
         [0000]    While this relationship may be used to calculate dose, it is necessary to perform two calibration steps to determine the values of the two calibration coefficients “a” and “b.” 
         [0052]    The total measured light signal may also be represented as a combination of the scintillation light and the {hacek over (C)}erenkov radiation light in each of the two spectral channels. Thus: 
         [0000]        B=B   s   +B   c , and 
         [0000]        G=G   s   +G   c   (2) 
         [0000]    where B s  is the scintillation light in the blue channel, B c  is the {hacek over (C)}erenkov radiation light in the blue channel, G s  is the scintillation light in the green channel and G c  is the {hacek over (C)}erenkov radiation light in the green channel. Inserting these terms into equation (1) yields: 
         [0000]        D=a ( B   s   +B   c )+ b ( G   s   +G   c )  (3) 
         [0000]    In the absence of scintillation, the value of the dose must be zero, so equation (3) must respect the condition: D=0 if B s =0 and G s =0, so: 
         [0000]      0 =aB   c   +bG   c , 
         [0000]    which, if put in terms of the calibration coefficient for the green channel, gives: 
         [0000]        b=−a ( B   c   /G   c )  (4) 
         [0000]    If this expression is then inserted back into equation (1), the result is: 
         [0000]        D=a[B −( B   c   /G   c ) G]   (5) 
         [0000]    As discussed below, this expression may be used for determining the calibration coefficients for when the blue channel represents the spectral emission window of the scintillator, i.e., when the scintillator is a “blue” scintillator. Similarly, when the green channel represents the spectral emission window of the scintillator (i.e., for a green scintillator), the value of the gain coefficient for the blue channel “a” may be isolated to give: 
         [0000]        a=−b ( G   c   /B   c ) 
         [0000]    and equation (1) rewritten as: 
         [0000]        D=b[G −( G   c   /B   c ) B]   (6) 
         [0053]    Equations (5) and (6) above represent alternative ways to write equation (1), and each still has two calibration coefficients. The first coefficient represents the gain factor of the detector (“a” or “b”), while the second represents the color ratio of the {hacek over (C)}erenkov light (B c /G c  or G c /B c ). Using a calibration method according to the present invention, the value of these coefficients may be determined from the system. Shown in  FIG. 9A  is a schematic view of a scintillating fiber segment  50  coupled to a non-scintillating collection fiber  52 . A radiation beam  54  is depicted as intersecting a portion of the collection fiber  52 , but not the scintillating fiber segment  50 . In the exemplary embodiment, the fiber is irradiated close to the scintillator, but far enough to avoid the production of any scintillation. Thus, with this manner of irradiation, only {hacek over (C)}erenkov radiation is produced, and no light from scintillation. The measurement of this signal allows the determination of the calibration coefficient B c /G c  or G c /B c  through the calculation of the ratio of luminosity collected in the two spectral windows (in this example, the blue and green channels). 
         [0054]    A second measurement, shown in  FIG. 9B , involves the location of the scintillating fiber segment  50  at a point of known dose D. The measurement of this signal thereby allows the determination of the gain coefficient “a” or “b” of the dosimeter. In this embodiment, scintillation light must be produced, although the size and characteristics of the radiation field are otherwise not important. It is not necessary to produce {hacek over (C)}erenkov radiation, although doing so will not significantly alter the measurement process. From equations (5) and (6) shown above, the value of the calibration coefficient (B c /G c  or G c /B c , respectively) is known for the spectral window in question. This acquired value may therefore be substituted into the appropriate one of equation (5) or (6), leaving the gain coefficient as the only unknown. For a blue scintillator, the value of the calibration coefficient B c /G c  may be determined from the first measurement, and that value used to solve for the gain coefficient “a” in equation (5), thereby yielding: 
         [0000]    
       
         
           
             a 
             = 
             
               D 
               
                 [ 
                 
                   
                     B 
                      
                     
                         
                     
                      
                     2 
                   
                   - 
                   
                     
                       ( 
                       
                         
                           B 
                           c 
                         
                         
                           G 
                           c 
                         
                       
                       ) 
                     
                     ⋆ 
                     
                       G 
                        
                       
                           
                       
                        
                       2 
                     
                   
                 
                 ] 
               
             
           
         
       
