Optimisation of eye plaque dosimetry using Monte Carlo method. J. Green, D. Cutajar, S. Guatelli, & Rosenfeld, A.B.

Transcription

1 Optimisation of eye plaque dosimetry using Monte Carlo method J. Green, D. Cutajar, S. Guatelli, & Rosenfeld, A.B.

2 Cancer of the eye is a rare and challenging disease Uveal melanoma is the most prevalent of ocular malignancies in adults Australia holds one of the highest rates of incidence worldwide In new cases in NSW 1 Projected outcomes set to rise to 100 cases per year in the new decade 1 Intraocular melanoma: Iris with nodular melanoma (left) and large choroidal melanoma (right) 2

3 Survival is generally poor Reported 25% five-year mortality 3 Metastasis is the main cause of death Almost 50% of patients will lose vision or the eye due to the disease and/or the treatment

4 Ocular melanoma is typically treated by resection of the surgical mass or for progressed large tumours, via enucleation (removal of the eye) Phototherapy Stereotactic radiosurgery Radiotherapy: - Proton Therapy - Episcleral eye plaque brachytherapy is the preferred treatment method

5 Surgical insertion (suturing) of a radioactive plaque to the sclera, close to the tumour Good results in terms of tumour control as well as improved vision retention I-125 is the most widely used isotope in eye brachytherapy 4 Survival rates for patients treated with I-125 plaque brachytherapy are high and compare favourably to those of external beam radiotherapy and enculeation Ru-106 is preferred for treatment of small tumours - 90% tumour control rate 5

6 I mm ROPES plaque Ru Bebig applicators

7 TLDs Extrapolation Chambers Film Fibre Optic Scintillators

8 I-125 Brachytherapy Source AAPM Task Group No. 43 (TG43) Report 6 Air Kerma Strength, Geometry, Radial Dose and Anisotropy Functions

9 Ru-106 Brachytherapy Source NIST traceable calibration protocol for dose rate calculation in Ru-106 eye plaque brachytherapy BEBIG supply the 11 point Protocol of Measurements 7 - depth-dose curve of 11 points from 0.6mm, then 1 10mm from the centre of the plaque surface - 33 dose rate points

10 Plaque Simulator (BEBIG GmbH, Berlin, Germany) Interactive 3D simulation of desired treatment plan A variety of sources and plaque types are available Integrates 3D CT information of the eye Allows the selection of any seed configuration and activity, providing depth dose and isodose curves

11 TG43 Protocol - the quality of the radial dose function fit for calculating dose data is reduced for values of r 0.5cm 8 - non-physical fluctuations and significant inaccuracy for values of r 0.25cm 8 - single Amersham Health model 6711 source data for radii between 0.5cm and 7cm Bebig Protocol of Measurements - measuring 1mm away from the surface of the source decreases resolution

12 Steep dose gradients Surrounding critical structures TPS inaccuracy around border of eye 9 Variations in seed activity Detector size

13 Three-dimensional semiconductor detectors such as the Medipix2 have been used successfully to provide high spatial resolution X-ray imaging Radiography, neutronography, and micro-tomography Single-particle-counting pixel detector with a high detection efficiency and an unlimited dynamic range 256 x 256 sensitive silicon pixel array real time intensity read-out

14 Three-dimensional dosimetric images of the dose distribution from a 15mm ROPES plaque loaded with I-125 seeds and the Ru-106 CCD plaque obtained using the Medipix2 detector placed below the plaque within a tissue equivalent phantom of eye matching dimensions 10

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16 The CMRP have developed dosimetry software, EyeCan, based on TG43 data, which provides: - Analytical calculation of the Medipix 2 output - Cross sectional dose colour map - Depth dose distribution - Isodose map We need to validate the dosimetry using Geant4

17 Monte Carlo methods of dosimetry: - handle complex geometry of the brachytherapy sources - can achieve even distribution of radioactivity from the source Both I-125 and Ru-106 Monte Carlo dosimetry have been performed with good results new experimental methods such as the Medipix2 developed at CMRP, necessitate Monte Carlo methods to verify dosimetry Geant4.9.3 Monte Carlo Toolkit for dose optimisation?

