Method for EPID-based verification, correction and minimization of the isocenter of a radiotherapy device

Abstract

A method for EPID-based verification, correction and minimization of the isocenter of a radiotherapy device includes the following: Positioning a measurement body; applying an irradiation field; capturing a common dose image of the measurement body; creating a dose profile on the basis of the captured dose image; determining an inflection point in a plot of the dose profile; linking positions of the inflection points to bodily limits of the measurement body; determining position of a center point of the measurement body relative to an EPID-center; determining a differential vector from a deviation in position of the center point of the measurement body from the EPID-center and from a deviation in position of the field center point of the irradiation field from the EPID-center; and correcting the current radiological isocenter.

Claims

1. A method for EPID-based verification, correction and minimization of an isocenter of a radiotherapy device, the radiotherapy device having at least one patient couch rotatable about a couch axis, a support arm rotatable about a support arm axis, a radiator head arranged on the support arm for generating a therapy beam, a rotatable collimator, a projection device for projecting a radiological isocenter at a projection position and a digital recording system (EPID) for acquiring dose images by the therapy beam, wherein the following steps are performed: a) a measurement body is positioned, by the projection device, at a projection position in a current radiological isocenter of the radiotherapy device, b) an irradiation field, limited by the collimator, is applied for at least one predefined angular setting of the support arm, the patient couch and the collimator and thereby c) at least one common dose image of the measurement body and the irradiation field is captured using the EPID, d) a dose profile for each direction within an EPID coordinate system is created on the basis of the common dose image and e) in a plot of the dose profile, an inflection point between a local dose minimum and a local dose maximum, and between a local dose maximum and a local dose minimum is determined at each of both expected bodily limits of the measuring body in an X-direction of the EPID coordinate system and at each of both expected bodily limits of the measuring body in a Y-direction of the EPID coordinate system, and f) positions of the inflection points determined in step e) are linked to the bodily limits of the measurement body in the X-direction and in the Y-direction, g) a position of a center point of the measurement body relative to an EPID-center is determined in the dose image on the basis of the bodily limits of the measurement body, the steps d) to g) being carried out in the same way for field limits and a field center point of the irradiation field, and h) a differential vector is determined from a deviation in the position of the center point of the measurement body from the EPID-center and from a deviation in position of the field center point of the irradiation field from the EPID-center, and i) vector components of the differential vector are used to correct the current radiological isocenter, wherein the irradiation field is applied taking into account minimum values of a field size, a relaxation time of the support arm, a dose per irradiation field, and/or a focus-EPID distance.

2. The method according to claim 1, wherein the method steps b) to h) are carried out with an increment of a maximum of 30° of angular freedoms of the patient couch, the support arm and the collimator.

3. The method according to claim 1, wherein differential vectors, which can be determined from the dose images taken from different angular positions of the patient couch, the support arm and the collimator, are used to determine a size and position of spatial isocenters, wherein vector components of associated position vectors of the spatial isocenters are used to correct the radiological isocenter.

4. The method according to claim 3, wherein the associated position vectors of the spatial isocenters are used for calibrating a patient positioning system.

5. The method according to claim 3, wherein the associated position vectors of the spatial isocenters are used to correct the projection device.

6. The method according to claim 3, wherein all device-specific parameters of the radiotherapy device influencing the spatial isocenters are optimized by minimizing a predetermined target function.

7. The method according to claim 3, wherein the radiotherapy device is a therapy simulator, and wherein the spatial isocenters are determined, corrected and minimized.

8. The method according to claim 1, wherein the inflection point(s) is/are determined in a range of a 50% dose point between a dose minimum and a dose maximum of the dose profile and/or in a range of a 50% dose point between a dose maximum and a dose minimum of the dose profile.

9. The method according to claim 8, wherein the inflection point(s) in the range of the 50% dose point is/are defined between two pixels, of which a first pixel represents a dose less than 50% and a second pixel adjacent to the first pixel represents a dose greater than 50%.

10. The method according to claim 1, wherein, when using a multi-leaf collimator (MLC), steps d) to h) are carried out for each pair of leaves of the MLC limiting the irradiation field.

11. The method according to claim 1, wherein a support arm angle of ≠0° is set when a patient couch angle is varied.

12. The method according to claim 1, wherein a global spatial isocenter of the radiotherapy device is determined from individual central beam deviations in three spatial directions X, Y and Z of three spatial isocenters of the support arm, the collimator and the patient couch.

13. The method according to claim 1, wherein the method steps d) to i) are carried out automatically under control of a software program.

