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WO2007138134A1 - Procédé pour l'obtention d'un noyau de convolution associé à un accélérateur pour radiothérapie, collimateur, moyens de traitement, programme informatique, support lisible par ordinateur, système pour la mise en oeuvre du procédé et accélérateur - Google Patents

Procédé pour l'obtention d'un noyau de convolution associé à un accélérateur pour radiothérapie, collimateur, moyens de traitement, programme informatique, support lisible par ordinateur, système pour la mise en oeuvre du procédé et accélérateur Download PDF

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Publication number
WO2007138134A1
WO2007138134A1 PCT/ES2007/000308 ES2007000308W WO2007138134A1 WO 2007138134 A1 WO2007138134 A1 WO 2007138134A1 ES 2007000308 W ES2007000308 W ES 2007000308W WO 2007138134 A1 WO2007138134 A1 WO 2007138134A1
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Prior art keywords
dose
accelerator
convolution
radiation
field
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PCT/ES2007/000308
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English (en)
Spanish (es)
Inventor
Javier Burguete Mas
Juan Diego Azcona Armendariz
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Instituto Científico Y Tecnológico De Navarra, S.A.
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Application filed by Instituto Científico Y Tecnológico De Navarra, S.A. filed Critical Instituto Científico Y Tecnológico De Navarra, S.A.
Publication of WO2007138134A1 publication Critical patent/WO2007138134A1/fr

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Classifications

    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N5/00—Radiation therapy
    • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
    • A61N5/103—Treatment planning systems
    • A61N5/1031—Treatment planning systems using a specific method of dose optimization
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N5/00—Radiation therapy
    • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/29—Measurement performed on radiation beams, e.g. position or section of the beam; Measurement of spatial distribution of radiation
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N5/00—Radiation therapy
    • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
    • A61N5/1042—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy with spatial modulation of the radiation beam within the treatment head

Definitions

  • This invention relates to a method for obtaining a convolution core associated with an accelerator for radiotherapy, a collimator, processing means, a computer program, a means readable by a computer, a system to implement The procedure already an accelerator.
  • Radiotherapy consists in exposing the patient to a radiation field, to administer a certain dose of radiation: by interacting with the emitted particles with a tumor and absorbing the latter energy, the death of the tumor cells is caused.
  • the dose is the magnitude used to characterize the effect of a beam of radiation when it interacts with a medium. Considering a portion of dm mass in the medium crossed by the radiation beam, the absorbed dose is defined as:
  • radiotherapy technique with modulated intensity in which the number of particles at each point of the radiation beam is controlled, that is, its creep.
  • the global beam of radiation can be broken down virtually into unit beams
  • a global beam of section 10 x 10 cm is composed of 100 unit beams of section 1 cm.
  • the creep within each unit beam is uniform, but the level of creep can vary between each unit beam of a modulated field.
  • intensity modulation allows varying the level of creep, and, therefore, of doses, within the same field.
  • the interest of this technique is that, using a large number of fields with modulated creep, we can obtain dose distributions that take the form of any tumor (i.e., with concave shapes).
  • the intensity modulation technique is used, due to the large number of degrees of freedom or configuration possibilities of the fields to radiate the tumor and protect the organs at risk, it is difficult to manually design an optimal configuration among all possible ones.
  • the complexity is greater the more complicated the shape of the tumor and the greater the number of healthy organs that can be damaged.
  • the dose could be shaped with the shape of the tumor, whatever it was.
  • a more complicated field configuration corresponds, that is, that it requires a greater number of fields, having these greater degree of modulation.
  • optimization methods are used: from a mathematical function of the absorbed dose, called the objective function, it is minimized (or maximized) by an optimization algorithm that results in the whole of creep maps to apply to a patient.
  • computer programs, called planners developed for this purpose are used.
  • TPR tissue-Phantom Ratio', tissue-dummy ratio
  • the filiform nucleus of convolution represents the dose distribution in a medium due to a beam of photons that interact along a straight line (filiform path).
  • Monte Carlo algorithm for this it is necessary to model the radiation beam and the absorbent medium, taking into account the physical and electronic densities as well as the effective sections of beam interaction for the energies and the environment considered; methods based on the algorithm of
  • Montecarlo will allow to calculate the dose with great accuracy, however, they require important human and computational resources, which implies that their use is not practical in the hospital environment. So far these methods have been used to calculate the cores to be applied in the convolution-overlay and filiform beam algorithms. In these cases, the convolution nuclei are calculated only once and stored in memory, which does not affect the speed required of these methods to calculate the dose routinely in a hospital.
