CN100432699C - Method for measuring photon beam energy spectrum of medical accelerator - Google Patents

Abstract

This invention discloses a medical accelerator measuring the photon beam spectroscopy method. steps are as follows: a. measuring medical accelerator exit collimator system in the three-dimensional photon beam dose distribution in water, access to data; b. Application different Monte Carlo method a number of group single-energy photon beam in the water to the dose distribution data; c. establishment of single-energy photon beam dose distribution in the water, the photon beam spectroscopy and measurement data between the dose of linear equations; by measuring the photon beam d. Die dose in the water data, the results of the application of energy photon beam weighted algorithm to solve linear equations available photon beam spectroscopy; advantage of this invention is: only in measuring photon beam dose distribution in water tanks, software reuse of this invention to be a photon beam spectroscopy data, the use of fast simpler; all radiotherapy units have radiotherapy water tanks, the invention will enable hospitals measured photon spectrum-free purchase equipment, reduce cost measurement; not photon spectrometer radiotherapy simplified the process and improve the efficiency of the treatment.

Description

A kind of method of measuring photon beam energy spectrum of medical accelerator

Technical field

The present invention relates to the assay method of the photon beam energy spectrum of medical accelerator in the radiation therapy, the mensuration of these gamma-spectrometric datas is bases of photon beam precise radiotherapy technology.

Background technology

Radiation therapy is one of main means of curing cancer, and for further improving radiocurable cure rate, the radiation therapy technology of new generation that development is a feature with accurate control beam dosage is crucial.The dose data that common radiation therapy is directly measured in the water mould based on the rectangular field photon beam, do simple body density correction and promptly estimate the dosage of photon beam in human body, do not need the photons spectrum data, but this method can produce than mistake concerning fitting the such precise radiotherapy technology of shape and Intensity Modulation Radiated Therapy (IMRT), is unacceptable.In order to realize that the high dose of photon beam is accurately thrown in the target area (tumour), must be determined at the photon beam energy spectrum of accelerator colimated light system exit (accelerator head).This is because be the photon beam of E megavolt for nominal energy, in fact it is beaten by the electronics of certain energy that the bremstrahlen photon that produces on the tungsten plate in the accelerator head forms, and the power spectrum of bremstrahlen photon is very wide, from low energy to high energy, have, the average energy of bremstrahlen photon is approximately E/3 (MeV), and the photon energy of nominal energy correspondence is E (MeV), in this case, dose distribution and the nominal energy of photon in human body is the difference that E (MeV) monoenergetic photon has essence, if do not know accelerator photon Shu Nengpu (energy composition), just can not accurately determine the dosage of photon in human body, be a necessary condition that realizes the photon beam precise radiotherapy so measure the photon beam energy spectrum of accelerator head.Can utilize one group of nucelonic instrument (as multiple tracks photon energy spectrometer) measurement of photon Shu Nengpu traditionally, but such measuring system is quite complicated, price is also very expensive, measuring system has very big influence to medical accelerator treatment patient's field layout, and surveying work is time-consuming disturbs clinical curing the disease again, wasting time and energy, is the thing that the hospital aspect is felt inconvenience deeply.So, how to determine that with short-cut method the power spectrum of photon beam becomes a key factor that realizes photon beam precise radiotherapy technology.

Summary of the invention

The purpose of this invention is to provide a kind of method of measuring the measurement photon beam energy spectrum of medical accelerator easy, that cost is low.

In order to achieve the above object, the present invention adopts following technical scheme.Its step is as follows:

A, the dose distribution data of measurement medical accelerator colimated light system exit photon beam in 3 d water tank;

B, application Monte Carlo method, the dose distribution data of some groups of monoenergetic photon beams in water of calculating different-energy;

C, set up the system of linear equations between the dose data of monoenergetic photon beam dose distribution data, photon beam energy spectrum and measurement in water;

D, by dose data in the measurement of photon Shu Zaishui mould, find the solution system of linear equations, obtain the power spectrum of photon beam;

E, the photon beam energy spectrum data that obtain are input to radiotherapy treatment planning system.

