Patent Number: 051665310
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 to 5, a multileaf collimator 2 is illustrated in accordance with the present invention. A gantry 4 is shown in FIG. 1. Gantry 4 includes a radiation head 6 for housing a radiation source 8. A patient support assembly 10 is positioned within a radiation beam 9. Multileaf collimator 2 is housed within a multileaf collimator system 12. A central axis 14 of radiation beam 9 is coincident with the central axis of multileaf collimator 2. A pair of conventional movable collimators, identified as jaw-blocks or jaws 7 is positioned to generally align the radiation beam with the treatment field to be irradiated, as seen in FIG. 2. The radiation treatment volume is dependent on the shape of the tumor, as seen from radiation source 8. It is desirable to generate a shaped treatment field to conform exactly to the shape of the tumor so as to permit a greater dose of radiation to be delivered to the tumor. Therefore a plurality of leaves 16 are independently movable with respect to jaws 7 in a longitudinal direction 22, oriented generally perpendicular to central axis 14. Referring to FIGS. 4 and 5, each leaf 16 includes a leaf end 24 generally transverse to longitudinal direction 22. The particular shape of the leaf ends ultimately determines the size of penumbra generated in conjunction with a radiation field. Furthermore, the shape of leaf end 24 determines the extent to which leaves 16 can be extended across central axis 14, while still producing a radiation field with acceptable penumbra. Leaves 16 are movable in longitudinal direction 22 from a fully retracted position as seen in solid lines in FIG. 5 to a fully extended position as seen in dashed lines in FIG. 5. In the preferred embodiment, the distance from central axis 14 to each of the fully withdrawn and fully extended positions is 20 cm, at a reference plane 23 positioned 100 cm from the source of radiation. Therefore, coverage may occur over the entire 40 cm range of motion, as measured at reference plane 23. This range of motion allows other conformal therapy methods to be employed with multileaf collimators employing leaf ends in accordance with the present invention. Isocenter 35 is positioned on reference plane 23 at its intersection with central axis 14. The difference in geometry between a proximal surface 39 of leaf 16 and a distal surface 40 of the leaf causes proximal surface 39 to produce penumbra values that are greater than distal surface 40 because radiation source 8 appears larger, as seen from the proximal position. The geometric penumbra is dependent on the size of radiation source 8, the distance from the radiation source to either distal surface 40 or proximal surface 39, and the distance from radiation source 8 to reference plane 23. The equation for geometric penumbra, by considering similar traingles, is: EQU penumbra=(source size)[(reference plane distance)-(leaf surface distance)]/(leaf surface distance) In the preferred embodiment, proximal surface 39 is 48.2 cm from the radiation source 8 while distal surface 40 is 53.4 cm from the radiation source. From the above equation, the geometric penumbra factor for the distal surface 40 will be: (source size) (0.873) while the geometric penumbra factor for proximal surface 39 will be (source size) (1.075). Therefore, in the preferred embodiment, the geometric penumbra for distal surface 40 will be (0.873/1.075) or 0.812 times as large as the geometric penumbra for proximal surface 39. The geometric penumbra factors calculated above must be compensated with transmission penumbra. Referring to FIG. 4, this is done by offsetting the axis of symmetry of the leaf end distally from the longitudinal axis 36 of leaf 16 by an offset distance 38. The amount of offset 38 is chosen such that the amount of radiation attenuation in offset 38 is equal to the geometric penumbra factor between proximal and distal surfaces 39, 40. For the preferred embodiment, the offset thickness is calculated as follows: EQU e.sup.-ux =0.812, where 0.812 is the geometric factor calculated above u is the linear attenuation coefficient PA1 x is the desired offset distance to solve for Table 1 lists calculated linear attenuation coefficients of several materials for common megavoltage x-ray beams. The calculated narrow-beam linear attenuation coefficients listed in Table 1 can be used for megavoltage x-ray beams to calculate the required material thickness to achieve the desired offset. TABLE 1 ______________________________________ Calculated Narrow-Beam, Linear Attenuation Coefficients. (inch.sup.-1) Atomic Density 10 25 Number (g/cm.sup.3) 4 MV 6 MV MV 15 MV MV ______________________________________ Al 4 2.70 0.365 0.316 0.256 0.215 0.186 V 23 6.11 0.782 0.687 0.581 0.505 0.462 Fe 26 7.87 1.06 0.924 0.812 0.708 0.667 Cu* 29 8.96 1.21 1.05 0.924 0.806 0.759 Sn 50 7.30 0.958 0.869 0.802 0.745 0.735 W 74 17.0 2.53 2.27 2.15 1.97 1.98 Pb* 82 11.3 1.60 1.51 1.43 1.31 1.32 ______________________________________ *Copper data scaled from iron data and lead data scaled from tungsten data, accounting for differences in density. For leaves constructed of tungsten, the linear attenuation coefficient can be taken as 2.0 inches.sup.