  • the absorbed dose for each patient is measured experimentally and verified to correspond with the dose calculated by the planner.
  • This verification is usually done with various types of detectors, such as ionization chambers, diodes or radiographic film.
  • detectors such as ionization chambers, diodes or radiographic film.
  • the measurements are made on homogeneous means and are compared with the calculation of planners on a homogeneous medium, not on the patient.
  • FIG. 1 An example of a large homogeneous field created by an accelerator is shown schematically in figure la.
  • This field 100 has a typical dimension 102 that is much larger in order of magnitude than a typical dimension, diameter 106, of the convolution core 104 of the throttle.
  • the diameter of the core is the diameter of the circumference formed when the core is cut, represented as a dose versus distance to the center, by a cylinder coaxial to the core, such that the volume enclosed under the surface of the core inside the cylinder is equal to volume enclosed under the surface of the core outside the cylinder.
  • the Chui and Mohán procedure is based on using an approach that consists of factoring the convolution nucleus into two components, each of which reproduces the behavior of the nucleus in one direction.
  • This approach called Fermi's approximation, provides results close to reality in large and homogeneous fields such as field 100, at points away from edges 108 and more especially corners 110.
  • FIG. 1 schematically shows an example of field with intensity modulation.
  • the radiation field 124 has a global shape that is variable and can be modified by moving sheets 122 and is also composed of unit fields 126.
  • Each unit field 126 corresponds to a unit beam ('beamlet') of the order of 1 x 1 cm 2
  • unit fields 126 ('beamlets') have a typical dimension 128 of the order of magnitude of the diameter 106 of the convolution core 104 of the accelerator.
  • the resolution of this problem is achieved with a procedure to obtain a convolution nucleus associated with a device, called an accelerator, that generates radiation fields for radiotherapy, fields whose characteristics include a dose of radiation applied and a creep distribution, the convolution of the convolution nucleus with a fluence distribution of a field the dose applied by that field, characterized in that it comprises the following stages:
  • step (f) obtain the convolution nucleus as a result of the application of a procedure for calculating the inverse transformation to the ratio of the transform of the dose obtained in step (c) between the transformation of the fluence of the test field obtained in the stage (d).
  • the accelerator generates radiation fields with photons.
  • the convolution core that is achieved according to the invention is a filiform beam core.
  • a convolution core linked to the accelerator is obtained, obtained in part experimentally.
  • a two-dimensional problem can be transformed into a monodimensional problem (in function of the distance to the center of each point of the treated field), without having to practice approximations such as the one made by Chui and Mohán in the aforementioned article (Fermi approximation) that are not valid for fields whose creep distributions have a scale of variation of the order of the convolution core diameter, and especially the modulated intensity fields.
  • the dose absorbed by each patient can be calculated alternately and independently of the methods currently in place (implemented by the planners), and the result of these methods can be verified, without using accelerator time to verify these calculations for each patient or for each different field since it is possible to use accelerator time only once to obtain the convolution nucleus of said accelerator.
  • a method according to this invention thus improves the performance of the accelerators by reducing the time required in experimental dose measurements and dedicating it to treating more patients.
  • the field with revolution symmetry is obtained by means of a collimator positioned between a radiation source of the accelerator, which emits a radiation beam, and the detector, the collimator comprising radiation attenuating material surrounding a gap in truncated cone shape through which the radiation beam passes, the generator of said cone passing through the radiation source, and following the cone the divergent path of the radiation beam.
  • each dose profile value is obtained by averaging different dose measurements that are at the same distance from the center, obtained in step (b) of the invention. Thanks to this embodiment, the effects of noise and random phenomena characteristic of the interaction between a beam of particles and matter are reduced.
  • the creep distribution is a step function with revolution symmetry.
  • the transformation used is the transformation of Fourier-Bessel.
  • an approximation to this Fourier-Bessel transformation is used in the calculation procedure, an approximation calculated by a finite series of terms of a discrete Fourier-Bessel transformation.
  • the procedure for calculating the inverse transformation may be the same procedure for calculating the transformation (property of the Fourier-Bessel transformation).
  • the accelerator generates intensity modulated fields.
  • the detector is a radiographic film.
