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BrachytherapyBrachytherapy

Physical Properties of SourcesPhysical Properties of Sources

A. Sam Beddar, Ph.D.A. Sam Beddar, Ph.D.Department of Radiation PhysicsDepartment of Radiation Physics

U.T. M.D. Anderson Cancer CenterU.T. M.D. Anderson Cancer Center

IntroductionIntroduction

Mainly two forms of radiation therapy for Mainly two forms of radiation therapy for treatments of cancer : treatments of cancer : teletherapyteletherapy (long(long--range) range) and and brachytherapybrachytherapy (short(short--range).range).

Teletherapy Teletherapy -- external beam radiation therapy in external beam radiation therapy in which a beam of photons/electrons is directed at the which a beam of photons/electrons is directed at the tumor from a medical linear accelerator.tumor from a medical linear accelerator.

Brachytherapy Brachytherapy -- radiation therapy in which sealed radiation therapy in which sealed radioactive sources are used to deliver radiation at radioactive sources are used to deliver radiation at short distances (< 10 cm).short distances (< 10 cm).

BrachytherapyBrachytherapy

Brachytherapy is also referred to as: Brachytherapy is also referred to as: Curietherapy or Endocurie therapyCurietherapy or Endocurie therapy

Treatment is delivered by placing the sources Treatment is delivered by placing the sources directly into or near the volume to be treated.directly into or near the volume to be treated.

The dose is delivered continuouslyThe dose is delivered continuouslyTemporary implants (over a short period of time)Temporary implants (over a short period of time)Permanent implants (over the life time of the source)Permanent implants (over the life time of the source)

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Types of brachytherapyTypes of brachytherapy

Intracavitary Intracavitary sources placed in a body cavity close to the tumor sources placed in a body cavity close to the tumor volume: e.g. vagina, uterus volume: e.g. vagina, uterus ……

Interstitial Interstitial sources implanted surgically within sources implanted surgically within the tumor the tumor volume: e.g. prostate, tissue sarcomas volume: e.g. prostate, tissue sarcomas ……

Intracavitary ImplantIntracavitary Implant

Intracavitary ImplantIntracavitary Implant

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Interstitial ImplantInterstitial Implant

Interstitial ImplantInterstitial Implant

Types of brachytherapyTypes of brachytherapy

IntraluminalIntraluminal -- sources placed in a lumen of the sources placed in a lumen of the body: bronchial tract, esophagusbody: bronchial tract, esophagus

IntraoperativeIntraoperative -- sources placed over the tumor or sources placed over the tumor or tumor beds during surgerytumor beds during surgery

Surface (mold)Surface (mold) -- sources placed over the tissue to be sources placed over the tissue to be treated: eyetreated: eye

IntravascularIntravascular –– single source placed into small or single source placed into small or large arterieslarge arteries

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Intraluminal ImplantIntraluminal Implant

Intraluminal ImplantIntraluminal Implant

Intraoperative ImplantIntraoperative Implant

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Intraoperative ImplantIntraoperative Implant

Types of Source LoadingTypes of Source Loading

Hot Loading:Hot Loading:The applicator is preThe applicator is pre--loaded and contains radioactive loaded and contains radioactive sources at the time of placement into the patientsources at the time of placement into the patient

Afterloading:Afterloading:The applicator is placed first into the target position The applicator is placed first into the target position and the radioactive sources are loaded later: and the radioactive sources are loaded later:

by hand (by hand (manual afterloadingmanual afterloading) ) by a machine (by a machine (automatic remote afterloadingautomatic remote afterloading))

Brachytherapy vs. EBRTBrachytherapy vs. EBRT

The physical advantage of brachytherapy The physical advantage of brachytherapy treatments is the improved localized delivery of treatments is the improved localized delivery of dose to the target volume of interest.dose to the target volume of interest.

The disadvantage is that brachytherapy can only The disadvantage is that brachytherapy can only be used in cases where the tumor is well localized be used in cases where the tumor is well localized and is relatively smalland is relatively small

Typical Rx Dept.: 10 to 20 % of all Rx patients areTypical Rx Dept.: 10 to 20 % of all Rx patients aretreated with brachytherapy.treated with brachytherapy.

