Abstract:
A method is provided for determining the positional accuracy of leaves of a multileaf collimator for delivering doses of radiation to a particular spatial location for treatment purpose. The method could be implemented as routine quality assurance check of the multileaf collimator leaf positioning errors. The method includes producing a first field and producing a second field, which is different from the first field. A dosimeter means is included for measuring a radiation dose difference or ratio between the first field and the second field at at least one spatial location. The dose difference or ratio is then used to determine the positional accuracy of the leaves by comparing with a known relationship between leaf positional errors and relative dosimeter outputs. The method provides a more simplified, accurate, efficient and reliable method over currently used methods.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is cross-referenced to and claims priority from U.S Provisional Application No. 60/306,736 filed on Jul. 19, 2001, which is hereby incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to multileaf collimators used to deliver therapeutic radiation doses. More particularly, the present invention relates to a method to check positional accuracy of the leaves of a multileaf collimator in intensity modulated radiation therapy.  
         BACKGROUND  
         [0003]    Intensity modulated radiation therapy (IMRT) is an advanced form of radiation therapy. In IMRT using multileaf collimators, it is critical for the MLC leaves to move accurately according to the pre-designed trajectories to achieve the planned radiation dose distribution with certain accuracy. In contrast to conventional methods, IMRT requires a much more stringent quality assurance (QA) to ensure the normal operation of the delivery system. One of the main quality assurance (QA) tasks is the assurance of positional accuracy of the multileaf collimator leaves. Several studies have indicated that the dose delivery accuracy of IMRT is highly sensitive to multileaf collimator leaf positioning error (See e.g. LoSasso T, Chui C S &amp; Ling C C (1998)  Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy,  Med. Phys. 25:1919-1927; Budgell G J, Mott J H L, Williams P C &amp; Brown K J (2000),  Requirements for leaf positioning accuracy for dynamic multileaf,  Phys. Med. Biol. 45:1211-1227). For instance, an error of about 1.0 mm in leaf position could result in more that 10% dose error. Multileaf collimator leaf errors can generally be classified into systematic errors and random errors. A systematic error is referred to as a constant error of all leaves at every leaf position. A random is referred to as an error that may occur at any leaf with an arbitrary value in a certain range. There are two main sources of systematic multileaf collimator leaf position errors, i.e. centerline mechanical offset and imprecise determination of radiation field offset in case of a rounded end MLC (See e.g. Zygmanski P &amp; Kung J H (2001),  Method of identifying dynamic multileaf collimator irradiation that is highly sensitive to a systematic MLC calibration error,  Med. Phys. 28:2220-2226).  
           [0004]    Several methods have been employed to identify and correct the systematic errors; accuracies of better than 3 mm can be obtained (See e.g. LoSasso T, Chui C S &amp; Ling C C (1998),  Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy,  Med. Phys. 25:1919-1927; Graves M N, Thompson A V, Martel M K, McShan D L &amp; Fraass B A (2001),  Calibration and quality assurance for rounded leaf-end systems,  Med. Phys. 28:2227-2233; Low W, Sohn J W, Klein E E, Markman J, Mutic S &amp; Dempsey J F (2001),  Characterization of a commercial multileaf collimator used for intensity modulated radiation therapy,  Med. Phys. 28:752-756). Sources that may affect the magnitude of a random leaf position error include the precision of the multileaf collimator control system, the absolute accuracy of calibration of the multileaf collimator leaf positions and the stability of leaf drive motors. For a multileaf collimator system, a systematic error is relatively easy to handle and once a systematic error is corrected in the system, it would not be necessary to check such an error if the conventional alignment of light fields and radiation fields is performed periodically. On the other hand, a random multileaf collimator leaf positioning error check should be performed on a more regular basis since it is unknown when such an error occurs in a particular leaf.  
