radiation therapy begins with the development of a treatment plan for delivering a prescribed dose of radiation to a tumor while minimizing the dose of radiation delivered to surrounding tissue . the treatment plan prescribes the amount of fluence each portion of target region should receive , but there are many leaf sequences ( i . e ., many combinations of leaf movements ) that can be used to produce a given fluence . moreover , treatment plans typically provide for irradiating the target from multiple angles . heretofore , treatment planning has proceeded on the basis that the treatment volume is stationary while the patient is being irradiated , i . e ., target motion was not taken into consideration . thus , prior art treatment planning made no effort to optimize leaf movements in relation to target motion . as discussed above , the present invention is directed to the problem which arises when the treatment volume moves while being irradiated . in one aspect , the present invention addresses the problem by taking into account the interplay effects between leaf movements and target movements , as discussed herein . as noted , the treatment volume may move in a periodic fashion along a trajectory . for purposes of the present invention the trajectory of motion is modeled as a single path , even if there is some variation in the actual motion . in most instances , the trajectory of any point in the treatment area can reasonably be approximated by a line segment , although the present invention is not limited to motion along linear trajectories . this trajectory can be projected onto the mlc plane , which is the plane is defined by the face of the mlc closest to the patient . this mlc plane is substantially orthogonal to the center of the radiation beam . fig1 a shows a “ snapshot ” of a portion of a treatment area 4 that moves ( sometimes referred to herein as a moving target ). area 4 may be thought of as a tumor or portion of a tumor which is prescribed to receive a certain fractional dose of radiation . the area surrounding target 4 may be , for example , a different region of the tumor for which a different fractional dose is prescribed . motion of the moving target 4 along a trajectory ( as illustrated by the arrows ) defines a larger region 2 through which the target moves in a periodic manner . such motion may be caused , for example , by respiration . in fig1 a , the trajectory is a line segment . in many instances , even if the moving target does not follow a linear trajectory , it is possible to obtain the benefits of the present invention by approximating the trajectory as a line . use of a linear trajectory is simple and convenient , and provides most of the advantages of the present invention when applied to actual treatment cases . more generally the trajectory can be curved , as discussed in further detail below . fig1 b is a “ snapshot ” representation of the radiation beam 8 projected onto a portion of region 2 through an aperture in an mlc created by opposing leaf pairs . in an unoptimized treatment plan radiation beam 8 may move in a direction that is parallel to the trajectory of the moving target ( as indicated by the arrow ). in other words , the mlc aperture is orthogonal to the trajectory of target motion . the movement of radiation beam 8 can be in connection with either a sliding window imrt technique or a point and shoot technique . fig2 a and 2b illustrate in further detail the interplay effects in an un - optimized sliding window imrt system . both figures show a target 4 moving within region 2 along a trajectory , where the sliding window aperture , and hence radiation beam 8 , moves at approximately the same rate . in both cases , direction of movement of beam 8 is generally parallel to the direction of movement of the target 4 and in both figures beam 8 moves from bottom to top . fig2 a , shows three positions of target 4 and the radiation beam 8 as they move through area 2 , indicated by time markers t 1 , t 2 and t 3 . in fig2 a , since the target 4 moves from top to bottom , while the beam 8 moves from bottom to top at approximately the same speed , the movement results in the target 4 and the beam 8 intersecting generally in the center of region 2 , as shown at time marker t 2 . in this case , movement of the aperture thus provides a dose to target 4 that is approximately equal to the prescribed dose . in contrast , fig2 b shows three positions of the target 4 as both it and beam 8 move from bottom to top at the same speed . thus , the motions of target 4 and beam 8 are “ in phase ” with one another . in comparison to fig2 a , the movement shown in fig2 b results in the continued overlap of beam 8 with target 4 throughout the entire time radiation is delivered to area 2 . this provides a dose that is much greater than the prescribed dose . indeed , in the situation depicted in fig2 b , the dose is several times the prescribed dose . fig2 a and 2b exaggerate the interplay effect due to the fact that the target and beam move at substantially the same speed . moreover , this makes the phase , i . e ., the starting position of the target relative to the beam , critical . nonetheless , those skilled in the art will understand that the problem of interplay effects due to target motion will cause unoptimized results in many other circumstances . unlike the unoptimized mlc orientation of fig1 b , 2 a and 2 b , the