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Radioactive DecayRadioactive Decay

N(t) = NN(t) = No o e e ––λλtt

NNoo is initial number of radioactive atomsis initial number of radioactive atomsN is the number of atoms at time tN is the number of atoms at time tλλ is the decay constantis the decay constant

Half lifeHalf life

The halfThe half--life of a radioactive element is the time life of a radioactive element is the time required to decay to half the initial number of required to decay to half the initial number of radioactive atomsradioactive atoms

N(t)/NN(t)/Noo = 0.5= 0.50.5 = e 0.5 = e --λλtt

ln(0.5) = ln(0.5) = --λλtt

t = Tt = T1/21/2 = 0.693/= 0.693/λλ

Characteristics of Isotopes used in Characteristics of Isotopes used in Brachytherapy Brachytherapy

IsotopeIsotope HalfHalf--lifelife AverageAverage HVL in LeadHVL in LeadEnergy (MeV)Energy (MeV) (mm)(mm)

226226Ra Ra 1620 years1620 years 0.8300.830 8.08.0

137137Cs Cs 30 years30 years 0.6620.662 6.56.5

198198Au Au 2.7 days2.7 days 0.4120.412 3.03.0

192192Ir Ir 74.2 days74.2 days 0.3800.380 2.52.5

125125I I 59.6 days59.6 days 0.0280.028 0.0250.025

103103Pd Pd 17 days17 days 0.0210.021 0.0080.008

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Mean lifeMean life

MeanMean--life of an isotope is defined as the life of an isotope is defined as the time taken to decay to 1/e of its original time taken to decay to 1/e of its original number of radioactive atomsnumber of radioactive atoms

N(t)/NN(t)/Noo = e= e--11

e e --λλtt = e= e--11 λλt = 1t = 1t = Tt = Tavgavg = 1/= 1/λλ

TTavgavg = T= T1/21/2/0.693 = 1.44 T/0.693 = 1.44 T1/21/2

Mean life vs. Half lifeMean life vs. Half life

Au198 half-life vs mean lifeHalf-life = 2.7 days

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half-lifemean life

Source Strength SpecificationsSource Strength Specifications

Activity:Activity:

the amount of radioactivity in a radioisotope. the amount of radioactivity in a radioisotope. 1 Ci = 3.7 x 101 Ci = 3.7 x 101010 dpsdps = 3.7 x 10= 3.7 x 101010 BqBq

1 1 BqBq (Becquerel) = 1 (Becquerel) = 1 dpsdps

Exposure rate constant Exposure rate constant ((ΓΓ)): :

exposure rate measured in air at a specified exposure rate measured in air at a specified distance d (typically 1m). distance d (typically 1m).

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Source Strength SpecificationsSource Strength Specifications

The exposure rate (dX/dt)The exposure rate (dX/dt)p p at a point at a point PP in air at a in air at a distance distance dd from a brachytherapy source is from a brachytherapy source is expressed as:expressed as:

(dX/dt)(dX/dt)p p = (= (AA . . ΓΓ) /) /d d 22

Where: Where: AA is the source activity (mCi or Ci)is the source activity (mCi or Ci)ΓΓ is the exposure rate constant (R.mis the exposure rate constant (R.m22.Ci.Ci--11.h.h--11))

Exposure rate constantsExposure rate constants

IsotopeIsotope ΓΓ (R(R--cmcm22/mCi/mCi--hr)hr)

226226RaRa 8.258.25137137CsCs 3.283.28192192IrIr 4.694.69198198AuAu 2.382.38125125II 1.511.51103103PdPd 1.481.48

Exposure rate constantsExposure rate constants

Example:Example:What is the exposure rate at a distance of 1 m for What is the exposure rate at a distance of 1 m for a 10 mCi a 10 mCi 137137Cs source?Cs source?