The present invention proposes the effective technology method measuring accelerator photon Shu Nengpu problem that solves.Promptly use the dose distribution of a 3 d water tank measurement of photon bundle in 3 d water tank, just can obtain the photon beam energy spectrum data from these dose distribution data then, measure quick, easy.3 d water tank is one of indispensable equipment of modern radiotherapy medical institutions.Therefore, utilize result of the present invention, the equipment of buying special measurement of photon power spectrum can be exempted in the hospital aspect, makes to measure the cost reduction.Simultaneously, owing to removed the medical link of using photon energy spectrometer measurement of photon Shu Nengpu, thus simplified the radiocurable flow process of photon beam.

Description of drawings

Fig. 1 is the process flow diagram of the inventive method;

Fig. 2 is by United States Medicine physicist association (American Association ofPhysiciats in Medicine, AAPM) the canonical measure curve that distributes of the percentage depth dose on the central shaft of 18MV photon in the water mould of 10cm * 10cm launched field of providing of 55# report;

Fig. 3 is the present invention calculates 6MeV monoenergetic photon beam dose distribution in water with Monte Carlo method data;

Fig. 4 is the power spectrum of the 18MV medical accelerator that obtains of the present invention.

Embodiment

A kind of method of measuring photon beam energy spectrum of medical accelerator includes following step (seeing Fig. 1, Fig. 2, Fig. 3, shown in Figure 4):

The first step is measured the dose distribution data of medical accelerator colimated light system exit photon beam in 3 d water tank; Specific practice is:

1. adjust the block (jaw) in the medical accelerator head, the launched field photon beam of a certain size that formation will be measured; This is the photon beam because of different launched fields, and its power spectrum is different.

2. with the photon beam irradiation 3 d water tank of this a certain size launched field,, measure the dose distribution of above-mentioned photon beam in water tank by the quantimeter in water tank; The sounding mark that the launched field central shaft is different is z ₁, z ₂... z _n, the photon beam dosage indicia at the launched field central shaft different depth place that measures with quantimeter is d (z ₁), d (z ₂) ... d (z _n), write a Chinese character in simplified form: d ₁, d ₂... d _n, perhaps write it as vector form

In second step,, calculate the dose distribution data of some groups of monoenergetic photon beams in water tank of different-energy with Monte Carlo method; Specific practice is:

1. establishing the photon beam nominal energy is E (MV), and E is divided into the n equal portions, gets the photon energy that every part average energy is represented this part, obtains E/2n, 3E/2n ... .. (2n-1) E/2n writes a Chinese character in simplified form it become: E ₁, E ₂... E _n, amount to n kind energy;

2. use Monte Carlo method, calculating energy is E _iDosage D (the z of unit fluence monoenergetic wide beam photon measurement point in water tank of [i.e. (2i-1) E/2n] ₁, E _i), D (z ₂, E _i) ... .D (z _n, E _i);

3. calculate the dosage of monoenergetic photon n measurement point in water tank of whole n kind energy, obtain following dose matrix D:

$D(z,E)=\left(\begin{array}{cccc}D(z_1,E_1)& D(z_1,E_2)& ...& D(z_1,E_n)\\ D(z_2,E_1)& D(z_2,E_2)& ...& D(z_2,E_n)\\ .& .& & .\\ .& .& & .\\ .& .& & .\\ D(z_n,E_1)& D(z_n,E_2)& ...& D(z_n,E_n)\end{array}\right)---\left(1\right)$

Because the energy of accelerator photon is a continuous spectrum from 0 to E, when photon beam being divided into n at interval the time, if space-number n is very big, then each at interval interior photon energy patibhaga-nimitta etc. nearby can be used average energy E _iRepresent the photon energy at this interval, and each total number of light photons that can distinguish just can be represented the power spectrum of this photon that can distinguish, it is designated as Ψ (E _i), or write a Chinese character in simplified form into Ψ _i, all n the photons spectrum that can distinguish is Ψ ₁, Ψ ₂... Ψ _n, perhaps write as vector form