18 The ultimate goal of this research and the collaborative research of CMRP, is to provide eye plaque quality assurance in a clinical setting in four refined steps: 1. Treatment planning based on three-dimensional (3D) imaging 2. Customised eye plaque preparation according to treatment plan 3. Dose calculation using TG43 and Monte Carlo derived data i 4. 3D dosimetric imaging with Medipix2 for quality assurance of the treatment plan and dose verification Imaging Plaque selection TPS(TG43/MC) QA (Medipix2)

19 Monte Carlo simulation of eye plaque brachytherapy sources using GEANT4 to obtain comprehensive dose data for TPS optimisation Simulation 1: I-125 single Amersham 6711 seed Simulation 2: Ru-106 Bebig CCD plaque

20 Monte Carlo simulation techniques were used to model a single Amersham Health model 6711 I-125 seed in water using the Geant4.9.3 Monte Carlo toolkit Obtain TG43 parameters

21 Cylindrical geometry Source encapsulated within a titanium shell Fattened endcaps as opposed to curved (Williamson et al. 1988) 11 Set of concentric cylinders with an inner silver rod

22 Eye diameter = 24mm Radial distances ranging from 1mm 25mm Sets of 25 concentric toroids Positioned every 1mm at an angle, θ, from the centre of the source (assuming radial symmetry), every five degrees

23 Geant4 visualisation of Monte Carlo simulation of model 6711 seed showing inner silver core (magenta), air gap (blue) and titanium shell (silver), and set of 50 concentric scoring toroids (red) positioned 1mm apart

24 To measure at zero degrees,scoring toroids were replaced with solid cylinders of varying radius and thickness to score the dose over the 25mm range Larger radius cylinders used for 1cm and greater distances Geant4 visualisation of Monte Carlo simulation of single 6711 seed with scoring tubes (red) positioned every 1mm on the source central axis

25 Random energy based on spectra Photon energy (kev) Photons per disintegration I-125 is a low energy photon emitter Half-life of 59.4 days 6 G4EMLOW.6.9 low-energy data set Range Cuts - 1μm

26 Generate random coordinate between zero and 2AE + AS Cylindrical Geometry considered for random particle generation Random Coordinate: - side of cylinder - two endcaps The y coordinate was defined by rcosθ and the x coordinate by rsinθ

27 Geant4 visualisation of Monte Carlo simulation of model 6711 seed

28 The results of the I-125 simulation were used to calculate the TG43 dosimetric parameters and were in good agreement with the published data Radial dose functions over the scoring range fell within 3% with a maximum deviation from the TG43 data of only 2.8% (occurring 1mm away from the source on the plaque central axis) Anisotropy functions were obtained within 5% uncertainty for all polar angles

29 g(r) 0.6 MC TG r/cm Monte Carlo derived radial dose function demonstrating the depth-dose delivered on the central axis compared with TG43 data

30 1.2 1 Anisotropy Function r = 0.5 (TG43) r = 1cm (TG43) r = 2cm (TG43) Poly. (r = 0.5cm (MC)) Poly. (r = 1cm (MC)) Poly. (r = 2cm (MC)) Polar Angle (deg) Monte Carlo anisotropy functions compared with tabulated data provided by the TG43 protocol

31 Upturn in dose from 90 to 70 degrees about the source for small values of r Lower anisotropy functions at small values of theta Attenuation and geometry effects seed endcaps 12 Seed geometry assumed by MC code did not have curved endcaps nor did it consider generating from beneath the silver surface of the inner rod Results verify the suitability of Geant4 based dosimetry methods for I-125 eye brachytherapy dose optimization

32 The BEBIG CCD Ru-106 plaque was modelled using the Geant4.9.3 toolkit BEBIG Ru-106 CCD plaque Two Monte Carlo simulations to obtain: 1. depth dose data on the central axis for calibration 2. dose rate array spanning the entire volume of the eye for future input into treatment planning software

