Dynamic radiation scanning device

The present invention is a dynamic radiation scanning system for detecting radiation dosimetry of a beam emitted along an axis from a radiotherapy treatment machine and a method for its use. The system contains a dosimetry probe to sense photons and electrons, a dynamic phantom body formed from a material having a density approximating that of the human body, a gantry mounting assembly rigidly attached to the radiotherapy machine for positioning of the phantom body, and a lead screw assembly rigidly affixed to the gantry for providing coplanar movement of the dynamic phantom within a plane perpendicular to the axis of radiation emission. Movement of the dynamic phantom through a series of locations is carried out at varying depths and angles so as to provide sufficient data to determine variations in beam uniformity, thereby providing for simple and reliable testing and calibration of the machine.

FIELD OF THE INVENTION
 This invention relates to a method and device for scanning of the radiation
 field emitted by a linear accelerator or other radiation producing device
 and particularly relates to the use of a movable small phantom carrying a
 radiation detector, usually an ion chamber, which moves together with the
 phantom.
 BACKGROUND OF THE INVENTION
 Various well-known medical techniques for the treatment of malignancies
 involve the use of radiation. Radiation sources, for example medical
 linear accelerators, are typically used to generate radiation to a
 specific target area of a patient's body. Use of appropriate dosimetry
 insures the application of proper doses of radiation to the malignant
 areas and is of utmost importance. When applied, the radiation produces an
 ionizing effect on the malignant tissue, thereby destroying the malignant
 cells. So long as the dosimetry of applied radiation is properly
 monitored, the malignancy may be treated without detriment to the
 surrounding healthy tissue. Accelerators may be utilized, each of which
 have varying characteristics and output levels. The most common type of
 accelerator produces pulse radiation, wherein the output has the shape of
 a rectangular beam with a cross-sectional area which is typically between
 16 and 1600 square centimeters. Rectangular or square shapes are often
 changed to any desired shape using lead or cerrobend blocks, using molds
 and casting procedures. More advanced accelerators use multileaf
 collimators. Other accelerators are continuous or nonpulsed such as cobalt
 radiation machines; and accelerators that utilize a swept electron beam,
 which sweep a very narrow electron beam across the treatment field by
 means of varying electromagnetic fields.
 To ensure proper dosimetry, linear accelerators used for the treatment of
 malignancies must be calibrated. Both the electron and photon radiation
 must be appropriately measured and correlated to the particular device.
 The skilled practitioner must insure that both the intensity and duration
 of the radiation treatment is carefully calculated and administered so as
 to produce the therapeutic result desired while maintaining the safety of
 the patient. Parameters such as flatness, symmetry, radiation and light
 field alignment are typically determined. The use of too much radiation
 may, in fact, cause side effects and allow destructive effects to occur to
 the surrounding tissue. Use of an insufficient amount of radiation will
 not deliver a dose that is effective to eradicate the malignancy. Thus, it
 is important to be able to determine the exact amount of radiation that
 will be produced by a particular machine and the manner in which that
 radiation will be distributed within the patient's body. In order to
 produce an accurate assessment of the radiation received by the patient,
 some type of pattern or map of the radiation at varying positions within
 the patient's body must be produced. These profiles correlate 1) the
 variation of dose with depth in water generating percent depth dose
 profiles and 2) the variation of dose across a plane perpendicular to the
 radiation source generating the cross beam profiles. These particular
 measurements of cross beam profiles are of particular concern in the
 present invention. Although useful for other analyses, the variation in
 the beam uniformity of the radiation field regardless of gantry
 orientation is the main purpose of this device.
 One existing system for measuring the radiation that is produced by medical
 linear accelerators utilizes a large tank on the order of
 50.times.50.times.50 cm filled with water. A group of computer controlled
 motors move the radiation detector through a series of pre-programmed
 steps beneath the water's surface. Since the density of the human body
 closely approximates that of water, the water-filled tank provides an
 appropriate medium for creating a simulation of both the distribution and
 the intensity of radiation which would likely occur within the patient's
 body. The aforementioned tank is commonly referred to as a water phantom.
