BRACHYTHERAPY SEED DESIGN

Transcription

1 BRACHYTHERAPY and SEED DESIGN MMII, Richard Taschereau René Roy Jean Pouliot

2 Outline Monte Carlo in radiation therapy Brachytherapy 3 projects permanent prostate implants { temporary prostate implants } { skin applicators investigation }

3 Monte Carlo usage Leitmotiv in radiation therapy «shape the dose distribution to conform to the target» Dose calculations Photons / electrons in heterogenous media Proton / neutron therapy Several Monte Carlo codes EGS4, MCNP, PTRAN, Peregrine GEANT4

4 GEANT4 versions used GEANT4.3.2 GEANT4.2.0 with Low Energy electromagnetic processes

5 Brachytherapy Tiny seeds containing a radioactive isotope A radiation therapy treatment modality 192 Ir 103 Pd 125 I 198 Au 137 Cs Radiation is delivered inside-out rather than the other way around Mostly gamma radiation

6 Brachytherapy Example: prostate implantation Seed implantation can be temporary (a few minutes) or permanent! prostate gland

7 Permanent implant seed design 1

8 Situation Half-life iodine 60 days days 105 palladium Biological effectiveness

9 The project Objective Design a seed with high effectiveness (lower energy photons) combined with a relatively long half-life Method Use 125 I & X-ray fluorescence from shell Try various materials and thicknesses Assessement of biological effectiveness

10 Effectiveness Can be assessed by calculating the point pair distance distribution between ionization locii Monoenergetic G4UserSteppingAction electron track and ionization points ε 1 ε 2 ε 3 Point-pair Post distance processing distribution ε 4 ε5 ε 6

11 Effectiveness Proximity functions Each curve is calculated from the average of 1000 monoenergetic electron tracks t(x) kev/µm kev distance / nm

12 Experiments 39 Z 45 The shell is varied in thickness and composition 20 µm 300 µm

13 X-ray energy Materials 15.0 kev

14 Results Up to 10% improvement 60 µm and molybdenum RBE mm 200 mm 100 mm 60 mm 20 mm 50 mm 150 mm Y Zr Nb Mo Ru Rh Element

15 New seed Molybdenum shell (60 µm) 125 I + Silver core (250 µm)

16 Measurements 125 I seed photon spectrum (kev) with and without a molybdenum wrap Te Ka Mo Ka Mo Kb Te Kb Ag Ka Ag Kb 125 I g

17 Seed/catheter shielding for temporary implantation 2

18 The treatment Remote controlled seed transport system Steel cable seed Temporary catheter implantation Isotope: 192 Ir

19 Prostate Transverse view of the prostate 50 % 75 % 100 % The sensitivity of the rectum limits the dose that can be delivered to the prostate Rectum Prostate

20 The project Objective Protect organs with anisotropic photon fluence Method Shield half of the seed and half of the catheter

21 Model of a 192 Ir seed A set of G4Tubs and G4Cons cable base steel shell tip 192 Ir pellet

22 Photon emission Isotropic Anywhere within the pellet volume... yield (%) according to 192 Ir 10 energy spectrum Energy / kev

23 The design A cross section view of the shielded seed inside the shielded catheter shielded seed shielded catheter Iridium 1.98 mm

24 Setup Large number of photons emitted (10 8 ) Seed and/or catheter transverse bisector plane ~6 h on a 733 MHz PC (corresponds to ~1 ms of irradiation time) World = water basin dose bin

25 Results Dose distribution at 1 cm from the center of the seed. Shielded seed Shielded catheter Normalization is relative to the «front» (90 o ) Shielded seed and catheter 270

26 Skin applicators 3

27 Situation Used to treat small lesions:skin, oral cavity (AIDS related Kaposi sarcoma) Leipzig applicators Has been little investigated

28 The project Objective Analyse the dose distribution (in depth and at the surface) Match and explain actual measurements Origin of electron contamination

29 The project Method Score dose in 1mm bins for the first few cm Tag e - from applicator to identify them

30 Leipzig applicators Stainless steel tube 6 standard applicators Tungsten alloy (92%) seed This is the model for the 1cm straight applicator 1 cm 6 mm

31 Setup Bin height can be 1 mm or 20 µm Collecting plane Cut for e - can be set as low as 5 µm! Water basin Dose bin

32 Dose distribution Depth / mm 0. uncertainty < 2% % 30% 20% 2-5% >10% % - 40.

33 Setup 1) tag e - with their origin when ejected from above water 2) when a tagged e - hits water: score its origin (2) (1) Water basin

34 Electron origin

35 Final word 4

36 Strengths Object oriented design Collaborative effort Learning curve not too steep User support Product in evolution

37 Thanks to UCSF, Dept. radiation oncology, San Francisco, USA Université Laval, Dept. physics, Quebec city, Canada CHUQ, Dept. radiation oncology, Quebec city, Canada Financial support from Natural Sciences and Engineering Research Council of Canada (NSERC) UCSF Prostate Cancer (SPORE grant)