Three dimensional dosimetry using solid array geometry

#The invention generally relates to radiation measurement equipment for radiation therapy and treatment, and in particular to systems and methods for measuring and localizing, spatially and/or temporally, a dose in a phantom for supporting quality assurance (QA) in radiation therapy beam delivery.

#There is a need for an accurate measurement of dose in a stationary object that simulates a patient, the stationary object herein referred to and well known as a phantom, while a radiation therapy delivery system moves with respect to the phantom, and that such a measurement results in a three dimensional (3-D) dose map that is coherent at any beam angle relative to the phantom. By coherent, it is meant that there is consistency with respect to a time and across time, with an inference to a geometrical projection (or property) of a detector array measurement that remains consistent as a radiation source moves relative to the detector array. A more detailed discussion of measurement coherence is addressed later in this document. Traditional or conventional radiation therapy delivery techniques have used treatment fields, where the beam axis remains stationary while the beam is on. This is true for the historical blocked fields of Cobalt and linear accelerator (LINAC) systems, and the more recent intensity modulated radiation therapy (IMRT) fields. Quality assurance (QA) methods have incorporated film, a passive array, and electronic active detector arrays (such as MapCHECK™, MatriXX™, and Seven29™)1, which provide two dimensional (2-D) planes orthogonal to the beam axis and result in a 2-D dose map of the field. With the evolution of delivery techniques where the source rotates (or moves) while the patient remains stationary (such as Rapid Arc™, HI-ART™, VMAT™, Single Arc Therapy™ (SAT), CyberKnife™, and Renaissance™)2, the 2-D array no longer provides the same coherent dosimetric information as it did when the beam projection was restricted to be normal to the array plane. At one beam angle, the 2-D array appears as a plane, but with a 90 degree rotation of the radiation source, the 2-D array appears as a linear array with many lines of detectors at different depths in the array. This creates a dose information weighting problem with the detector sampling dose at depths and densities that change significantly at and near the vicinity of the beam axis as the beam rotates around the array.

#There is also a need, in this 3-D dosimetry system, to measure and store the dose during specific time segments throughout the duration of the radiation delivery to the phantom, and to have no significant measurement limit on the total dose delivery. With movement of the source comes a temporal feature to the dose delivery because the position of the source is time dependent and the position of the source is a factor in the dose distribution. Any rigorous QA solution that verifies the dose delivery should do so with a number of finite “time segment” dose measurement distributions that can be compared to the desired dose distribution during any particular segment in time, or over a beam angle which is a function of time in the delivery system. Furthermore, with time segmented dose data in three dimensions and beam edge proximities to detectors, which is provided by embodiments of the present invention, it is possible to determine the source angle by ray tracing through 3-D dose distributions, and verify the source angle with the intended angle during that time segment. Without time segment data, the measured dose distribution becomes a composite of the entire dose delivery from all angles, which in itself, can be compared to the intended dose distribution, but with limited QA benefit. The composite blurs the delivery error that occurred at any given angle, just as it does in conventional IMRT QA when all fields are summed together into a composite. A current American Association of Physicists in Medicine (AAPM) task group activity (TG119) has discussed recommending against composite QA and recommending field measurement QA, but is not published at the time of this writing. Therefore, comparison of time segment measurements of dose delivery with planned dose delivery during the time segment is analogous to field QA in conventional IMRT.

#A preferred embodiment of the invention, as well as alternate embodiments are described by way of example with reference to the accompanying drawings and photographs in which:

#FIG. 1 is a diagrammatical illustration of a dosimetry measurement system including a detector array embedded within a cylindrical phantom according to the teachings of the present invention;

#FIG. 2 is a partial cross sectional view of the phantom and array taken through lines 2-2 of FIG. 1;

#The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, the embodiments herein presented are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