-1. Substitution of the linear attenuation coefficient into the above equation yields an offset of 0.104 inches. Thus, in the preferred embodiment having leaves 16 constructed of tungsten, the axis of symmetry of the leaf ends 24 should have an offset 38 on the distal side of longitudinal axis 36 of leaves 16 of 0.104 inches. A first flat end 28 and a second flat end 30 are directed at the edge of the radiation source when leaf 16 is fully extended or fully retracted, respectively, as seen in FIG. 5. A central portion 26 of leaf end 24 is an arc of fixed radius, as measured from the point P. The radius of central portion 26 influences the overall penumbral performance of the leaf end. In the preferred embodiment, the radius of curvature R is 8 cm. In operation, jaw-blocks 7 are positioned relative to patient support assembly 10 at the generally desired location such that the radiation field is positioned in the approximate desired area for treatment. Leaves 16 are then driven on rails 20 to locate each respective leaf in the desired position for exactly defining the treatment volume of the tumor. First and second flat ends 28, 30 and central portion of leaf end 24 allow leaves 16 to be positioned on either side of central axis 14, as seen in FIG. 5, while maintaining equal penumbra values for positions equidistant from central axis 14. The calculated multileaf penumbra is shown in FIG. 7. The figure represents leaf ends 16 retracted away from central axis 14 as positive position values, while leaf positions extended beyond central axis 14 are represented by negative position values. FIG. 7 shows the calculated penumbra for a simple 8 cm constant radius, and an asymmetric 8 cm radius with flat surface tangents. (The asymmetric configuration can be seen in FIG. 4.) In both cases, the calculated penumbra values show the largest differences at the largest displacements from central axis, as is expected. From these positions at long displacements from central axis, the difference between the penumbra values smoothly diminish. At central axis, there is no difference between the simple 8 cm radius and the asymmetric 8 cm radius with tangents. Although the penumbra generated by distal portion 40 of leaf 16 has been increased at large retractions from central axis, the penumbra for leaf positions near central axis has not increased. The asymmetric leaf end shape of the present invention has an additional benefit. Second flat end 30 is shorter when leaf end 24 has an asymmetric configuration than if the leaf end 24 were symmetric about longitudinal axis 36 of leaf 16. As leaf 16 is retracted away from central axis 14, second flat end 30 defines the edge of the radiation field. It also rescatters charged secondary particles back into radiation beam 9. These charged secondary particles (typically electrons and positrons) increase the surface dose in the reference plane 23 and cause the depth of the maximum dose in the reference plane to vary as a function of the size of radiation beam 9. It is desirable to minimize both of these effects, and providing a shorter leaf second flat end 30 accomplishes this. The shorter surface makes it less probable for charged secondary particles to be rescattered into the radiation beam 9. ALTERNATE EMBODIMENTS FIG. 8 shows an alternate embodiment of the asymmetric leaf end where the asymmetric end is approximated by a polygon. Clearly, the limit of a multifaceted polygon is a curved surface. The advantage of using a polygon is increased manufacturability, at the expense of increased transmission penumbra at the positions where the field edge is defined by a corner of the polygon. FIG. 9 shows an alternate embodiment of the asymmetric radius leaf end where the leaf extends in the full lateral extent of the radiation field, what is normally called a jaw-block. FIG. 10 shows an alternate embodiment of the asymmetric radius leaf end in which leaves 16 vary, dependent upon position. In the preferred embodiment, leaves 16 are identical for ease of fabrication. FIG. 10 shows leaves 16 with variable width in the direction parallel with rays from source 8. The embodiment of FIG. 10 would be required for multileaf collimators positioned close to an extended source, such that each leaf projected the same width onto the reference plane. FIG. 11 shows an alternate embodiment of the asymmetric radius leaf end in which a linear radiation source is oriented perpendicularly to the motion of collimator leaves 16, rather than the generally circular sources typically used. As discussed above, the first and second flat ends and the asymmetric leaf end of the present invention provides uniform and minimized penumbra over the full range of travel of the collimator leaves, including travel across the central axis of the radiation source. Furthermore, the equal penumbra values for points equidistant from the central axis simplifies operation of multileaf collimator requiring no differentiation between retraction and extension of the individual leaves. However, variations and modifications can be made to the preferred embodiment without departing from the scope of the present invention, which is limited only by the following claims.