A monte carlo investigation of the effect of source geometry variations on brachytherapy seed sources

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A MONTE CARLO INVESTIGATION OF THE

EFFECT OF SOURCE GEOMETRY VARIATIONS

ON BRACHYTHERAPY SEED SOURCES

**B.CAMGÖZ, G.YEĞİN*, M.N. KUMRU**

Ege University, Institute of Nuclear Science-Izmir,Turkey *Celal Bayar University, Physics Department, Manisa

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Brachytherapy

 Low dose rate (LDR)

brachytherapy seed sources are widely used for cancer treatment in recent years.

 There are many diferent

seed models manifactured by different componies

 In prostate cancer

treatment 50-130 sources can be used together in an implant

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Brachytherapy

 In current practice, dose distribution around seed sources is

calculated based on a formalism described in AAPM Task Group 43 report.

 This formalism requires the usage of a number of dosimetry

parameters for the calculation of dose value at certain point around a brachytherapy source.

 Those parameters should be determined previously for every

single source model either experimentally or using Monte Carlo techniques in water medium.

 TG-43 prptocol assumes that every seed model is cylindrical TG-43 prptocol assumes that every seed model is cylindrical

symmetric. But this is not valid in real practice

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TG-43 dosimetry formalism

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Problem



This talk presents our preliminary results about the

calculation of the effect of source geometry

variations on brachytherapy seed sources



In this work only the

Amersham 6711

model seed

was investigated. But we are going to extend this

work to all seed models currently being used in

patient treatments

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Former Works in Literature

For DraxImage 125I seed source,

Williamson showed that

increasing the diameter of the spherical resins in a seed about

1% has an effect on the dose value at a point about 5%

(Williamson Med.Phy., 2002)

Rivard and Melhus showed that the movement of inter-seed

components affect calculated dose values considerably (Rivard

and Melhus, Med.Phy. 2004)

Mobit and Badragan took X-ray pictures of several

brachytherapy seed sources of the same madel and showed that

they are different in length and diameter about 5%. (Mobit and

Badragan, Phy.Med.Biol., 2004)

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The Movement of Resins in

ProstaSeed Brachytherapy

Source

X-Ray images (left), Geometry models (right) (Rivard and Melhus, Med. Phys. 2004)

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Seed Geometry Variations for

6711 Amersham Source

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BrachyDose

A Monte-Carlo dose calculation engine for brachy-therapy

sources

BrachyDose is developed in a joint project between

Carleton University in Canada

and

Celal Bayar University in

Turkey

The code can m

odel brachytherapy seeds in a voxel

geometry

.

C

ode is based on

the

M

ulti-

G

eometry

package and

EGSnrc

C

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Models Different seeds

Amersham Model 6711

Theragenics Model 200

ProstaSeed

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Calculations

We investigated Amersham 6711 seed source in this work.

The position of the core is shifted in different scenarios

Monte Carlo calculations were performed on high speed

computers

Each calculation requires a computer to be run several days

in order to get reasonable dose results

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Amersham 6711 seed source

ideal (i.e. cylindrical symmetric) geometry

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Different scenarios

2090 (ideal) 2092 2094 ID

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Dose ratios

Dose ratio of two models (2092 to 2090) at

points on the transverse axis of both seeds

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Dose ratios

Dose ratio of two models (2094 to 2090) at

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Dose ratios

Dose ratio of two models (2100 to 2090) at

points on the trasnverse axis of both seeds

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Conclusion

The

movement of core affect considerably dose values at distances

less then 1 cm from the seed center.

The effect of the movement of the seed core on dose values

changes

1

1 % to 5

% to 5

% at points 1 cm from the seed center.

This dose difference increases exponentially while getting closer the

source center.

The maximum discrepancy is observed at the seed surface, which is

~40%.

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