#With reference initially to FIGS. 1 and 2, a system 10 for measuring radiation dose according to the teachings of the present invention comprises a dosimeter 12 including an ionizing radiation detector array 14, wherein the array encompasses a three dimensional geometric shape such as the cylindrical phantom 16, herein described by way of example. The array 14 may comprise a passive detector array or an active detector array. For the embodiment herein described by way of example, the active detector array 14 comprises each detector 18 within the array 14 formed from diodes, but may also comprise ionization chambers, luminescent sensors, or amorphous silicon. In addition, while a cylindrical shape is illustrated by way of example, the dosimeter 12 may be a three dimensional geometric shape defined by a closed directrix, and wherein each of a plurality of detectors 18 within the ionizing detector array 14 is within an envelope defined by a generatrix of the directrix, as will be addressed later in this disclosure. By way of example, the closed directrix may include the cylindrical shape illustrated with reference to FIGS. 1 and 2, or any regular polygon shape, as will be addressed later in this section. Yet further, and for the embodiment of FIGS. 1 and 2, the plurality of detectors 18 is positioned only within the envelope 20, but as will come to the mind of those skilled in the art, detectors may be located at other desirable positions without departing from the essence and teachings of the present invention.

#Before proceeding further with regard to features and embodiment of the present invention, and to aid the reader, the following definitions are provided: a. Coherent=1) marked by an orderly, logical, and aesthetically consistent relation of parts; 2) logically connected, consistent; having a natural or due agreement of parts; 3) harmonious; 4) having the same property at a time and across time. Consider one particular beamlet emitted from the source at some angle from the beam axis. Now consider the measurement of this beamlet, as the source moves relative to the dosimetry array. A detector's location when the beamlet is measured determines the coherence of the measurement relative to measurements of the same beamlet at other points in time. Here it is assumed that the response of all detectors is invariant, i.e., the same measurement result, in identical measurement geometry, would occur, regardless of which detector makes the measurement. i. For example, uniform dosimetric film forming a cylindrical geometry will provide a coherent dose measurement as the beam axis rotates on a circle around the cylinder while remaining normal to the cylinder axis. At each angle of rotation, the particular beamlet will be measured by the film at the same source to detector distance (SDD), the same angle of incidence to the film detector, the same depth in the phantom, but at a different point on the film. If the film response is invariant to location on the film, then the beamlet will be measured at each angle as if there were no beam axis or cylinder rotation, providing a coherent measurement at a time and across time as the rotation progresses. ii. As an example of an incoherent measurement, the film forms a spiral around an axis, as in the Paliwal spiral phantom. At each angle of rotation, a particular beamlet will be measured by the film at a different SDD, at a different angle of incidence to the film detector since the radius changed, and at a different depth in the phantom. The film response may be invariant (normal to the film “plane”) to location on the film, but each angle of measurement provides a unique measurement geometry for the given beamlet, i.e., the beam axis rotation provides a different view of the detector for a given beamlet across time, thus destroying the measurement coherence. iii. The film examples provide illustration of an array with nearly a continuum of detectors, i.e., very high density. Now consider an array that has a repeating geometric pattern of detectors with inactive regions between the detector locations. For example, on the circumference of the cylinder there exists a detector at every one cm increment, and there are circumferential rows along the length of the cylinder spaced every one cm. Again, consider one particular beamlet emitted from the source at some angle from the beam axis, and again consider the measurement of this beamlet, as the source moves relative to the dosimetry array. Only at given repeating angles will this particular beamlet be measured, when one of the circumferential detectors is aligned to the beamlet, which occurs every 1 cm along the length of the circumference. At each of these given angles, the given particular beamlet will be measured by a detector positioned at the same source to detector distance (SDD), the same angle of incidence to the detector, the same depth in the phantom, but by different detectors on the circumference. If the detector's response is invariant by proper calibration in the array, then the beamlet will be measured at each angle as if there were no beam axis or cylinder rotation, thus providing a coherent measurement at a time and across time as the rotation progresses. iv. As an example of an incoherent measurement with a repeating detector pattern, a detector array exists in a flat plane in a phantom and the plane is initially normal to the beam axis. As the source rotates in a circle around the array center, the beam axis is no longer normal to the detector plane and eventually is parallel to the plane at a 90 degree rotation. Considering again one particular beamlet, only at given non-repeating angles will this particular beamlet