 The radiation produced by the linear accelerator will be directed into the
 water in the phantom tank at which point the intensity of the radiation at
 varying depths and positions within the water can be measured with the
 radiation detector. As the radiation penetrates the water the direct or
 primary beam is scattered by the water, in much the same way as when the
 radiation beam impinges upon the human patient. Both the scattered
 radiation as well as the primary radiation are detected by the
 ion-chamber, which is part of the radiation detector. The ion-chamber is
 essentially an open air capacitor which produces an electrical current
 that corresponds to the number of ions produced within its volume. The
 detector is lowered to a measurement point within the phantom tank and
 measurements are taken over a particular time period. The detector can
 then be moved to another measurement point where measurements are taken as
 the detector is held in the second position. At each measuring point a
 statistically significant number of samples are taken while the detector
 is held stationary.
 Several prior art devices are known to teach systems for ascertaining the
 suitable dosimetry of a particular accelerator along with methods for
 their use.
 U.S. Pat. No. 5,621,214, issued Apr. 15, 1997, to Sofield, is directed to a
 radiation beam scanner system which employs a peak detection methodology.
 Except for the peak detection, this system operates like any other
 conventional scanning system, using two ion chamber detectors, a signal
 and a reference detector. In use, the reference detector remains
 stationary at some point within the beam while the signal detector is
 moved continuously by the use of electrical stepper motors.
 U.S. Pat. No. 4,988,866, issued Jan. 29, 1991, to Westerlund, is directed
 toward a measuring device for checking of radiation fields from treatment
 machines for radiotherapy. This device comprises a measuring block that
 contains radiation detectors arranged beneath a cover plate and provided
 with field marking lines and an energy filter. The detectors are connected
 to a read out unit for signal processing and presentation of measurement
 values. Westerlund arranges the dose monitoring calibration detectors in a
 particular geometric pattern to determine homogeneity of the radiation
 field. In use, the measuring device is able to simultaneously check the
 totality of radiation emitted by a single source of radiation at varying
 positions within the measuring block. Although Westerlund's does not use a
 water phantom, his device is nevertheless limited in that all of the
 ionization detectors are in one plane. This does not yield an appropriate
 threedimensional assessment of the combination of scattering and direct
 radiation which would normally impinge the human body undergoing radiation
 treatment. Thus, accurate dosimetry in a real-life scenario could not be
 readily ascertained by the use of the Westerlund device.
 U.S. Pat. No. 5,006,714, issued Apr. 9, 1991, to Attix utilizes a
 particular type of scintillator dosimetry probe which is manufactured from
 a material that approximates water or muscle tissue in atomic number and
 electron density. Attix indicates that the use of such a detector
 minimizes perturbations in a phantom water tank. While recognizing the use
 of a polymer material which is similar to water or muscle tissue in atomic
 number and electron density, Attix nevertheless requires the use of a
 cumbersome phantom water tank.
 Additionally, there is an apparatus called a Wellhofer "bottle-ship" which
 utilizes a smaller volume of water than the conventional water phantom.
 The Wellhofer device still utilizes a timing belt and motor combination to
 move the detector through the water, thus requiring a long initial set-up
 time. Lastly, the Wellhofer device still operates on the principle of
 moving the detector through the phantom body, while the instant device
 moves a substantially smaller (15.times.15.times.15 cm) plastic phantom
 body through the radiation field.
 Thus, there exists a need for a device that is capable of quickly and
 accurately detecting both scattering and direct radiation components from
 radiation devices without requiring the use of a large and cumbersome
 water phantom.
 SUMMARY OF THE INVENTION
 The present invention is based upon the general principle of scanning
 radiation by the use of a radiation detector attached to a moving phantom.