33 Plaque Geometry: - Rim diameter = 17.9mm - Height = 4.3mm - Radius of curvature =12mm Silver spherical cap : - Inner radius = 12.1mm - Inner height = 4.3mm - Two eyelets excluded - 0.1mm added to radius to account for silver window

34 Calibration simulation performed using a series of scoring cylinders (G4Tubs) 0.5mm thick and 0.2mm in radius positioned at 90 degrees, 1mm from the centre of the plaque Total of 25 cylinders, positioned 1mm apart along plaque central axis to provide comparison data with the BEBIG Protocol of Measurements

35 Sets of 25 concentric scoring rings were positioned at a variable distance along the central axis of the plaque Rings were 0.2mm thick and of radius ranging from 1mm 25mm When combined with the central axis calibration data, a total of 650 sampled regions encompassed the eye

36 Ruthenium-106 decays by beta emission via Rhodium-106 Max energy keV Rhodium-106 half life - only 30seconds Main electron contribution from the decay of Rh-106 Photon contamination contribution from Rh-106 is significant Rh-106 beta energy emission: - max 3.54MeV - average 1.41MeV *Decay data for Ru-106 from the National Nuclear Data Center (2001) was obtained from Appendix A of the ICRU Report 72 (2004) (see Tables 4 & 5) 13

37 Spherical cap uniformly coated with Ru-106 Particles may be emitted from any point on surface Surface area is proportional to h Random Height: - random height, z generated between zero and h - consider series of rings located at any point, z - ring radius determined by height in plaque Random Coordinate: - generated at any point x,y on ring - determine radius of ring

38 Geant4 visualisation of Monte Carlo simulation of Ru-106 plaque showing silver curved surface

39 Geant4 simulation successfully generated particles from a uniform distribution of Ru-106 on the surface of the plaque Scoring over wide range was possible 650 dose point array obtained Monte Carlo uncertainty within 10% for small distances Improved statistics required for large distances

40 Reletive Dose 10 1 MC Bebig Depth (mm) Monte Carlo derived Ru-106 CCD plaque central axis depth dose compared with central axis dose generated from the Plaque Simulator

41 Monte Carlo results differed considerably from TPS generated depth dose data Greatest deviation occurring at very close distances to the plaque surface A significantly higher dose at small distances (r 1cm) in comparison with the TPS results, likely due to: - source thickness - depth of particle generation

42 Simulation generated particles from the surface of an infinitesimally thin layer of silver 0.1mm beneath the plaque surface (Bebig) Astrahan (2003) determined that the radioactivity of the plaque is a distribution through 0.2mm of the silver Indicates a mm thick gap between where the radioactive part begins and the edge of the plaque itself

43 Cross et al. (2001) 14 performed Monte Carlo dosimetry of various Ru-106 plaque types and compared the results with experimental measurements performed in Soares et al. (2001) 15 Findings: - planar source results within 10% - curved plaque results differed by about 30% Conclusions: - while limited, experimental approach is more accurate - likely to be a result of differences in source geometry definitions and specifications

44 Producing unifrom distribution of activity on complex curved surface Replicating the complicated energy spectrum of Ru-106 Results emphasise the importance of experimental 2D and 3D dosimetric imaging of Ru-106 plaques with devices such as the Medipix2 used in the recent experimental work at the CMRP

45 Geant4 is a useful tool for dosimetry of plaque brachytherapy sources used in the treatment of ocular melanoma The dose point array obtained for the 6711 seed can be input into the dose planning software for dose optimisation and the quality assurance of eye plaque brachytherapy treatment Ru-106 results highlight the complexity of eye brachytherapy source dosimetry, consistent with current literature

46 Revised source models and scoring babushka 3D dose imaging and reconstruction of Ru- 106 using Medipx2 Monte Carlo modeling of Medipix2 with eye plaque and brachytherapy sources Monte Carlo modeling of 15mm ROPES plaque Effect on surrounding critical structures using Geant4

47 And so we say Hooray for eyes! Hooray, hooray, hooray... for eyes! (Theo. LeSieg)

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