Abstract:
A radiation therapy method that includes directing a beam along a beam path toward a treatment area. Performing a correction process on the beam, the process includes selectively collimating the beam based on a dose that takes into account bremsstrahlung interactions caused by the beam.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to radiation therapy devices, and more particularly, to a removable electron multileaf collimator for use in a radiation therapy device. 
   2. Discussion of Related Art 
   Conventional radiation therapy typically involves directing a radiation beam at a tumor in a patient to deliver a predetermined dose of therapeutic radiation to the tumor according to an established treatment plan. This is typically accomplished using a radiation therapy device such as the device described in U.S. Pat. No. 5,668,847 issued Sep. 16, 1997 to Hernandez, the contents of which are incorporated herein for all purposes. 
   The radiotherapy treatment of tumors involves three-dimensional treatment volumes which typically include segments of normal, healthy tissue and organs. Healthy tissue and organs are often in the treatment path of the radiation beam. This complicates treatment, because the healthy tissue and organs must be taken into account when delivering a dose of radiation to the tumor. While there is a need to minimize damage to healthy tissue and organs, there is an equally important need to ensure that the tumor receives an adequately high dose of radiation. Cure rates for many tumors are a sensitive function of the dose they receive. Therefore, it is important to closely match the radiation beam&#39;s shape and effects with the shape and volume of the tumor being treated. 
   Either primary photon or primary electron beams may be used in radiation therapy. Currently, clinical practice requires substantial manual intervention to use conformal electron treatment. Conformal photon fields typically are shaped using one or more collimating devices positioned between the source and the treatment area. Many of these photon beam collimating devices (multi-leaf collimators or MLCs) are positioned automatically to deliver a desired photon field shape to a treatment area on a patient. Little manual intervention is required to administer photon radiation therapy. A new type of therapy is also emerging, which involves using both photon beams and electron beams in the same treatment, here called “Mixed Beam Radiotherapy”. To be practical, Mixed Beam Radiotherapy requires advances in electron delivery, such as an automatic collimating device designed explicitly to shape electrons such as disclosed in U.S. patent application Ser. No. 09/909,513, filed on Jul. 20, 2001, the entire contents of which are incorporated herein by reference. The photon MLC and the new electron MLC need to be coordinated in an optimal way. 
     FIG. 1  schematically shows a radiation therapy machine  10  that includes a gantry  12  which can be swiveled around a horizontal axis of rotation  14  in the course of a therapeutic treatment. A treatment head  16  is fastened to a projection of the gantry  12 . A linear accelerator (not shown) is located inside gantry  12  to generate the high energy radiation required for the therapy. The axis of the radiation bundle emitted from the linear accelerator and the gantry  12  is designated by beam path  18 . Electron, photon or any other detectable radiation can be used for the therapy. 
   During a course of treatment, the radiation beam is trained on treatment zone  20  of an object  22 , for example, a patient who is to be treated and whose tumor lies at the isocenter of the gantry rotation. Several beam shaping devices are used to shape radiation beams directed toward the treatment zone  20 . For example, a multileaf photon collimator and a multileaf electron collimator can be arranged to shape the radiation beams. Each of these collimators may be separately controlled and positioned to shape beams directed at treatment zone  20 . 
   For example, when the electron beam source is used, the multileaf photon collimator may be fully retracted and the multileaf electron collimator is designed specifically to stop the primary electrons. However, a few electrons in the beam have bremsstrahlung radiation interactions with high atomic number materials in the head of the accelerator that result in a low percentage photon component (3-5%) to the beam that are not stopped by the electron collimator. This component may be considered “leakage” since it may not be noticeably attenuated by the multileaf electron collimator and will cause an unmodulated background component to the distribution. This is not a significant problem for single electron fields, in fact it may be considered useful since it is possible to get an image of the field from this component with an extremely sensitive portal imaging system such as described in U.S. patent application Ser. No. 09/910,526, the entire contents of which are incorporated herein by reference. If electron modulation is introduced, however, the number of segments or individual fields in an Intensity Modulated Radiation Therapy (IMRT) sequence is increased. A significant increase to the integral dose may result if many segments are used because the photon leakage through the multileaf electron collimator is summed from each segment. 
   One possible solution is to make the leaves of the multileaf electron collimator thick enough to attenuate the photon component, but this increases the size and weight of the accessory considerably. A second possible solution is to use the multileaf photon collimator in such a way that it acts as a “back up” attenuator. This technique will nearly eliminate the photon component outside the field, but the effect of the multileaf photon collimator  116   a  and jaws  116   b  on the electron field itself must be considered. 