14. The method according to claim 1, wherein an isocenter of the patient couch is alternatively determined at a support arm angle=0° by means of a radiological patient positioning system independent of the radiotherapy device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is to be described in more detail by way of example on the basis of the following figures.

(2) The following are shown:

(3) FIG. 1 a two-dimensional dose image of a spherical measuring body in an MLC-limited irradiation field,

(4) FIG. 2a a dose profile of a two-dimensional dose image for explaining an exemplary embodiment of the method,

(5) FIG. 2b the dose profile from FIG. 2a for explaining the definition of the 50% dose points,

(6) FIG. 3 a schematic diagram for the verification in terms of measurement technology of the achieved spatial resolution,

(7) FIG. 4 dose profiles of the Winston-Lutz pointer as the measuring body,

(8) FIG. 5 additional dose profiles of an MLC-limited irradiation field in the X-direction,

(9) FIG. 6 additional dose profiles of an MLC-limited irradiation field in the Y-direction,

(10) FIG. 7 a schematic diagram of the geometry of an isocentroid dependent on the support arm angle of a radiotherapy device,

(11) FIG. 8 a graphical representation of a global isocentroid.

DETAILED DESCRIPTION

(12) For explaining the method, FIGS. 1, 2a and 2b are to be considered together.

(13) With the method for the EPID-based verification and correction of the isocenter of a radiotherapy device, which comprises at least one patient couch rotatable about at least one couch axis, a support arm rotatable about a support arm axis, a radiator head arranged on the support arm for applying the therapy beam, a collimator for limiting a radiation field, a device for projecting the radiological isocenter and a digital recording system (EPID) for generating dose images by means of the therapy beam, the following steps are carried out:

(14) a) A measuring body 13 is positioned at the projection position of the radiological isocenter of a radiotherapy device,

(15) b) Subsequently, an irradiation field is applied for at least one predetermined angle setting of the support arm, the patient couch and the collimator, and thereby

(16) c) Using the EPID, a dose image of the measuring body 13 is taken, as shown in FIG. 1. The dose image in FIG. 1 comprises a two-dimensional grayscale dose image of an MLC-limited irradiation field of 15×15 mm.sup.2 in MATLAB® (version R2007a). The EPID used has pixels with an edge length of 0.392 mm. The reference sign 13 indicates a spherical measuring body (Winston-Lutz pointer) with a known diameter. The reference sign 14 indicates a limit of the irradiation field caused by the collimator. In the dose image, dark areas have a lower applied absorbed dose compared to light areas. To determine the sphere center point of the measuring body 13 by means of the crossed, orthogonal measuring planes 15, initially, both sphere boundaries 1 and 2 are determined in the X direction and both sphere boundaries 16 and 17 are determined in the Y direction. The sphere boundaries 1, 2, 16 and 17 can be determined from the dose profiles, which are generated on the basis of the recorded dose image in step (d) of the method. FIG. 2a and FIG. 2b show a dose profile 5 in the X-direction of the dose image shown in FIG. 1. The curve marked with the reference sign 6 in FIG. 2a represents the gradient of the dose profile over the X-axis.

(17) Further in step e) of the method, initially, a central position of the measuring body 13 is determined by calculating, in the plot of the dose profile 5, at at least one expected limit of the measuring body 13, an inflection point 29 between a dose maximum 19 and a dose minimum 28, and an inflection point 30 between a dose minimum 28 and a dose maximum 20. The inflection points 29 and 30 can be determined in the area of the 50% dose point, wherein the 50% dose point comprises the position in the plot of dose profile 5, at which the dose is 50% between the dose minimum 28 and the dose maxima 19 and 20. Preferably, the 50% dose point is determined between two pixels, of which a first pixel represents a dose less than 50%, and a second pixel adjacent to the first pixel represents a dose greater than 50%.

(18) An analogous procedure can be used to determine the field limits of the irradiation field, wherein, in the plot of the dose profile 5, at at least one expected limit of the irradiation field, an inflection point 3 is determined between a dose minimum 18 and a dose maximum 19, and an inflection point 4 is determined between a dose maximum 20 and a dose minimum 21. The inflection points 3 and 4 can be determined in the area of the 50% dose point, wherein the 50% dose point comprises the position in the plot of dose profile 5, at which the dose is 50% between the dose minima 18 and 21 and the dose maxima 19 and 20. Preferably, the 50% dose point is determined between two pixels, of which a first pixel represents a dose less than 50% and a second pixel adjacent to the first pixel represents a dose greater than 50%.