           [0005]    Currently, in most clinics, the routine QA of MLC is performed using radiographic films with specifically designed MLC leaf sequences as described by Chui et al. (Chui C S, Sprirou S &amp; LoSasso T (1996),  Testing of dynamic miltileaf collimation,  Med. Phys. 23, 635-641; LoSasso T, Chui C S &amp; Ling C C (2001),  Comprehensive quality assurance for the delivery of intensity modulated radiotherapy with a multileaf collimator used in the dynamic mode,  Med. Phys. 28:2209-2219). In the method taught by Chui et al., a film is exposed to a dynamically delivered multileaf collimator field that produces a matrix of high intensity regions, for instance about 1 mm wide and 2 cm apart. Subsequently, the film is evaluated for irregularities of the high intensity regions caused by potential leaf positioning inaccuracy. With this method, it is believed possible to visually detect leaf positioning errors as small as about 0.2 mm. However, such a QA performance test is time consuming due to the overhead associated with film irradiation and processing. Furthermore, the film measurement results are difficult to interpret and quantify. Therefore it would, for instance, be difficult to detect errors less than 0.2 mm, which renders the method of film measurement inadequate for QA in IMRT. Accordingly, there is a need to develop new and more accurate methods to improve QA in IMRT.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides a method for determining the positional accuracy of leaves of a multileaf collimator. The method includes producing a first field and producing a second field, which is different from the first field. A dosimeter means is included for measuring a radiation dose difference or ratio between the first field and the second field at at least one spatial location. The dose difference or ratio is then used to determine the positional accuracy of the leaves by comparing the dose difference or ratio with a known relationship between leaf positional errors and relative dosimeter outputs.  
           [0007]    The present invention teaches different embodiments to obtain the first and second reading. A first embodiment shows the first field as is an open field and the second field as a partial field based on the settings of the leaves of the multileaf collimator. The open field is used as a reference for the partial field. Once the open field measurement is known, it would also be possible and sufficient to simply provide this open field as a measurement to determine the difference or ratio with the partial field. Another embodiment shows the first field being produced based on a first setting of the leaves of the multileaf collimator and the second field being produced based on a second setting of the leaves of the multileaf collimator whereby the second setting being inverse from the first setting. In this case both leaves settings are used as references to each other. Yet another embodiment in which both fields are used as reference to each other shows the first field being a wedge field and second field being the inverse of that wedge field. Still another embodiment shows the first field being determined using a calibrated collimator whereby the calibrated collimator provides a reference to the second field which is based on a setting of the leaves of a multileaf collimator.  
           [0008]    In view of that which is stated above, it is the objective of the present invention to provide a method that determines the positional errors of leaves in a multileaf collimator.  
           [0009]    It is another objective of the present invention to provide a method for checking the positional errors of leaves in a multileaf collimator that can be used as a routine check for quality assurance measurements.  
           [0010]    The advantage of the present invention is that is provides a more simplified and more accurate method over currently used methods. The method is efficient and reliable in determining possible positional errors of leaves in a multileaf collimator. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0011]    The objectives and advantages of the present invention will be understood by reading the following detailed description in conjunction with the drawings, in which:  
         [0012]    [0012]FIG. 1 shows an overview of the method according to the present invention;  
         [0013]    [0013]FIG. 2 shows an exemplary relationship of relative output of the dosimeter means and leaf positional errors according to the present invention; and  
         [0014]    FIGS.  3 - 7  show exemplary embodiments of methods for determining positional accuracy of one or more leaves of a multileaf collimator according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Although the following detailed description contains many specifics for the purposes of illustration, anyone of ordinary skill in the art will readily appreciate that many variations and alterations to the following exemplary details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.  
