Abstract:
A sensor is inserted into a closed water tank filled with water to the brim and the closed water tank is fixed directly to a radiation beam irradiating section, and the sensor is moved freely with respect to a mounted frame used for fixation. Therefore, a rapid and accurate prediction of the actual dose distribution of radiation beam prior to radiation therapy can be conducted, even when the irradiating section is attached to a rotation gantry.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a water phantom type dose distribution determining apparatus, particularly to a water phantom type dose determining apparatus allowing a rapid and accurate determination of the dose distribution in water which is suitably used for predicting in advance the dose distribution of radiation beam, before radiation therapy dependent on the use of a proton radiation therapy device or the like is undertaken. 
     2. Description of the Prior Art 
     Conventional cancer therapy based on radiation of active rays uses X-rays, gamma rays, electron beams, fast neutron beams, etc. These active rays, as shown in FIG. 6, become the strongest at sites close to the surface of a patient, and thus may inflict damages on normal tissues close to the body surface when those rays are directed towards a cancer in a deeper part of the body. By the way, a proton or a particle which comes into being when a hydrogen atom has been removed of the electron, has a positive charge, and has a mass 1836 times as large as that of electron, can be accelerated under a high energy state by an accelerator to give a proton beam. The proton beam is characterized by having the maximum dose peak or a Bragg peak P at a certain depth from the body surface, and then declining rapidly to zero. 
     This is because, as the electric force a proton A exerts on electrons becomes large in proportion to its proximity to the latter, when the proton has a high kinetic energy and runs at a high speed, the time for the proton to interact with nearby electrons is short, and ionization is small in magnitude, but, when it loses the kinetic energy to nearly make a stop, the time for interaction becomes long and ionization rapidly increases in magnitude. 
     Thanks to this nature peculiar to protons, it is possible to apply proton beams for cancer therapy keeping normal cells other than a cancer comparatively free from damages, even if the cancer lies in a deeper part of the body. Further, as the radiation-based biological effect (RBE) of a proton beam is nearly equal to that of x-rays, the proton radiation therapy is advantageous in that it can make the most of knowledge and experience accumulated in the field of conventional X-ray radiation therapy. With these features, the proton radiation therapy device is being introduced as a therapy means to treat a cancer without removing any functional organs and encroaching on the quality of life. 
     In the radiation therapy of cancer, it is ideal to concentrate a lethal dose of active rays onto the cancer alone without inflicting any irreversible damages to nearby normal tissues. The Proton radiation therapy, as shown in FIG. 6, exploits the feature characteristic with protons that a proton beam incident on a substance gives the maximum dose or Bragg peak P just before it ceases to move. Namely the therapy in question aims at achieving this ideal by covering only the cancerous lesion with that Bragg peak. 
     By the way, protons obtained from an accelerator are in the form of a slender beam, and its energy is constant (the depth of Bragg peak is also constant). On the other hand, cancerous lesions are varied in size and have complex shapes, and their depths in the body are not constant. Further, the density of tissues through which a proton beam must pass is not constant neither. Accordingly, to achieve an effective radiation therapy, it is necessary to (1) enlarge the proton beam to have a sufficient width to cover the whole cancer lesion in one radiation; (2) adjust the beam energy according to the depth of lesion; (3) give a sufficient energy distribution in depth so that the whole cancer lesion having a certain depth can receive a uniform irradiation; and (4) make corrections according to the irregularities in contour of the lesion, and in density of the tissues through which the proton beam must pass. To meet these requirements, a device as shown in FIG.  7  is introduced whereby an irradiation field is formed in accordance with the shape of a lesion to be radiated. To put it more specifically, a slender proton beam  20  transmitted to an irradiating section is passed through a scattering body  22  made of lead with a thickness of several millimeters to be converted into a wide beam  24  extending crosswise. Out of the wide beam  24  which widens in a conical form with the summit at the scattering body  22 , picked up by a collimator described below is a portion which is close to the central axis and comparatively uniform in dose distribution. This beam gives an irradiation field of about ten and several centimeters in diameter necessary for therapy on a therapeutic platform below (not illustrated here). The widened beam  24  is incident on a fine degrader  26  which adjusts the maximum attainable depth in accordance with the depth of a lesion to be treated (for example, a tumor  12  in the patient&#39;s body  10 ). The fine degrader  26  is composed, for example, of two wedge-shaped acryl blocks  26 a and  26 b placed opposite to each other, and adjustment of overlaps of the two blocks  26   a  and  26   b  enables a continuous alteration of the thickness through which the proton beam must pass. The proton beam loses energy in accordance with the thickness through which it must pass, and thus the depth it can reach varies in accordance therewith. Thus, adjustment by means of this fine degrader  26  makes it possible for Bragg peak P shown in FIG. 6 to fall at the same depth at which the lesion requiring therapy lies. 
