Photon Sources for Photon Sources for

BrachytherapyBrachytherapy

Alex RijndersEurope HospitalsBrussels, Belgium

a.rijnders@europehospitals.be

Topics of this lecture:

� Terminology

� Sources and source types

� Definition of some relevant physical quantities

PHOTON SOURCES for PHOTON SOURCES for

BRACHYTHERAPYBRACHYTHERAPY

Brachytherapy

Brachytherapy is a treatment method in which sealed radioactive source(s) are used to deliver radiation dose at a short distance.

A high dose can be delivered locally to the tumor with a rapid dose fall-of to the surrounding healthy tissue

AL

PL

AL

EL

EL

s

s 1/2 s

PL

PL

Tube

Needle

Wire

Seed Ribbon

Source Train

Stepping source

AL Active Length ; PL Physical Length ; EL Equivalent Active Length

EL = n x s (Number of seeds x distance between source centres)Condition: equal activity

Sealed sources

Temporary ImplantsTemporary Implants

LDR

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hours

Dose R

ate

PDR

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Dose R

ate

Low Dose Rate:● Continuous

irradiation

● 0.40 – 2 Gy/h

Pulsed Dose Rate:● mimic low dose rate ● short pulses, same average dose rate

High Dose Rate:● >0.2 Gy/min ● One/a few fractions

LDR

PDR

HDR

HDR

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Dose R

ate

Important milestonesImportant milestones

Early 1900s: use of Radium for BT

1930s: Manchester System

End 1950s: artificial isotopes (60Co – 137Cs)

1960s: 192Ir wire sources – Manual afterloading techniques – Paris System

1970s-1990s: Remote Afterloading Devices

2000s: Imaging Assisted Brachytherapy

2010s : Improved dose calculation algorithms

SOURCES in BRACHYTHERAPYSOURCES in BRACHYTHERAPYRadium needles and tubes (original design)Radium needles and tubes (original design)

Radium sources : UnitsRadium sources : Units

226Ra source strength specification in terms of contents:

mass, expressed in milligrams (mg Ra)

Drawbacks of 226Ra associated with safety(possible Radon-leaks, too long half time)

End 1950’s : Replacement by artificially produced sources such as 137Cs, of which the strength was expressed as radium-equivalent

=> mg Ra-equivalent

1950s ARTIFICIEL ISOTOPES1950s ARTIFICIEL ISOTOPESReplacementReplacement of Radiumof Radium

Example of a 2 cm length tube source, 137Cs

RReplacement of Radiumeplacement of Radium

Other new sources and source types were developed: leading to a need for other source strength specification, based on “contents”, activity, # of disintegrations per time unit

Definition of Curie:1 Ci (3.7 x 1010 s-1) = activity of 1 g 226Raactually 1g 226Ra = 3.655 x 1010 S-1 or .988 Ci

SI-units: 1 Bq = 1 disintegration per secMBq, GBq

- Number of nuclei prone to decay:

N(t) = N0 ���� e-λλλλt

- Activity:

A(t) = -∆∆∆∆N(t) / ∆∆∆∆t ≈≈≈≈ N(t)

- Half life of the source:

A(t = T½) = ½ ���� A0

- Decay constant:

λλλλ = ln(2) / T½ = 0.693 / T½

Radioactivity, decay Radioactivity, decay

calculationcalculation

� Reference Air Kerma Rate (µµµµGy/s)

� Kerma rate to air, in air, at reference distance 1 m, corrected for air attenuation and scattering (ICRU 38 and 58)

RK

•

Specification of source Specification of source

strength for dosimetrystrength for dosimetry

Practical units: µµµµGy/h (LDR),

mGy/h or µµµµGy/s (HDR)

Activity – MBq (mCi)

Air Kerma Rate -

µGy/s (Exposure Rate – C/kg.s)

Kδ

R2

refapp

)(Γ

KdA

•

=

( )Γδ K : air kerma rate constant (source construction, encapsulation and ene

dref : reference distance (1m)

137Cs: from 3 to 3.31 192Ir: from 4.0 to 5.0

Jack Venselaar

SOURCES in BRACHYTHERAPYSOURCES in BRACHYTHERAPY

Examples of source test certificates

Example of TPS output

Reference Air Kerma Rate is the reference

unit for the clinical physicist

Important milestonesImportant milestones

Early 1900s: use of Radium for BT

1930s: Manchester System

End 1950s: artificial isotopes (60Co – 137Cs)