  • the test field has a diameter between 20 and 400 mm at the distance in which the detector is positioned.
  • this invention concerns a method for determining a dose applied by a radiation field generated by an accelerator, characterized in that the convolution core associated with the accelerator is obtained according to an embodiment of a method for obtaining a core of convolution according to the invention and additionally, the convolution between the creep function associated with said radiation field and said convolution core associated with the accelerator is calculated.
  • the dose that will correspond to a certain field generated by the accelerator can be determined by knowing its flow distribution.
  • this invention concerns a method for verifying an expected dose value applied by a radiation field generated by an accelerator, characterized in that:
  • an applied dose verification value corresponding to said field is determined by an embodiment of a procedure to determine a dose applied according to the second aspect of the invention
  • this invention concerns a collimator of a radiation beam generated by a radiation source of a device for generating a radiation field for radiotherapy, called an accelerator, characterized in that it is specially designed for implementation. of a previously described embodiment of a process for obtaining a convolution core according to the first aspect of the invention.
  • the collimator comprises radiation attenuating material, arranged around a truncated cone-shaped hole that passes through the collimator, cone that is designed to be able to follow the divergence of the accelerator's radiation beam and so that the generator of said cone may pass through the radiation source.
  • said attenuating matter may be substantially opaque to radiation.
  • the attenuating material comprises lead, depleted uranium, steel, tungsten or any alloy comprising these materials.
  • this invention concerns processing means, characterized in that they comprise means for applying a method of calculating a transformation to a set of dose values and to a creep distribution according to steps (d) and (e) of a procedure to obtain a convolution core according to the first aspect of the invention.
  • These processing means may comprise means for applying a procedure for calculating an inverse transformation and / or means for calculating a quotient, whereby they can obtain the convolution core as a result of the application of a procedure for calculating the inverse transformation to the quotient of the dose transform obtained in step (c) between the creep transform of the test field obtained in step (d) (step (f) of the invention).
  • these processing means are specially designed to implement a process for obtaining a convolution core according to the first aspect of the invention.
  • the transformation used is the zero-order Hankel transformation, also called Fourier-Bessel transformation and the calculation procedure is based on an approximation of the discrete Fourier-Bessel transform, within an allowable error range. Then, the procedure of calculation of the inverse transformation can be equal to the procedure of calculation of the transformation (property of the Fourier-Bessel transformation).
  • these processing means comprise radiographic film scanning means.
  • these means comprise radiographic film interpretation means to obtain at least relative dose values of certain points of a film irradiated by a radiation field.
  • these processing means also calculate the average of the dose measurements when each dose profile value is obtained by averaging different dose measurements that are at the same distance from the center, obtained in step (b) of the invention.
  • the processing means comprise additionally means to calculate a convolution to implement a procedure to determine a dose applied according to the second aspect of the invention.
  • the processing means further comprise means for obtaining an expected value of applied dose. Thanks to this embodiment, the processing means can include a scheduler.
  • this invention concerns a computer program for obtaining a convolution core, characterized in that it comprises program code means for carrying out steps (d), (e) and, preferably, (f) of the invention according to any one of the embodiments of a method according to the invention, when said program operates on a computer.
  • this computer program is specially designed to implement a method for obtaining a convolution core according to the first aspect of the invention.
  • the transformation used is the zero-order Hankel transformation, also called Fourier-Bessel transformation and the calculation procedure is based on an approximation of the discrete Fourier-Bessel transform.
  • this program is included in the processing means according to the fifth aspect of the invention.
  • the computer program for obtaining a convolution core is copied into a medium readable by a computer.
  • this invention concerns a computer-readable medium, characterized in that it contains a computer program copied comprising program code means for carrying out steps (d), (e) and, preferably, (f ) of the invention according to any one of the embodiments of a method according to the invention, when said program operates on a computer.
  • this invention concerns a system for obtaining a convolution core associated with a device, called an accelerator, for generating a radiation field for radiotherapy, characterized in that it comprises (I) an accelerator that includes a source of a radiation beam, (II) a collimator according to an embodiment of the fourth aspect of the invention, (III) collimator fixing means and (IV) processing means according to an embodiment of the fifth aspect of the invention, and because it is especially designed to implement a method according to any one of the embodiments of the first aspect of the invention when the field with revolution symmetry is obtained by means of a collimator positioned between a radiation source of the accelerator, which emits a beam of radiation, and the detector, the collimator comprises radiation attenuating material surrounding a cone-shaped hole truncated by the which passes the radiation beam, the generator of said cone passes through the radiation source, and the cone follows the divergent path of the radiation beam.