 This principle states that the dynamic component of the scanning system
 becomes the phantom instead of the radiation detector used in conventional
 scanning methods. The theory behind conventional scanning is that the use
 of a large water phantom results in the scattering of the directly applied
 radiation in the large water tank in a manner similar to that which occurs
 when this direct radiation impinges upon the human body being treated. It
 has now been observed that the majority of the scatter contribution of
 this radiation component, in actuality, comes from a comparatively small
 volume very near to the ion-chamber itself. Therefore, moving this small
 volume together with the radiation detector provides radiation scanning
 measurements equivalent to those achieved when a large water phantom is
 employed.
 A specially designed gantry mounting assembly for the linear accelerator
 holds a scanning guide which is attached to a phantom body made of a
 polymer material such as an acrylic, or the like material of similar and
 appropriate density. It has been determined that the acrylic material has
 a density similar to that of water and that the scatter which occurs in
 the enclosed material is substantially equivalent to that which occurs
 within the human body. It should be noted that the measurements taken are
 not absolute values, but relative measurements expressed in percent
 relative to a selected value. An appropriately sized block of acrylic or
 similar material, having a plurality of recesses adapted to receive the
 radiation detector, is affixed to the gantry mount assembly and at least
 one radiation detector is inserted therein. The recesses which are not
 being utilized are filled with a plug of the acrylic or similar material
 during the scanning procedure. The "dynamic phantom", including the ion
 chamber therein, is then moved through the radiation field. Movement is
 accomplished via the use of a lead screw assembly, which is essentially a
 worm gear rotated via a stepper motor and to which the dynamic phantom is
 operatively attached. Rotation of the worm gear results in coplanar
 movement of the dynamic phantom, in a plane which is perpendicular to the
 axis of emission of the radiant beam, and irrespective of the angle of the
 radiotherapy machine relative to the patient's body. This construction
 allows for rapid setup and calibration of the system, often in as little
 as about seven minutes as opposed to the twenty to thirty minutes needed
 to calibrate a water phantom apparatus. Scanning and analyzing of the
 various radiation parameters of interest is carried out at varying angles
 and depths, wherein the only movement which occurs is the movement of the
 dynamic phantom itself, in and around the linear accelerator. The device
 is capable of utilizing isocentric scanning, for example scans are
 performed at a Source Axis Distance of 100 cm (SAD); or non-isocentric
 scanning techniques may be employed, e.g. 100 cm SSD (Source Surface
 Distance). The device is able to scan at virtually any arbitrary depth for
 field sizes up to 40.times.40 centimeters, at any radiotherapy machine
 angle, and in either the radial or transverse direction in simplified
 models.
 As in conventional radiation scanning techniques, the process is conducted
 by remote control utilizing computer programs which control the motion of
 the system and produce analyses of the data produced thereby. The dynamic
 phantom of the present invention is capable of measuring both photons and
 electrons and is further adaptable to any linear accelerator.
 Thus, it is an objective of the present invention to provide a convenient
 technique for measuring the most commonly verified parameters of radiation
 beams.
 It is a further objective of the present invention to describe a procedure
 for utilizing a small dynamic phantom which allows direct measurement of
 cross plots for photons and cross plots and percentage depth dose for
 electrons.
 It is yet another objective of the present invention to provide a method
 and apparatus for radiation detection and measurement which utilizes rapid
 and accurate setup, and significantly reduces the measurement time
 required by traditionally used scanning systems.