present invention orients the mlc to use apertures that are generally parallel to the trajectory of the target movement . one consequence of this orientation is that the apertures are generally longer in the direction of target movement , and preferably extend substantially the length of area 2 . according to an embodiment of the present invention , fig3 shows a radiation beam 10 reaching area 2 through an mlc aperture that extends parallel with and moves perpendicularly to the trajectory that defines region 2 . ( region 2 in fig3 is the same as in fig1 a , 1 b , 2 a and 2 b .) fig4 a and 4b are similar to fig2 a and 2b , showing the same target 4 movement at times t 1 , t 2 and t 3 ; however , they illustrate an optimized sliding window imrt according to an embodiment of the present invention , wherein the beam motion and target motion are orthogonal . in fig4 a , as target 4 moves from top to bottom in region 2 , beam 10 moves across the region from left to right from time t 1 to t 3 . thus , the movement of the beam ( and hence the mlc slice or aperture ) is orthogonal to the target trajectory . as a result , at all times throughout the delivery period at least some portion ( e . g ., 12 a , 12 b , 12 c ) of the target 4 is receiving radiation . similarly , as shown in fig4 b , when the target 4 moves from bottom to top , sections 14 a , 14 b , 14 c receive radiation as aperture 10 moves across region 2 . thus , regardless of the phase relationship between the target and the beam , i . e ., where target 4 is at the beginning of the radiation delivery , beam 10 will deliver approximately the prescribed dose to the target . moreover , the dose delivered to the entire region 2 is the same in both cases . fig5 shows an exemplary mlc plane having a plurality of leaves 16 , arranged in opposing pairs , and an aperture 15 created by selected leaf movements , in well known fashion . radiation passes through and is shaped by aperture 15 to create beam 10 . in imrt , aperture 15 is moved continuously ( sliding window ) or periodically ( point and shoot ) across the face of the mlc in either of the directions indicated in fig5 . as the aperture is moved its shape may be adjusted to control the fluence to different portions of the treatment volume , i . e ., the combined leaf movements may be used , in known fashion , to vary the fluence delivered to the treatment area . an embodiment of the present invention uses leaf sequences wherein the beam through the mlc aperture is generally parallel to the motion of target 4 and substantially equal to the length of region 2 . as beam 10 moves , the configuration of aperture 15 , may be adjusted , thereby allowing additional control over the fluence delivered to the treatment area . fig6 a depicts the fluence delivered to a treatment field in the form of a topographical fluence map 20 , in accordance with an illustrative embodiment of the present invention . the lowest fluence represented in the map is in the generally annular region 18 a , and the fluence increases to a maximum fluence region 18 n . fig6 b shows the fluence received in the portion of the treatment area through the slice b - b of fig6 a . slice b - b is aligned with the trajectory of movement . all of the points along slice b - b will receive a fluence that is between the lowest fluence 18 a and a middle fluence 18 c . using the method of the present invention , the slices are aligned with the trajectory of the target &# 39 ; s motion , thus even if a point in slice b - b were moving , it would not receive a higher fluence than the maximum for its slice , the middle fluence 18 c . accordingly , the present technique has limited the range over which the delivered dose may vary . the mlc leaf movements of the present invention can be achieved with an algorithm similar to the one described in the co - pending , co - assigned application entitled “ leaf sequencing algorithm to reduce tongue and groove effects ” ( u . s . patent application ser . no . ______ ), the disclosure of which is incorporated by reference . according to the current invention , the leaf sequence is optimized to produce the field of radiation delivery for a moving target that more closely matches the intended dose . to form mlc slices parallel to the target region &# 39 ; s movement , the leaves may most easily be oriented to move perpendicularly to that movement , but other orientations are possible . the leaf motion need not be oriented in the same direction as the movement of the aperture , although such orientation may simplify planning . however , there may be competing treatment considerations that make it necessary or desirable orient the mlc such that the angle of leaf movement is not orthogonal to the trajectory . according to an aspect of the present invention , the angle between the elongate apertures formed in the mlc and the trajectory is minimized . the leaf movement may be perpendicular to the tumor movement ( as shown in fig5 ), allowing the leaves to modulate the fluence in all positions in one slice relatively quickly . ideally , the orientation of the leaves is chosen based on the longest direction of the target field , such that they move perpendicularly to that direction as has been described . however , deciding on the orientation of the leaves is also influenced by the shape of the apertures that need to be formed in view of the shape of the tumor undergoing