  • this system is specially designed to implement a method according to the second aspect of the invention. Furthermore, preferably, this system is specially designed to practice a method according to the third aspect of the invention.
  • the collimator fixing means comprise an accelerator block holder tray.
  • this invention concerns an accelerator for generating a radiation field for radiotherapy characterized in that it comprises processing means according to one of the embodiments of the fifth aspect of the invention.
  • Figure 2a Shows a schematic flow chart of a procedure for obtaining a convolution core according to the invention.
  • FIGS 2b, 2c, 2d, 2e and 2f.- Show, in relation to the stages of the diagram of Figure 2a, schematic representations of what was obtained in some of those stages.
  • Figure 3a Shows a schematic representation of a mold for manufacturing a collimator according to the invention.
  • Figure 3b. Schematically shows an arrangement of a collimator in relation to a radiation source of an accelerator and a detector for carrying out the invention.
  • Figure 3 c Shows a schematic representation of a collimator according to the invention according to a view.
  • Figure 3d.- Shows a schematic representation of a collimator according to the invention according to another opposite view.
  • Figure 3e Shows a schematic representation of what is obtained in a detector positioned in a field when performing a procedure according to the invention.
  • Figure 4. Shows a schematic diagram of a procedure for determining a dose applied by a radiation field according to the invention.
  • Figure 5. Shows a schematic diagram of a procedure to verify a calculation of an expected value of applied dose according to the invention.
  • the convolutions are two-dimensional. The calculated dose D represents the dose distribution in one plane.
  • the transform used is the Fourier transform and its related, the Hankel transform zero order, also called Fourier-Bessel transform.
  • the convolutions are transformed into point-to-point products:
  • the convolution core is obtained from a two-dimensional deconvolution, using dose measurements of a relatively small circular field (diameter of the order of 5 cm).
  • a relatively small circular field diameter of the order of 5 cm.
  • the zero-order Hankel transform or Fourier-Bessel transform
  • This transform is more complicated to handle than the ordinary Fourier transform, since we develop the function in terms of Bessel functions of first class and zero order, whose zeros are no longer equally spaced, such as the zeros of the sine and cosine functions.
  • the processing means and the computer program for obtaining a convolution core according to the invention use the Fouiier-Bessel transform in this embodiment.
  • the Fourier-Bessel transformation is introduced below, also called a zero-order Hankel transform or simply Bessel transform.
  • f (x, y) in Cartesian coordinates
  • equation (2) is analogous to the definition of the Fourier transform in one dimension, with one exception.
  • equation (1) develops as an infinite sum of sines and cosines.
  • the discretization of equation (2) should be developed as an infinite sum of Bessel functions (first class and zero order).
  • the Fourier transform - Bessel continuous retains a fundamental property: the transformation of a convolution of functions is equal to the product of The transformed functions. If TH 0 represents the Fourier-Bessel transform then:
  • the discrete Fourier transform - Bessel is analogous to the discrete Fourier transform. Historically, equation (2) has been calculated, even when the fifteen function is discrete, as a numerical approximation of the continuous transform. Today it is possible to calculate it using a discrete analog.
  • serial development is in terms of trigonometric functions (sines and cosines). The zeros of these functions are equally spaced (that is, they are periodic functions).
  • the functions that appear naturally when using the symmetry of revolution to represent the two-dimensional information through a single variable are the Bessel functions of first class and zero order (Jo ). The zeros of these functions are not equally spaced.
  • the inverse transformation operation coincides with the direct operation, so that the variables of the continuous case r and q are similar to the names r ⁇ and 7- 2 .
  • the functions / and F are denoted as /; Y/:
  • R] yi? 2 are the radii of the interval where the respective functions f ⁇ and T 2 are non-zero, that is, for r ⁇ > R] and? - 2 > i? 2 :
  • FIG. 2a shows a schematic flow chart 200 of an embodiment of a method for obtaining a convolution core according to the invention and Figures 2b, 2c, 2d, 2e and 2f show, in relation to the steps of the diagram of the figure 2a, a schematic representation of what was obtained in some of those stages.
  • an accelerator is used that generates fields with modulated intensity and that includes a multilayer collimator (different from the collimator of the invention).