         [0016]    The present invention provides a method for determining the positional accuracy of leaves of a multileaf collimator for delivering doses of radiation to a particular spatial location for treatment purposes, e.g., for delivering radiation doses to treat tumors. The method of the present invention could be implemented as a method for routine QA of the multileaf collimator leaf positioning. FIG. 1 shows an overview  100  of the steps of the present method for determining the positional accuracy of the leaves of a multileaf collimator. The method includes two different radiation dose readings or measurements, i.e.  110  and  120 , of a field that are compared to each other. A dosimeter means is used for measuring a radiation dose difference or ratio  130  between the first field reading  110  and the second field reading  120  at at least one spatial location in the field. In general, the difference could be determined from a fluence or an intensity. Different dosimeter means can be used such as, for instance, but not limited to, a single dosimeter, an array of dosimeters or an ion chamber. Other dosimeters known in the art to measure radiation doses could also be used. Once the difference or ratio between the two radiation dose readings is established the positional accuracy  140  of the leaves could be determined. For instance, the positional accuracy could be determined by looking up in a data set, data plot or a graph the relative output  210  from the dosimeter means with the leaf positional errors  220 . Such a data set, data plot or a graph could have been established by previous calibration tests or is known for the particular multileaf collimator that is being tested. An example of such a graph is shown in FIG. 2 by graph  200  in which data points of the relative output and leaf positional errors were fitted to a linear relationship  230 . However, any other form or relationship would also be possible and the present invention is not limited to a linear relationship, since the relationship is dependent on the type of multileaf collimator and type of detector. Once the relative output  220  or difference between the two radiation dose readings has been established, graph  200  could then be used to determine and quantify the leaf positional error as is shown, for illustrative purposes only, for relative output  240  which then determines leaf positional error  250 .  
         [0017]    [0017]FIG. 3 shows one embodiment according to the present invention in which a volumetric field  310  is used to determine a first reading  300 A and a second reading  300 B. For illustrative purposes, field  310  includes one dosimeter  320  that is used to determine the radiation dose from radiation source  330 . In this particular embodiment shown by  300 A, the leaves of a multileaf collimator are set in a position or sequence such that an open field could be produced. For the second reading as shown in  300 B, leaf  340  of a multileaf collimator is set in a position or sequence such that a partial open field could be produced. In this particular example, leaf  340  is positioned to verify the positional accuracy around the central axis  350  of dosimeter  320 . However, as a person of average skill in the art would readily appreciate, the positional accuracy of leaf  340  could also be determined at any position of leaf  340  that is off the central axis  350  of dosimeter  320 . Dosimeter  320  obtains two different readings or radiation dose measurements, which could be compared as a difference, ratio or other mathematical means. If the position of leaf  320  is inaccurate, then the difference will indicate whether the second reading in  300 B is higher or lower than it ought to be. In this particular embodiment, the open field is used as a reference field. As a person of average skill in the art would readily recognize, instead of measuring the open field, the open field could also be known based on a previous measurement. In that case there would be no need for measuring the open field radiation dose. In that case, one only needs to measure a partial field, which is then compared with the open field value to determine the positional errors of the leaves.  
         [0018]    The following is an example of how one could determine the positional error based on the embodiment of FIG. 3. Assume that the previously determined calibration curve for a given multileaf collimator and detector is linear and is given by R=a E+b, with a=0.5702 (cm −1 ) and b=0.0866, where F is positional error and R is the relative output of a field shaped by leaf. When performing QA check of leaf  340  according to the setup in FIG. 3, one measures the (first) open field  300 A and the (second) leaf field  300 B. The ratio of the two readings of dosimeter means  320  gives the R in the above formula. Substitute the value R into the above formula and one will obtain the positional error E=(R−0.0866)/0.5702. If R=0.0866, E=0, which means that the leaf is in the desired position. Otherwise, there is an error in the positioning of the leaf. For example, if R=0.0800, there is an error E=0.0116 cm.  
