Mosfets

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MOSFET dosimetry in radiotherapy Joanna E.Cygler1 and Paolo Scalchi2 1The Th Ottawa Ott Hospital H it l R Regional i lC Cancer C Centre, t Ottawa, Ott C Canada d 2Department of Medical Physics, San Bortolo Hospital, Vicenza, Italy The Ottawa L’Hopital L Hopital Hospital d’Ottawa Regional Cancer Centre Disclosure The authors have received research support from Thomson-Nielsen, Best Medical Canada and d Si Sicell T Technologies, h l i I Inc. Cygler, MOSFET dosimetry, AAPM Summer School 2009 Outline • Principles of MOSFET dosimetry • Brief description of commercially available MOSFET systems • Dosimetric characteristics of MOSFET detectors – – – – – Temperature p dependence p Energy dependence Dose and dose rate dependence Time dependence (other than the dose-rate dependence) Angular dependence • Advantages and disadvantages • Clinical dosimetry applications • Summary Cygler, MOSFET dosimetry, AAPM Summer School 2009 MOSFET structure • Metal Oxide Semiconductor Field Effect Transistor • Capable of dose measurements immediately after irradiation or can be sampled in predefined time intervals (on-line dosimetry) • Can operate in active (negative bias on gate during radiation) or passive mode Soubra, Cygler, Mackay, Med. Phys. 21(4) 21(4),, 567567-572, 1994 Cygler, MOSFET dosimetry, AAPM Summer School 2009 Threshold voltage shift / ΔVT Before exposure After exposure • VT is a function of absorbed dose • That function is linear when the MOSFET operates in the biased m d during mode d in th the irradiation • Absorbed dose linearity y region increases with the increase of the bias voltage Soubra, Cygler, Mackay, Med. Phys. 21(4) 21(4),, 567567-572, 1994 Cygler, MOSFET dosimetry, AAPM Summer School 2009 Types of MOSFETs available Single bias, single MOSFET • Temperature dependence • Instability of response Dual-bias, dual-MOSFET • Proposed P db by S Soubra, b Cygler C l et. t al. l in i Med. M d Ph Phys. (1994) • Two MOSFETs on same silicon chip operating at two different gate biases • Better sensitivity, reproducibility, and stability than single MOSFET • Minimal temperature effects Unbiased single MOSFET • Temperature dependence • Instability of response, response frequently used as disposable detectors • Shorter linearity range than biased MOSFETs Cygler, MOSFET dosimetry, AAPM Summer School 2009 DualDual -MOSFET MOSFET-dual dual-bias detector Courtesy of Ian Thomson Cygler, MOSFET dosimetry, AAPM Summer School 2009 Commercial MOSFET systems available il bl on th the market k t • BEST Medical (Thomson-Nielsen) – Mobile MOSFET system • S Sicel cel Technologies echnolog es Inc. – OneDose system – DVS (Dose Verification System) Cygler, MOSFET dosimetry, AAPM Summer School 2009 Mobile MOSFET system Reader / bias box MOSFET MOSFET array TN detectors come in two physical sizes: standard and microMOSFETs, see Table 29-I Cygler, MOSFET dosimetry, AAPM Summer School 2009 Wireless setup Cygler, MOSFET dosimetry, AAPM Summer School 2009 Nominal sensitivities in highhighenergy photon beams of various TN detector/bias combinations. combinations TN MOSFET type Bias Nominal sensitivity (mV/cGy) Standard sensitivity Standard 1 Standard Sta da d sensitivity se s t v ty High g 3 High sensitivity Standard 3 High sensitivity High 9 Cygler, MOSFET dosimetry, AAPM Summer School 2009 OneDose MOSFET system OneDose MOSFET buildup cap OneDosePlus MOSFET OneDose reader Cygler, MOSFET dosimetry, AAPM Summer School 2009 OneDose MOSFET detectors Courtesy of Sicel Technologies Dose Verification System Components 11-gauge needle Implantable l bl D Dosimeter Plan and Review Software Reader DVS Reader DVS Database Cygler, MOSFET dosimetry, AAPM Summer School 2009 Courtesy of Sicel Technologies Implantable p MOSFET detector (DVS) ( ) • Electronics assembly contains 2 MOSFETs and support circuitry • Bi-directional