Calcifications in LDR Brachytherapy

Stuart George1,2, Susanna Guatelli2, Dean Cutajar2, A.B.Rosenfeld3

1 University of Sheffield2 CMRP University of Wollongong3 Chair, CMRP University of Wollongong

Summary Of Talk

• Quick review of Prostate Brachytherapy and Calcifications

• Implementation of Brachytherapy Seeds and Calcifications in Geant4

• Results

Prostate Brachytherapy

• LDR Brachytherapy used for prostate treatment

• Seeds implanted in the prostate

• Can be used independently or with beam therapy

What is a Calcification?

• Good question!

• Small lumps of Calcium based material in the prostate

• Rare for men under 40, common over 50

• Tend to be asymptomatic and associated with benign prostate enlargement

Why study Calcifications?

• Prostate Brachytherapy uses low dose rate sources (I 125 - 28 keV)

• Mainly Photoelectric Interactions

• Cross section proportional to Z4 - 100 times larger for Calcium!

• Represents a significant inhomogenity in the prostate.

Implementation in Geant4

• Building a Brachytherapy Seed

• Setting up the Physics

• Getting the source radioactive

• Validation against Tg43

Geometry Implementation

• Implemented an Oncura 6711 (Amersham) Oncoseed

• Water phantom - high tissue equivalence, useful for Callibration

1. Regulations

1.1 IntroductionBefore ordering or using radiation sources or anyother radioactive material customers must takewhatever actions are necessary to ensure thatthey are complying with their national, state andlocal regulations governing the use of suchmaterials. In most countries regulations areclosely related to the International AtomicEnergy Agency (IAEA) regulations and codes ofpractice. Customers should consult theirregulatory agency for guidance on the medicaluse of these radiation sources and the rules inforce for radiological protection, receiving,storing, using and disposing of radioactivematerials.

1.2 European Medical Device Regulations Sterile OncoSeeds and EchoSeeds are ActiveImplantable Medical Devices as defined by theEuropean Directive 90/385/EEC. They havebeen assessed and comply with therequirements of the Directive and carry the markof conformity granted in 1999 and 2002,respectively.

2. Product Description2.1 Sterile Convenience PacksThe Sterile Convenience Pack contains fifteen I-125 Seeds loaded into a Mick® DisposableCartridge and steam sterilised for Ready to Use.See Figure 1.

2.2 OncoSeed™ Sterile Convenience PacksThe OncoSeed Sterile Convenience Packcontains fifteen Model 6711 I-125 Seeds. Model6711 OncoSeed consists of a welded titaniumcapsule containing iodine-125 adsorbed onto asilver rod. See Figure 2.

2.3 EchoSeed™ Sterile Convenience PacksThe EchoSeed Sterile Convenience Packcontains fifteen Model 6733 I-125 Seeds. Model6733 EchoSeed consists of a welded titaniumcapsule containing iodine-125 adsorbed onto asilver rod. The specifically designed grooves onthe outer surface of the capsule allow enhancedvisualization under ultrasound. See Figure 3.

2.4 Sterile Convenience Packs contain eitherfifteen Model 6711 OncoSeeds or fifteen Model6733 EchoSeeds loaded into a Mick DisposableCartridge (see Figure 1). The cartridge is sealedinto a pouch and steam sterilised. Up to tenpouches may be placed into a lead shieldingcontainer.2.5 Labeling supplied with the product includes:Order I.D. number, apparent activity per seed,reference date, total apparent activity, expirationdate, manufacturing date.2.6 Total air kerma strength specified in units ofmicroGray meter squared per hour (µGym2/h) isgiven in the certification report.

3. Physical Characteristics• Principle Radionuclide: 125I (Iodine)

• Radionuclide Purity: > 99.9% 125I< 0.005% 126I

• Half-life of 125I: 59.43 days

• Types of Radiation: X-ray and Gamma

• Energy Level: X-ray 27.4 keV and31.4 keV

Gamma 35.5 keVFluorescent X-Rays 22.1 keV andfrom the Silver Rod 25.2 keV

• Decay Mode: 125I decays by electron capture with theemission of characteristic photons andelectrons. The electrons are absorbedby the titanium wall of the seed.

3.1 Shelf Life of SourceThe shelf life is six months after date ofmanufacture and is given on the label.

0.8 mm

I-125 adsorbed�on silver rod��

4.5 mm

0.05 mm�titanium

0.5 mm

3.0 mm

Figure 2: OncoSeed Iodine-125

0.05 mm�titanium

0.5 mm0.8 mm

I-125 adsorbed�on silver rod��

4.5 mm

3.0 mm

Figure 3: EchoSeed Iodine-125

00086

3 of 13190-6740B, English, Rev. November 2005

Figure 1: I-125 Seeds loaded into a Mick Cartridge

Calcification

Brachytherapy Seed

Sensitised Volumes

Physics

• Physics List

• Cuts

• GPS

Results

• Validation of the source against Tg43

• What the energy deposition around a Brachytherapy seed looks like

• Effects of Calcifications

• Calcifications made of lots of little spheres

Validation

• What is Tg43?

• 1mCi Seed

• Allows us to make our dose rates statistics independent

Nath et al, Medical Physics, Jan 1995

Edep Around a Brachytherapy Seed

15mm

Dose Isolines at105 Gy25 Gy10 Gy5 Gy

Effect of Calcifications

• Lets put some Calcifications in there

• 1mm, 2mm, 3mm diameter spheres of elemental calcium

• Substantial effect

Effect of Calcifications

Isodose Curves at 105, 25, 10, 5 Gy

15mm

Hot and Cold Spots

Isodose Curves Every 5 Gy from -5Gy to -70Gy

5mm

Random Spheres

• Why look at spheres?

• Lack of structural and compositional information about Calcifications - look at Calcium and Water

• Calcium Apatite ~40% calcium by mass, the rest is lower Z elements, Phosphorus, Oxygen, Hydrogen

A Few Spheres ...

100 micron 50 micron

10 micron

Results

• Cuts set to 1um.

• Roughly linear relationship

• Lots of runs(250,000,000x10)

• Size appears to have no effect

Conclusions

• Millimeter scale Calcifications cause significant radiation attenuation

• Attenuation through calcium/water mixes is linearly dependent on the amount of calcium

• Next steps...?

Any Questions?