  • the dose generated by a test field that has revolution symmetry and is circular is measured by means of a detector.
  • a collimator according to the invention is used, an experimental device consisting of a lead block with a hole or hole constructed in the shape of a truncated cone, where the generator of said cone passes through the radiation source and follows the divergent trajectory of the radiation beam.
  • the collimator according to the invention is located in the throttle head, on the block holder tray, as close as possible to the multi-sheet throttle collimator.
  • the geometric penumbra is as similar as possible to a case of clinical treatment with radiotherapy of modulated intensity.
  • Figure 3 shows schematically a mold used to make an embodiment of a lead collimator according to the invention.
  • the collimator can be made with a material (or mixture of materials) that attenuates radiation.
  • said material can have a physical density greater than 7.86 gr / cm 3 and an atomic number Z greater than 26. Lower physical densities require using collimators with a greater thickness to obtain the desired attenuation.
  • Cerrobend®, depleted uranium, steel, tungsten or any alloy comprising these materials can also be used as examples.
  • Figure 3b schematically shows the arrangement in this embodiment of the collimator in relation to the radiation source 302 of the accelerator and in relation to a detector, which is in this embodiment a radiographic film 304.
  • the detector can be a diode array (also called
  • the throttle tray 314 of the accelerator is located at a distance 316 from the radiation source 302.
  • Figure 3 c is a representation of the collimator of the invention according to a view 308 in which the diameter 320 of the opening 305 of the hollow or conical hole 306 of the collimator is seen.
  • Figure 3d is a representation of the collimator according to a view 310 in which the diameter 322 of the opening 307 of the conical hole 306 of the collimator, diameter 322, which is greater than the diameter 320 is appreciated.
  • the collimator has a thickness 312.
  • the distance 316 is 56 cm.
  • the lower opening 307 of the collimator 300 has a diameter 322 of 28 mm.
  • the radiation beam when passing through the collimator 300, generates an image 332 with a diameter 334 in a plane located at a distance 318 from the radiation source 302 according to the following relationship: (Diameter 322) (Distance 31 6 )
  • the detector used to obtain the dose distribution in this embodiment is a radiographic film 304.
  • the radiographic film has a high resolution, which depends on the size of the silver bromide grain.
  • a radiographic film is used that saturates towards 100 cGy, of relatively high sensitivity, because it has a higher signal in the low dose area, than another film with a lower sensitivity.
  • the distance 318 between the film 304, and more precisely the center 330 of the image 332 obtained in the film 304, and the radiation source 302 is 100 cm.
  • Image 332 which can be seen in Figure 3e which is a schematic representation of what was obtained in film 304 according to view 331 ( Figure 3b), then has a diameter 334 of 50 mm.
  • this image 332 is large enough that there is lateral electronic balance in the center of the field, and small enough to be comparable to the segments used in radiotherapy with modulated intensity.
  • the thickness 312 chosen from the collimator 300 is 8 cm, sufficient to achieve an attenuation of the photon beam even greater than that used in radiotherapy.
  • the opening 305 of the hole in the face according to view 308 of the collimator 300 has a diameter 320 of 24 mm.
  • the lateral dimensions 301 and 303 of the collimator 300 are 10 cm and 10 cm, so that the weight of the collimator 300 is not very high and can be maintained on the block holder tray 314 without it undergoing appreciable deformation.
  • the radiographic film is irradiated with photons of energy whose core of filiform beam we want to get.
  • the so-called monitor unit contains the minimum exposure time that the accelerator can manage.
  • the film can be irradiated from 10 to 500 monitor units.
  • the relative dose distribution obtained in the radiographic film is independent of the irradiation time (provided that this time is not so high that it causes the radiographic film to saturate).
  • the radiographic film is irradiated between 20 and 100 monitor units, at a depth of 15 cm in white polystyrene, which is radiologically equivalent to water.
  • the radiographic film is revealed in a developer for later scanning. It is necessary during development to keep the temperature constant and to carry out a careful and rigorous calibration of the scanning process.
  • Figure 2b shows a schematic representation of the dose distribution 250 obtained in the radiographic film (i.e., image 332 of Figure 3e).
  • one or more radial profiles 252 are generally extracted. To this end, the center 254 of the distribution is first located. In this embodiment, 360 profiles 252 are taken (only some of them are shown in Figure 2b, namely profiles p 36 or
  • FIG. 2c shows a radial profile without averaging, that is, the value of the dose in ordinates 255 as a function of the distance to the center 254 r in abcisses 257.