   Some of the electrons that contribute to the field at the patient plane originate from scattering off of secondary “sources” along the beamline, such as the scattering foils and the air column just outside of the beam. Thus, the multileaf photon collimator and the jaws block part of the field if they are fitted to the same size and shape as the field defined by the multileaf electron collimator alone. The result is a broadened penumbra and reduced output due to the scattered electrons. 
   Accordingly, when bremsstrahlung leakage is generated through a multileaf electron collimator, it is desirable to reduce dosage applied to a patient while providing as clean a beam as possible for the mixed beam treatment. The ideal margin for the photon multileaf collimator for each electron field is a compromise between these two competing interests. In general the margin is a function of the secondary electron energy of the secondary electrons generated from the scattered primary electrons. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention regards a radiation therapy device that includes a radiation source that directs a beam along a beam path toward a treatment area and a beam shaping device controllable to selectively collimate the beam. A treatment planning system is connected to the beam shaping device for simulating a beam shape delivered to a treatment zone. The treatment planning system includes a memory that stores treatment data and a correction device that receives the data from the memory and calculates a proper dose that takes into account bremsstrahlung interactions when the beam is present. The correction device controls the beam shaping device based on the calculated proper dose. 
   A second aspect of the present invention regards a radiation therapy method that includes directing a beam along a beam path toward a treatment area. A correcting process on the beam includes selectively collimating the beam based on a dose that takes into account bremsstrahlung interactions caused by the beam. 
   Each aspect of the present invention provides the advantage of reducing dosage applied to a patient in the case when bremsstrahlung leakage is generated through a multileaf electron collimator. 
   Each aspect of the present invention provides the advantage of maintaining a beam with small penumbra and a constant and high output factor. Further characteristics and advantages of the present invention ensue from the following description of exemplary embodiments by the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an embodiment of a radiation therapy machine; 
       FIG. 2  shows an embodiment of a radiation therapy machine in accordance with the present invention; 
       FIG. 3  shows a block diagram illustrating portions of the radiation therapy machine of  FIG. 2 ; 
       FIG. 4  shows a flow chart that shows a mode of a correction process for the radiation therapy machine of  FIG. 2  in accordance with the present invention; and 
       FIG. 5  schematically shows a collimator system to be used with the radiation therapy machine of  FIG. 2  in order to execute the method of  FIG. 4  in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A radiation therapy machine  100  that incorporates a number of the elements of the radiation therapy machine  10  of  FIG. 1  is shown in  FIGS. 2 and 3 . The radiation therapy machine  100  includes a gantry  102  which can be swiveled around a horizontal axis of rotation  104  during the course of a therapeutic treatment. A beam source  106  is used to generate radiation beams in any of a number of ways well-known to those skilled in the art. For example, the beam source  106  may include a dose control unit  108  used to control a trigger system generating injector trigger signals fed to an electron gun in a linear accelerator (not shown) located inside the gantry  102  to produce the high energy radiation, such as an electron beam or photon beam, required for the therapy. The beam source  106  may include separate sources of radiation  300  and  302  for photons and electrons, respectively, as schematically shown in FIG.  5 . The axis of the radiation bundle emitted from the linear accelerator and the gantry  102  is designated by beam path  110 . 
   During a course of treatment, the radiation beam is trained on treatment zone  112  of an object  114 , for example, a patient who is to be treated and whose tumor lies at the isocenter of the gantry rotation. Several beam shaping devices are used to shape radiation beams directed toward the treatment zone  112 . In particular, a set of photon jaws  116   a  multileaf photon collimator  116   b  and a multileaf electron collimator  118  are provided. Each of these collimators, as will be described further below, may be separately controlled and positioned to shape beams directed at the treatment zone  112 . 
   The plates or leaves of the collimators  116   b  and  118  are made of a material, such as brass, tungsten or lead, substantially impervious to the emitted radiation. The collimator leaves or plates are mounted between the radiation source and the patient and positioned in order to delimit (conform) the field. Areas of the body, for example, healthy tissue, are therefore subject to as little radiation as possible and preferably to none at all. 
   Note that the plates or leaves of the collimators  116   b  and  118  are movable such that the distribution of radiation over the field need not be uniform (one region can be given a higher dose than another). In particular, the leaves of each collimator are individually driven by a drive unit  120 ,  122  and are positioned under the control of electron collimator control  124 , photon collimator control  126  and sensor(s)  128  and  130 . Drive units  120 ,  122  move the leaves of each collimator in and out of the treatment field to create a desired field shape for each type of beam. In one embodiment, where an electron beam is to be generated and primary electrons are to be used in a treatment, photon collimator control  126  operates to retract individual leaves of photon collimator  116   b , while electron collimator control  124  operates to position individual leaves of electron collimator  118  across the path of the electron beam to generate a desired electron field shape at the isocenter. 
