DEVELOPMENT OF A PRIMARY STANDARD IN TERMS OF D w FOR 125 I BRACHYTHERAPY SEEDS

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1 DEVELOPMENT OF A PRIMARY STANDARD IN TERMS OF D FOR 25 I BRACHYTHERAPY SEEDS I Aubineau-Lanièce, P Aviles-Lucas 2, J-M Bordy, B Chauvenet, D Cutarella, J Gouriou, J Plagnard CEA, LIST, Laboratoire National Henri Becquerel, 99 Gif-sur-Yvette Cedex, France 2 CIEMAT, Laboratorio De Metrología De Radiaciones Ionizantes, Avda Complutense, Madrid, Spain This study is part of the European Joint Research Project T2J06 on "Increasing cancer treatment efficacy using 3D brachytherapy"

2 Contents JRP6 objective to develop a primary standard in D W The LNE-LNHB experimental set-up Design and characterization in current and volume Correction factors

3 Primary standards for LDR brachytherapy Use of the AAPM protocol, folloing the TG43 formalism R ( m) Dose-rate constant, Λ D ater ( 0 % at patient level JRP6 Increasing cancer treatment efficacy using 3D brachytherapy D ater ( 5 % at patient level D ater ( 2 % ithin reference conditions JRP6 partners involved in developing a primary standard for LDR BT sources: PTB, ENEA-INMRI, LNE-LNHB

4 Objective: Determination of D (cm, 90 ) around a single LDR source D?? Air cm 30 cm

5 Objective: Determination of D (cm, 90 ) around a single LDR source D?? Air cm 30 cm

6 Objective: Determination of D (cm, 90 ) around a single LDR source D?? Air cm 30 cm

7 Objective: Determination of D (cm, 90 ) around a single LDR source Vacuum cm

8 Objective: Determination of D (cm, 90 ) around a single LDR source Vacuum cm

9 Objective: Determination of D (cm, 90 ) around a single LDR source Vacuum cm

10 Objective: Determination of D (cm, 90 ) around a single LDR source col, ( cm,90 ) Vacuum cm Back scattering correction factor needed! F bscatt ( cm,90 ) = (cm,90 ) D col, F bscatt

11 Objective: Determination of (cm, 90 ) around a single LDR source col, col, phant Vacuum cm col, = col, phant Fphant

12 col, phant Vacuum P PMMA ( µ / ρ) en 063 Air P 2 r P / r ' P 2 ( 600) Vacuum col, ref Vacuum P P col, phant = col, ref F med _ geo

13 col, ref Objective: Determination of around a single LDR source col, phant d vacuum col, ref col, phant d col, V : collection volume V: interaction volume (included in the collection volume) To difficulties to overcome: the equatorial anisotropy of emission of the LDR brachytherapy seeds the small subtended solid angle that causes a poor S/N ratio

14 col, ref Objective: Determination of around a single LDR source col, = i k i e ρ ( g) I V col, phant col, phant d col, d vacuum col, ref = col, F ref col, ref F ref

15 30 cm D ( cm,90 ) = F F F bscat phant med _ F geo ref i k i e ρ ( g) I V

16 Signal to noise and volume characterization I Noise = fe 0-4 A I Source(0,5- GBq) = fe 0-2 A S B u( I ) 00, I 02% Hexagon Metrology (LNHB) Robot alloing to locate the probe position in space Electrodes have been added to straighten up the field lines and homogenize the electric field ithin the collection volume (Quickfield 5) Probe localized on the point to locate in space Interaction volume V = (233 ± 22) cm 3 (093 %)

17 Correction factors assessed by MC calculations for I25S6 Bebig seed Bebig I25S6 photon emission spectrum (Ag vs I/Te) BEBIG I25S6 [TG 43, Hedtjärn et al] Characteristic x rays originating from the silver (AgI) ere considered as initial emission ithin the source: ( ( + α, Ag α2, Ag 2 α, Te + α2, Te ) = ) 50 (5 %) Numerical modeling using the PENELOPE code of the experimental set-up Ag Te I Energy (kev) ,202 27,4726 3, 3,76 35,499 Intensity Energies and intensities of photons taken into account ithin simulation

18 Correction factors Recapitulation and first estimate D = F F F bscat phant med _ F geo ref i k i e ρ ( g) I V vacuum vacuum Vacuum Air Vacuum Air vacuum vacuum col, Correction Mean value u rel (%) F F ref med geo F phant F bscat Note: Statistical uncertainties, do not include uncertainties in cross section or geometry

19 Correction factors - Recapitulation D = F global I e ρ ( g) V i k i col, F global = 209 ± 3 (0%) (only statistical uncertainty)

20 First estimate of the uncertainty budget D = F global I e ρ ( g) V i k i Quantity Relative uncertainty (%) F global 0 I 02 (W/e) 0004 ρ 0008 V 093 k pol k rec k trans (only statistical uncertainty) u( D ) ( D ) = u( D D ) = 0 % u + ( F F ( ) 2 B type global global ) 2

21 Conclusion - LNE-LNHB : free-in- ionization chamber - PTB and ENEA-INMRI : extrapolation chambers Those devices and methods are different This ill enhance the quality of the knoledge of the final dose-to-ater estimate by providing different ays of identifying possible biases introduced by the different methods studied

22 Thank you for your attention