(19) In the subsequent step f) of the method, the determined inflection points are linked to a field limit or a measuring bodily limit, as the case may be. In the example shown, the inflection point 29 can be linked to the bodily limit 1 of the measuring body 13, which is located in the negative X-direction, and the inflection point 30 can be linked to the bodily limit 2 of the measuring body 13, which is located in the positive X-direction. In the same manner, the bodily limits of the measuring body 13 in the Y-direction can be determined by means of a dose profile in the Y-direction. In FIG. 1, the bodily limits in the Y-direction are marked with the reference signs 16 and 17. The inflection point 3 can be linked to the field limit located in the negative X-direction and the inflection point 4 can be linked to the field limit located in the positive X-direction. In the same manner, a determination of the field limits in the Y-direction can be carried out on the basis of a dose profile in the Y-direction.

(20) After determining the two bodily limits of the measuring body 13, the center position of the measuring body 13 can be determined through the arithmetic averaging of the two spatial coordinates of the boundary points. Preferably, the distances of the bodily limits in the X-direction and in the Y-direction are used to determine the center point position. In the same manner, the central beam position can be determined through the arithmetic averaging of the spatial coordinates of the boundary points after defining the two field limits 3 and 4. Preferably, the distances of the field limits in the X-direction and in the Y-direction are used to determine the central beam position.

(21) In the further step g) of the method, the positions of the center point of the measuring body 13 and of the irradiation field relative to the EPID center are determined on the basis of the linked bodily limits of the measuring body 13 and the linked field limits of the irradiation field. Steps d) to g) are carried out for the directions X and Y. In step h), the difference vector in the EPID plane, which points from the center of the measuring body 13 to the central beam penetration point through the EPID plane, is projected into the isocenter plane in accordance with equation (1). Steps b) to h) are carried out for all prescribed support arm angles, collimator angles and couch angles (of the patient couch).

(22) Finally, the vector components of the differential vector are used to correct the current radiological isocenter.

(23) The method achieves a spatial resolution of 0.01 mm with a standard clinical EPID, which corresponds to a resolution 39.2 times better than the standard EPID.

(24) The measurement conditions for the dose profile shown in FIG. 2 were determined as follows: support arm angle=0°, collimator angle=0°, couch angle=0°, nominal field size=15×15 mm.sup.2, focus-EPID distance=1.5 m (magnification factor=1.5), photon energy=6 MeV and irradiated dose=12 MU. The dose D is measured by the EPID in the unit [CU] (calibrated dose unit), where 1 CU=1 Gy under calibration measurement conditions. The sizes ax and CAX.sub.X are the field width or central beam position, as the case may be, in the X direction. For the same irradiation field without a Winston-Lutz pointer, the same values were determined for such sizes. The graphic was created using MATLAB® (version R2007a).

(25) FIG. 2b shows the definition of all four 50% dose points of the dose profile in the X-direction of FIG. 2a, which are required to determine the measuring bodily limits, irradiation field limits and the positions of the center point of the measuring body and the central beam. All 50% dose levels are defined as the arithmetic mean of a local dose minimum and a local dose maximum. The smallest of all local dose maxima is specified as a 100% dose. The smallest possible values for the measuring body and for the irradiation field are used as dose minima. If both the respective dose maximum and the respective dose minimum for calculating the 50% dose point are minimal, its dose value will also be minimal. Thereby, every 50% dose point is in the area of the inflection point there. Both aspects increase the resolution of the method. The fact that a low 50% dose point is advantageous can be physically explained as follows: The lower the 50% dose point of the field, the less interference is caused by the scattered photons generated in the measuring body, since the distance of the measuring body to the 50% dose point increases. And vice versa; the lower the 50% dose point of the measuring body, the less interference is caused by the scattered photons generated by the field limitation (block apertures, leaves of an MLC or a circular collimator), since the distance of the field limitation from the 50% dose point increases. FIG. 2b shows a dose profile for determining the geometric properties of the irradiation field and the measuring body in the X-direction. However, in the method, at least two different dose profiles are cut out of a dose image, which profiles have different Y-coordinates. In the Y-direction, the procedure is the same. This also contributes to the good resolution or error minimization, as the case may be, of the method. The equations of determination are as follows:
D.sub.100%=min[D.sub.max(−X),D.sub.max(+X),D.sub.max(−Y),D.sub.max(+Y)],
D.sub.min(field)=0[CU],
D.sub.50%(MK)=[D.sub.100%+D.sub.min(MK)]/2,
D.sub.50%(field)=[D.sub.100%+D.sub.min(field)]/2=D.sub.100%/2,
ΔX(MK)=[X.sub.1(MK)+X.sub.2(MK)]/2,
ΔY(MK)=[Y.sub.1(MK)+Y.sub.2(MK)]/2,
ΔCAX.sub.X=[X.sub.1(field)+X.sub.2(field)]/2,
ΔCAX.sub.Y=[Y.sub.1(field)+Y.sub.2(field)]/2,
ΔX.sub.ISO=ΔCAX.sub.X−ΔX(MK),
ΔY.sub.ISO=ΔCAX.sub.Y−ΔY(MK).