   Radiation therapy machine  100  also includes a central treatment processing or control unit  132  that is operated by a user. A mass storage device  134  stores data used and generated during the operation of the radiation therapy machine device including, for example, treatment data as defined by an oncologist for a particular patient. This treatment data is generated, for example, using a treatment planning system  136  which may include manual and computerized inputs to determine a beam shape prior to treatment of a patient. Treatment planning system  136  is typically used to define and simulate a beam shape required to deliver an appropriate therapeutic dose of radiation to treatment zone  112 . 
   In accordance with the present invention, data is stored in the treatment planning system  136  that allows a proper dose of electron radiation to be calculated that takes into account bremsstrahlung interactions as explained previously. The stored data is determined by following the correction process shown in FIG.  4 . The correction process  200  is performed prior to applying electron radiation to a patient and may be performed at the manufacturer&#39;s premises or the therapy institution. The correction process  200  involves first retracting the leaves of the multileaf photon collimator  116   b  per step  202  so that the electron radiation beam is solely formed by the multileaf electron collimator  118 . 
   The retraction for each leaf is dependent on a number of factors, such as beam energy, field size and the position of the leaves within the field. As an approximation, the retraction for each leaf of the collimator  116   a  is the same. At this stage, the penumbra and the electron output factor are ideal but the leakage outside the electron radiation field is not. The penumbra and electron output factor, M i , are measured by a detector  138  per step  204 . The penumbra may be defined as the perpendicular distance between the 50% and 80% isodose lines. In addition, the output factor is measured by “counting” the number of photons or electrons (or measuring the dose they deposit) crossing a plane at the isocenter. Signals representative of the detected penumbra and electron output factor, M i , are sent to the computer  140 . 
   Next, the computer  140  sends the representative control signals to the drive  122  via dose control unit  108  that causes each leaf of the multileaf photon collimator  116   b  to be moved into the electron radiation field by one unit per step  206 . Once the leaves have been moved, the penumbra and output of the electron radiation field are measured per step  204  by the detector  138  as schematically shown in FIG.  5 . In step  208 , the penumbra M i  measured in step  204  is compared with the penumbra M i−1  previously measured in step  204  by the computer  140  by calculating the ratio M i /Mi −1 . When the electron radiation field is perturbed enough that the penumbra measured in step  204  is significantly degraded, such as at least 2 millimeters or 5% or more of M i−1 , when compared with the penumbra M i−1  previously measured in step  204 , the computer  140  sends control signals to the drive  122  via dose control unit  108  that causes the leaves of the multileaf photon collimator to retract per step  210 . The leaves may retract by an amount that may range from 1-2 centimeters. The exact amount is dependent upon electron energy and field size and shape. 
   The amount of retraction needed depends on the degree of lateral scatter of the electrons in the electron radiation beam. The amount of retraction may be experimentally derived from a number of factors as mentioned previously. However, since the predominant factor affecting the amount of retraction is the energy spectrum of the secondary electrons, that factor alone (calculated by physics theories or mathematical modeling known to one of ordinary skill in the art) can be used to determine the amount of retraction. The electron output factor with the leaves of the multileaf photon collimator in this configuration is noted and stored in a table  142  stored in computer  140 . The table  142  stores the configuration of the photon collimator  116   b  that will reduce leakage and penumbra while maintaining an adequate electron output factor. The table  142  is accessible to the treatment planning computation programs used by the treatment planning system  136  so that the proper dose can be calculated. 
   The table  142  will need to be multi-dimensional because the degree of lateral scatter of the electrons is known to vary with both the energy of the beam and the field size. An individual electron field may have a complex shape (although most do not) and may be approximated by an equivalent field size in the table  142 . 
   After the configuration of the photon collimator is determined per step  200 , the photon collimator  116   b  is retained or moved to the determined configuration. A patient is placed in proper position via computer  140 , gantry control  144  and table control  146  that controls table  146  in a manner similar to that described in U.S. patent application Ser. No. 09/910,526, filed on Jul. 20, 2001. After the patient is properly positioned, the electron and photon collimators are positioned based on the correction for bremsstrahlung interactions and the electron radiation beam is applied to the treatment area to generate a desired dosage. The computer  140  is operatively coupled to the dose control unit  108  which includes a dosimetry controller which is designed to control the beam source  106  to generate a desired beam achieving desired isodose curves. 
   Those skilled in the art will appreciate that various adaptations and modifications of the just described preferred embodiments can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.