WO2010017218A2

WO2010017218A2 - Method and apparatus to discriminate out interference in radiation dosage measurements

Info

Publication number: WO2010017218A2
Application number: PCT/US2009/052735
Authority: WO
Inventors: Toshihide Ushino
Original assignee: Mirion Technologies (Gds), Inc. (Formerly Known As Global Dosimetry Solutions, Inc.)
Priority date: 2008-08-06
Filing date: 2009-08-04
Publication date: 2010-02-11
Also published as: WO2010017218A3

Abstract

A method and apparatus to mitigate interference, also known as the stem effect, from Cerenkov radiation and fluorescence photons generated in optical fiber radiation detectors, provide for real-time ionizing radiation measurement. The fiber optic detector has a radioluminescent scintillator at its tip. Data collection from the fiber optic dosimeter is gated so that photons from the scintillator radioluminescence are collected after Cerenkov radiation and fiber fluorescence photons disappear. Data collection is striggered by detecting the presence of a scattered radiation field when the pulsed radiation beam is on.

Description

METHOD AND APPARATUS TO DISCRIMINATE OUT INTERFERENCE IN RADIATION DOSAGE MEASUREMENTS

BACKGROUND Field

The described technology pertains generally to radiation detection, more particularly to fiber optic radiation detectors, and most particularly to eliminating interference in fiber optic radiation detectors. More narrowly, this writing describes at least a method and appartus to discriminate out photon interference from fiber optic radiation monitor in pulsed radiation beam using scattered radiation.

Description of Related Art

There are many different types of radiation detectors or dosimeters for monitoring exposure to hazardous ionizing radiation, such as x-rays, gamma rays, electrons and neutrons. These range from simple colorimetric film or badge dosimeters to complex electronic devices. Some devices are real-time; others show a cumulative exposure over a long period of time. A wide range of dosages may be detected. There are many applications, from safety monitoring to industrial process monitoring to imaging. One particular application of great interest is in the medical field to monitor radiation dosage applied to a patient during radiotherapy.

Many detectors use luminescent materials, including phosphors that produce an optical signal when exposed to ionizing radiation. These include thermoluminescent (TL) materials, as exemplified by U.S. Patent 5,585,640 to Huston et al., and optically stimulable luminescent (OSL) materials, as exemplified by U.S. Patent 5,811 ,822 to Huston et al. In both TL and OSL materials, charge trapping occurs upon exposure to radiation. Charge recombination and light emission occurs when heat is applied to the TL material and optically stimulating light is applied to the OSL material after they have been exposed to radiation.

U.S. Patents 5,391 ,320; 5,122,671 ; and 5,108,959 to Buchanan et al. are directed rbium activated silicate luminescent glasses. These glasses are useful for converting x-ray radiation into visible radiation, and can be used for both detection and imaging applications.

A particular type of dosimeter that is very advantageous for medical applications, e.g. for monitoring radiation dosage applied to a patient, as well as other remote radiation monitoring applications, is a fiber optic dosimeter, as exemplified by U.S. Patents 6,087,666 and 5,606,163 to Huston et al. In a fiber optic dosimeter, the luminescent dosimeter material is placed at the tip of an optical fiber. This small dosimeter can then be placed precisely on or in a patient at a point to be monitored during radiation exposure, or at another location for other monitoring. The system of U.S. Patent 6,087,666 uses an OSL material while the system of U.S. Patent uses a TL material. Both systems include a light source to provide an activating light signal through the optical fiber to the dosheter tip. For the OSL system, the activating light signal is of the right wavelength to produce an output signal from the OSL material, while in the TL system the activating light source heats the TL material to produce an output signal. In both cases the output signal from the luminescent dosimeter tip passes back through the optical fiber to a detector.

One problem with fiber optic dosimeters is that they are affected by interference, Le. the optical fiber itself may emit photons upon exposure to radiation, known as the stem effect. When the dosimeter is positioned to measure radiation dosage, at least a portion of the fiber near the dosimeter will also generally be exposed to the radiation. The interference is typically caused by two sources, fluorescence produced by the fiber and Cerenkov radiation generated in the fiber. Fluorescence photons are emitted by the fiber with absorption of radiation by the fiber material; fluorescence generally persists in the nanosecond range. When high energy particulate ionizing radiation travels through the fiber medium, the particulate radiation may travel faster than the speed of light in the medium, generating Cerenkov radiation, which typically has a lifetime on the order of picoseconds. The photons from the fiber may interfere with measurement of photons from the dosimeter, Le. the detector signal will be a composite of both signals and it is impossible to know how much is from the dosimeter alone and how much is interference. Thus the radiation dosage cannot be accurately determined.