In the 3rd step, set up the system of linear equations between the dose data of monoenergetic photon beam dose distribution data, photon beam energy spectrum and measurement in water; Because each z that fathoms _iThe photon dosage at place all is the sum total that is produced dosage by n kind monoenergetic photon, so obtain the system of linear equations of following form:

$\left(\begin{array}{cccc}D(z_1,E_1)& D(z_1,E_2)& ...& D(z_1,E_n)\\ D(z_2,E_1)& D(z_2,E_2)& ...& D(z_2,E_n)\\ .& & & \\ .& & & \\ .& & & \\ D(z_n,E_1)& D(z_n,E_2)& ...& D(z_n,E_n)\end{array}\right)\left(\begin{array}{c}\mathrm{\Ψ}\left(E_1\right)\\ \mathrm{\Ψ}\left(E_2\right)\\ .\\ .\\ .\\ \mathrm{\Ψ}\left(E_n\right)\end{array}\right)=\left(\begin{array}{c}d\left(z_1\right)\\ d\left(z_2\right)\\ .\\ .\\ .\\ d\left(z_n\right)\end{array}\right)---\left(2\right)$

Can be written as in form:

$D\stackrel{\→}{\mathrm{\Ψ}}=\stackrel{\→}{d}---\left(3\right)$

The 4th step, by the dose data in the quantimeter measurement of photon Shu Zaishui mould, find the solution system of linear equations, obtain the power spectrum of photon beam; We directly obtain photons spectrum by common direct matrix of no use $\stackrel{\→}{\mathrm{\Ψ}}=D^{-1}\stackrel{\→}{d},$ Because D is an ill-condition matrix, can obtain incorrect result with the inverse matrix method, might obtain the non-physical result of the higher-order of oscillation.We propose a kind of new method and can obtain correct available result in the present invention.Its step is as follows:

1. use approximate equation (4) to replace equation (3)

$F\stackrel{\→}{\mathrm{\Ψ}}=(D+\mathrm{\αI})\stackrel{\→}{\mathrm{\Ψ}}=\stackrel{\→}{d}---\left(4\right)$

Here I is a unit matrix, I _{I, j}=δ _{I, j}, δ _{I, j}Be the Kronecker symbol, α is a positive number customized parameter, and when α was very little, equation (4) was with regard to convergence equation (3), because α I has increased the value of diagonal element in the matrix, thereby will significantly improve the pathosis of equation (3), the power spectrum that it is obtained

Be rational;

2. choose any one very big positive number earlier as initial alpha ₀(iterative initial value of α) because the pathosis of F is overcome, therefore satisfies in power spectrum $\left|\right|D\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}-\stackrel{\&RightArrow;}{d}\left|\right|<\mathrm{\&delta;}$ Under the precision conditions, can try to achieve with direct matrix again ${\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}}_0={(D+{\mathrm{\&alpha;}}_{0}I)}^{-1}\stackrel{\&RightArrow;}{d};$

3. the error of calculation ${\mathrm{\&Delta;}}_0=\left|\right|D{\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}}_0-\stackrel{\&RightArrow;}{d}\left|\right|$

If Δ ₀＞δ then selects a littler α ₁Repeat the aforementioned calculation process, up to the m time, the α that makes selection _mObtained ${\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}}_m={(D+{\mathrm{\&alpha;}}_{m}I)}^{-1}\stackrel{\&RightArrow;}{d},$ Can make ${\mathrm{\&Delta;}}_m=\left|\right|D{\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}}_m-\stackrel{\&RightArrow;}{d}\left|\right|<\mathrm{\&delta;}$ Set up for the first time, then iterative process is finished, and so this has just obtained an available photons spectrum

4. with photons spectrum

Be stored in the database storage

5. in the time of will calculating photon beam dosage, can call these photons spectrum data

Patient is implemented pre-irradiation, and the doctor is input to radiotherapy treatment planning system with these photon beam energy spectrum data.And radiotherapy treatment planning system is used these data, and photon beam accurate dose distributed intelligence in the patient body just can be provided.

