brachytherapy

Lecture 21Lecture 21 Ahmed GroupAhmed Group

General Lecture in

BRACHYTHERAPY

Outline

Introduction

Technical

Dose calculations

Clinical

Introduction

Brachytherapy -is the internal radiation treatment achieved by implanting radioactive

material directly into the tumor or very close to it .

Sometimes called internal radiation therapy.

Prefix “brachy” – from Greek for “short range” Greek “brachys” meaning short, contrast to “tele”

meaning distant (teletherapy)

Can be interstitial, intracavitary or surface

High local radiation dose with fast dose fall off

History

Discovered in 1898, radium was most common brachytherapy source

Not used today due to high -ray energy (shielding problems)

Chemical and radioactive toxicity of radium and byproducts

Typical artificial, -ray emitting isotopes used e.g. cesium, iridium, gold, iodine

History

X-rays discovered before radium – radium used for treatment first

Alexander Graham Bell 1903Suggests encapsulation in needles for direct

insertion into cancer lesions

Advantages

Usually out-patient no hospitalization

Highly conformal dose distribution and good healthy tissue sparing

Since source is implanted less concern for localization and target motion

Very high local doses

Advantages

• Usually out-patient no hospitalization

• Highly conformal dose distribution and good healthy tissue sparing

• Since source is implanted less concern for localization and target motion

• Very high local doses

Types of Brachytherapy

• Intracavitary implant, in which sources are placed into body cavities close to the tumour volume.

• Interstitial implant, in which sources are implanted surgically within the tumour volume.

• Surface (mould) implant, in which sources are placed over the tissue to be treated.

• Intraluminal implant, in which sources are placed in a lumen.

• Intraoperative implant, in which sources are implanted into the target tissue during surgery.

• Intravascular implant, in which a single source is placed into small or large arteries.


Interstitial• Surgical placement of sources within the tissue

• Temporary or permanent

• Most common application for permanent implant of prostate cancer

• Other sites include: some gynecological tumours, breast tumours and head and neck


Interstitial Brachytherapy

Interstitial brachytherapy can be either temporary or permanent.

Temporary brachytherapy.

Most widely used radionuclide at the present time is iridium-192


Temporary Brachytherapy.

The relatively short half-life of iridium-192 (70 days) means that a range of dose rates is inevitable. It is important, therefore, to correct the total dose rate.

Iridium-192 has two advantages:

1) The source size can be small, and2) Its lower photon energy makes radiation

protection easier than with radium or cesium-137


Temporary Brachytherapy.

Sources of thisradionuclide are ideal for use with computer-controlled remote afterloadersintroduced in 1990s.


Permanent Interstitial Implants

Encapsulated sources with relatively short half-life can beleft in place permanently. There are two advantages for thepatient:

1) An operation to remove the implant is not needed,2) the patient can go home with the implant in place.

Iodine-125 has been used most widely to date for permanentimplants.The total prescribed dose is usually about 160 Gy at the peripheryof the implanted volume, with 80 Gy delivered in the firsthalf-life of 60 days.A major advantage of iodine-125 is the low energy of the photonsemitted (about 30 keV).


Permanent Interstitial Implants

A number of other new radionuclides are under considerationas sources for brachytherapy that share with iodine-125 theproperties of a relatively short half-life and low-energy photonemission to reduce problems of radiation protection.

Types of Brachytherapy Intracavitary

• Sources placed within body cavity via special applicator

• Intracavitary treatments are always temporary

• Most common implant worldwide

• Most gynecological tumours of the uterine cavity and vagina

• Intraluminal: sources through lumen of vessel; e.g. bronchus, esophagus or bile duct


Surface Molds• Radioactive sources placed in specially

designed surface molds

• Molds placed on surface of target to treat superficial tissue

• Less commonly used

Loading systems

Manual hot loading• Initial technique with sources placed by staff directly

into patient, high dose to staff

Manual Afterloading• Needles, catheters, or applicators inserted into tumour

• Confirm position, then load radioactive seeds by hand

Remote Afterloading• As above, but seeds are loaded remotely under

computer control to minimize dose to staff

Duration of treatmentPermanent implants

• Many radioactive sources implanted and left to decay within patient

• Distribution of sources cannot change after implant

• Isotopes with low energy (tens of keV) and short half lives (~days)