     The proton beam, after having passed the fine degrader  26 , is incident on a ridge filter  28  which is introduced to confer an energy depth distribution AP to the proton beam in accordance with the thickness of tumor  12 . The ridge filter  28  consists of metal rods placed in parallel like a series of steps which have different thickness with each other. Proton beams passing through the metal rods different in thickness have Bragg peaks P at different depths. Thus, expansion of the range of peaks or AP can be achieved by adjusting the width and height of those “steps” to give appropriate overlaps. 
     The proton beam, after having passed through the ridge filter  28 , is incident on a block collimator  30  which roughly adjusts the planar form of proton beam. The reason why the block collimator  30  is introduced here for the adjustment of beam shape, in addition to a final collimator described later, is to prevent secondary radiation due to the block collimator from occurring close to the patient&#39;s body. 
     The proton beam, after having passed through the block collimator  30 , is incident on a bolus  32  or a resin-made irregularly formed filter, for example, and receives corrections in accordance with the cross-sectional shape of tumor  12  at the maximum depth, and the irregularities of involved tissues. The shape of bolus is determined on the basis of the electron densities of nearby tissues determined from the contour line of tumor  12  and, for example, X-ray CT data of that tumor. 
     The proton beam, after having passed through the bolus  32 , is incident on a final collimator  34  made of brass, for example, receives a final correction in accordance with the contour of planar shape of the tumor  12 , and strikes the patient  10  as a therapeutic proton beam  36 . 
     Prior to treatment, firstly, to check that the irradiation field is formed as initially designed, it is necessary to predict the actual dose distribution using a water phantom type dose distribution determining apparatus as shown in FIG. 8, which includes a water tank  42  equipped with a sensor  46  and filled with water  44  to simulate the absorption of active rays by the human body. 
     The conventional proton radiation therapy device is based on a horizontal or vertical static radiation, and, as the entire device including the irradiating section  120  is fixed rigidly, positioning of the device is determined manually each time experiment is undertaken: the water tank  42  has its top opened, and is placed on the treatment bed, or is placed on a wheeled cart  48 . 
     With the conventional device, however, it is cumbersome to make a proper positioning, and properly maintain the level of water  44 , and further water may spill while the water tank is being carried, to cause the water level to change. When the radiation therapy device is operated for actual treatments, it will be probably used at a frequency of once for every 20 minutes. Thus, checking the dose distribution by means of a water phantom type dose distribution determining apparatus must proceed rapidly. Further, as the device is handled by a physician or a radiological technician instead of an engineer, the apparatus must not require special techniques for its operation and must be easily manipulated. With the irradiating section  120  as shown in FIG. 1 (not publicly known) the inventors are designing in order to irradiate a properly shaped proton beam, the irradiating section  120  is mounted on a rotary irradiation chamber (to be referred to as gantry) rotatable round a therapeutic bed  200  on which a patient is fixed, and the irradiating section rotates 360° round the patient during use. Thus, as shown by the broken line of FIG. 8, the incident angle θ of proton beam with respect to the surface of water  44  in the water tank  42  varies during rotation, but the device must be so constructed as not to allow those altered angles to affect the measurement results of dose distribution. 
