E x p er i m en tal ev al u ati o n s o f M O S F E T d etec to r i n a 62 A M ev p r o to n b eam f o r c l i n i c al ap p l i c ati o n s

Transcription

1 AZI E NDA OSPE DAL I E R O-UNI VE R SI TAR I A POL I C L I NI C O G. R ODOL I C O UNI V ER SI TA DE G L I STUDI DI C ATANI A I S T I T U T O NA Z I O N A L E D I FI S I C A NU C L E A R E L A B O R A T O R I NA Z I O N A L I D E L SU D - C A T A N I A C E N T R O SI C I L I A N O F I SI C A N U C L E A R E E ST R U T T U R A D E L L A M A T E R I A - C A T A N I A E x p er i m en tal ev al u ati o n s o f M O S F E T d etec to r i n a 62 A M ev p r o to n b eam f o r c l i n i c al ap p l i c ati o n s S. Pittera, G.A.P. Cirrone, G. Cuttone, S. Lo Nigro, L. Raffaele, V. Salamone, L. Valastro QuickTimeª e un decompressore sono necessari per visualizzare quest'immagine. 11th Topical Seminar on Innovative Particle and Radiation Detectors 1-4 October 2008 Siena, Italy

2 Summary Physics of MOSFET detectors Treatment proton beam line Characterization of the detector in the proton beam line

3 M OSFE T and microm OSFE T M etal Oxide Semiconductor Field E ffect Transistor W h y a M O S F E T an d m i c r o M O S F E T d o si m et er : S tab l e, r ep r o d u c i b l e, p er m an en t sto r ag e; L i n ear v s ad so r b ed d o se, n o f l u en c e p er tu r b ati o n ; D o se r ate, tem p er at u r e i n d i p en d en t ; P r ati c al o n p ati en t an d easy t o u se, o n l i n e r ead i n g, S m al l si z e, i m m ed i ate r eu se, p o r tab l e ID EA L FO R SM AL L FI E L D DOSI M ETR Y

4 M OSFET M etal Oxide Semiconductor Field E ffect Transistor S c h em ati c c r o ss sec ti o n p -c h an n el M O S F E T d etec to r : R eg i o n ac ti v e: M O S str u c tu r e (M etal -O x i d e-s em i c o n d u c to r ) N o i r r ad i ati o n, n eg ati v e p o l ar i z ati o n o f G ate (V G ) w i th r esp ec t S i b u l k su b str ate p er m i t th e f o r m ati o n o f p -c h an n el c o n d u c ti o n b etw een S o u r c e an d D r ai n c o n tac t. E l ec tr i c P ar am eter o f o b ser v ati o n : Threshold Voltage Vth : G ate V o l tag e t o p er m i t a c o n stan t c u r r en t I D S i n th e p c h an n el, i f V D S i s f i x ed.

5 I R R ADI ATI ON OF DOSI M ETER : I nteraction of incident ionizing radiation with active region (SiO 2) Positive Polarization Bias G ate V oltage VI C r eati o n el ec tr o n -h o l e p ai r s i n si l i c o n d i o x i d e: B u i l d -u p o f tr ap p ed c h ar g e i n th e o x i d e I n c r ease i n th e n u m b er o f i n ter f ac e S i O 2 /S i tr ap s I n c r ease i n th e n u m b er o f b u l k o x i d e S i O 2 tr ap s E l ec tr o n, q u i c k l y m o v e t o w ar d c o n tac t b i ased p o si t i v el y ; so m e f r ac ti o n w i l l r ec o m b i n e w i t h h o l es, d ep en d i n g o n t h e f i el d el ec t r i c ap p l i ed an d th e en er g y an d k i n d o f th e i n c i d en t p ar t i c l e; H o l es u n d er g o a st o c h ast i c h o p p i n g tr an sp o r t t h r o u g h t h e o x i d e an d, c l o se th e S i O 2 -S i i n t er f ac e, ar e c ap tu r ed i n l o n g -t er m tr ap p i n g si tes.