One approach has been to filter the input to the detector, e.g. using narrowband pass filters, to remove the Cerenkov interference, as illustrated by KJ. Jordan, "Evaluation of Ruby as a Fluorescent sensor for Optical FiberBased Radiation Dosimetry," Proceedings of SPIE - Vol. 2705, Fluorescent Detection IV, E. R. Menzel, Abraham Katzir, Editors, March 1996, pp. 170-178. This will only work if the filter removes all the interference and none of the dosimeter signal. Finding a suitable filter may be difficult.

Another approach has been to use electronic gating to only collect photons from a dosimeter between the accelerator beam pulses when there is no Cerenkov radiation, as illustrated by B. L. Justus et al., "Gated Fiber-OpticCoupled Detector for In Vivo Real-Time Dosimetry," Applied Optics, Vol. 43, Issue 8, March 2004, pp. 1663- 1668. While in principle this is effective, in practice it is not. The timing signal to trigger data collection is generated either by a synchronizing signal from the linear accelerator (linac), or by detecting the presence of prompt Cerenkov radiation and fluorescence photons in the optical fiber. This necessitates either connecting a cable to the linac power supply, or placing a separate trigger fiber in the therapy beam, or using a beam splitter to extract Cerenkov and fluorescence photons from the optical fiber. None of these approaches are entirely satisfactory. The difficulty is that synchronizing pulses from the linac is machine dependent, the placement of a separate trigger fiber in the therapy beam is cumbersome for the end user and may be critical, and splitting the output from the fiber dosimeter results in a more complicated optical configuration and reduces the photons available for the dose measurement.

Accordingly it is desirable to provide a fiber optic dosimeter with an improved method and apparatus for producing a gating pulse to trigger data collection to eliminate interference caused by fluorescence and Cerenkov radiation in the optical fiber. BRIEF SUMMARY

An aspect of the present technology is an apparatus for gating data collection from a fiber optic dosimeter positioned to measure dosage of a radiation pulse from a radiation source to eliminate stem effect interference in the dosimeter, including at least one scattered radiation detector positioned to receive scattered radiation produced by the pulse of radiation; and a gating circuit connected to the at least one scattered radiation detector for producing a gating pulse to gate collection of data from the dosimeter.

Another aspect of the present technology is a method for gating data collection from a fiber optic dosimeter positioned to measure dosage of a pulse from a radiation source to eliminate stem effect interference, by detecting scattered radiation produced by the pulse of radiation; and producing a gating pulse from the measured scattered radiation for gating the collection of data from the dosimeter.

A further aspect of the present technology is a dosimetry apparatus, including an optical fiber dosimeter, made up of a scintillator, and an optical fiber having the scintillator attached to one end thereof; a dosimeter detector connected to the other end of the optical fiber; at least one scattered radiation detector; and a gating circuit connected to the at least one scattered radiation detector for producing a gating pulse and connected to the dosimeter detector to apply the gating pulse to the dosimeter detector.

Also an aspect of the present technology is a method of measuring dosage of radiation pulses directed from a radiation source to a target, by providing an optical fiber dosimeter, made up of a scintillator, and an optical fiber having the scintillator attached to one end thereof; positioning the dosimeter scintillator at the target; positioning at least one scattered radiation detector to detect scattered radiation produced by the radiation pulses; producing gating pulses from the output of the at least one scattered radiation detector; and applying the gating pulses to a dosimeter detector to gate the collection of data from the dosimeter scintillator to eliminate the stem effect from the fiber.

Further aspects of the present technology will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the present technology without placing limitations thereon.

The present technology attempts to discriminate out or eliminate interference in radiation dosage measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

The present technology will be more fully understood by reference to the following drawings which are for illustrative purposes only:

FIG. 1 is a simple block diagram of a fiber optic dosimeter of the present technology.

FIG. 2A is a diagram of detector output with interference produced by a radiation pulse.

FIG. 2B is a timing diagram of a fiber optic dosimeter of the present technology.

FIG. 3 is a simple block diagram of a radiotherapy system including the fiber optic dosimeter of the present technology.

FIGs. 4A-C illustrate several embodiments of the data collection triggering system of the present technology.