  • the center 254 of Figure 2b corresponds to the intersection of the ordinates 255 and the abcisses 257 of Figure 2c.
  • the average of these profiles 252 is calculated to obtain the average profile.
  • the average profile value for a given radius is the average of the values with that radius in all radial profiles. Obtaining this average, the influence of noise and random phenomena associated with the interaction between the radiation beam and matter is diminished.
  • the dose measurement points have been taken spaced 0.2 mm.
  • Figure 2d shows a schematic representation of the radial average profile 258.
  • the analysis of the radiographic film is done with a computer program which in one embodiment may be included in a computer program to obtain a core according to the invention.
  • This computer program used works with relative distributions, that is, it is imposed that a point corresponds to the reference dose, and the rest of the points are shown with a relative value to the reference.
  • the program works with absolute dose values.
  • step 201 the dose distribution is obtained.
  • step 202 the dose transformed function is calculated using a computer program to obtain a core according to the invention using a method of calculating the Fourier-Bessel transformation using a quasi-discrete Fourier transform.
  • step 204 a two-dimensional step fluence is generated are revolution symmetry.
  • a filiform beam algorithm is used to express the dose in a plane such as the two-dimensional convolution of the creep and the convolution nucleus.
  • this convolution product is factored using Fourier-Bessel transform from a dose distribution with revolution symmetry. It has been verified that it is possible to generate with this technique two-dimensional convolution cores sufficiently precise to adequately represent the absorption of the dose under conditions of intensity modulation.
  • the flow distribution profile 260 (see Figure 2e) is a step function, such that the width of the step function is equal to that of the averaged dose profile 258.
  • the width of the profile is defined. doses such as the distance between the dose level of 50% on either side of the center, assuming that the dose level of the center is 100%.
  • step 206 using the processing means of the invention and the program to obtain a core, the Fourier-Bessel transform of the creep distribution profile is calculated. For this, it is necessary to have the zeroes of the first-class and zero-order Bessel functions, J 0 .
  • step 210 the convolution core is obtained by applying the inverse transformation, which in the case of the Fourier-Bessel transform is equal to the direct transformation.
  • Figure 2f shows a schematic representation of a part of the convolution core 262 obtained.
  • a new and important advantage in relation to the background of the invention is that a method according to the invention uses experimentally obtained dose measurements. Not only analytical formulas, or numerical simulation, are used as in the background of the invention. From a measured dose distribution, and knowing the creep, we calculate the convoluted filiform beam core using the two-dimensional deconvolution technique.
  • the process for obtaining a convolution core of the invention is valid for any field size, which makes it particularly interesting to obtain valid convolution nuclei under conditions of intensity modulation.
  • the convolution nucleus obtained is totally independent of the convolution nucleus used in any commercial planner (obtained following the Monte Carlo algorithm).
  • One of the advantages of a method according to the invention is that it performs a complete calculation of the dose distributions obtaining experimental convolution nuclei, and avoiding the complexity of using the Monte Carlo algorithms, which makes it an easily transportable algorithm to any another hospital
  • Monte Carlo algorithms either for the calculation of the convolution core or for the calculation of the accelerator head dispersion, implies having both large computational resources to perform the calculations, as well as scientific personnel trained to execute the Monte Carlo algorithm modeling, among other things, the geometry of the accelerator or its energy spectrum.
  • the means required to implement this invention, once an accelerator is installed, have the advantage of being easily transportable to any hospital: it is only necessary to make a dose measurement on a radiographic film and have a collimator with revolution symmetry according to the invention of a computer program according to the invention capable of calculating the zero order Hankel transforms and subsequently, in one embodiment, of calculating the dose using the resulting core. It clearly implies in particular an advantage over the Monte Carlo algorithms to generate the convoluted filiform beam nuclei, since these require, apart from the computational means capable of executing a Monte Carlo algorithm efficiently and quickly, from scientists who know these well algorithms and be able to simulate accelerators and radiation beams.
  • Figure 4 shows a schematic flow chart 400 of a method for determining a dose applied by a radiation field according to the invention.