(26) Legend for the Equations:

(27) MK=measuring body or Winston-Lutz pointer or tungsten sphere (the Winston-Lutz pointer used for explanation in the exemplary embodiment of the method is a commercial pointer manufactured by BRAINLAB AG, Feldkirchen, Germany)

(28) Field=irradiation field with specific field width (X direction) and field length (Y direction)

(29) Coordinate system=EPID coordinate system

(30) D.sub.100%=100% substitute dose (compare 100% dose of the IEC 60976 standard)

(31) D.sub.50%(field)=50% dose to determine the field width and field length by means of the inflectional tangent localized there

(32) D.sub.50%(MK)=50% substitute dose for determining the measuring bodily limits by means of the inflectional tangent localized there

(33) D.sub.max(−X)=local dose maximum of the X-profile with a negative spatial coordinate

(34) D.sub.max(+X)=local dose maximum of the X-profile with a positive spatial coordinate

(35) D.sub.max(−Y)=local dose maximum of the Y-profile with a negative spatial coordinate

(36) D.sub.max(+Y)=local dose maximum of the Y-profile with a positive spatial coordinate

(37) D.sub.min(MK)=local dose minimum of both dose profiles in the area of the measuring body

(38) D.sub.min(−X)=local dose minimum of the X-profile at the field edge with a negative spatial coordinate

(39) D.sub.min(+X)=local dose minimum of the X-profile at the field edge with a positive spatial coordinate

(40) D.sub.min(−Y)=local dose minimum of the Y-profile at the field edge with a negative spatial coordinate

(41) D.sub.min(+Y)=local dose minimum of the Y-profile at the field edge with a positive spatial coordinate

(42) D.sub.min(field)=uniform dose minimum of both dose profiles at all field edges

(43) X.sub.1(MK)=limit of the measuring body in the negative X-direction

(44) X.sub.2(MK)=limit of the measuring body in the positive X direction

(45) Y.sub.1(MK)=limit of the measuring body in the negative Y-direction

(46) Y.sub.2(MK)=limit of the measuring body in the positive Y-direction

(47) X.sub.1(field)=position of the field limit in the negative X-direction

(48) X.sub.2(field)=position of the field limit in the positive X direction

(49) Y.sub.1(field)=position of the field limit in the negative Y-direction

(50) Y.sub.2(field)=position of the field limit in the positive Y-direction

(51) ΔX(MK)=position of the center point of the measuring body in the X-direction relative to the EPID center

(52) ΔY(MK)=position of the center point of the measuring body in the Y-direction relative to the EPID center

(53) ΔCAX.sub.x=position of the central beam of the irradiation field in the X-direction relative to the EPID center

(54) ΔCAX.sub.Y=position of the central beam of the irradiation field in the Y-direction relative to the EPID center

(55) ΔX.sub.ISO=central beam deviation relative to the center point of the measuring body in the X-direction (measured in the EPID plane)

(56) ΔY.sub.ISO=central beam deviation relative to the center point of the measuring body in the Y-direction (measured in the EPID plane)

