Abstract:
A system and method of radiation planning. The system and method are configured to implement or include the steps of (a) obtaining an image of a patient encompassing tumorous and non-tumorous tissue, (b) applying an encompassing field to the image having an area covering the tumorous and non-tumorous tissue, (c) using a graphical user interface to subtract subset fields from the encompassing field corresponding to radiation sensitive non-tumorous tissues to define a treatment area, and (d) inputting the treatment area to a computer program to generate a radiation treatment plan based on at least one prescribed dose to the treatment area.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based on provisional application Ser. No. 60/618,846 filed Oct. 14, 2004 and entitled Radiation Treatment Planning Using Conformal Avoidance. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   This invention was made with United States government support awarded by the following agencies; NIH CA88960. The United States has certain rights in this invention. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to radiation therapy planning for the treatment of tumors, and in particular, radiation treatment planning for radiation therapy machines that provide independent intensity modulation of many narrow beams of radiation. 
   External beam radiation therapy involves the treatment of tumorous tissue with high-energy radiation according to a treatment plan. The treatment plan controls the radiation&#39;s placement and intensity, and thus the dose level to volume elements of tissue within a treatment volume so that the tumorous tissue within the treatment volume receives a sufficient dose of radiation while radiation to the surrounding and adjacent non-tumorous tissue is minimized. 
   Intensity modulated radiation therapy (IMRT) treats a patient with multiple smaller rays of radiation, each substantially smaller than the treatment volume and independently controllable in intensity and/or energy. The rays are directed at different angles at the patient and combined to produce the desired dose pattern. Typically, the radiation source consists of either high energy x-rays, electrons from certain linear accelerators, or gamma rays from highly focused radioisotopes such as Co 60 . 
   Radiation plays a central role in the treatment of head and neck (H&amp;N) cancer patients. Conventional H&amp;N radiation treatment is associated with substantial toxicity to normal tissue, and IMRT provides the opportunity to deliver radiation in H&amp;N with enhanced conformance to tumor targets while diminishing the radiation dose to the surrounding normal tissue structures. Unfortunately, tumor target definition in H&amp;N cancer is complex and the delivery of IMRT requires accurate and thorough target definition. Experienced H&amp;N cancer specialists commonly consume several hours to fully contour and refine desired targets for a single H&amp;N IMRT case. A substantial part of the complexity arises from the need to define not only the gross tumor volume (GTV) containing the tumor, but also to describe a contour containing surrounding lymph nodes. The lymph nodes can be very difficult to image and thus to distinguish from other tissue. 
   The complexity of the planning process associated with IMRT, a particularly in H&amp;N IMRT may discourage the use of highly effect IMRT techniques. 
   SUMMARY OF THE INVENTION 
   The present invention provides a radiation treatment planning method which reverses the process normally used for treatment planning from that of describing the tumor volume and in some cases the surrounding lymph node areas, to one of describing areas of normal tissue and subtracting those areas from an encompassing field covering both normal and tumorous tissue types. Normal tissue structure in H&amp;N treatment such as salivary glands and the spinal cord are more clearly defined in image studies and therefore easier to contour reproducibly than less well defined elective nodal or “at risk regions”. Physician planning time, using this method, can be reduced by as much as three times, and preliminary analysis in H&amp;N planning shows that salivary gland and spinal cord protection is equivalent to that achieved with a conventional “target definition” approach. The method of the present invention may also have application outside of the head and neck area. 
   Specifically then, the present invention provides a method of radiation planning comprising steps of obtaining an image of the patient encompassing tumorous and non-tumorous tissue and applying an encompassing field to the image having an area covering the tumorous and non-tumorous tissue. A graphical user interface is used by a physician to define a subset field of normal tissue for subtraction from the encompassing field. The resulting treatment area may then be input to a computer program (being separate or the same as that providing for the graphical user interface) to generate a radiation treatment plan using at least one prescribed dose to the treatment area. 
   It is thus one object of at least one embodiment of the invention to improve the acceptance of IMRT in head and neck cancer treatment or any treatment where complex treatment areas must be created by simplifying treatment planning. 
   It is another object of at least one embodiment of the invention to allow treatment area definition based on the more easily identified structure of normal tissue rather than the often more difficult to identify tumor tissues and surrounding elective areas. 
   The method may include the step of using the graphical user interface to subdivide the treatment area into at least two zones and applying different prescribed doses to the zones. 
   Thus it is another object of at least one embodiment of the invention to allow rapid treatment planning while allowing a set of different doses within the treatment area. 
   The encompassing field may be a slice through the head and neck region defined by the exposure area of two counterdirected beams from the particular radiation therapy equipment, the beams extending along a lateral access through the head and neck region. 
   Thus it is one object of at least one embodiment of the invention to provide an extremely simple definition of the encompassing field for H&amp;N treatment using a beam configuration familiar to H&amp;N. 
   The image may be a medial slice through the head and neck region and the subset fields may be selected from the group covering a spinal cord, parotid gland, mandible, and area outside the mandible. 
   Thus it is another object of at least one embodiment of the invention to use subset fields matched to readily identifiable anatomical structures. 
   The encompassing field may be applied to the image by tracing a periphery of the desired encompassing field. This tracing may be done manually or by automatic techniques following, for example, an isodose line of the unmodulated beams. 
   Thus it is another object of at least one embodiment of the invention to provide a simple method of rapidly defining an encompassing field. 
   Alternatively, the encompassing field may be a predefined pattern from a library of encompassing fields for different procedures and the step of applying the encompassing field may be the step of fitting one encompassing field from the library to the patient image based on anatomical landmarks. For example, the fitting may be a translation and rotation of the selected encompassing field or a warping of the selected encompassing field. A similar procedure may be adopted for defining the subset fields. 
   Thus it is another object of at least one embodiment of the invention to provide a rapid method of creating treatment plans by reusing general encompassing fields and/or subset fields that will fit a wide variety of patients with simple modification. 
   These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a simplified diagram of a transverse cross-section of the head and neck region showing the position of laterally opposed IMRT radiation sources whose unmodulated beams describe an encompassing area; 