• 125I, 198Au and 103Pd

Duration of treatment

Temporary implants• Radioactive source placed near treatment site for short

time period (up to 20 minutes) then removed when desired dose is delivered

• Better control of dose distribution

• Usually fractionated

• Can alter insertion in subsequent fractions

Dose RatesLow dose rate: LDR (0.4 to 2.0 Gy/hour)

• Permanent, manual afterloading techniques• Temporary treatments over days requiring hospitalization• Most common historically

Medium dose rate (2 to 12 Gy/hour)• Rarely used, pulsed dose rate (high dose rate exposed for

5 – 10 min / hour) ~LDRHigh dose rate: HDR (>12 Gy/hour)

• High activity 10 CI 192Ir source, remote afterloading• Out-patient treatment, time ~ minutes• Requires increased radiation shielding • Renewed interest in brachytherapy

Technical

Sources

Brachytherapy sources are usually encapsulated; the capsule serves several purposes:

Containing the radioactivity;

Providing source rigidity;

Absorbing any and, for photon emitting sources, radiation produced through the source decay.

SourcesAlthough 226Ra was original brachytherapy source –

not idealCharacteristics

• Energy of -ray (shielding)• Half life• Toxicity• Machinability• Specific activity• Source strength;• Inverse square fall-off of dose with distance from the

source

Brachytherapy sources

Isotope <Energy> (keV) HVL (mmPb) Production T1/2 (Rcm2mCi-1hr-1) Activity DecayPd-103 21 0.008 neutron 17d 1.5 5 mCi EC, I-125 28 0.025 neutron 60d 1.5 0.5 mCi EC, Ir-192 360 2.5 neutron 74d 4.7 10 Ci Cs-137 662 5 fission 30y 3.3 20 mCi Ra-226 1030 8 natural 1600y 8.25 Co-60 1250 12 neutron 5.25y 13 5-10 kCi

192Iridium

Complicated -ray spectrum with average energy ~ 360 keV

Moderate -ray energy requires less shielding than some isotopes

Short half life means sources replaced routinely (3 months)

High specific activity allows for small source size (0.6 x 5 mm) with high activity (10 Ci)

Brachytherapy sources(a) active length, the

distance between the ends of the radioactive material; (b) physical length, the distance between the actual ends of the source; (c) activity or strength of source, (mg radium content); and

(d) filtration, transverse thickness of the capsule wall, (mm Pt)

Varian (VariSource)

0.6 diameter, 5mm long iridium seed welded to Nitinol (Ni/Ti) flexible wire

Wire mechanically drive out of afterloader through one of 20 channels

Program range of treatment positions and dwell times

Typically 0.5 cm step sizes (up to 20 steps)

Stepping of source

150 cm147 cm

to machine

OperationSource stored within unit in shielded safeReceives command to send out sourceSends out ‘dummy’ wire to check path clearStepper motor drives active wire out to

most distal desired position of 1st channelRetracts wire in planned steps and dwells at

each position for required timeRetract source completed and repeat in

other channels if required

VariSource Afterloader

Calibration of Brachytherapy Sources

Specification of Source Strength • Activity

– Number of dis/s (1 Ci = 3.7 1010 dps) • Exposure rate (ER) at a specified distance

– NCRP recommendation, usually at 1 m • Equivalent mass of radium

– Divide ER by the ER constant for Radium • Apparent activity

– Bare point source (no filtering) • Air kerma strength

– AAPM recommendation

Historically source strength specified by exposure, X [R]

Currently source strength specified by air kerma strength (SK)

Relating the two SK = X(R/h) (W/e)

= X(R/h) (8.76 x 103 m2 mGy/R)• (W/e) average energy absorbed per unit charge

of ionization in air (0.876 cGy/R)

Exposure Rate Calibration

Exposure is being phase-out and Air kerma is replacing it.