antenna coil provides dosimeter power and communications channel • Hermetically sealed in biocompatible compat ble glass capsule • Filled with medical grade epoxy p y Cygler, MOSFET dosimetry, AAPM Summer School 2009 MOSFETS CMRP MOSFET Dosimetry y System y MOSkin detectors, thickness th ckness 0.07 mm, see Table 29-III MOSFET Clinical Dosimetry System: designed and distributed by CMRP Cygler, MOSFET dosimetry, AAPM Summer School 2009 Courtesy of Anatoly Rosenfeld MOSFET calibration • Purpose p – to establish calibration coefficient of the detector D0 (Q) CF (Q)  M det ( D0 , Q) Units: cGy/mV 1 S AD , w (Q)  CF (Q) Units: mV/cGy M det ( D0 , Q)  V ( D0 , Q) D(Q)  CF (Q)  Vth  i k i Cygler, MOSFET dosimetry, AAPM Summer School 2009 Calibration process Calibration process d detectors dosimetric characterization of calibration lib i coefficient ffi i • In principle, the users are responsible for the calibration of new detectors • Some companies, e.g. Sicel sell pre-calibrated detectors. – the user is still responsible for checking the calibration coefficients, so errors are avoided in patient dosimetry Cygler, MOSFET dosimetry, AAPM Summer School 2009 Dosimetric characterization of MOSFET detectors d t t • MOSFET detectors should be fully characterized before use • How it is done, depends on the intended use of the detector • Calibration C lib ti in i full f ll b buildup ild conditions diti s – in phantom, at a standard depth, e.g. dmax, 5 cm, etc. etc – in a linac beam, simultaneous measurement of the detector and ionization chamber signals g Cygler, MOSFET dosimetry, AAPM Summer School 2009 SSD calibration setset-up SSD1 = 80 cm (Co-60) SSD2 = 100 cm (linac) Field Size = 10x10 cm2 MOSFET depth = 5.0 cm Ion chamber depth = 11.3 cm Backscatter = 12.3 cm *Diagrams not to scale Cygler, MOSFET dosimetry, AAPM Summer School 2009 Clinical calibration process • 50-100 MU delivered several times, threshold voltage g recorded before and after each trial • Simultaneous ion chamber measurement used to Q ) for d t min th determine the d dose s D0 ((Q f each h ttrial i l Mraw TG-51 TG 51 Dose @ depth = 11.3 cm D 0 (Q ) CF   V th ( D 0 , Q ) Cygler, MOSFET dosimetry, AAPM Summer School 2009 PDD curves Dose to water at MOSFET location  cGy   mV    Calibration Process for DVS • Calibration performed by the manufacturer f •The response (or radiation sensitivity) of each dosimeter is first determined using a 60Co Sicel 60Co Irradiator source • Calibration is performed in a phantom (“in vitro” ) at body ttemperature mp ratur ((37°C) 7 ) Cygler, MOSFET dosimetry, AAPM Summer School 2009 In Vitro Water Tank Testing In-Vitro Courtesy of G. Beyer Calibration Process for DVS (cont.) • A cumulative dose response Radiation Sensitiv vity (mV/cGy) Lot Dose Response - RADFET Radiation Sensitivity 0.5 0.45 0.4 0.35 0.3 0.25 02 0.2 201 1004 2008 3008 4009 5011 6018 7026 8036 C um ul a t i v e D o se ( c Gy ) Dose response curve calibration curve is obtained for a specific lot by irradiating a statistically significant representative sample from the lot up to 80Gy (maximum dose range of the dosimeter). • Verified by UW-ADCL UW ADCL (sample lot sent for testing) • Calibration is valid for use with daily doses of 150-250 cGy • Reported accuracy for each lot has a calibration certificate with values within: •<5.5% (2σ) up to 20 Gy •<6.5% (2σ) up to 74 Gy (accuracy decreases for doses > 74 Gy). Cygler, MOSFET dosimetry, AAPM Summer School 2009 Courtesy of G. Beyer Correction factors for MOSFETs • Environmental – temperature (no pressure c correction) cti n) • Energy dependence – beam m energy gy – modality (photons, electrons, particles) • Accumulated dose • Dose D rate • Field size • SSD • Directional dependence Cygler, MOSFET dosimetry, AAPM Summer School 2009 Temperature dependence • TN dual-MOSFET-dual-bias detector – temperature p independent p • Other currently available MOSFET detectors need corrections to be applied when used at a temperature different from the one at calibration Cygler, MOSFET dosimetry, AAPM Summer School 2009 Temperature dependence DVS dosimeter is approximately 3.3% more sensitive (higher dose reading g for same applied pp dose)) when irradiated at 37oC vs. 23oC 23C 0.46 Average sen nsitivity (mV/cGy) The DVS is calibrated at 37oC for use at body temperature. 