       FIG. 2  is a lateral view of the head and neck region of  FIG. 1  showing a series of treatment planes that may be obtained by movement of the IMRT beams of  FIG. 1  vertically and showing preliminary beam collimation describing the modulated areas of the beams of  FIG. 1  and a tumor outline; 
       FIG. 3  is a block diagram of a standard electronic computer that may store images of the patient per  FIGS. 1 and 2  and may execute a stored program in accordance with the present invention; 
       FIG. 4  is a flow chart of the steps of the present invention as may be executed using the computer of  FIG. 3  executing the stored program; 
       FIG. 5  is a transverse image slice of the patient of  FIGS. 1 and 2  showing an encompassing field derived by a tracing of the 85% isodose line of the collimated beams of  FIG. 2 ; 
       FIG. 6  is a figure similar to that of  FIG. 5  showing the application of subset fields to the encompassing field of  FIG. 5  over the mandible, salivary glands, and spinal cord; 
       FIG. 7  is a figure similar to that of  FIG. 6  showing trimming of the encompassing area to eliminate regions outside of the mandible and to apply different doses to the refined treatment area; and 
       FIG. 8  is a detail of a display on the graphical interface of the computer of  FIG. 3  showing application of a standard library area to an image structure; 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a radiation source for an IMRT radiation therapy system provides a beam  12  comprised of multiple rays  14  each of which may be independently modulated as to intensity and/or energy according to methods well known in the art. 
   For head and neck radiation therapy, the radiation source  10  may be positioned alternately on left and right lateral sides of a patient&#39;s head and neck  16  at lateral positions  18   a  and  18   b  opposed about the head and neck  16  to irradiate a tumor volume  20  and the region  22  surrounding the tumor. 
   Referring also to  FIG. 2 , the radiation beam  12  from the lateral positions  18   a  and  18   b  may expose a single transverse slice  26  of the patient, and the entire vertical height of the treatment area  24  may be covered by a series of successive vertically repositionings of the source  10  at each of lateral positions  18   a  and  18   b.    
   Referring now to  FIG. 3 , the present invention generally provides a target definition program  30  that may define a dose map providing a desired dose distribution in each of the slices  26 . The target definition program  30  may be stored in the memory  32  of an electronic computer  34  and executed by a processor  36 . The electronic computer  34  includes interfaces  38  and  42  allowing it to communicate with a graphic monitor  40  and cursor control device  44  such as a mouse or track ball, light pen or other device well known in the art. 
   The memory  32  may also hold a series of CT slice images  46  taken of the patient along each of the slices  26  per  FIG. 2  as will be understood in the art using a conventional CT machine. Optionally, the memory  32  may also include a treatment planning program  35 , also of the type well known in the art, that takes the dose map produced by the target definition program  30  for each of the slices  26  to calculate the necessary intensities and sequences of the rays  14  of the beams  12  for a particular radiation therapy machine. The memory  32  may also include a template library  50  holding subset areas as will be described below. 
   Referring now to  FIGS. 2 and 4 , in a first step of the target definition program  30  as indicated by process block  60 , the tumor volume  20  and a lateral field  28  (shown in  FIG. 2 ) are defined. 
   Generally, the tumor volume may be identified by tracing on each CT slice image  46  a boundary surrounding the tumor. The tracings for each slice are then joined into the tumor volume  20  by interpolation between slices. 
   The lateral field  28  describes for each of the slices  26 , a collimated anterior-posterior width of the beam  12 . The beam width is set to amply cover the tumor volume  20  and a margin of tissue around the tumor volume  20  sufficient to cover any elective treatment area. So long as the lateral field  28  is reasonably generous, it need not be precisely set because actual dose within the lateral field  28  will be further controlled by the modulation of the radiation beams. 
   The lateral field  28  is most easily defined by creating a lateral image  48  (shown in  FIG. 3 ) of the orientation of  FIG. 2  showing the tumor volume  20 , and using the lateral image  48  as a basis for tracing the lateral field  28  on the lateral image  48 . The lateral image  48  may, for example, be generated from the CT images  46  (shown in  FIG. 3 ) by a simple rebinning of the data. 
   Referring now to  FIGS. 4 and 5  at succeeding process block  62  for each of the CT images  46 , an encompassing field  65  may be defined and displayed superimposed on the CT slice image  46 . The encompassing field  65  may, for example, be defined as the 85% isodose line from the source  10  positioned at positions  18   a  and  18   b  collimated to produce the lateral fields  28  but otherwise unmodulated. 
   As will be seen from  FIG. 5 , the encompassing field  65  in the case of H&amp;N treatment will generally be a horizontal band extending laterally across the transverse CT slice image  46  from a point posterior to the back of the mandible  66  to a point anterior to the spinal column  68 . This encompassing field  65  will thus cover the tumor volume  20  as well as nodal areas or at risk areas surrounding the tumor volume  20  which can be extremely difficult to define and outline. 
   Other methods of defining the encompassing field  65  may be used including a simple tracing or painting process well known to those in the graphical imaging art or application of a library of pre-defined fields from a library as will be described below. 
   Referring now to  FIGS. 4 and 6  at succeeding process block  64 , subset fields may be subtracted from the encompassing field  65 . These subset fields, in this example, will include subset fields  70   a  and  70   b  covering the salivary glands  67 , subset field  72  covering the spinal column  68 , and subset fields  74   a  and  74   b  covering the mandible. As mentioned before, these normal tissue structures covered by the subset fields  70 ,  72 , and  74  are relatively easy to identify in the CT slice images  46  and may be quickly outlined through the use of the cursor control device  44  by a physician. 
   Alternatively, a standard library subset field may be used and library subset fields fit to the anatomical structures of the CT slice images  46  as will be described below. 
   Referring now to  FIGS. 4 and 7  at process block  78 , the encompassing field  65  is further trimmed, this time manually using the cursor control device  44  to remove the regions outside of the mandibles  66 . 
   At this time, the encompassing field  65  may be partitioned by the drawing of a partition line  73  to create two distinct dose regions  80   a  near the tumor volume  20  and  80   b  further removed from the tumor volume  20 . Each of these regions may be assigned a different dose. The tumor volume  20  may be individually contoured and given a separate dose definition. 
   Referring again to  FIG. 4  at the final step of process block  82 , the refined encompassing field  65 , now termed a dose map, may be provided to the treatment planning program  35  of  FIG. 3  to generate a treatment plan controlling the intensity of the rays  14  of the beam  12  as shown in  FIG. 1  for positions  18   a  and  18   b.    
   A preliminary analysis of the use of this technique as shown in the following Table 1 indicates that the “subtractive” approach of the present invention is a substantial improvement over conventional non-IMRT treatment and is comparable to “target definition” IMRT which is substantially more time consuming than the present invention. 
   