ΓΓ (R(R--cmcm22/mCi/mCi--hr) = 3.28hr) = 3.28

(dX/dt)(dX/dt)p p = (= (AA . . ΓΓ) /) /d d 2 2

= (10 = (10 mCimCi x 3.28 R.cmx 3.28 R.cm22.mCi.mCi--11.h.h--11)/ (100 cm))/ (100 cm)22

= 0.00328 R/h = 3.28 = 0.00328 R/h = 3.28 mR/hmR/h

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Exposure rate constantsExposure rate constants

Exposure rate at different distances:Exposure rate at different distances:

((dX/dt)dX/dt)pp = 3.28 x 10= 3.28 x 10--33 R/hR/h at 1 mat 1 m

((dX/dt)dX/dt)pp = 32.8 R/h= 32.8 R/h at 1 cmat 1 cm

((dX/dt)dX/dt)pp = 0.328 R/h= 0.328 R/h at 10 cmat 10 cm

your best FRIEND is the inverse square lawyour best FRIEND is the inverse square law

Radiation ProtectionRadiation Protection

TimeTime -- Dose is proportional to exposure time. Half the Dose is proportional to exposure time. Half the time results in half the dose.time results in half the dose.

DistanceDistance -- Radiation Exposure is inversely proportional to Radiation Exposure is inversely proportional to the square of the distance.the square of the distance.

ShieldingShielding -- Proportional to the energy of the photons and Proportional to the energy of the photons and the HVL. For example a 1 mm of Pb can provide enough the HVL. For example a 1 mm of Pb can provide enough shielding for Ishielding for I--125 but not Ir125 but not Ir--192192

The The ““3R3R’’ss”” for Radiation Protectionfor Radiation Protection

137137CsCs137137Cs decays with a halfCs decays with a half--life of 30 yr via life of 30 yr via ββ decay followed decay followed by emission of by emission of γγ rays of energy 0.662 MeV.rays of energy 0.662 MeV.

Source activities must be corrected annually to account Source activities must be corrected annually to account for radioactive decay (2% per year).for radioactive decay (2% per year).

Most commonly used brachytherapy source for Most commonly used brachytherapy source for intracavitary insertions especially for LDR Gyn.intracavitary insertions especially for LDR Gyn.

Available in several forms, such as needles, tubes and Available in several forms, such as needles, tubes and pellets (doubly encapsulated stainless steel).pellets (doubly encapsulated stainless steel).

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137137Cs Intracavitary TubeCs Intracavitary Tube

192192IrIr192192Ir decays with a halfIr decays with a half--life of 74.2 d.life of 74.2 d.

Complicated Complicated γγ ray spectrum with ray spectrum with an average energy of an average energy of 0.380 MeV.0.380 MeV.

Available in the form of: Available in the form of:

wires, the radioactive core being an iridiumwires, the radioactive core being an iridium--platinum alloy with platinum alloy with on outer sheath of 0.1 mm thick Pt.on outer sheath of 0.1 mm thick Pt.

seeds, doubly encapsulated with an outer sheath of stainless steseeds, doubly encapsulated with an outer sheath of stainless steel.el.

strands of nylon ribbon containing iridium seeds 3mm long and strands of nylon ribbon containing iridium seeds 3mm long and 0.5 mm diameter, spaced with their center 1 cm apart.0.5 mm diameter, spaced with their center 1 cm apart.

192192IrIr192192Ir because of its characteristics and its form Ir because of its characteristics and its form availability is used for almost all types of implants.availability is used for almost all types of implants.

Most common Isotope for HDR brachytherapy remote Most common Isotope for HDR brachytherapy remote afterloadersafterloaders, specially designed with typical activities of , specially designed with typical activities of 10 Ci. 10 Ci.

Short halfShort half--life life ⇒⇒ new sources must be orderednew sources must be ordered

Possible to reuse a Possible to reuse a ““ used lotused lot”” of of 192192Ir sources for several casesIr sources for several casesFor HDR, a new For HDR, a new ““10 Ci10 Ci”” source is ordered every 3 monthssource is ordered every 3 months

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198198AuAu198198Au decays with a halfAu decays with a half--life of 2.7 d and emits life of 2.7 d and emits monoenergetic monoenergetic γγ rays of energy 0.412 MeVrays of energy 0.412 MeV

Available in the form of encapsulated seeds or Available in the form of encapsulated seeds or ““grainsgrains””, , implanted surgically into the tumor volume using special implanted surgically into the tumor volume using special delivery delivery ““gunsguns””. (grains sheathed in . (grains sheathed in Pt(IrPt(Ir), typically 2.5 ), typically 2.5 mm long with an outer diameter of 0.8 mm).mm long with an outer diameter of 0.8 mm).