0.47 37C 0.45 Linear (23C) 0.44 0.43 0.42 0.41 0.40 0.39 y = -1.0054E-02x + 4.5635E-01 0.38 0.37 0 1 2 3 4 5 6 Irradiation session # A multiplicative correction factor of 1.033 can be used for room temperature phantom measurements Courtesy of G. Beyer Cygler, MOSFET dosimetry, AAPM Summer School 2009 7 8 MOSFET energy dependence Wang et al, MC, 2005 Air-kerma sensitivity Edwards et al 1997 Wang et al MC 2005 Edwards et al, 1975 Kron et al 1998 Air-kerma sensitivity Kron et al, 1998 Absorbed dose ensitivity photon energy p gy / keV Cygler, MOSFET dosimetry, AAPM Summer School 2009 Wang et al Radiat. Prot. Dos. 2005 MOSFET energy dependence TNTN -502 RDM Calibration Factor (c cGy/mV)) 1.09 1.07 1.05 1.03 1 01 1.01 0.99 0.97 0.95 C0-60 4 MV Beam Energy Cygler, MOSFET dosimetry, AAPM Summer School 2009 6 MV Courtesy of T. Woods Energy dependence - TN TN-502 502-RD * experiment MC high energy X-rays ● MC mono-energetic beams Panettieri et al Phys Med Biol. 52(1):303-16.2007 Cygler, MOSFET dosimetry, AAPM Summer School 2009 Absorbed dose linearity – dual MOSFETMOSFET -duald l-bias dual bi 6 MV Ramaseshan et al, Phys. Med. Biol. 49 , 4031– 4048, 2004 Cygler, MOSFET dosimetry, AAPM Summer School 2009 Consorti et al, Int. J. rad. Onc. Biol. Phys. 63, 952-60, 2005 Effect of accumulated dose Ramani et al, al Int. J. Rad. Onc. Biol. Phys., 37, 959-64, 1997 Cygler, MOSFET dosimetry, AAPM Summer School 2009 Effect of Eff f accumulated m dose – unbiased MOSFET - DVS Rad diation Sen nsitivity (mV/cGy y) Lot Dose Response - RADFET Radiation Sensitivity 0.5 0.45 0.4 0.35 0.3 0.25 0.2 201 1004 2008 3008 4009 5011 C u m u l a t i v e D o se ( c Gy y) Cygler, MOSFET dosimetry, AAPM Summer School 2009 6018 7026 8036 Directional dependence • Ideally it should be isotropic • In p practice,, angular g response p of the detector depends on - its design • It can be different for different energies • Should be evaluated in-phantom to derive correction factors Cygler, MOSFET dosimetry, AAPM Summer School 2009 Directional dependence of microMOSFET Rowbottom & Jaffray. Med. Phys. 31, 609-15, 2004 Cygler, MOSFET dosimetry, AAPM Summer School 2009 Directional dependence phantom Example of a spherical phantom that can be used to p measure angular dependence of MOSFET response Cygler, MOSFET dosimetry, AAPM Summer School 2009 Directional dependence Energy: 100 kVp , FS:10x10 cm2 ,FSD:50 cm, Depth:1.5 cm Xray Tube Rotating insert Relaative re sp o on se 1.10 1.05 1.00 0.95 0.90 0.85 0.80 3 cm Polystyrene phantom (25 cm x 25 cm x 3 cm ) Cygler et al, Radiother. Onc. 80, 296–301, 2006 Cygler, MOSFET dosimetry, AAPM Summer School 2009 0 30 60 90 120 150 180 210 240 270 300 330 Angle / deg Isotropic within 2.5% (1SD) Time dependence - Cyberknife • Cyberknife: long treatment times • High High--dose DVS dosimeter initial design significant g dependence p of response p on irradiation time: 11-h and 2.5 2.5--h multiple irradiations caused mean overover-responses of 4% and 8%, respectively, when compared to a single irradiation of 1.5 min. • Improved p DVS design g - no time dependence p (response within 2 %) Cygler, MOSFET dosimetry, AAPM Summer School 2009 MOSFET detectors Advantages vs. vs disadvantages Advantages dvantages • Very small active volume • Dual-MOSFET-dual bias system eliminates most correction factors • Instantaneous readouts (on-line dosimetry) • Permanent dose storage g (Can be read multiple times) • Waterproof • Efficient in use