In order further to clearly demonstrate the inventive method, illustrate below by specific embodiment: as Fig. 1, Fig. 2, Fig. 3, shown in Figure 4, this embodiment is for obtaining the power spectrum of 18MV photon beam on the basis of using the invention described above method, its method is as follows: implementation process such as Fig. 1

1) select a station symbol to claim that energy is the medical accelerator of 18MV, carries out according to the described method of the step a in the inventive method.

2) utilize 3 d water tank to measure the dose data (see figure 2) of photon beam in water tank.Carry out according to the described method of the step a in the inventive method.

3) utilize Monte Carlo method to calculate 0.5MeV to the dose distribution data (see figure 3) of 20MeV monoenergetic photon beam in water.Carry out according to the described method of the step b in the inventive method.

4) set up the system of linear equations that concerns between the dose data of monoenergetic photon beam dose distribution, photon beam energy spectrum and measurement; Carry out according to the described method of the step c in the inventive method.

5) find the solution system of linear equations, obtain the power spectrum of photon beam.(see figure 4).Carry out according to the described method of the steps d in the inventive method.

6) the photon beam energy spectrum data that obtain are input to radiotherapy treatment planning system.When patient was implemented to shine, radiotherapy treatment planning system was used these data, and photon beam accurate dose distributed intelligence in the patient body is provided.

Claims (1)

1, a kind of method of measuring photon beam energy spectrum of medical accelerator has following step:

The first step is measured the dose distribution data of medical accelerator colimated light system exit photon beam in 3 d water tank; Specific practice is:

1. adjust the block in the medical accelerator head, the launched field photon beam of a certain size that formation will be measured;

2. with the photon beam irradiation 3 d water tank of a certain size launched field,, measure the dose distribution of above-mentioned photon beam in water tank by the quantimeter in water tank; The sounding mark that the launched field central shaft is different is z ₁, z ₂... .z _n, the photon beam dosage indicia at the launched field central shaft different depth place that measures with quantimeter is d (z ₁), d (z ₂) ... .d (z _n), write a Chinese character in simplified form: d ₁, d ₂... .d _n, perhaps write it as vector form

In second step,, calculate the dose distribution data of some groups of monoenergetic photon beams in water tank of different-energy with Monte Carlo method; Specific practice is:

1. establishing the photon beam nominal energy is E (MV), and E is divided into the n equal portions, gets the photon energy that every part average energy is represented this section, obtains E/2n, 3E/2n ... .. (2n-1) E/2n writes a Chinese character in simplified form it become: E ₁, E ₂... .E _n, amount to n kind energy;

2. use Monte Carlo method, the energy of unit of account fluence is E _i=(2i-1) dosage D (z of monoenergetic wide beam photon measurement point in water tank of E/2n ₁, E _i), D (z ₂, E _i) ... .D (z _n, E _i);

3. calculate the dosage of monoenergetic photon n measurement point in water tank of whole n kind energy, obtain following dose matrix D:

$D(Z,E)=\left(\begin{array}{cccc}D(z_1,E_1)& D(z_1,E_2)& \&CenterDot;\&CenterDot;\&CenterDot;& D(z_1,E_n)\\ D(z_2,E_1)& D(z_2,E_2)& \&CenterDot;\&CenterDot;\&CenterDot;& D(z_2,E_n)\\ \&CenterDot;& \&CenterDot;& & \&CenterDot;\\ \&CenterDot;& \&CenterDot;& & \&CenterDot;\\ \&CenterDot;& \&CenterDot;& & \&CenterDot;\\ D(z_n,E_1)& D(z_n,E_2)& \&CenterDot;\&CenterDot;\&CenterDot;& D(z_n,E_n)\end{array}\right)---\left(1\right)$

Because the energy of accelerator photon is a continuous spectrum from O to E, when photon beam being divided into n at interval the time, if space-number n is very big, then each at interval interior photon energy patibhaga-nimitta etc. nearby can be used average energy E _iRepresent the photon energy at this interval, and each total number of light photons that can distinguish just can be represented the power spectrum of this photon that can distinguish, it is designated as Ψ (E _i), or write a Chinese character in simplified form into Ψ _i, all n the photons spectrum that can distinguish is Ψ ₁, Ψ ₂.... Ψ _n, perhaps write as vector form