be measured, when one of the detectors on the plane is aligned to the beamlet, which occurs more frequently or less frequently, depending upon the beamlet angle from the beam axis. On the beam axis, the center array detector always measures the same beamlet, and parallel to the array, all detectors on the array aligned to the rotation measure the beam axis, each at different SDD and depth. Off the beam axis, when a beamlet is measured by a detector, it is at a different SDD, a different angle of incidence and a different phantom depth. The detector response may be invariant to location on the array by proper calibration, but each angle of incidence measurement provides a unique measurement geometry for the given beamlet, i.e., the beam axis rotation provides a different view of the detector for a given beamlet across time, thus destroying the measurement coherence. v. Finally, in the above example (iii) of the repeating detector that possesses measurement coherence, the particular beamlet that is measured at repeated beam angles have a neighborhood of other beamlets that are also measured, each having their own unique but consistent SDD, angle of incidence, and phantom depth. Then at angles where these particular beamlets are not measured, there is another neighborhood of beamlets that are being measured. With a continuous rotation of the source around the cylinder, there is a near continuous measurement of beamlet neighborhoods, each measurement repeating in a coherent manner. b. Invariant=1) a quantity or expression that is constant throughout a certain range of conditions, 2) Unaffected by a designated operation, as a transformation of coordinates, 3) unchanged by specified mathematical or physical operations or transformations c. Cylinder12 i. Merriam-Webster Online Dictionary “1 a: the surface traced by a straight line moving parallel to a fixed straight line and intersecting a fixed planar closed curve b: a solid or surface bounded by a cylinder and two parallel planes cutting all its elements ii. Microsoft Encarta 12 “Three-dimensional geometric figure” Quoting Further . . . . A circular cylinder consists of two circular bases of equal area that are in parallel planes, and connected by a lateral surface that intersects the boundaries of the bases. More generally, a cylinder need not have circular bases, nor must a cylinder form a closed surface. If MNPQ is a curve in a plane (reference being made to FIG. 3A), and APB is a line that is not in the plane and that intersects the curve at a point P, then all lines parallel to AB and intersecting MNQ when taken together form a cylindrical surface. If the curve MNPQ is closed, the volume enclosed is a cylindrical solid. The term cylinder may therefore refer to either the solid or the surface. The line APB, or any other line of the surface that is parallel to APB, is called a generatrix or element of the cylinder, and the curve MNPQ is called a directrix or base. In a closed cylinder, all the elements taken together form the lateral surface. A closed cylinder is circular, elliptical, triangular, and so on, according to whether its directrix is a circle, ellipse, or triangle. In a right cylinder, all elements are perpendicular to the directrix; in an oblique cylinder, the elements are not perpendicular to the directrix. In general, the volume of a closed cylinder between the base and a plane parallel to it is given by B′h, in which B′ is the area of the base and h is the perpendicular distance between the two parallel planes, reference being made to FIG. 3A. d. Polygon 12, Again quoting from Microsoft Encarta 11 . . . in geometry, a simple closed two-dimensional figure formed by the joining of three or more straight line segments, called sides. A figure is simple if it extends in no more than two directions from any point and closed if its starting point is the same as its endpoint. These restrictions require that the sides of a polygon not cross each other and that pairs of sides intersect at their endpoints, which are called vertices. As illustrated with reference to FIGS. 4A and 4B, all polygons have an equal number of sides and vertices, and the sum of the interior angles of a polygon with n sides is 180° ×(n−2). If the sides of a polygon are of equal length and the angles are equal, the polygon is regular; otherwise it is irregular. As illustrated with reference to FIG. 5, the distance from the center of a regular polygon to a side is called its apothem. One-half the apothem times the number of sides times the length of a side provides the area of a regular polygon: Area ½ a(n)(s). Polygons are either convex or concave, as illustrated with reference to FIGS. 6A and 6B. Every interior angle of a convex polygon is less than 180° , while at least one angle of a concave polygon is greater than 180° . An easy way to tell if a polygon is convex is to lay a ruler along each side in turn. If the ruler never juts into the inside of the polygon, the polygon is convex. If it does jut inside, however, the polygon is concave. A polygon's name reflects the number of sides it has. The best-known polygons are the triangle, which has three sides; the quadrangle, which has four sides and includes squares, rectangles, and parallelograms; the pentagon, which has five sides; the hexagon, which has six sides; the heptagon, which has seven sides; the octagon, which has eight sides; the nonagon, which has nine sides; and the decagon; which has ten sides. e. Solid Geometry 12 i. Solid (geometry) 12=three-dimensional figure along with the volume it encloses. ii. Solid Geometry 12=branch of geometry that deals with the properties and measurement of geometric figures in three-dimensional space.