 Other objects and advantages of this invention will become apparent from
 the following description taken in conjunction with the accompanying
 drawings wherein are set forth, by way of illustration and example,
 certain embodiments of this invention. The drawings constitute a part of
 this specification and include exemplary embodiments of the present
 invention and illustrate various objects and features thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Now referring to FIGS. 1 and 2, a linear accelerator 10, emits a beam of
 radiation from the collimator area 12. The radiation beam is directed
 toward the dynamic phantom 14, which in a particular embodiment is a
 15.times.15.times.15 cm acrylic block. The dynamic phantom contains a
 dosimetry probe 16, usually an ion chamber, which may be inserted in one
 of several recesses 18, which are positioned so as to enable the user to
 alter the depth of the dosimetry probe within the block. Exemplary depths
 are 1.5 cm (depth of maximum radiation intensity for 6MV), 3 cm (depth of
 maximum radiation intensity for 18MV), 5 cm and 10 cm. Those recesses
 which are not being utilized during the scanning procedure are plugged
 with an insert of material equal to that which forms the block. The gantry
 20 is mounted on the accelerator and rigidly supports the dynamic phantom
 on a horizontal rail 22 which has a computer controlled stepper motor 24
 attached to one end thereof. In operation, the linear accelerator is
 activated and the beam of radiation is emitted therefrom and impinges the
 dynamic phantom acrylic block. Direct radiation permeates the block and
 impinges upon the probe. Simultaneously, scattering radiation is also
 produced by virtue of the radiation beam impinging the acrylic block,
 which simulates the scattering radiation which occurs in a human patient
 being treated by such a radiation beam, and is also detected by the probe.
 The computer which controls the stepper motor is operatively attached to a
 lead screw assembly which comprises a worm gear 26 which is operatively
 associated with the dynamic phantom support bracket 28. As the stepper
 motor receives instructions to rotate the worm gear, this rotary movement
 is translated into horizontal displacement of the dynamic phantom through
 a series of locations designed to gather data regarding the variation of
 the radiation intensity through the field. While the field is
 characterized as "horizontal" as depicted in the figures, the displacement
 is in any plane that is perpendicular to the axis of emission of radiation
 from the radiotherapy machine, said plane being at a distance defined by
 positioning of the probe within the dynamic phantom block. The block is
 sized in such a way that the total of direct and scattering radiation
 represents virtually 100 percent of the radiation to which a patient will
 be exposed. In a particularly preferred embodiment, the block is
 15.times.15.times.15 cm. Any additional radiation which occurs due to
 scattering outside the area of the dynamic phantom block has been
 determined to be of no clinical significance. After having been traversed
 through a first series of movements directed by the stepper motor, the
 depth of the probe within the block may be changed by moving it to an
 alternative recess. The block may then be stepped through a similar series
 of movements and further data is gathered. After moving the dynamic
 phantom through several such series at varying depths and radiotherapy
 machine angles, the computer will have gathered sufficient data to form a
 graphical analysis of the actual radiation to which a patient would be
 exposed during radiation therapy. The elimination of the large water tank
 utilized in the prior art allows the present device to be set up very
 quickly without the need for movement of a large tank, filling the tank
 with water and subsequently having to empty the water from the tank. The
 use of the lead screw assembly provides an accuracy of positioning of 0.1
 mm and an accuracy of percentage dose measurement better than 0.5%. In
 addition, this device does not suffer from the mechanical failures which
 have often occurred in the past, when the stepper motors are utilized
 underwater. Failures due to corrosion, short circuiting and build up of
 dirt and sediment due to the underwater environment in which the probe was
 utilized are eliminated by the instant invention.
 The present invention thus provides a reliable means of accurately and
 quickly determining the efficacy of a radiation producing device. By
 utilizing the dynamic phantom of the present invention a radiation
 physicist may accurately and repeatedly determine the beam radiation
 uniformity at any gantry angle. Thus the viability and accuracy of the
 machine may be easily determined by the clinical physicist as well as by
 the manufacturers of the accelerators.
 It is to be understood that while a certain form of the invention is
 illustrated, it is not to be limited to the specific form or arrangement
 of parts herein described and shown. It will be apparent to those skilled
 in the art that various changes may be made without departing from the
 scope of the invention and the invention is not to be considered limited
 to what is shown in the drawings and described in the specification.