     SUMMARY OF INVENTION 
     This invention aims at providing a water phantom type dose distribution determining apparatus meeting above requirements and suitably applied to a proton radiation therapy device for medical use. 
     This invention solves above problems by providing a water phantom type dose distribution determining apparatus for determining the dose distribution in water of a radiation beam irradiated from a radiation beam irradiating section using a sensor placed in the water, which comprises: a closed water tank filled with water to the brim and containing the sensor therein; a mount means to attach the closed water tank to the radiation beam irradiation section; and a moving means to move at least the sensor with respect to the mount means. The moving means may move the sensor together with the closed water tank in the direction vertical to the radiation direction of radiation beam, while it moves only the sensor in directions in parallel with the radiation direction of radiation beam. 
     According to this invention, it is possible to rapidly and easily determine the dose distribution of radiation beam prior to treatment. Particularly, as the closed water tank filled with water is used, the distance between the water surface and the sensor position remains constant even when the radiation beam irradiating section is mounted to a rotation gantry and is revolved to meet an actual radiation condition, and thus it is possible to accurately determine the dose distribution in accordance with the lesion of a patient to be treated. Further, water spilling does not occur while the water tank is moved; the water level therefore remains constant; and no monitoring of water surface is needed. Furthermore, the apparatus is advantageous in that attaching and detaching it to and from the radiation beam irradiating section is easy, and thus it is simple for handling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments will be described below with reference to the drawings, wherein like elements have been denoted throughout the figures with like reference numerals, and wherein: 
     FIG. 1 is a perspective view illustrating how to the rotation gantry is mounted the radiation beam irradiating section to which is attached the water phantom type dose distribution determining apparatus of this invention; 
     FIG. 2 is a front view illustrating how an embodiment of this invention is attached to the tip of the radiation beam irradiation section. 
     FIG. 3 is an enlarged front view of the dose distribution determining apparatus of the above embodiment. 
     FIG. 4 is an enlarged side view of the same apparatus. 
     FIG. 5 is a sectional view of the same apparatus. 
     FIG. 6 gives graphs for explaining the principle underlying proton radiation therapy. 
     FIG. 7 is a perspective view illustrating the principle underlying the formation of an irradiation field in proton radiation therapy. 
     FIG. 8 is a sectional view illustrating the composition of a conventional water phantom type dose distribution determining apparatus, and problems inherent therewith. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment of this invention will be detailed below with reference to figures which are applied to a proton radiation therapy device having a rotation gantry  100  whereby an irradiating section  120  of a proton beam  36  rotate around a treatment bed  200  as shown in FIG.  1 . In FIG. 1,  150  stands for a preparation room set up in front of the rotation gantry  100 ; and  160  for a bed moving unit to carry the bed  200  freely along six axes (X, Y, and Z, and θx, θy, and θz) from the preparation room  150  into the rotation gantry  100 . 
     FIG. 2 shows how a water phantom type dose distribution determining apparatus  50  of this embodiment is attached to the tip (the lowermost end in the figure) of the irradiating section  120 . 
     The water phantom type dose distribution determining apparatus  50  of this embodiment comprises a cylindrical closed water tank  52  filled with water to the brim and receiving a sensor  46  inserted from the bottom; a mount frame  54  to attach the closed water tank  52  to the irradiating section  120 ; and a moving mechanism  60  which moves the sensor  46  together with the closed water tank  52  with respect to the mount frame  54  in directions vertical to the radiation direction of proton beam  36  (lateral directions, and directions vertical to the surface of FIG.  2 ), and moves only the sensor  46  in the closed water tank  52  in directions in parallel with the radiation direction of proton beam  36  (up and down direction of FIG.  2 ). 