In the 3rd step, set up the system of linear equations between the dose data of monoenergetic photon beam dose distribution data, photon beam energy spectrum and measurement in water; Because each z that fathoms _iThe photon dosage at place all is the sum total that is produced dosage by the photon of n kind energy, so obtain the system of linear equations of following form:

$\left(\begin{array}{cccc}D(z_1,E_1)& D(z_1,E_2)& \&CenterDot;\&CenterDot;\&CenterDot;& D(z_1,E_n)\\ D(z_2,E_1)& D(z_2,E_2)& \&CenterDot;\&CenterDot;\&CenterDot;& D(z_2,E_n)\\ \&CenterDot;& & & \\ \&CenterDot;& & & \\ \&CenterDot;& & & \\ D(z_n,E_1)& D(z_n,E_2)& \&CenterDot;\&CenterDot;\&CenterDot;& D(z_n,E_n)\end{array}\right)\left(\begin{array}{c}\mathrm{\&Psi;}\left(E_1\right)\\ \mathrm{\&Psi;}\left(E_2\right)\\ \&CenterDot;\\ \&CenterDot;\\ \&CenterDot;\\ \mathrm{\&Psi;}\left(E_n\right)\end{array}\right)=\left(\begin{array}{c}d\left(z_1\right)\\ d\left(z_2\right)\\ \&CenterDot;\\ \&CenterDot;\\ \&CenterDot;\\ d\left(z_n\right)\end{array}\right)---\left(2\right)$

Can be written as in form:

$D\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}=\stackrel{\&RightArrow;}{d}---\left(3\right)$

The 4th step, by dose data in the quantimeter measurement of photon Shu Zaishui mould, find the solution system of linear equations, obtain the power spectrum of photon beam; Because D may be an ill-condition matrix, adopts following method to handle:

1. use approximate equation (4) to replace equation (3)

$F\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}=(D+\mathrm{\&alpha;I})\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}=\stackrel{\&RightArrow;}{d}---\left(4\right)$

Here I is a unit matrix, I _{I, j}=δ _{I, j}, δ _{I, j}Be the Kronecker symbol, α is a positive number customized parameter, and when α was very little, equation (4) was with regard to convergence equation (3), because α I has increased the value of diagonal element in the matrix, thereby will significantly improve the pathosis of equation (3), the power spectrum that it is obtained

Be rational:

2. choose any one very big positive number earlier as initial alpha ₀(iterative initial value of α).Because the pathosis of F is overcome, therefore satisfy in power spectrum $\left|\right|D\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}-\stackrel{\&RightArrow;}{d}\left|\right|<\mathrm{\&delta;}$ Under the precision conditions, can try to achieve with direct matrix again ${\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}}_0={(D+{\mathrm{\&alpha;}}_{0}I)}^{-1}\stackrel{\&RightArrow;}{d};$

3. the error of calculation ${\mathrm{\&Delta;}}_0=\left|\right|D{\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}}_0-\stackrel{\&RightArrow;}{d}\left|\right|,$ If Δ ₀＞δ then selects a littler α ₁Repeat the aforementioned calculation process, up to the m time, the α that makes selection _mObtained ${\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}}_m={(D+{\mathrm{\&alpha;}}_{m}I)}^{-1}\stackrel{\&RightArrow;}{d},$ ... ... can make ${\mathrm{\&Delta;}}_m=\left|\right|D{\stackrel{\&RightArrow;}{\mathrm{\&Psi;}}}_m-\stackrel{\&RightArrow;}{d}\left|\right|<\mathrm{\&delta;}$ Set up for the first time, then iterative process is finished, and so this has just obtained an available photons spectrum

4. with photons spectrum

Be stored in database;

5. in the time of will calculating photon beam dosage clinically, call these photons spectrum data to radiotherapy treatment planning system.

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