treatment . this is because some shapes present greater difficulties and can only be formed by a limited number of leaf arrangements . in some cases there will have to be a trade - off between the ideal orientation for the optimized leaf sequence based on the target movement and the physical constraints on the system for the leaves to form an aperture in the shape that is required . in such cases , the dose should be built up using the largest aperture possible that provides the precision needed . in another embodiment of the invention , the optimization is fine - tuned by controlling the time each movement slice is exposed to radiation through the aperture . for example , if each slice intersects with the open aperture for about the duration of one movement cycle , the dose distribution is closer to the planned average distribution . in a further embodiment , a variable dose rate could be used to better control the exposure time for groups of slices . while timing the dose in this way is effective in conjunction with the present invention , and timing or gating in general is useful in a variety of applications , timing alone is not sufficient to avoid the interplay effects in an un - optimized system . due to the periodic nature of the target movement and the difficulties in determining the precise location of the target 4 within region 2 at any given time , it is not practical to attempt to avoid phase problems , such as the overlapping situation shown in fig3 b , by simply timing the dose . in a further embodiment of the invention , and as mentioned above , the trajectory of the target region may be curved . when the trajectory is not a simple line segment , but is curved or has some other shape , additional considerations must be taken into account in order to determine the proper orientation for the aperture and the related optimized leaf sequence . the best technique for handling a curved trajectory will depend on the exact shape and other aspects of the system , such as the physical constraints on leaf movement . in some cases , it may be sufficiently accurate and most efficient to simply map the trajectory to a predominant direction — a major axis of the trajectory curve , for example . in other cases , a rigid translation can be used to reduce the trajectories to a single direction , again , such as a predominant direction . this approach might be particularly useful for multiple , non - linear trajectories . where the trajectory is more complex , it can be obtained from deformable image registration , in which imaging is used to find a best fit . there , features of the target region are matched in order to align the trajectory with a template image . with other complex trajectories , a point - specific movement model might be used to identify the trajectory , such as the use of boundary conditions to model the target region . once the trajectory has been obtained , if necessary , the trajectory can be mapped or translated into a predominant direction as discussed above . ideally , as shown in fig7 , optimization of the sliding window imrt system produces a fluence 6 that has the same value at all positions where it is possible for the tumor to be . because target 4 only occupies a portion of region 2 at any one time , this ideal fluence may result in unnecessary irradiation of some portions of region 2 . specifically , in addition to target 4 , region 2 may include healthy tissue which will be irradiated by the same dose as target 4 . while this is not the most desirable situation , in many situations it is more important to ensure an adequate and even dose to the target 4 than to avoid irradiating the surrounding tissue . in this regard , it should be noted that this unnecessary irradiation of portions of region 2 also occurs using the un - optimized system . for example , in fig3 a the entire region 2 is irradiated , although target 4 only occupies a portion of the total area . thus , except for when target 4 is exposed to the aperture 8 at time t 2 , the remainder of the radiation delivery to region 2 is unnecessary . accordingly , the present invention provides an improved method for delivering radiation to a moving target , but further improvements to reduce the unnecessary radiation delivered to regions in the trajectory of the target would be beneficial . the embodiments described above are illustrative of the present invention and are not intended to limit the scope of the invention to the particular embodiments described . accordingly , while one or more embodiments of the invention have been illustrated and described , it will be appreciated that various changes can be made therein without departing from the spirit or essential characteristics thereof . accordingly , the disclosures and descriptions herein are not intended to be limiting of the scope of the invention , which is set forth in the following claims .
a method of providing intensity modulated radiation therapy to a moving target is disclosed . the target moves periodically along a trajectory that is projected onto a multi - leaf collimator plane . the mlc plane is divided into thin slices parallel to the movement of the target . the present invention optimizes the leaf sequence such that , within each slice , if a point receives radiation , all other points in that slice that receive the same amount or more fluence are also receiving radiation at the same time .