1960s: 192Ir wire sources – Manual afterloading techniques – Paris System

1970s-1990s: Remote Afterloading Devices

2000s: Imaging Assisted Brachytherapy

2010s : Improved dose calculation algorithms

1960s MANUAL AFTERLOADING1960s MANUAL AFTERLOADING

Interstitial cathetersInterstitial catheters

Fletcher-Suit-DelclosVaginal cylinder

Ring applicator

Important milestonesImportant milestones

Early 1900s: use of Radium for BT

1930s: Manchester System

End 1950s: artificial isotopes (60Co – 137Cs)

1960s: 192Ir wire sources – Manual afterloading techniques – Paris System

1970s-1990s: Remote Afterloading Devices

2000s: Imaging Assisted Brachytherapy

2010s : Improved dose calculation algorithms

Cs afterloader (LDR)Cs afterloader (LDR)

137Cs pellets – Free composition of trains of active/inactive pellets

IrIr afterloadersafterloaders (HDR(HDR--

PDR)PDR)

192Ir stepping source, HDR or PDR

SOURCES in BRACHYTHERAPYSOURCES in BRACHYTHERAPY

Example of tip of a pulsed dose rate (PDR) source, Ir-192,

welded to the end of a drive cable

Advantage of HDR technology

� One single source (costs)

� Half life 74 days => usable for 3-4 months (costs)

� Afterloader system => radiation protection

� Stepping source technology allows dose optimisation => optimisation algorithms

� Short irradiation time (10-15 minutes)

Stepping sourceStepping source

Advanced Optimisation Technology Advanced Optimisation Technology

(example :SWIFT(example :SWIFTTM)TM)

Permanent ImplantsPermanent Implants

Radioactive sources

remain in the patient

and decay ● Relative short half life

● Low energy (radiation

protection)

Permanent Implant 125

I

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

0 30 60 90 120 150 180 210 240 270 300 330 360

Days

Do

se

Ra

te

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

To

tal D

os

e

Permanent ImplantsPermanent Implants

e.g., for prostate, brain

these sources should combine a relative short half life with low energy:

=> half life determines dose rate/radiobiology

=> patient should be able to continue life as usual

Examples:

I-125 (60 days; 28 keV)

Pd-103 (17 days; 21 keV)

Cross-Sectional drawings of sources with a Rod, Wire, or Cylinder internal core design; (a) Amersham 6711 OncoSeed, (b) Syncor PharmaSeed, (c) UroMed Symmetra, (d)SourceTech Medical 125Implant, (e) Med-Tec I-Plant, (f) International Brachytherapy, Inc. InterSource125, (g) Best Medical Model 2301 (h) Amersham 6702, (i) UroCor ProstaSeed, (j)Imagyn IsoSTAR, (k) Mentor's IoGold, (l) DraxImage BrachySeed.

From: Heintz et al, Comparison of I-125 sources used for permanent interstitial implants. Med Phys 2001

Some examples of seeds:

Special presentations of Special presentations of

sourcessources

“Rapidstrand®” seed ribbon technique with

the 125I sources connected in a suture

“Isocord®”: comparable technique with the 125I sources connected in a bio-absorbable suture

And there are many more …

NewerNewer system: system: SourceLinkSourceLink(Bard(Bard)

Afterloader for Afterloader for 125125I seedsI seeds

Important milestonesImportant milestones

Early 1900s: use of Radium for BT

1930s: Manchester System

End 1950s: artificial isotopes (60Co – 137Cs)

1960s: 192Ir wire sources – Manual afterloading techniques – Paris System

1970s-1990s: Remote Afterloading Devices

2000s: Imaging Assisted Brachytherapy

2010s : Improved dose calculation algorithms

MOTIVATION

Apply also to modern

brachytherapy

Apply also to (modern)

brachytherapy

� ‘Modern Radiotherapy’ seems to be driven

by significant developments in EBT

� 3D Conformal Radiotherapy

� Stereotactic Radiotherapy: High Precision

� Intensity Modulated Radiotherapy: Dose Shaping

� Imaging for GTV/PTV, OAR (structure segmentation)