6 EFFEC T ON THE THR ESHOL D VOL TAG E V th T h e h o l es t r ap p ed r ed u c i n g th e c u r r en t f l o w i n t h e p -c h an n el, f o r el ec tr o stati c c o u l o m b i an r ep u l si o n. N E G A T IV E SH IFT T H R E SH O L D V O L T A G E V th Vth = Vot + Vit = Qot Qit Cox Cox T h e v o l tag e sh i f t i s p r o p o r t i o n al to th e to tal q u an ti ty o f t r ap p ed c h ar g ed i n th e o x i d e Q o x an d i n th e i n ter f ac e Q i t w i c h i s p r o p o r ti o n al to ab so r b ed d o se. (C o x o x i d e c ap ac i ty ) T h e g ate v o l tag e af ter ex p o su r e i s, i n ab so l u te v al u e, g r eat er w i th r esp ec t b ef o r e ex p o su r e.

7 M OSFET R ESPONSE VS DOSE Vth ( D) = α VI (1 e β D ) In radiotherapy range: Vth(D) = α β VI D k = αβvi sensitivity o f d etec to r. I t d ep en d s o n th e B i as S u p p l y V I, o n th e th i c k o x i d e l ay er t o x an d tr ap p i n g p ar am eter s, i.e. tr ap su p er f i c i al d en si ty N o x. D etec to r S en si ti v i ty S atu r at i o n w i t h B i as V o l tag e: Vth proportional to dose up to 100 Gy

8 INSTRUMENTATION A u to sen se P ati en t D o se V er i f i c ati o n S y stem (T N -R D 1 3) D ual Sensitivity Bias Supply (u p to 5 M O S F E T ) 2 S en si ti v i ty : High e Standard M O S F E T T N -502R D an d m i c r o M O S F E T T N -502R D M AutoSense PCSoftware V. 1.1

9 PROTON BEAM FOR RADIOTHERAPY MAXIMUM DOSE AT DEPTH (BRAGG PEAK) LOW ENTRANCE DOSE (SKIN SPARING) RAPID DISTAL DOSE FALL- OFF LOW INTEGRAL DOSE SMALL LATERAL PENUMBRA INCREASE OF DOSE TO TUMOUR (TCP>) TCP> FOR AN EQUAL DOSE TO SURROUNDINGS HEALTHY TISSUE (NTCP ) REDUCE DOSE TO HEALTHY TISSUES (NTCP<) FOR THE SAME DOSE TO THE TUMOUR (TCP )

10 PASSIVE RANGE MODULATION SYSTEMS FOR PROTONS AND LIGHT IONS SOBP RELIES ON TIME MODULATION PRESENTING DIFFERENT THICKNESESSES TO BEAM FOR DIFFERENT FRACTIONS OF IRRADIATION TIME WEEL

11 THE ONLY ITALIAN PROTON THERAPY FACILITY

12 In Catania we developed a facility (CATANA) for the treatment of ocular tumours with 62 AMeV proton beams

13 Centro di AdroTerapia ed Applicazioni Nucleari Avanzate 62 MeV proton beam Range 3 cm; Ocular tumor treated (90% Uveal Melanoma) 170 patients treated from 2002 up to now; Total dose 60 CGE in 4 days;

14 DOSIMETRIC CHARACTERIZATION OF MOSFET ON CLINICAL PROTON BEAM Riproducibility in High Sensitivity Fading Linearity Sensitivity dependence with the accumulated dose Energy dependence Dose rate dependence Output Factor in small fields

15 Riproducibility in High Sensitivity Irradiation setup Full energy beam Isocentro 100 cgy Fantoccio XWU-IMRT e 200 cgy MOSFET Collimator: φ=25 mm Lato piatto esposto al fascio, strato epossidico nell incavo del fantoccio 5 irraggiamenti consecutivi, intervallati da 1 minuto di attesa per evitare effetto Creep-up

16 Riproducibility in High Sensitivity Detector MOSFET 3303 MOSFET 3304 Dose [cgy] Riproducibility 1σ Riproducibility 1σ T&N (60Co) 100 2,03 % 3,33 % 0,71 % < 1,2 % 200 0,52% 0,75% 0,73% 0,93% micromosfet 0132 MOSFET 3302 MOSFET 3303 MOSFET 3304 micromosfet 0110 * dopo valutazione fading < 0,8 %