     As shown in detail in FIG. 3, the mount frame  54  has positioning pins  56  which establish a proper positioning by penetrating holes (not illustrated in FIG. 3) prepared on the mounting surface of the irradiating section  120 , and one-touch levers  58 , for example, four in number, which fix the mount frame  54  to the irradiating section  120  by clamping through simple one-way operations. 
     As shown in detail in FIGS. 3 to  5 , the moving mechanism  60  comprises an X-axis driving device  62  to carry an X-direction moving frame  64  along X-axis (lateral directions of FIG. 3) with respect to the mount frame  54 ; a Y-axis driving device  72  to carry a Y-direction moving frame  80  along Y-axis with respect to the X-direction moving frame  64  (directions vertical to the surface of FIG.  3 ), and a Z-axis driving device  90  to move the sensor  46  from the bottom surface of the closed water tank  52  attached to the Y-direction moving frame  80  to a specified depth (for example to a level flush with the iso-center position). 
     As shown in detail in FIG. 3, the X-axis driving device  72  has an electric motor  65  containing, for example, a decelerator, and fixed to the mount frame  54 , a feed screw  66  which is driven into rotation by the electric motor  65 ; and a nut  68  to move the X-direction moving frame  64  along X-axis direction by engaging with the feed screw  66 . 
     As shown in detail in FIG. 4, the Y-axis driving device  72  has an electric motor  74  fixed to the X-direction moving frame  64  and containing, for example, a decelerator, a feed screw  76  which is driven into rotation by the electric motor  74 , and a nut  78  which moves the Y-direction moving frame  80  along Y-axis direction by engaging with the feed screw  76 . As shown in detail in FIG. 4, the Z-axis driving device  90  has an electric motor  92  fixed to the Y-direction moving frame  80  and containing, for example, a decelerator, a feed screw  94  which is driven into rotation by the electric motor  92 ; and a nut  96  to move the sensor  46  along Z-axis direction by engaging with the feed screw  94 . 
     In FIG. 3,  98  stands for a cylinder to adjust the volume of water in the closed water tank  52  which varies according to how deep the tank is moved along Z-axis. 
     Prior to the dose distribution measurement, the mount frame  54  is properly positioned with respect to the irradiating section  120  by means of the positioning pins  56 , and then the mount frame  54  is attached to the irradiating section  120  through the works of one-touch levers  58 . 
     Then, electric motors  64 ,  74  and  92  connected with X-, Y- and Z-axis driving devices  62 ,  72  and  90  respectively are put into rotation to move the sensor  46  at desired positions, and the dose distribution measurement is undertaken. 
     In this embodiment, thanks to the positioning pins  56 , the mount frame  54  can be accurately attached with respect to the irradiating section  120 , and thus highly reproducible results can be obtained. 
     Further, as the mount frame  54  is fixed to the irradiating section  120  through the works of one-touch levers  58 , attaching and detaching the mount frame  54  to and from the irradiating section  120  are easy to manipulate. 
     Furthermore, as the sensor  46  can be freely moved to a desired depth in the closed water tank  52  by means of the Z-axis driving device  90 , it is quite easy to alter the distance between the water surface and the sensor, or the distance corresponding with that from the surface of a patient&#39;s body to his/her lesion to be treated. 
     Still further, as X- and Y-axis driving devices  62  and  72  are introduced to move the closed water tank  52  itself, the constitution of X- and Y-axis driving devices is simple. Moreover, it is also possible to freely move only the sensor  46  throughout the closed water tank  52  along three coordinate axes of X-, Y- and Z-axis. 
     In the above-described embodiment, this invention is applied to a proton radiation therapy system including an irradiating section installed in a rotation gantry  100 , but the applicable field of this invention is not limited to above, but apparently can be applied with the same profit to an irradiating section rigidly fixed to a fixed beam chamber, or to other radiation therapy systems based on the use of X-rays, electron beams, or the like.