Calculation of air-kerma strength and dose rate constant for new BEBIG 60Co HDR brachytherapy...

Dr. Mir Md. Akramuzzaman1, G.A. Zakaria2, G.H. Hartmann3

1Department of Physics, Jahangirnagar University, Savar, Dhaka, Bangladesh.2Gummersbach Academic Teaching Hospital, University of Cologne, Germany.3Department of Medical Physics in Radiotherapy, German Cancer Research Center, Heidelberg, Germany.

M. Anwarul IslamDepartment of PhysicsJahangirnagar University

Calculation of air-kerma strength and dose rate Calculation of air-kerma strength and dose rate constant for new BEBIG constant for new BEBIG 6060Co HDR brachytherapy Co HDR brachytherapy

source: an EGSnrc Monte Carlo studysource: an EGSnrc Monte Carlo study

To calculate the Air kerma strength for BEBIG 60Co source with Monte Carlo codes

To calculate the dose rate constant for BEBIG 60Co source with the protocol of TG-43 (AAPM)

To compare the calculated Air kerma strength with published or measured value

ObjectivesObjectives

BEBIG 60Co HDR brachytherapy source model

Using EGSnrc Monte Carlo codes developed by National Research Council (NRC) of Canada

Calculating the photon fluence rate in air with the source

Using the AAPM TG-43 protocol

Materials and Method Materials and Method

Monte Carlo Procedure

Filtering

Apply physical/ statistical

law/ theory

Group of Random Events

Analysis Apply Physical Law/theory

Apply Boundary Condition

Apply Statistical

Law

Apply Probability theory

Applications of Monte Carlo

Computational Physics Physical Chemistry Quantum CromodynamicsHeat ShieldingAerodynamics Statistical Physics Molecular modeling Particle Physics Radiation Physics Energy Transport Stochastic Financial Modeling Telecommunication Mathematical Solution Weather Forecast Light transport in biological tissueReliability Engineering etc

Monte Carlo code for Radiation TransportThe following codes are available for the simulation of radiation transport : GEANT: simulation of high energy particles interacting with a detector.

CompHEP, PYTHIA: Monte-Carlo generators of particle collisions

MCNP(X): radiation transport codes

MCU: particles (electrons, photons, neutrons) transport code

Cont.

PEREGRINE: code for radiation therapy dose calculations

PENELOPE: code for coupled transport of photons and electrons

BEAMnrc: code system for modeling radiotherapy sources (Linac)

EGSnrc: code for coupled transport of electrons and photons

EGSnrc Monte Carlo codes

The EGSnrc Monte Carlo codes are used in this work. This code is very powerful research tool for radiotherapy field in brachytherapy.

The EGS (Electron-Gamma Shower) system of computer codes is a general purpose package for Monte Carlo simulation of the coupled transport of electrons and photons.

The EGSnrc codes developed by National Research Council of Canada

The EGSnrc is capable to calculate fluence, dose, stopping power ratio, kerma etc.

BEBIG 60Co HDR source models

Real geometry of the source

Model geometry of the source

Monte Carlo input of source model

Sagittal Section

Fig: Equal sagittal section for Monte Carlo source input

TG-43 Formalism

General 2D formalismThe general, two-dimensional ~2D dose-rate equation from the TG-43 protocol is retained,

Where,

= dose rate at the point (r, θ)

Fig: Coordinate system used for brachytherapy dosimetry calculations

TG-43 Formalism

Using Formula for fluence calculation

ΔE .

ρ

Eμ..E Eφ.101.602=(d)K ien

i

E

Ei

10air

max

min

And finally, Sk/A = 2 × K΄air(d) × d2

Where,

(d)Kair is the total air-kerma at the distance, d and the unit is Gy/Photon

iEφ is the photon fluence per unit energy and it’s unit MeV-1 cm-2

ρ

Eμ ien is the mass energy absorption coefficient and it’s unit cm2 gm-1

Ei is the energy spectrum and ΔE is the energy bin size

The factor 1.602×10-10 is required to convert Kair(d) from MeV gm -1 into Gy

Sk/A is the air-kerma strength per unit source activity.

The unit is μGym2h-1.Bq-1 or UBq-1

D = 100 cm according to the TG-43 formalism

Calculation grid: Slab and radial thickness = 1-2 mm

Photon cutoff energy = 0.001 MeV Electron cutoff energy = 0.521 MeV No. of history simulated for every point = 109 Dose for photon contribution simulated Considered one decay will result in the emission

of 2 photons Average time per simulation is 5 hours for photon

Dose calculation formulaDose calculation formula

Dose calculation formulaDose calculation formula

The user-code DOSRZnrc is used to calculate, Dphotons

where Dphotons is the total dose by photons

Sk/A is air-kerma strength per source activity into [U Bq-1]

is the true dose rate per unit air-kerma strength for 60Co source in [cGy h-1 U-1]

)//(2106.3 5 ASD kphotontrueD

Phantom ModelPhantom Model

Geometric Preview Window

Calculated point

Tissue basis absorbed dose calculationTissue basis absorbed dose calculation

Tissue Water Combone

Breast Blood Lung Adiposetissue

Muscle Softtissue

Testese

Densityg/cm3 1.00 1.85 1.02 1.06 0.26 0.92 1.12 1.0 1.04

Some body equivalent tissues (shows in table below) are simulated to investigate the relative absorbed dose with different distances in respective tissue phantom and also in vacuum phantom with 5 cm distance.

Tissue basis density table

ResultsResults

Energy fluence vs. SpectrumEnergy fluence vs. Spectrum

Calculated Calculated 6060Co fluence dataCo fluence data (MeV-1 cm-2)(MeV-1 cm-2)

Calculated Calculated 6060Co fluence dataCo fluence data (MeV-1 cm-2)(MeV-1 cm-2)

Calculated Calculated 6060Co fluence dataCo fluence data (MeV-1 cm-2)(MeV-1 cm-2)

Air-kerma strengthAir-kerma strength

Article Air-kerma strengthPer unit source

activity(cGy.cm2.h-1.Bq-1 )

This work 3.035×10-7

±0.15%

T. Palani Selvam et al.(2010), India

3.04×10-7 ±0.05%

Dose Rate ConstantDose Rate Constant

Article Dose rate constant, Λ

(cGy h-1 U-1)

This work 1.097 1.097 ± 0.12%± 0.12%

T. Palani Selvam et al.(2010), India

1.086 1.086 ± 0.06%± 0.06%

Tissue basis relative absorbed doseTissue basis relative absorbed dose

Relative absorbed dose with distanceRelative absorbed dose with distance

Lung

Comp. bone

Conclusion

In the calculation of air-kerma strength and dose rate constant, uncertainty were 0.15% and 0.12% which is acceptable limit.

Published data of Palani Selvam, the uncertainty were 0.05% and 0.06% which are lower than our values.

In this study, EGSnrc Monte Carlo code was used but Palani Selvam used MCNP code

Relative absorbed dose in different tissues in 5 cm distance are approximately same accept lung tissue. 22% less dose in lung tissue with water

The relative absorbed dose with different distances in respective tissue phantom, the lung doses are higher than the compact bone