DETAILED DESCRIPTION

Referring more specifically to the drawings, for illustrative purposes the present technology is embodied in the apparatus generally shown in FIG. 1 through FIG. 4C. It will be appreciated that the apparatus may vary as to configuration and as to details of the parts, without departing from the basic concepts as disclosed herein.

The present technology is a method and apparatus to mitigate interference, also known as the stem effect from Cerenkov radiation and fluorescence photons generated in optical fiber radiation detectors, for real-time ionizing radiation measurement. The fiber optic detector has a radioluminescent scintillator at its tip. Data collection from the fiber optic dosimeter is gated so that photons from the scintillator radioluminescence are collected after Cerenkov radiation and fiber fluorescence photons disappear. Data collection is triggered by detecting the presence of a scattered radiation field when the pulsed radiation beam is on.

FIG. 1 shows a fiber optic dosimeter (fiber optic radiation detector) 10 of the present technology, having a radiation-sensitive scintillator (radioluminescent phosphor) or dosimeter element 12 attached to one end of an optical fiber 14. The optical fiber 14 and dosimeter element/scintillator 12 are surrounded by a light-tight cladding 16. The opposite end of the optical fiber 14 is connected to a photon detector 18. The output of detector 18 is input into a processor or other device 20.

When a beam of ionizing radiation 22 is incident on scintillator 12, the scintillator 12 produces radioluminescence photons (dosimeter signals) 24 that travel down the optical fiber 14 to the detector 18. The detector output from this dosimeter signal is related to the incident radiation dosage. Processor 20 may be used to provide the radiation dosage data from the detector outputs. Processor 20 may also be or include a display unit, an alarm device, or other external device.

The fiber optic dosimeter 10 of the present technology is made with a dosimeter element 12 formed of a scintillator material. Scintillators, or radioluminescent phosphors, are materials, including plastics, crystals, glass, and quartz, that emit radioluminescent photons with characteristic wavelengths upon exposure to ionizing radiation. Scintillators are preferred because they emit the photons spontaneously. Other materials such as thermoluminescent and optically stimulable luminescent materials could be used but require an optical source to activate their outputs so the system is more complex. Terbium doped scintillator materials are particularly preferred, for example the terbium activated silicate luminescent glasses shown in U.S. Patents 5,391 ,320; 5,122,671; and 5,108,959 to Buchanan et al., which are herein incorporated by reference.

The scintillator should be chosen so that its radioluminescence lifetime is much less than the minimum time between successive radiation pulses to prevent pulse pile- up. For example, if the minimum time between linac pulses is three milliseconds, then a scintillator should be used having a radioluminescence lifetime of much less than three milliseconds. The detector 18 is a light sensitive electronic detector, e.g. a photomultiplier (PMT), charge coupled device (CCD) or other similar sensitive electronic detector designed to detect photons. Detector 18 counts the photons transmitted through the optical fiber 14. Radiation intensity and dosage at the location of the scintillator 12 is measured by counting photons from the scintillator.

Unfortunately, the ionizing radiation 22 also generates an interference signal 26 in optical fiber 14. Interference signal 26 is generally made up of Cerenkov radiation and fluorescence photons from the optical fiber itself. Interference signal 26 also travels down optical fiber 14 to detector 18 so that detector 18 measures the combination of the desired dosimeter signal 24 and the undesired interference signal 26. Thus the radiation dosage data obtained from the measured detector output will be inaccurate.

As shown in Fig. 2A, a radiation pulse 30 produces a dosimeter output pulse or signal 32. The dosimeter output signal 32 includes a main or initial dosimeter pulse 34 that occurs substantially simultaneously with the radiation pulse 30. Main dosimeter pulse 34 is made up of a portion 35 produced by the scintillator and a portion 36 produced by the interference. While the total main dosimeter pulse 34 can be measured, the scintillator portion 35 cannot be determined since the amount of interference is variable.

However, dosimeter output 34 also includes an afterglow signal 38 that occurs after the radiation pulse 30. Fortunately, the afterglow signal 38 is only from the scintillator. Also, the interference signal is made up of very short lived phenomena. Cerenkov radiation is only produced when the radiation is passing through the fiber and fiber fluorescence is also very rapid. Thus the afterglow signal 38 is a true measure of the radiation dosage. The present technology is directed at triggering the detector to only measure this afterglow signal.

In accordance with the present technology, and as shown in Fig. 1 , a gating pulse to trigger the detector 18 to measure the afterglow signal from scintillator 12 is produced by measuring scattered radiation from the radiation beam 22. Scattered radiation will inevitably occur as the beam 22 passes from the radiation generator to the target, from objects struck by the beam. High energy radiation is always accompanied by scattered radiation at lower energy. Scattered radiation from high energy radiation tends to be highly directional. Successive scatter will result in lower energy radiation being scattered in all directions.