     
         [0000]    where B2 and G2 are the luminosities measured during the second measurement in the blue and green channels, respectively. Since the dose value, D, is known, and the value of the calibration coefficient B c /G c  is determined from the first measurement, the gain coefficient “a” can be determined. The same process may be used for the case of a green scintillator, such that the gain coefficient “b” may be determined using the relation: 
         [0000]    
       
         
           
             b 
             = 
             
               D 
               
                 [ 
                 
                   
                     G 
                      
                     
                         
                     
                      
                     2 
                   
                   - 
                   
                     
                       ( 
                       
                         
                           G 
                           c 
                         
                         
                           B 
                           c 
                         
                       
                       ) 
                     
                     ⋆ 
                     
                       B 
                        
                       
                           
                       
                        
                       2 
                     
                   
                 
                 ] 
               
             
           
         
       
     
         [0000]    where B2 and G2 are the luminosities measured during the second measurement in the blue and green channels, respectively, D is the known dose value and the value of the calibration coefficient G c /B c  is determined from the first measurement. 
         [0055]    Those skilled in the art will understand that the method described above is not specific to “blue” or “green” channels, and that it may be thought of more generically in terms of two optical wavelength spectral windows. A first spectral window should fit the emission spectrum of the scintillator used. In general, a scintillator may be described as having a known emission spectrum S(λ), where λ is optical wavelength. A second spectral window may then be defined as the integral of this emission spectrum plus the {hacek over (C)}erenkov emission spectrum C(λ) in the second spectral window (delimited by the wavelengths λ1 and λ2). The luminosity in the second channel may therefore be represented by: 
         [0000]    
       
         
           
             L 
             = 
             
               
                 ∫ 
                 
                   λ 
                    
                   
                       
                   
                    
                   1 
                 
                 
                   λ 
                    
                   
                       
                   
                    
                   2 
                 
               
                
               
                 
                   ( 
                   
                     
                       S 
                        
                       
                         ( 
                         λ 
                         ) 
                       
                     
                     + 
                     
                       C 
                        
                       
                         ( 
                         λ 
                         ) 
                       
                     
                   
                   ) 
                 
                  
                 
                     
                 
                  
                 
                    
                   λ 
                 
               
             
           
         
       
     
         [0000]    Moreover, while the calibration method is described with reference to a single scintillator/collection fiber combination, those skilled in the art will recognize that the process may be applied to a system having any number of scintillator/fiber elements. In addition, when using fiber components having the same material and construction, and the same signal optical detection for each of the scintillator/collection fibers of the system, the calibration of one scintillator/collection fiber may be applied to the others of the system. 
         [0056]    An earlier proposed calibration method using a subtraction of the {hacek over (C)}erenkov contribution relied on the assumption that the attenuation of the optical fiber was the same in the two channels. However, while being a reasonable approximation, this is not true, and the result was errors in the extraction of the calibration coefficients. In the method of the present invention, two very extreme conditions are used to extract the calibration factors. This results in robust parameters that can be used in any clinical configurations. Since B c /G c  (or G c /B c ) has to be measured only once for a given fiber, this reduces the number of steps necessary for performing a calibration, and better agrees with certain calibration protocols, which are often formulated in term of one specific point of dose. The method also simplifies the calibration protocol for certain brachytherapy and radiosurgery applications as it eliminates the influence of the radiation field dimension. As mentioned above, in the measurement shown in  FIG. 9A , the size of the radiation field is of no importance and, in the second measurement shown in  FIG. 9B , it is only important to provide a dose at the scintillator in order to produce scintillation light. Removing the need to control field dimension for calibration is highly beneficial to brachytherapy and radiosurgery applications, which can produce only a small field size (typically less than a five centimeter diameter). As an example of the improved accuracy using the calibration method described above,  FIG. 10  shows the same in-plane beam profiles as are depicted in  FIG. 8A . These profiles were measured using the same scintillation fiber detectors in the same conditions as in  FIG. 8A , but made use of the new calibration method. 
         [0057]    The present invention enables rapid and accurate measurements of the most complex radiation therapy treatment modalities such as IMRT, tomotherapy and radiosurgery. Due to the unique properties of the plastic scintillating fiber, the total time required for mandatory quality assurance tests of these complex treatment modalities is minimized. The dosimeter described herein possesses the accuracy and precision of a single point detector without requiring time-consuming processing such as the development or scanning of films. The scintillating fiber detectors of the invention are also fully water equivalent so as to not cause perturbation of the beam even when the detectors are closely distributed in three-dimensional space. 
         [0058]    While the invention has been shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.