(57) To prove the spatial resolution of 0.01 mm achievable by the method, the leaf positions of an MLC are varied with the smallest possible step size of 0.01 mm. Thereby, the determination of the respective central beam position is carried out in an MP3 large water phantom with a high-resolution dosimetry diode E type 60012 as a dose detector, a TANDEM two-channel electrometer and the MEPHYSTO® mc.sup.2 software program made by PTW GmbH (Freiburg, Germany). In contrast to the method, in which the focus-EPID distance amounts to 150 cm, the focus-detector distance for the detection of the spatial resolution is 100 cm. When carrying out the verification of the spatial resolution, a photon energy=6 MeV, a dose rate=400 MU/min, a dose integration time per measuring point=1 s and a detector step size=0.2 mm to 1 mm are set. FIG. 3 shows the verification of the spatial resolution. The radiological determination of the central beam deviations ΔX.sub.CAX or ΔY.sub.CAX, as the case may be, from the theoretical isocenter of a radiation therapy device in the large water phantom represents the gold standard in radiation therapy. The black dots mark the central beam displacements between a centrally located pair of leaves (here, no. 31) in the X-direction, as determined by the Winston-Lutz method. For this purpose, the pairs of leaves limiting the irradiation field were shifted in a defined manner; the smallest adjustable step size for the “High-Definition 120” MLC used amounts to 0.01 mm. The associated regression line has the equation X.sub.ISO=1.094.Math.ΔX.sub.CAX−0.217 mm. The correlation coefficient according to Pearson amounts to r=0.9991 at p=0.0000 (probability of non-correlation) and thus indicates an almost ideal linear relationship. The measurement results in the large water phantom MP3 are shown as black circles. With both measuring methods, the smallest leaf displacement±0.01 mm can be resolved. The slightly increasing plot of the central beam deviation Y.sub.ISO during the Winston-Lutz test, which lasted 15 min in this case, proves the relaxation of the support arm at 0° as a result of a positive bending moment around the spatially fixed X-axis acting on the support arm and radiator head. At |ΔX.sub.CAX|≥0.05 mm, the black dots of the Winston-Lutz analysis no longer correspond to the black circles of the MP3 measurement, since, in the first case, the medium at the field edge and in the field is not homogeneous: air, plastic and tungsten versus homogeneous water. The graphic in FIG. 3 was generated using MATLAB® (version R2007a).

(58) FIG. 4 shows dose profiles of the Winston-Lutz pointer as the measuring body in the X-direction 7 and Y-direction 8 with the inflection points for defining the measuring bodily limits in the X-direction 9 and 10 and in the Y-direction 11 and 12. The dotted vertical lines 7.1 and 8.1 indicate the position of the sphere center point in the X-direction (7.1) and in the Y-direction (8.1). The stretch factor amounts to k.sup.−1>1 with the definition in equation (1). The graphic in FIG. 4 was created by means of MATLAB® (version R2007a).

(59) FIG. 5 shows four dose profiles of an MLC-limited irradiation field of the size 15×15 mm.sup.2 in the X-direction under the leaf pairs no. 29 to no. 32, with the inflection points 22 and 23, for defining the field limits of the irradiation field and with the positions of the local central beams (dashed vertical lines 24). The stretch factor amounts to k.sup.−1>1 with the definition in equation (1). The graphic in FIG. 5 was created by means of MATLAB® (version R2007a).

(60) FIG. 6 shows an additional two dose profiles of an MLC-limited irradiation field of the size 15×15 mm.sup.2 in the Y-direction between the closed leaf pairs no. 27 and no. 34, with the inflection points 25 and 26, for defining the field limits and the positions of the local central beams in the Y-direction (dashed vertical lines 27). The stretch factor amounts to k.sup.−1>1 with the definition in equation (1). The graphic in FIG. 6 was created by means of MATLAB® (version R2007a).

(61) FIG. 7 shows a schematic diagram of the geometry of an isocentroid dependent on the support arm angle of a Novalis radiation therapy device powered by TrueBeam™ STx (VARIAN Medical Systems, Inc., Palo Alto, Calif., U.S. and BRAINLAB AG, Feldkirchen, Germany). The solid lines in the colors of black, dark gray and gray represent the central beam deviations relative to the measuring body in the directions X, Y or Z, as the case may be, of the spatially fixed coordinate system. The dashed lines mark the spatial coordinates of the isocentroid in such directions. The diameters in the axes of the inertial system, their maximum and the spatial coordinates are also output as numerical values. In addition, the amount of the spatial radius vector as a function of the support arm angle is shown (light gray line) and its maximum is indicated. Analog result representations are generated by means of MATLAB® (version R2007a) for the solitary isocentroids dependent on the collimator and couch angle along with the global isocentroid, which combines all three solitary isocentroids.

(62) FIG. 8 shows a graphical representation of the geometry of the global isocentroid of a Novalis medical electron linear accelerator powered by TrueBeam™ STx using MATLAB®. The solid lines in the colors of black, dark gray and gray represent the central ray deviations dependent on the support arm angle relative to the Winston-Lutz pointer in the directions X, Y or Z, as the case may be, of the inertial system. The limits of the additional negative and positive scattering bands for the superimposed collimator rotation are shown as thin dash-point lines or dashed lines, as the case may be. If the central beam deviations dependent on the couch angle are added, the corresponding lines are shown as thick lines. The light gray line shows the plot dependent on the support arm angle of the maximum radius of the global isocentroid. The point lines mark the spatial coordinates of the global isocentroid. The diameters in the axes of the inertial system, their maxima and the spatial coordinates are also output as numerical values. In addition, the diagram below right shows the spatial distances of the rotation axes of the collimator and the couch relative to the support arm rotation axis.