The scattered radiation is measured by a scattered radiation detector 21. The output of scattered radiation detector 21 coincides with radiation pulse 22 and is input into a gating circuit 25 which produces a gating signal to detector 18. The detector 18, processor 20, and gating circuit 25 may be included in a monitor unit 28. The fiber optic dosimeter 10, monitor unit 28, and scattered radiation detector 21 together form a fiber optic dosimetry system 11 of the present technology.

Fig. 28 is a timing diagram for the fiber optic dosimeter. A linear accelerator produces a sequence of radiation pulses 40. The occurrence of these pulses is measured by detecting scattered radiation produced by these radiation pulses. The sequence of radiation pulses 40 produces a sequence of output pulses 42 from the fiber optic dosimeter. These output pulses include an initial output pulse 44 that includes scintillator radioluminescence and stem effect interference generated in the fiber, and an afterglow 46 that is only scintillator luminescence. The radiation pulses 40, as measured by the scattered radiation detectors, are used to produce a sequence of gating pulses 48 that are applied to the detector to control data collection so only the afterglow pulses 46 are measured. The detector is only turned on during the gating pulse so the initial dosimeter pulse 44 that includes the interference signal, and precedes the gating pulse, is not measured. The gating pulses 48 may occur right after the radiation pulses 40 or after a small delay "0" to ensure that all photons from the stem effect have decayed.

The present technology can be utilized in a radiotherapy treatment room, as shown in Fig. 3. An optical fiber dosimetry system 50 of the present technology is used to monitor radiation dosages applied from a linear accelerator (linac) 52 to a patient 54 positioned on a table 56. Dosimetry system 50 includes a fiber optic dosimeter 58 similar to fiber optic dosimeter 10, i.e. formed of an optical fiber with a scintillator tip, as shown in Fig. 1. The scintillator tip can be positioned on or even inside the patient at a location where it is desired to monitor radiation dosage. Multiple dosimeters 58 can be used to map out an area.

Dosimetry system 50 includes an in vivo monitor 60 similar to monitor 28 of Fig. 1 , i.e. it includes a detector and gating circuit. Fiber optic dosimeter 58 is connected to monitor 60. A plurality of radiation detectors 62 are positioned around the treatment room where they can detect scattered radiation produced when radiation pulses fromlinac 52 are transmitted toward the patient. More than one radiation detector 62 will typically be used to insure that a reliable scattered radiation signal is detected. The radiation detectors 62 are connected to monitor 60, and their outputs are summed, to generate a gating pulse so that data collection from fiber optic dosimeter 58 can be triggered to eliminate stem effect interferences.

Any fast response radiation detector may be used to detect the scattered radiation to generate the data collection triggering pulse. Figs. 4A-C illustrate several embodiments of the data collection triggering system. In Fig. 4A, fiber optic dosimeter 10 made up of scintillator 12 at the tip of optical fiber 14 is connected to detector 18, as in Fig. 1. A scattered radiation detector 70 is formed of a scintillator crystal 72, e.g. Na-I₁ connected to an associated photomultiplier (PMT) 74. The output of PMT 74 is connected by cable 76 to gating circuit 25 which generates the gating pulses and applies the gating pulses to detector 18.

In Fig. 48, the scattered radiation detector is a scintillator 80 connected by an optical fiber 82 to gating circuit 25. Scintillator 80 may be the same as scintillator 12 of fiber optic dosimeter 10, or it may be made of a different scintillator material. However, it is positioned to receive scattered radiation and is used to generate the trigger pulse. In Fig. 4C, the scattered radiation detector is similarly a scintillator 84 but it is connected into the same optical fiber 14 to which the dosimeter scintillator 12 is connected. In this case scintillator 84 must be made of a different scintillator material than scintillator 12, one that emits radioluminescence at a different wavelength. Scintillator 84 is positioned along fiber 14 so that it receives scattered radiation, while scintillator 12 is positioned directly in the radiation beam path. Fiber 14 will then carry two signals, the dosimeter signal from scintillator 12 and the trigger generating scattered radiation signal from scintillator 84. Optical fiber 14 is connected to detector/gating circuit unit 86 which includes both the detector and gating circuit. The two signals are separated by wavelength and the signal from scintillator 84 is used to generate the gating pulse so that the detector can collect data from scintillator 12. It may be possible to eliminate the scintillator 84 as a discrete element and make the fiber 14 of the scintillator material.