         [0019]    [0019]FIG. 4 shows another embodiment according to the present invention in which a volumetric field  410  is used to determine a first reading  400 A and a second reading  400 B. For illustrative purposes, field  410  includes one dosimeter  420  that is used to determine the radiation dose from radiation source  430 . In this particular embodiment shown by  400 A, leaf  440  of a multileaf collimator is set in a position or sequence such that a partial field could be produced. For the second reading as shown in  400 B, leaf  450  of a multileaf collimator is set in a position or sequence such that a partial open field could be produced which is the inverse of the partial field in  400 A produced by leaf  440 . In this particular example, leaves  440 ,  450  are positioned to verify the positional accuracy around the central axis  460  of dosimeter  420 . However, as a person of average skill in the art would readily appreciate, the positional accuracy of leaves  440 ,  450  could also be determined at any position of leaves  440 ,  450  that is off the central axis  450  of dosimeter  420 . The key idea here is that the first reading and the second reading are not only different from each other but also have an inverse relationship as shown in FIG. 4. The determination of the positional accuracy here takes advantage of the symmetry and asymmetry of the leaves. Dosimeter  420  obtains two different readings or radiation dose measurements, which could be compared as a difference, ratio or other mathematical means. If the position of leaves  440 ,  450  is inaccurate, then the difference will indicate the degree of positional inaccuracy of the leaves of the multileaf collimator.  
         [0020]    [0020]FIG. 5 shows yet another embodiment according to the present invention in which a multileaf collimator wedge field is used in a volumetric field  510  to determine a first reading  500 A and a second reading  500 B. For illustrative purposes, field  510  includes one dosimeter  520  that is used to determine the radiation dose from radiation source  530 . In this particular embodiment shown by  500 A, wedge  540  is set in a position or sequence such that a partial field could be produced. For the second reading as shown in  500 B, wedge  550  of a multileaf collimator is set in a position or sequence such that a partial open field could be produced which is the inverse of the partial field in  500 A produced by wedge  540 . The key idea here is that the first reading and the second reading are not only different from each other but also have an inverse relationship as shown in FIG. 5. The determination of the positional accuracy here takes advantage of the symmetry and asymmetry of the wedges. Dosimeter  520  obtains two different readings or radiation dose measurements, which could be compared as a difference, ratio or other mathematical means. If the position of wedges  540 ,  550  is inaccurate, then the difference will indicate a positional inaccuracy of the leaves of the multileaf collimator. Different wedges could be used such as a phantom material wedge, metal wedge, step-wise field or the like.  
         [0021]    [0021]FIG. 6 shows another embodiment according to the present invention in which a volumetric field  610  is used to determine a first reading  600 A and a second reading  600 B and determine the position accuracy of a plurality of leaves  620  in a multileaf collimator with an array of dosimeters  610  and radiation source  630 . In this particular example of FIG. 6 the first reading pertains an open field reading and the second reading pertains a partial open field reading according to the position of the leaves.  
         [0022]    [0022]FIG. 7 shows another embodiment according to the present invention in which a volumetric field  710  is used to determine first readings  700 A and second readings  700 B and determine the position accuracy of a plurality of leaves  720  in a multileaf collimator with an array of dosimeters  710 . In this particular example of FIG. 7 the first reading includes a calibrated collimator  750  to obtain a calibrated reading and the second reading pertains a partial open field reading according to the position of the leaves. Calibrated collimator  750  is, for instance, a calibrated metal strip with different thickness, that allows collimating the radiation from radiation source  730  to dosimeters  740  through the thinner part of calibrated collimator  750 . The key idea here is that the first readings and the second readings are not only different from each other but also have an inverse relationship as shown in FIG. 7. The determination of the positional accuracy here takes advantage of the symmetry and asymmetry of the calibrated collimator and the leaves. Dosimeters  710  obtained different readings or radiation dose measurements, which could be compared as differences, ratios or other mathematical means. If the position of leaves  720  is inaccurate, then the difference will indicate the degree of positional inaccuracy of leaves  720  of the multileaf collimator.  
         [0023]    The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. For example, the present invention could be modified to check gantry angle accuracy in conformal arc therapy or intensity modulated arc therapy. Yet another way that the present method could be modified is to check dynamic wedge positional accuracy. All such variations are considered to be within the scope and spirit of the present invention as defined by the following claims and their legal equivalents.