  • D (x, y, z o ) is the dose for a field with intensity modulation of dimensions L xA
  • D 0 (x, y, z Q ) is the dose for the corresponding unmodulated field
  • the quotient of integrals is the correction by the intensity modulation at each point: in the numerator the convolution of the convolution core k (characteristic of a depth in the middle z 0 , corresponding to a distance to the source 318 in Figure 3b) with the creep of the modulated field and in the denominator with a 2D step function, representative of the non-modulated field creep.
  • L x A is the minimum rectangular field that completely covers the set of segments.
  • the homogeneous field dose Do is calculated using the TPR algorithm. Equation (10) can alternatively be seen as a convolution integral where the convolution core size has been renormalized according to the dose value at an open field point (not modulated).
  • the dose distribution due to a known flow map is calculated at the entrance of the absorbent medium.
  • the distribution is calculated at a depth of 15 cm in water, with the beam entry surface in the middle completely flat.
  • the convolution cores used are obtained in this embodiment from dose distributions measured with radiographic film at a depth of 15 cm in solid water (white polystyrene medium, radiologically equivalent to water). Be it also assumes that the calculation plane is at a distance from the source equal to 318. Implicitly, it is assumed that the radiation beam is not divergent, but parallel. Each of the fields that affect the patient are calculated separately, under the conditions just described.
  • a creep is generated. Creep characterization can be carried out through the concept of modulation index (see Xing L, Chen Y, Luxton G, Li JG and Boyer AL 2000 "Monitor unit calculation for an intensity modulated photon field by a simple scatter-summation algorithm "Phys. Med. Biol, 45, N1-N7).
  • modulation index see Xing L, Chen Y, Luxton G, Li JG and Boyer AL 2000 "Monitor unit calculation for an intensity modulated photon field by a simple scatter-summation algorithm "Phys. Med. Biol, 45, N1-N7).
  • the field consists of K segments, each of which is open (radiating) a time MU k .
  • the modulation index is defined as:
  • a k represents the shape of segment k.
  • s m ⁇ l if I A k
  • ⁇ m ⁇ 0 if m £ A k .
  • the modulation index describes the level of creep in a unit beam m.
  • the matrices containing the creep maps have 400 x 400 points. Yes, as in the vast majority of cases, the L xA fields are less than 40 x
  • step 404 the uncorrected dose is calculated using the filiform beam algorithm consisting of calculating the following integral:
  • ⁇ ⁇ x ', y' is the distribution of creep at the entrance surface S of the patient or dispersion medium in general and k is the convoluted filiform beam core obtained according to a method of the invention to obtain an associated convolution core to an accelerator
  • Both the filiform beam core and the resulting dose distribution D ⁇ refer to a fixed depth in water ⁇ o, which in one embodiment is 15 cm.
  • This convolution core is a function that represents how the absorbed dose is distributed in water due to an infinitely fine virtual beam of photons that interacts along a straight path, in a semi-infinite medium of water and with normal incidence to the surface ( flat) entrance in the middle.
  • the convolution core is poly-energetic (averaged for the energy spectrum of each accelerator).
  • step 406 Parallel to obtaining the uncorrected dose, in step 406, a rectangular field is generated from the most extreme positions of the sheets. From there, on the one hand, in step 408, a two-dimensional step function U (x, y) is generated, and then in step 410 calculate the convolution of U (x, y) with the convolution core k associated to the accelerator On the other hand, in a step 412, the dose, called A (x, y), is calculated for this rectangular field without intensity modulation.
  • Figure 5 shows a schematic flow chart 500 of an embodiment of a method for verifying an expected dose value according to the invention.
  • the verification has, in this embodiment, its starting point in the file generated by a planner and that is sent to the accelerator to execute the treatment. From this file the positions of the plates of all the segments are read. With this information, the creep map is reconstructed, also called a modulation map, since it does not include creep due to dispersion in the throttle head.
  • the convolution integral is used to calculate the dose.
  • the nucleus of spatially invariant convolution is assumed, which is a good approximation in homogeneous media (Mohán R and Chui C 1987 "Use of fast Fourier transforms in calculating dose distributions for irregularly shaped fields for three-dimensional treatment planning" Med. Phys. 14 70-77).
  • the basic idea is to compare a dose value calculated by a planner, called an expected dose value, with another dose value, called a dose verification value, determined by a procedure to determine a dose according to the invention.
  • a flow distribution is generated by optimizing a dose function (expected value), called the objective function.
  • a dose function expected value
  • This process is particular for each patient and is carried out by a planner.