The present technology thus provides a system that accurately measures realtime and near real-time ionizing radiation and dosage rate. The system is particularly advantageous for radiation measurement of patients undergoing medical radiotherapy, but can be used for any other monitoring application. Because of the increase in the number of radiation therapies for cancer patients, there is a greater need to accurately measure the real time dose to target tissues and other critical organs. The present technology facilitates the use of fiber optic dosimeters by providing a simple way to gate data collection to eliminate interference. The present technology eliminates the need to physically connect the dosimeter to the linac, eliminates the placement of trigger fibers in the radiation beam, and simplifies the dosimetry system optics and electronics.

Broadly, this writing discloses a method and apparatus to discriminate out photon interference from fiber optic radiation monitor in pulsed radiation beam using scattered radiation. In greater detail, this writing discloses a method and apparatus to mitigate interference, also known as the stem effect, from Cerenkov radiation and fluorescence photons generated in optical fiber radiation detectors, provide for realtime ionizing radiation measurement. The fiber optic detector has a radioluminescent scintillator at its tip. Data collection from the fiber optic dosimeter is gated so that photons from the scintillator radioluminescence are collected after Cerenkov radiation and fiber fluorescence photons disappear. Data collection is triggered by detecting the presence of a scattered radiation field when the pulsed radiation beam is on.

As short summaries, this writing has disclosed at least the following broad concepts.

CONCEPT 1. Apparatus for gating data collection from a fiber optic dosimeter positioned to measure dosage of a radiation pulse from a radiation source to discriminate stem effect interference in the dosimeter, comprising: at least one scattered radiation detector positioned to receive scattered radiation produced by the pulse of radiation; a gating circuit connected to the at least one scattered radiation detector for producing a gating pulse to gate collection of data from the dosimeter.

CONCEPT 2. A method for gating data collection from a fiber optic dosimeter positioned to measure dosage of a pulse from a radiation source to eliminate stem effect interference, comprising: detecting scattered radiation produced by the pulse of radiation; producing a gating pulse from the measured scattered radiation for gating the collection of data from the dosimeter.

CONCEPT 3. Dosimetry apparatus, comprising: an optical fiber dosimeter, comprising: a scintillator; an optical fiber having the scintillator attached to one end thereof; a dosimeter detector connected to the other end of the optical fiber; at least one scattered radiation detector; a gating circuit connected to the at least one scattered radiation detector for producing a gating pulse and connected to the dosimeter detector to apply the gating pulse to the dosimeter detector. CONCEPT 4. The dosimetry apparatus of concept 3 wherein the scintillator is a terbium doped material.

CONCEPT 5. The dosimetry apparatus of concept 4 scintillator is a terbium doped silicate luminescent glass.

CONCEPT 6. The dosimetry apparatus of concept 3 wherein the scintillator has a radioluminescence lifetime substantially less than the minimum time between successive radiation pulses.

CONCEPT 7. The dosimetry apparatus of concept 3 wherein the dosimeter detector is a light sensitive electronic detector.

CONCEPT 8. The dosimetry apparatus of concept 7 wherein the dosimeter detector is a photomultiplier or a charge coupled device.

CONCEPT 9. The dosimetry apparatus of concept 3 further comprising a processor connected to the dosimeter detector.

CONCEPT 10. The of dosimetry apparatus concept 3 wherein the at least one scattered radiation detector comprises a plurality of scattered radiation detectors.

CONCEPT 11. The dosimetry apparatus of concept 3 wherein each scattered radiation detector comprises a scintillator crystal and a photomultiplier connected to the crystal; and the photomultiplier is connected to the gating circuit by a cable.

CONCEPT 12. The dosimetry apparatus of concept 3 wherein each scattered radiation detector comprises a scintillator; and the scintillator is connected to the gating circuit by an optical fiber. CONCEPT 13. The dosimetry apparatus of concept 3 wherein each scattered radiation detector comprises a scintillator that is connected to the optical fiber of the dosimeter, the dosimeter scintillator and scattered radiation detector scintillator being made of different materials producing different radioluminescent wavelengths.