17 Fading MOSFET at the entrance of a PMMA phantom, full energy beam, φ=25mm 1,040 Reading relative to 10 s 1,020 Negligible fading 1,000 0,980 Best reproducibility if the reading is always at the same time after irradiation 0,960 0,940 0,920 normalizzata a 10 s Lettura 0, tempo della lettura dopo l'irraggiamento [s] Timeprima after irradiation (s) We choose to read 10 s after the irradiation

18 Linearity Detector at the entrance of PMMA phantom

19 Sensitivity with accumulated dose 200 cgy, full energy beam, Φ=25mm, PMMA phantom, entrance MOSFET ,02 Decresing of sensitivity due to the radiation damage in the proton beam MOSFET 3302 MOSFET MOSFET 3304 micromosfet ,98 micromosfet ,96-3,0% 0,94-4,8% -6,2% 0,92-6,4% -9,8% 0,9 Sensibilitˆ relativa 0,88 0,86-14,3% 0, Dose accumulata [mv] It is needed to recalibrate

20 Energy Dependence 5 points of measurement in a modulated Bragg peak; MOSFET measurements in the 5 points: 200 cgy in High Sensitivity S= x x x x Dose (%) Absolute Dosimetry in each point with Markus chamber in a PMMA phantom; mean reading MOSFET [mv] Markus Dose [cgy] Rres Zref x Rp(10% ) Depth in water (mm) (E P )eff b = a ( Rres ) + c ( Rres ) : Rres = Rp(10%) - zref a = , b = , c =

21 Energy Dependence 2,20 2,00 R2 = 0,93 1,80 1,60 MOSFET High Sensitivity 2 R = 0,89 1,40 MicroMOSFET High Sensitivity 1,20 MOSFET1 Standard Sensitivity 1,00 MOSFET2 Standard Sensitivity R2 = 0,99 0,80 0,60 Sensibilitˆ [mv/cgy] R2 = 0,99 0,40 0,20 0, Energia Effettiva [MeV] Calibration of MOSFET in every modulated beam configuration

22 Energy Dependence BRAGG PEAK FULL ENERGY WITH MOSFET MOSFET IN STANDARD SENSITIVITY MOSFET φ = 25 mm, PMMA phantom, plastic water LET dependence Peak - Plateau ratio: 2,1 (4,5 Markus)

23 Dose Rate Dependence Beam current : 1 7 na Evaluation of the MOSFET signal with respect of the Markus Chamber signal Grafic of the Markus Signal MOSFET output Markus Chamber zref = 5 mm H2O MOSFET at the entrance full energy beam

24 Dose rate Dependence 270 Dose rate in the ocular treatments 267,5 265 (15 40 Gy/min) Comparable with the reproducibility of detector 262, ,5 255 Markus/MOSFET signal [nc/mv] 252, Dose Rate [Gy/min] Max Var. (7 Gy/min) 2%

25 Output Factor for small field MOSFET e micromosfet in a modulated Bragg (zref=8,7mm PMMA) 3 irradiations with 200 cgy. MOSFET underevaluetes up to 9% 110 OF(φ) = 105 L (φ) L (φ =25) 100 ) [υνιτ ρελατιϖε] φ 95 Σι διοδε ΣΧΑΝ ΙΤΡΟΝΙΞ Γ αφχηροµ ιχ Μ 55 2 ΜΟΣΦΕΤ ΤΝ 502Ρ 90 µ ιχρομοσφετ Output Factor ( Diametro Collimatore [mm] In routine micromosfet used to evaluate Output Factor in fields 120 mm2 or with a diameter < 6 mm

26 Conclusion 100 cgy Riproducibility > 1,2% Negligible Fading Sensitivity dependence with accumulated dose Energy dependence (LET), calibration in every modulator configuration. No dose rate dependence Output Factor measured with micromosfet

27 THANK YOU