Cygler, MOSFET dosimetry, AAPM Summer School 2009 Disadvantages g • Finite lifetime(~100 Gy) • Energy dependence • Temperature dependence for single-MOSFETdetector • Sensitivity change with accumulated dose for unbiased MOSFETs Clinical applications • In-phantom measurements – Build-up curves for high energy photon beams – Interface dosimetry – Small field output factors (radiosurgery) • In-vivo dosimetry – External beam: entrance and exit doses, skin dose, peripheral dose, tumor dose – TBI – IMRT – IGRT – IORT – brachytherapy Cygler, MOSFET dosimetry, AAPM Summer School 2009 Build-up depth dose curves 6 MV Build2 SSD=100 FS 10 10 cm2, FS=10x10 SSD 100 cm MOSkin chip developed at CMRP allows measurements of the surface dose at a depth of 0.07mm Courtesy of Anatoly Rosenfeld Cygler, MOSFET dosimetry, AAPM Summer School 2009 Entrance and exit dosimetry • Use of build-up p caps p is recommended • OneDose Plus detectors – come equipped with caps • TN MOSFETs – one can fit inside special buildup caps Cygler, MOSFET dosimetry, AAPM Summer School 2009 Radiosurgery dosimetry o u ttp u t faccto r 1 0.9 0.8 clinical li i l 0.7 MOSFET 0.6 0 20 40 60 80 100 cone size i / mm Cygler, MOSFET dosimetry, AAPM Summer School 2009 Courtesy of J. Wojcicka IMRT in vivo dosimetry y Marcie et al, Int. J. Rad. Onc. Biol. Phys., 61, 1603–6, 2005 Cygler, MOSFET dosimetry, AAPM Summer School 2009 • In some cases cases, BB BB’ss placed on top of patient’s mask for original CT Results from in vivo measurements Tomotherapy h Example: Patient E Cherpak et al Radiother. Oncol. 86, 242-250, 2008 Cygler, MOSFET dosimetry, AAPM Summer School 2009 IORT Pancreas treatment with Novac7 Consorti et al, Int. J. rad. Onc. Biol. Phys. 63, 952-60, 2005 Cygler, MOSFET dosimetry, AAPM Summer School 2009 Detector calibration for brachytherapy -TG TG-43 formalism  D(r,)  SK G(r,) / G(r0 ,0 ) g(r)F(r, ) •Solid Solid water phantom Reference distance 1 cm  D( r ,  )  S K  f cal Dose (cGy )  Vth (mV ) 5.5 cm MOSFET 1.0cm 125 I (6702) Source 6 cm Solid W ater Energy Dependence: Sensitivity (25I/ 60Co) ~ 3 Cygler et al, Radiother. Onc. 80, 296–301, 2006 Cygler, MOSFET dosimetry, AAPM Summer School 2009 In Vivo Measurements – prostate implants Setscrew seed 1 2 3 4 US probe MOSFET position MOSFET •MOSFET reading taken every 1cm Cygler, MOSFET dosimetry, AAPM Summer School 2009 Fluoroscopy image of the prostate after implant 7 Use of MOSFET detectors during prostate implant i l procedure d calculated l l t d iinitial iti l pre-plan l () measured d postt iimplant l t (()) 12 Initial Dose Rate (cGy//h) 150% mPD 10 8 mPD 6 Prostate Base 4 90% mPD Prostate Apex Prostate length g = 50 mm 2 0 0 10 20 30 40 50 60 70 Distance (mm) 80 90 100 110 Cygler et al Radiotherapy and Oncology 80: 296-301; 2006 Cygler, MOSFET dosimetry, AAPM Summer School 2009 BNCT at BNL medical research reactor t Re elative boron dose 1.00 0.80 0.60 0.40 0.20 0.00 0 2 4 6 8 Depth in phantom (cm ) 10 12 Thermal neutron depth dose distribution in a perspex phantom in a BNCT epithermal neutron beam facility at BNL obtained with paired MOSFET detectors with 10B converter. converter Subtracting on on-line line the response of paired MOSFET eliminate effect of gamma dose. Cygler, MOSFET dosimetry, AAPM Summer School 2009 Courtesy of Anatoly Rosenfeld Conclusions • MOSFET detectors are very useful for dosimetry, dosimetry especially – High dose gradient fields • Accurate if properly characterized and used • Phantom (in vitro) dosimetry • In vivo dosimetry in external beam and brachytherapy treatments m Cygler, MOSFET dosimetry, AAPM Summer School 2009 Acknowledgements • • • • • • • Anatoly Rosenfeld Ian Thomson Nuria Jornet G. Beyer, y , Sicel Technologies g Inc. A. Hallil, Best Medical Canada Amanda Cherpak Tara Woods Cygler, MOSFET dosimetry, AAPM Summer School 2009 Thank you Merci Cygler, MOSFET dosimetry, AAPM Summer School 2009