  • planners use different versions of the filiform beam and convolution algorithms — overlay to perform calculations; Monoenergetic convolution nuclei are calculated by a Monte Carlo algorithm. Subsequently, the spectrum with some monoenergetic components is represented or a poly-energetic convolution core is adjusted to an analytical expression using the monoenergetic convolution cores to adjust the expression parameters.
  • a flow map optimized for the patient is obtained in step 504, which allows: in step 506 to obtain a calculated water dose result by the planner, - to obtain in step 508 the dose over the patient by the planner (expected value) and obtain in step 510 a water dose verification value by a procedure to calculate a dose applied according to the invention.
  • the calculated water dose result obtained in step 506 is compared with the water dose verification result obtained in step 510. The result of the comparison is analyzed in 514. If the difference between both results is greater than a predetermined threshold value, then (option 516) the dose is measured experimentally with the accelerator for the field provided for the patient in step 522.
  • the calculation made by the 508 planner is validated and the patient is treated with the accelerator without the need to experimentally check the dose associated with the field in a step 520, so that accelerator use time is gained and its performance is increased.

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Abstract

L'invention concerne un procédé pour l'obtention d'un noyau de convolution d'un accélérateur, qui génère des champs pour la radiothérapie, la convolution du noyau de convolution avec une distribution de fluence de champ étant proportionnelle à la dose appliquée par ce champ. Selon l'invention, ledit procédé est caractérisé en ce qu'il consiste : (a) à créer un champ d'essai à symétrie de révolution; (b) à obtenir des mesures de dose; (c) à obtenir un profil de dose; (d) à obtenir la transformée du profil de dose; (e) à obtenir la transformée de la fluence; (f) à obtenir le noyau de convolution, et la transformation inverse au quotient de la transformée de la dose entre la transformée de la fluence du champ d'essai.
PCT/ES2007/000308 2006-05-26 2007-05-25 Procédé pour l'obtention d'un noyau de convolution associé à un accélérateur pour radiothérapie, collimateur, moyens de traitement, programme informatique, support lisible par ordinateur, système pour la mise en oeuvre du procédé et accélérateur WO2007138134A1 (fr)

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ES200601398A ES2296523B1 (es) 2006-05-26 2006-05-26 Procedimiento para obtener un nucleo de convolucion asociado a un acelerador para radioterapia, colimador, medios de procesamiento, programa de ordenador, medio legible por un ordenador, sistema para poner en practica el procedimiento y acelerador.

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030068009A1 (en) * 2001-02-16 2003-04-10 Lei Xing Verification method of monitor units and fluence map in intensity modulated radiation therapy
WO2005052721A2 (fr) * 2003-09-24 2005-06-09 Radion Technologies Calcul deterministe de doses de rayonnement delivrees a des tissus et a des organes d'un organisme vivant
WO2005119295A1 (fr) * 2004-06-04 2005-12-15 Bc Cancer Agency Procede et appareil permettant de verifier des distributions de doses de rayonnement
WO2006004933A2 (fr) * 2004-06-29 2006-01-12 Wayne State University Soustraction numerique adaptative pour verification de radiotherapie a intensite modulee

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030068009A1 (en) * 2001-02-16 2003-04-10 Lei Xing Verification method of monitor units and fluence map in intensity modulated radiation therapy
WO2005052721A2 (fr) * 2003-09-24 2005-06-09 Radion Technologies Calcul deterministe de doses de rayonnement delivrees a des tissus et a des organes d'un organisme vivant
WO2005119295A1 (fr) * 2004-06-04 2005-12-15 Bc Cancer Agency Procede et appareil permettant de verifier des distributions de doses de rayonnement
WO2006004933A2 (fr) * 2004-06-29 2006-01-12 Wayne State University Soustraction numerique adaptative pour verification de radiotherapie a intensite modulee

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REICH P. ET AL.: "The prediction of transmitted dose distributions using a 3D treatment planning system", AUSTRALIAN PHYSICAL O ENGINEERING SCIENCE IN MEDICINE, vol. 29, no. 1, March 2006 (2006-03-01), pages 18 - 29, XP008090509 *
YOICHI WATANABE: "Point dose calculations using an analytical pencil beam kernthe for IMRT plan checking", PHYSICS IN MEDICINE AND BIOLOGY, vol. 46, no. 4, 2001, pages 1031 - 1038, XP008090983 *

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