     
       
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
                 
               GTV 
                 
               Spinal 
               Parotid 
             
             
                 
               Contoured 
               Dose 
               CTV Dose 
               Cord 
               Gland 
             
             
                 
               CTV Volume 
               Volume 
               Volume 
               Dose 
               Dose 
             
             
               Method 
               (CM 2 ) 
               (CM 2 ) 
               (CM 2 ) 
               (Max) 
               (Mean) 
             
             
                 
             
           
           
             
               Conventional 
               N/A 
               421 
               838 
               49.1 
               63.1 
             
             
               Target 
               350.2 
               123 
               628 
               40.0 
               21.0 
             
             
               Drawing 
             
             
               Avoidance 
               579.4 
               138 
               699 
               39.8 
               22.1 
             
             
               Drawing 
             
             
                 
             
           
        
       
     
   
   Referring now to  FIG. 8 , both the encompassing field  65  and the subset fields  70 ,  72 ,  74  may be applied by adapting predefined templates  90  to a particular anatomical area  92 . A template  90  may be taken from the template library  50  of  FIG. 3  and may represent encompassing areas or subtracting areas that have been predefined for use by physicians, for example, by experienced practitioners looking at particular patients or composites of patients. Each template  90  may have a manipulation bar  94  surrounding it allowing it to be expanded or contracted, rotated or warped so as to best fit the anatomical area  92 . This manipulation of the template  90  may be done while the template  90  is superimposed on a CT slice image  46  to greatly simplify this process. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.