WATER EQUIVALENT DEPTH MEASUREMENT

• US 20120228493A1
• Filed: 03/06/2012
• Published: 09/13/2012
• Est. Priority Date: 03/07/2011
• Status: Active Grant

• Current Assignee: Ion Beam Applications s.a.
• Original Assignee: Ion Beam Applications s.a.
• Inventors: Jongen, Yves, GOTTSCHALK, Bernard
• Granted Patent: US 9,480,862 B2
• Time in Patent Office: Days
• US Class Current: 250/307
• CPC Class Codes: A61N 2005/1087 Ions; Protons
• CPC Class Codes: A61N 5/103 Treatment planning systems
• CPC Class Codes: A61N 5/1075 for testing, calibrating, or quality assurance of the radiation treatment apparatus

Abstract

A method for determining a water equivalent depth between an entrance point and a reference point is disclosed. The method may comprise sending to a charged particle beam detector placed at a reference point within or beyond a body a charged particle beam whose energy is modulated between a minimum and maximum energy value, acquiring the time dependent response of said charged particle beam detector, determining from said time dependent response a value of a statistical parameter, providing a calibration curve expressing a relationship between values of said statistical parameter and water equivalent depths, and extracting from this calibration curve the water equivalent depth corresponding to the value of the statistical parameter determined from the time dependent response of the charged particle beam detector placed at the reference point.

Claims

1. A method for determining a water equivalent depth between an entrance point and a reference point, said entrance point lying on an external surface of a body, said reference point being a point at which a charged particle beam detector is placed, the method comprising:

providing an apparatus for delivering a charged particle beam to said body in a beam direction defined from said entrance point to said reference point;

choosing a maximum energy value such that a charged particle beam having said maximum energy value and being sent in said beam direction penetrates the body from said entrance point up to at least said reference point;

choosing a minimum energy value such that a charged particle beam having said minimum energy value and being sent in said beam direction penetrates the body from said entrance point and is stopped in said body before reaching said reference point;

sending in said beam direction a charged particle beam that is time modulated in energy between said minimum energy value and said maximum energy value;

acquiring a time dependent response of the charged particle beam detector;

determining a value of a statistical parameter of said time dependent response;

providing a pre-determined calibration curve expressing a relationship between values of said statistical parameter and water equivalent depths; and

determining from said calibration curve the water equivalent depth between the entrance point and the reference point corresponding to the value of the determined statistical parameter.

2. The method of claim 1, wherein said statistical parameter is a root mean square width of said time dependent response.

3. The method of claim 1, wherein said reference point is positioned inside said body.

4. The method of claim 1, wherein said reference point is positioned outside said body.

5. The method of claim 1, wherein the charged particle beam detector comprises a semiconductor diode for detecting the charged particle beam.

6. The method of claim 5, wherein said semiconductor diode is connected to an input of a transimpedance amplifier, and wherein said time dependent response is measured at an output of said transimpedance amplifier.

7. The method of claim 6, wherein said transimpedance amplifier includes a first transimpendance amplifier, an inverting voltage amplifier with a gain ranging between 35 and 45, and an output stage whose gain is set to obtain a gain between the input and the output of said transimpedance amplifier ranging between 0.4 V/nA and 0.6 V/nA.

8. The method of claim 1, wherein the pre-determined calibration curve is obtained by:

(1) positioning said charged particle beam detector at a given position in a water or water-equivalent calibration phantom;

(2) sending to said charged particle beam detector the charged particle beam that is time modulated in energy between said minimum and said maximum energy value;

(3) acquiring a time dependent response of said charged particle beam detector;

(4) determining a value of said statistical parameter from said time dependent response;

(5) determining a water equivalent depth, said water equivalent depth being the distance between the entrance point of said charged particle beam in said calibration phantom and said given position;

(6) repeating steps (1) to (5) more than one time; and

(7) establishing a calibration curve that relates the determined values of the statistical parameter and the determined water equivalent depth.

9. The method of claim 1, wherein the charged particle beam detector comprises an array of semiconductor diodes placed transversally with respect to the charged particle beam.

10. A device for determining a water equivalent depth between an entrance point and a reference point, said entrance point lying on an external surface of a body, said device comprising:

a charged particle beam detector that is placed at the reference point;

a subunit for acquiring a time dependent response of said charged particle beam detector;

a subunit for determining a value of a statistical parameter of said time dependent response;

a subunit for loading a pre-determined calibration curve expressing a relationship between values of said statistical parameter and water equivalent depths; and

a subunit for determining from said calibration curve the water equivalent depth corresponding to the value of the statistical parameter of the time dependent response of the charged particle beam detector placed at the reference point.

