Angelica Perez-AndujarUniversity of Puerto Rico, Mayagüez Campus

Fermi National Accelerator LaboratoryNeutron Therapy Facility

Experimental Analysis of Wedge Factors for Neutron Therapy

Outline

NEUTRON THERAPY AND TREATMENT PLANNING

THE USE OF TEFLON WEDGE FILTERS

WEDGE FACTOR MEASUREMENTS

RESULTS

CONCLUSION

RECOMMENDATIONS

Dose Distribution

An isodose chart presents the percent of dose that is going to be delivered at an specific depth at the body.

The dose distribution is presented as isodose curves.

Teflon Wedge Filter

Wedge Effect on the Isodose Curves

Wedges tilts the isodose curves.

no wedge 45º wedge 60 º wedge

Wedges Characteristics

Wedge Isodose tilt Angle

(º)

Height(cm)

Width(cm)

Length(cm)

Physical Wedge angle

(º)

1 45 5.3 8.9 17.2 31

2 45 8.9 15.0 17.5 31

3 60 10.2 10.2 10.2 45

4 60 15.2 15.2 15.2 45

h

wl

Teflon wedge filter without aluminum mount

Each wedge has a physical angle that tilts the dose that the patient is going to received.

More about Wedge Filters

They absorb some beam

What other changes can wedges produce?

What can be done to ensure the delivery of the correct amount of dose?

A wedge factor could be calculated.

What is a wedge factor?

The wedge factor is the ratio of the dose delivered with and without wedge.

If the wedge factor is known it is possible to calculate the amount of beam necessary to deliver the desired dose.

Experimental Setup

Linac Protons

BerylliumTarget

Heavy ConcreteShielding WallNeutrons

SDD= 109cm SAD=190cm

Collimator

Transmission Chamber

Isoplane

Isocenter

Polyethylene Phantom

Ionization Chamber in the Phantom

Spokas Thimble Chamber

Collimators

Wedges and Collimators Used

Wedge Collimators used (cm x cm)

1(45º)

3x3, 4x4, 6x6, 8x8, 10x10, 12x12, 14x14

2(45º)

10x10, 12x12, 14x14, 16x16, 18x18, 20x20, 24x24

3(60º)

3x3, 4x4, 6x6, 8x8, 10x10, 12x12, 14x14

4(60º)

10x10, 12x12, 14x14, 16x16, 18x18, 20x20, 24x24

Not all wedges were used for all collimators.

Wedges 1 and 3 do not cover the big collimator aperture.

Wedge 2 and 4 absorb more beam than necessary when small collimators are used.

Wedge Factor Calculations

Wedge Factor =

(Normalized Charge x TPCOR) wedge

(Normalized Charge x TPCOR )no wedge

The wedge factors obtained were compared with the ones that were calculated years ago

The amount of charge received by the ionization chamber at the phantom was normalized to the one received at the transmission chamber.

The normalized charge is multiplied by a temperature-pressure correction factor.

TPCOR=T + 273.15

295.15x 1013.25

P

The wedge factor is given by:

Wedge Factor vs. Depth Wedge Factor vs. Depth (3 cm x 3 cm collimator)

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 5 10 15 20 25 30

Depth (cm)

Wed

ge F

acto

r

wedge 1 measured

wedge 1 calculated

wedge 3 measured

wedge 3 calculated

Wedge Factor vs. Depth(24 cm x 24 cm collimator)

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0 5 10 15 20 25 30

Depth (cm)

We

dg

e F

ac

tor

wedge 2 measured

wedge 2 calculated

wedge 4 measured

wedge 4 calculated

Wedge Factor vs. Depth(10 cm x 10 cm collimator)

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 5 10 15 20 25 30

Depth (cm)

Wed

ge F

acto

r

wedge 1 measured

wedge 1 calculated

wedge 2 measured

wedge 2 calculated

wedge 3 measured

wedge 3 calculated

wedge 4 measured

wedge 4 calculated

Wedge Factor vs. Square Root of Collimator Aperture

Wedge Factor vs. Square Root of Collimator Aperture(4cm deep)

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 5 10 15 20 25 30

Square Root of Collimator Aperture (cm)

Wed

ge

Fac

tor

wedge 1 measured

wedge 1 calculated

wedge 2 measured

wedge 2 measured

wedge 3 measured

wedge 3 calculated

wedge 4 measured

wedge 4 calculated

Wedge Factor vs. Square Root of Collimator Aperture(10cm deep)

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 5 10 15 20 25 30

Square Root of Collimator Aperture (cm)

Wed

ge

Fac

tor

wedge 1 measured

wedge 1 calculated

wedge 2 measured

wedge 2 calculated

wedge 3 measured

wedge 3 calculated

wedge 4 measured

wedge 4 calculated

Wedge Factor vs. Square Root of Collimator Aperture(22cm deep)

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 5 10 15 20 25 30

Square Root of Collimator Aperture (cm)

Wed

ge

Fac

tor

wedge 1 measured

wedge 1 calculated

wedge 2 measured

wedge 2 calculated

wedge 3 measured

wedge 3 calculated

wedge 4 measured

wedge 4 calculated

Conclusions and Recommendations

The wedge factor is a function of the collimator size and the depth in the phantom.

It increases as depth and collimator size increase. The relationship between depth and collimator size is not linear

it suggests a curvature as depth increases.

Conclusions

Recommendations New measurements have to be made. It is necessary to use a tissue equivalent material phantom. Measurements need to be performed with longer beam exposure. An improved phantom positioning system should be developed.

Acknowledgements

Thomas Kroc Arlene Lennox Gordon Holmblad Ruth DeJerld Marc Austin Miguel Morales All my new friends SIST Program you

Questions ?