� Computerized treatment plan optimization

� Image guided RT

From Poetter et al

=> Evaluate potential of Brachytherapy based on modern technology

3D Treatment Planning Prostate BT

D 90

V 100

V 150

CTV, Urethra, Rectum

- CT

- MRI

- US

- PET

- Functional…

=> the role of ‘imaging’in RT increases rapidly

� Better understanding of anatomy

� Better understanding of pathology

� More appropriate contouring

Multi modalityImaging

Examples of CT/MRI compatible Examples of CT/MRI compatible

applicatorsapplicators

• Long Half Life=> Economical

• High Specific activity: activity per unit of mass

=> physical size of source)

• Low mean energy of radiation

=> less penetration in tissue

• Small half value layer in lead or concrete

=> radiation protection

Ideal source/isotope Ideal source/isotope (hospital use)(hospital use)

Physical properties of nuclides

λ=ln2 / T½

IsotopeAverage photon energy*

Half-life T½Half value layer in lead

Treatment room wall

Cobalt-60 Co-60 1,25 MeV 5,26 years 12 mm(Concrete)typical values

Caesium-137 Cs-137 662 KeV 30,1 years 6,5 mm

Iridium-192 Ir-192 380 KeV 73,8 days 3,0 mm (40 cm)

Ytterbium-169 Yb-169 93 KeV 32,0 days 0,23 mm

Thulium-170 Tu-170 66 KeV 128.6 days 0,17 mm (12 cm)

Iodine-125 I-125 28 KeV 59,4 days 0,025 mm (4.5 cm)

Palladium-103 Pd-103 21 KeV 17,0 days 0,01 mm (1 cm)

Caesium-131 Cs-131 30 KeV 9,7 days -

* Approximate values, depending on the source make and filtration

⇒ Radioprotection:

� Reduced Mean Energy (<= 100 KeV)

� Yb-169, Tu-170, I-125, Pd-103

⇒ Increased half-life (source exchange)

� Co-60

« New » Isotopes

X-ray source tip detail

Miniature x-ray source inserted into a flexible cooling catheter� High vacuum x-ray tube technology � 50 kV max. operating potential� Water cooled� Fully disposable device

X-Ray Tube HV Cable

Cooling connections

HV connection

miniature

x-ray

source

Electronic BT sourcesElectronic BT sources

Xoft Inc.

Diameter (approx. 3 mm) larger than existing sources

(catheter approx. 5 mm)

Dose rate stability? High Voltage dependence?

Dose rate/energy variation in function of life

time/use/between different sources?

Usage for specific applications…...

(APBI-Mammosite)

Some comments

Training in Brachytherapy

� As treatment techniques and delivery systems become more complex

� Need for better formed/trained staff

� ! Few centres specialised in “ high end BT “(certainly in Western Europe)

Brachytherapie ���� Teletherapie

Investments

€ 300.000 € 2.400.000

(+ € 13.000 / source) (+ maintenance)

Workload ++ Workload +++

References / more reading

� ICRU, International Commission on Radiation Units and Measurements, Dose and volume specification for reporting intracavitary therapy in gynecology, ICRU Report 38, 1985.

� ICRU, International Commission on Radiation Units and Measurements. “Dose and volume specification for reporting interstitial therapy”. ICRU Report 58, 1997.

� The GEC ESTRO Handbook of Brachytherapy, Editors: Alain Gerbaulet, Richard Pötter, Jean-Jacques Mazeron, Erik van Limbergen,

available at the ESTRO web site: www.estro.org� A Practical Guide to Quality Control of Brachytherapy

Equipment, ESTRO Physics Booklet No. 8, Editors: Jack Venselaar, José Pérez-Calatayud,

available at the ESTRO web site: www.estro.orghttp://www.estro.org/school/articles/publications/publications

Still more reading

� Nath, R., Anderson, L.L., Meli, J.A., Olch, A.J., Stitt, J.A. and Williamson, J.F. “Code of practice for brachytherapy physics: Report of the AAPM Radiation Therapy Committee Task Group No. 56”. Med. Phys. 24, 1557-1598, 1997.

� Radiation Oncology Physics: a Handbook for Teachers and Students, Editor E.B.Podgorsak, available at the IAEA web site: www.iaea.org

� Dosimétrie en Curiethérapie, A. Dutreix, G. Marinello and A. Wambersie, Masson, Paris, 1982

� The Physics of Modern Brachytherapy for Oncology, D. Baltas, L. Sakelliou and N. Zamboglou, Taylor and Francis, New York London, 2007