SShorthort--lived radioisotopelived radioisotopeused for interstitial, permanent implants.used for interstitial, permanent implants.within a few days after implantation, activity is sufficiently lwithin a few days after implantation, activity is sufficiently low ow that patient may be released from radiation safety supervision.that patient may be released from radiation safety supervision.

125125II125125I decays with a half life of 59.6 d with the emission of I decays with a half life of 59.6 d with the emission of 0.035 MeV 0.035 MeV γγ rays.rays.

125125I has generated interest for brachytherapy because of I has generated interest for brachytherapy because of its low energy (35.5 keV) photons.its low energy (35.5 keV) photons.

Available in the form of encapsulated seeds. Many Available in the form of encapsulated seeds. Many 125125I I seed models have been manufactured. Dosimetry is seed models have been manufactured. Dosimetry is much more complex than other interstitial sources.much more complex than other interstitial sources.

Used for interstitial implants and Eye Plaques (Surface Used for interstitial implants and Eye Plaques (Surface implants), and has less radiation safety problems.implants), and has less radiation safety problems.

125125I seedsI seeds

Model 6702

Model 6711

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125125I seedsI seeds

125125I Model 6711I Model 6711

103103PdPd103103Pd decays with a half life of 17 d, by electron capture Pd decays with a half life of 17 d, by electron capture with the emission of characteristic xwith the emission of characteristic x--rays in the range of rays in the range of 20 to 23 keV (average energy 20.9 keV).20 to 23 keV (average energy 20.9 keV).

103103Pd have recently become available for use and may Pd have recently become available for use and may provide a biological advantage in permanent implants as provide a biological advantage in permanent implants as the dose is delivered at much faster rate.the dose is delivered at much faster rate.

Available in the form of encapsulated seeds. Few Available in the form of encapsulated seeds. Few 125125I seed I seed models have been manufactured. models have been manufactured.

Used mainly for interstitial implants (Prostate) and has Used mainly for interstitial implants (Prostate) and has less radiation safety problems than any other isotope.less radiation safety problems than any other isotope.

103103PdPd

Model 200

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Implant DosesImplant Doses

Permanent ImplantPermanent ImplantD = (dDD = (dD00/dt/dt ))TTavav

Temporary Implant with TTemporary Implant with T1/21/2>>T>>TD = (dDD = (dD00/dt/dt )) TT

Temporary Implant with TTemporary Implant with T1/21/2 not >> Tnot >> TD = (dDD = (dD00/dt/dt )) TTavav[1 [1 -- exp(exp(--T/TT/Tavav)])]

AAPM Task Group 43AAPM Task Group 43

Dosimetry of interstitial brachytherapy sources:Dosimetry of interstitial brachytherapy sources:

Recommendations of the AAPM Radiation Recommendations of the AAPM Radiation Therapy Committee Task Group 43, Med Phys Therapy Committee Task Group 43, Med Phys 2222, 209 , 209 -- 234, 1995.234, 1995.

Task Group 43Task Group 43

Incorporates latest dataIncorporates latest dataIncorporates SI unitsIncorporates SI units

Becquerel (Bq)Becquerel (Bq)1 Bq = 1dps = 2.7*101 Bq = 1dps = 2.7*10--1111 CiCi

Air Kerma Strength (U)Air Kerma Strength (U)1U = 11U = 1μμGy mGy m22/hr = 1cGy cm/hr = 1cGy cm22/hr/hr

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Task Group 43Task Group 43

(dD(r,(dD(r,θθ)/dt) = S)/dt) = SkkΛΛ[G(r,[G(r,θθ)/G(r)/G(r00,,θθ00)]g(r)F(r,)]g(r)F(r,θθ))

Air Kerma StrengthAir Kerma Strength

SSkk = (dK(d)/dt)d= (dK(d)/dt)d22, , UU1 U = 1 1 U = 1 μμGy mGy m22/h = 1cGy cm/h = 1cGy cm22/h/h

Brachytherapy source strength specified in Brachytherapy source strength specified in terms air kerma rate at a point in air along the terms air kerma rate at a point in air along the perpendicular bisector of the source. Product perpendicular bisector of the source. Product of air kerma rate times distance (usually 1 of air kerma rate times distance (usually 1 meter) to point.meter) to point.