11. The device of claim 10, wherein the charged particle beam detector comprises a semiconductor diode.

12. The device of claim 11, wherein said semiconductor diode is connected to an input of a transimpedance amplifier, and in that said time dependent response is measured at an output of said transimpedance amplifier.

13. The device of claim 12, wherein the charged particle beam detector comprises an array of semiconductor diodes placed transversally with respect to the charged particle beam.

17. The device of claim 10, further comprising:

computational means for determining statistical probabilities of skewness and/or kurtosis for said time dependent response; and

computational means for computing a range mixing parameter which is an indicator of observed deviations in said statistical probabilities of skewness and/or kurtosis when compared with values obtained during a calibration measurement using a water phantom.

14. A method for determining a water equivalent depth between a number of entrance points and a number of corresponding reference points, said entrance points being located on an external surface of a body, said corresponding reference points being points where a corresponding charged particle beam detector is placed, the method comprising, for each entrance point and corresponding reference point, the steps of:

providing an apparatus for delivering a charged particle beam to said body in a beam direction defined from an entrance point to the correspond reference point;

choosing a maximum energy value such that a charged particle beam having said maximum energy value and being sent in said beam direction penetrates the body from said entrance point up to at least said reference point;

choosing a minimum energy value such that a charged particle beam having said minimum energy value and being sent in said beam direction penetrates the body from said entrance point and is stopped in said body before reaching said reference point;

sending in said beam direction a charged particle beam that is time modulated in energy between said minimum energy value and said maximum energy value;

acquiring a time dependent response of the charged particle beam detector;

wherein, for each time dependent response curve obtained for each of the said corresponding detectors, further comprising the steps of;

determining a value of a statistical parameter of the time dependent response;

providing a pre-determined calibration curve expressing a relationship between values of said statistical parameter and water equivalent depths; and

determining from said calibration curve the water equivalent depth between the entrance point and the reference point corresponding to the determined value of the statistical parameter.

15. The method of claim 14, further comprising, for each time dependent response acquired for each of the corresponding charged particle beam detectors, the steps of:

determining statistical probabilities of skewness and/or kurtosis with respect to the time dependent response curve;

comparing statistical probabilities of skewness and/or kurtosis obtained in the previous step with values obtained during a calibration measurement using a water phantom;

computing a range mixing parameter which is an indicator of the observed deviations in probabilities of skewness and/or kurtosis; and

classifying the beam detectors according to the range mixing parameter.

16. A method for determining range mixing when a water equivalent depth between a number of entrance points and a number of corresponding reference points is being measured using a charged particle beam, said entrance points being located on an external surface of a body, said corresponding reference points being points where a corresponding charged particle beam detector is placed, the method comprising, for each entrance point and corresponding reference point, the steps of:

providing an apparatus for delivering a charged particle beam to said body in a beam direction defined from an entrance point to the correspond reference point;

choosing a maximum energy value such that a charged particle beam having said maximum energy value and being sent in said beam direction penetrates the body from said entrance point up to at least said reference point;

choosing a minimum energy value such that a charged particle beam having said minimum energy value and being sent in said beam direction penetrates the body from said entrance point and is stopped in said body before reaching said reference point;

sending in said beam direction a charged particle beam that is time modulated in energy between said minimum energy value and said maximum energy value;

acquiring a time dependent response of the charged particle beam detector;

wherein, for each time dependent response curve obtained for each of the said corresponding detectors, further comprising the steps of;

determining statistical probabilities of skewness and/or kurtosis with respect to the time dependent response curve;

comparing statistical probabilities of skewness and/or kurtosis obtained in the previous step with values obtained during a calibration measurement using a water phantom;

computing a range mixing parameter which is an indicator of the observed deviations in probabilities of skewness and/or kurtosis; and

classifying the beam detectors according to the range mixing parameter.

18. A device configured to determine a water equivalent depth between an entrance point and a reference point, said entrance point lying on an external surface of a body, said reference point being a point at which a charged particle beam detector is placed, the device comprising:

a first software module configured to acquire a time dependent response of said charged particle beam detector;

a second software module configured to determine a value of a statistical parameter of said time dependent response;

a third software module configured to load a pre-determined calibration curve expressing a relationship between values of said statistical parameter and water equivalent depths; and

a fourth software module configured to determine from said calibration curve the water equivalent depth corresponding to the value of the statistical parameter of the time dependent response of the charged particle beam detector placed at the reference point.