CONCEPT 14. A method of measuring dosage of radiation pulses directed from a radiation source to a target, comprising: providing an optical fiber dosimeter, comprising: a scintillator; an optical fiber having the scintillator attached to one end thereof; positioning the dosimeter scintillator at the target; positioning at least one scattered radiation detector to detect scattered radiation produced by the radiation pulses; producing gating pulses from the output of the at least one scattered radiation detector; applying the gating pulses to a dosimeter detector to gate the collection of data from the dosimeter scintillator to eliminate the stem effect from the fiber.

Having described this present technology in connection with a preferred embodiment, modification will now certainly suggest itself to those skilled in the art. As such, the technology is not to be limited to the disclosed embodiments except as required by the appended claims.

Broadly, this writing has disclosed the following. A method and apparatus to mitigate interference, also known as the stem effect, from Cerenkov radiation and fluorescence photons generated in optical fiber radiation detectors, provide for real-time ionizing radiation measurement. The fiber optic detector has a radioluminescent scintillator at its tip. Data collection from the fiber optic dosimeter is gated so that photons from the scintillator radioluminescence are collected after Cerenkov radiation and fiber fluorescence photons disappear. Data collection is striggered by detecting the presence of a scattered radiation field when the pulsed radiation beam is on.

Although the description above contains many details, these should not be construed as limiting the scope of the present technology but as merely providing illustrations of some of the presently preferred embodiments of this present technology. Therefore, it will be appreciated that the scope of the present technology fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present technology is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device to address each and every problem sought to be solved by the present technology, for it to be encompassed by the present claims. Furthermore, no element or component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U. S. C. 112, sixth paragraph, unless the element is expressly recited using the phrase "means for."

Claims

CLAIMS What is claimed is:

1. Apparatus for gating data collection from a fiber optic dosimeter positioned to measure dosage of a radiation pulse from a radiation source to eliminate stem effect interference in the dosimeter, comprising: at least one scattered radiation detector positioned to receive scattered radiation produced by the pulse of radiation; a gating circuit connected to the at least one scattered radiation detector for producing a gating pulse to gate collection of data from the dosimeter.

2. A method for gating data collection from a fiber optic dosimeter positioned to measure dosage of a pulse from a radiation source to eliminate stem effect interference, comprising: detecting scattered radiation produced by the pulse of radiation; producing a gating pulse from the measured scattered radiation for gating the collection of data from the dosimeter.

3. Dosimetry apparatus, comprising: an optical fiber dosimeter, comprising: a scintillator; an optical fiber having the scintillator attached to one end thereof; a dosimeter detector connected to the other end of the optical fiber; at least one scattered radiation detector; a gating circuit connected to the at least one scattered radiation detector for producing a gating pulse and connected to the dosimeter detector to apply the gating pulse to the dosimeter detector.

4. The dosimetry apparatus of claim 3 wherein the scintillator is a terbium doped material.

5. The dosimetry apparatus of claim 4 scintillator is a terbium doped silicate luminescent glass.

6. The dosimetry apparatus of claim 3 wherein the scintillator has a radioluminescence lifetime substantially less than the minimum time between successive radiation pulses.

7. The dosimetry apparatus of claim 3 wherein the dosimeter detector is a light sensitive electronic detector.

8. The dosimetry apparatus of claim 7 wherein the dosimeter detector is a photomultiplier or a charge coupled device.

9. The dosimetry apparatus of claim 3 further comprising a processor connected to the dosimeter detector.

10. The of dosimetry apparatus claim 3 wherein the at least one scattered radiation detector comprises a plurality of scattered radiation detectors.

11. The dosimetry apparatus of claim 3 wherein each scattered radiation detector comprises a scintillator crystal and a photomultiplier connected to the crystal; and the photomultiplier is connected to the gating circuit by a cable.

12. The dosimetry apparatus of claim 3 wherein each scattered radiation detector comprises a scintillator; and the scintillator is connected to the gating circuit by an optical fiber.

13. The dosimetry apparatus of claim 3 wherein each scattered radiation detector comprises a scintillator that is connected to the optical fiber of the dosimeter, the dosimeter scintillator and scattered radiation detector scintillator being made of different materials producing different radioluminescent wavelengths.

14. A method of measuring dosage of radiation pulses directed from a radiation source to a target, comprising: providing an optical fiber dosimeter, comprising: a scintillator; an optical fiber having the scintillator attached to one end thereof; positioning the dosimeter scintillator at the target; positioning at least one scattered radiation detector to detect scattered radiation produced by the radiation pulses; producing gating pulses from the output of the at least one scattered radiation detector; applying the gating pulses to a dosimeter detector to gate the collection of data from the dosimeter scintillator to eliminate the stem effect from the fiber.