Source GeometrySource Geometry

(c)

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Air Kerma StrengthAir Kerma Strength

K = X(W/e)[(K = X(W/e)[(μμtrtr//ρρ)/()/(μμenen//ρρ)])]μμenen//ρ = (μρ = (μtrtr//ρ)(ρ)(11--g)g)g = 0g = 0K = X(W/e)K = X(W/e)SSkk = (dX= (dXdd/dt)(W/e)d/dt)(W/e)d22

SSkk = (dX(R/h)/dt) (0.876 cGy/R)(1m= (dX(R/h)/dt) (0.876 cGy/R)(1m22))

Dose Rate ConstantDose Rate Constant

ΛΛ = (dD(r= (dD(r00, , θθ00)/dt)/S)/dt)/Skk

Dose rate to water at a point along Dose rate to water at a point along perpendicular bisector of source 1 cm perpendicular bisector of source 1 cm from the source for source strength of 1U.from the source for source strength of 1U.

Geometry FactorGeometry Factor

G(r,G(r,θθ) = 1 / r) = 1 / r22 point sourcepoint sourceG(r,G(r,θθ) = ) = β β / (L r sin / (L r sin θθ) line source) line sourceL = active lengthL = active lengthβ β = = θθ22 -- θθ11

Accounts for variation in relative dose due to Accounts for variation in relative dose due to distribution of activity within the source, distribution of activity within the source, ignoring photon absorption and scattering.ignoring photon absorption and scattering.

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Radial Dose FunctionRadial Dose Function

g(r) = (dD(r,g(r) = (dD(r,θθ00)/dt)G(r)/dt)G(r00,,θθ00)/ (dD(r)/ (dD(r00,,θθ00)/dt) G(r,)/dt) G(r,θθ00))

Accounts for the effects of absorption and scatter Accounts for the effects of absorption and scatter in tissue along the perpendicular bisector of the in tissue along the perpendicular bisector of the source. Similar in concept to Meisberger source. Similar in concept to Meisberger technique, but radial dose function is normalized technique, but radial dose function is normalized at 1 cm.at 1 cm.

Anisotropy FunctionAnisotropy Function

F(r,F(r,θθ) = (dD(r,) = (dD(r,θθ)/dt) G(r,)/dt) G(r,θθ00)/ (dD(r,)/ (dD(r,θθ00)/dt) G(r,)/dt) G(r,θθ))

Accounts for anisotropy of dose distribution Accounts for anisotropy of dose distribution around the source, including effects of around the source, including effects of absorption and scatter in medium, i.e., self absorption and scatter in medium, i.e., self filtration in source, oblique filtration in walls, filtration in source, oblique filtration in walls, scattering and absorption in tissuescattering and absorption in tissue

Anisotropy ConstantAnisotropy Constant

Anisotropy factor, Anisotropy factor, φφanan(r), averaged over distance (r), averaged over distance yields anisotropy constant yields anisotropy constant φφanan. Used at all . Used at all distance and angles for point sources considered distance and angles for point sources considered in Task Group 43 report.in Task Group 43 report.

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The choice of an appropriate radioactive source The choice of an appropriate radioactive source for a specific brachytherapy treatments depends for a specific brachytherapy treatments depends on several relevant physical and dosimetric on several relevant physical and dosimetric characteristics.characteristics.

Photon energies and beam penetration in tissue and Photon energies and beam penetration in tissue and in shielding materials.in shielding materials.Half life.Half life.Source Strength.Source Strength.Inverse square fallInverse square fall--off of dose with distance from the off of dose with distance from the source.source.

Summary of brachytherapy sourcesSummary of brachytherapy sources