ON-LINE MONITORING OF AN HADRON BEAM FOR RADIOTHERAPEUTIC TREATMENTS

Transcription

1 ON-LINE MONITORING OF AN HADRON BEAM FOR RADIOTHERAPEUTIC TREATMENTS INFN-Laboratori Nazionali del Sud Via S. Sofia 44, Catania, Italy Patient positioned for treatment System under consideration (experimental configuration) G.A.P. CIRRONE, S.COCO, G.CUTTONE, C. DE MARTINIS, D. GIOVE, P.A. LOJACONO, M. MAURI, R. MESSINA

2 AT LABORATORI NAZIONALI DEL SUD (CATANIA), CATANA FACILITY (Centro di AdroTerapia e Applicazioni Nucleari Avanzate) employs a 62 MeV proton beam, produced by a superconducting cyclotron, for the radiotherapeutic treatment of choroid and iridal melanoma. The main requirement for a successful treament is: ACCURACY in the spatial distribution of thedosereleased to skin guaranteed by numerous experimental checks Patient positioned for the treatment

3 ROUTINE BEAM MONITORING SYSTEMS DIODE: transversal scanning of the beam by means of a silicon diode which measures the current and displays its distribution along the two coordinates of a Cartesian system, transversal to the beam direction; GAFCHROMIC FILM: determination of the lateral dose distribution released by the beam from the darkening of an irradiated gafchromic film; both systems, though very accurate, are subject to long acquisition and processing times Acquisition period ~30 minutes Acquisition period ~24 hours

4 DEVELOPED SYSTEM HADRON BEAM SCINTILLATING SCREEN 45 MIRROR monitoring system of a hadron beam based on a fluorescent screen coupled with an analog CCD camera; acquisition and processing of the image reflected by the mirror by means of a software application implemented with LabVIEW 6.1; CCD LabVIEW Acquisition period ~ 1 minute

5 TESTED SCINTILLATORS Alumina (Al2O3) Caesium Iodide (CsI) Plastic scintillator BC 400 Alumina: poor in light efficiency; Caesium Iodide: excellent light efficiency, but scarce homogeneity; BC 400: good brightness and homogeneity; BC 400 has been chosen for our experimental purposes.

6 FINAL CONFIGURATION Mirror forming 45 with the beam direction. Scintillating screen lodged on a support, perpendicularly to the beam axis. Camera forming 90 with the beam axis, framing the image reflected by the mirror.

7 DATA ANALYSIS High quality monochrome image acquisition board: IMAQ PCI-1409 (National Instruments) Acquisition and memorization of signal in the form of 10 bits matrices Spatial coordinates of each pixel of the camera Corresponding grey-scale values

8 SOFTWARE FOR DATA ANALYSIS LabVIEW Environment Spatial calibration Cartesian reference system Calibration factor: mm/pixel Selection of the Region Of Interest

9 SOFTWARE FOR DATA ANALYSIS LabVIEW Environment NOISE REDUCTION: average of N images EXPERIMENTAL SIGNAL ACQUISITION BACKGROUND SUBTRACTION

10 SOFTWARE ANALYSIS Acquisition and analysis of lateral profiles signal % W 50 % 60 H 90/50 % Psx distance from axis (mm) Pdx Remote communication, through the intranet, between the application and the accelerators console

11 FRONT PANEL OF THE VI LabView Environment

12 BLOCK DIAGRAM OF THE VI LabView Environment

13 3D-GRAPH

14 CHARACTERIZATION OF THE SYSTEM LINEARITY SIGNAL TO NOISE RATIO SNR vs sqrt(frames) Equazion: y = A*x + B A = ± B = ± R 2 = SNR Experimental data linear FIT sqrt (frames) Linearity in the treatment range (10 20 Gy/min) Castleman says: SNR (N)= N^0.5*SNR(1) Experimental results are in good agreement with theoretical considerations

15 CHARACTERIZATION OF THE SYSTEM CONTRAST SPATIAL RESOLUTION Background subtraction upgrades image contrast of a factor 2 Given: MTF = 0.3 Spatial resolution is : 2.22 mm/lp

16 EXPERIMENTAL RESULTS AND COMPARISONS signal % CCD - DIODE COMPARED PROFILES diode CCD Peculiar parameters of a lateral beam profile, calculated with the two systems,are in a good agreement CCD diode ratio 0 profile ratio distance from axis (mm) X AXIS W50% (mm) H 90/50% Psx (mm) Pdx (mm) CCD 50 FRAMES DIODE DIFF.%

17 EXPERIMENTAL RESULTS AND COMPARISONS

18 CONCLUSIONS AND EXPECTATIONS Comparable results with those obtained through currently used systems Remarkably reduced times of data acquisition and processing Potential employment of the system as a fast and accurate instrument of: beam centering (directly used by cyclotron operators) beam monitoring In the future, the use of thinner scintillating materials is expected, in order to overcome some scattering phenomena which affect spatial resolution.

19 CHARACTERIZATION OF THE SYSTEM MODULATION TRANSFER FUNCTION INPUT MODULATION OUTPUT MODULATION

20 CHARACTERIZATION OF THE SYSTEM MODULATION TRANSFER FUNCTION PATTERN METHOD IDEALLY: SINUSOIDAL WAVES OF SAME AMPLITUDE AND DIFFERENT FREQUENCY ACTUALLY THE IMAGING SYSTEM MODIFIES THE SIGNAL SINUSOIDAL WAVES OF DIFFERENT AMPLITUDE AND FREQUENCY

21 CARATTERIZZAZIONE DEL SISTEMA MODULATION TRANSFER FUNCTION FOURIER TRANSFORM GIVEN A FUNCTION: FOURIER TRANSFORM IS: EXAMPLE: FOURIER TRANSFORM IS: GIVEN THE FOURIER TRANSFORM H(f), THE INVERSE OPERATION, CALLED ANTITRANSFORMATION, GIVES THE BEGINNING FUNCTION: SUM OF INFINITE SINUSOIDAL AND COSINUSOIDAL FUNCTIONS OF DIFFERENT AMPLITUDES AND VARYING FREQUENCIES

22 CHARACTERIZATION OF THE SYSTEM MODULATION TRANSFER FUNCTION SLIT METHOD: IDEALLY: AN IDEAL SLIT CAN BE SEEN AS THE SUM OF INFINITE COSINUSOIDAL FUNCTIONS OF DIFFERENT FREQUENCIES, BUT SAME AMPLITUDE CONSTANT: ACTUALLY: GAUSSIAN CURVE: SUM OF INFINITE COSIDUSOIDS OF DIFFERENT FREQUENCIES AND AMPLITUDES DECREASING WITH FREQUENCY:

23 CARATTERIZZAZIONE DEL SISTEMA MODULATION TRANSFER FUNCTION EDGE METHOD: IDEALLY: ACTUALLY:

24 CHARACTERIZATION OF THE SYSTEM MODULATION TRANSFER FUNCTION Pattern Edge

25 CHARACTERIZATION OF THE SYSTEM CONTRAST Definizione e figura Johns

26 DEVELOPED SYSTEM HADRON BEAM MIRROR 45 SCINTILLATING SCREEN CCD LabVIEW