• Open air measurements

– Hard to measure output, used to calibrate sources

• Well-type ionization chambers

– Calibrated against a calibrated source

– More suitable for routine work

Exposure rate calibrationNIST calibrates in air at 1m with spherical graphite

chamber

Used to calibrate well type ionization chamber

These are used for routine calibration

Recalibrated every two years at accredited dose lab

Specific to isotope of interest

Activity calibration

Well-type chamber to measure activity and compare to manufacturer specifications

Correct for temperature and pressurePerform measurements at chamber “sweet

spot” – highest dose responseagree to within 0.5%

Well-type ion chamber

Dose Calculations

Dose is determined by three factors

(a) Inverse square law ( (

(b) Attenuation in tissue

(c) Scatter in tissue

(1) Point Source:

1/r2

)e-r(

Ď(P) = A Γ (1/r2) (e-r) Ks

Task Group -43 Modular method of dose calculation using tabular

data as a function of position

)(),()2/,1(

),(),( rgrF

G

rGSrD Krate

SK = air kerma strength [U = cGy cm2 h-1]

= dose rate constant (type of source, its construction and encapsulation), dose rate per unit SK at 1 cm along the transverse axis of source; =Doserate(1,/2)/SK

G(r, ) = geometry factor (accounts for falloff of photon fluence from source; 1/r2 for a point source

F(r, ) = anisotropy factor normalized to =/2

g(r) = radial dose function; radial dependence of scatter and absorption in the medium along transverse axis

TG - 43

For 192Ir, anisotropy less significantSimplify by assuming point source= 1.11 cGy h-1 U-1

g(r) available in tabular form

2

)(11.1),(

r

rgSrD K

Source LocalizationOrthogonal Imaging

Applicator positions digitized on each filmy coordinate common to bothApplicator position stored as x, y, z and

corrected for magnification using known length or geometry

Can also be done at smaller angles (stereo shift method ± 20°), larger errors

Difficulties: mag factor, many coplanar seeds and patient motion

Source LocalizationOrthogonal Imaging

Orthogonal Images

PA image LAT image

Tip of marker seed #7 in catheter #2

Source LocalizationCT Imaging

More complex implants require CT imagingProstate uses up to 20 lines insertedImplant applicators, CT image of areaDigitize applicator locations in 3DDigitize organs at risk and define tumour

volume to be treatedBetter visualization of anatomyIncreased time and transport patient to CT

Source LocalizationUltrasound Imaging

Trans-rectal ultrasound allows for visualization prostate, urethra, bladder etc

Planning ultrasound allows for generation of pre-plan

Planned needle positions referenced to grid template sutured to patient’s skin

Implant done under GA with ultrasound guidance; monitor needle placement

Clinical sites

Clinical sitesSiteTypeDose

rate

Dose/#fx

)cGy(

Imaging

EndometriumIntracavitaryHDR*1800 / 3X-ray

CervixIntracavitaryHDR*2500 / 5

2600 / 4

X-ray / CT

ProstateInterstitialHDR2100 / 3CT

InterstitialLDR (125I)

14 400US

EsophagusIntralumenalHDR1800 / 3X-ray

LungIntralumenalHDR2100 / 3X-ray

*previously LDR Cs-137

Carcinoma of the endometrium~3600 cases / year (Canada); JCC 100Standard treatment hysterectomy - curativeTreatment to dome of vagina recurrenceCombined with external beam radiationTarget vaginal mucosal lining Prescribe dose to surface of cylinderSimple application, no sedation requiredDiameter conform to patient anatomy

Vaginal treatments

Cervical cancer~ 1400 cases / year; JCC 200 Combined with external beam radiation and

chemo as curative treatmentTarget vaginal fornices, cervix and

endometriumPrescribe dose to point ATolerance dose uterine vessels cross ureterTandem length variable Sedation required, sterile procedure

Uterine Cervix

Uterine Cervix

Seed #7 cervical os

PA image LAT image

Rectal retractor

Uterine Cervix

Foley catheter

Dose to bladder and rectum

Lung cancerSurgery, external beam, chemo and brachyBrachytherapy palliative treatment onlyObstructive lung tumours – open airwayPrescribe dose 1 cm radial to catheter or to

volumePlace catheter(s) with bronchooscope under

conscious sedation Treat multiple obstructions if necessary

Prostate cancer~18000 cases/yearTreat with surgery, ext. radiation, brachyBrachy limited to early stage (I and II)

disease confined to prostateHDR implants and LDR permanent

implants usedBoth require high dose to prostate volume

and sparing of urethra and rectum

HDR procedureNeedles and catheters inserted into prostate with

template under US guidanceCT of volume; contouring of prostate and organs

at riskTreatment plan created and treatment deliveredCatheters can be removed and reinserted for

subsequent fractions, orCatheters remain in place and treatment repeated

later (Day 1: CT am treat pm; Day 2: treat am and pm

Dose to prostate ~2000 cGy (2x10 or 3x7)

Misc. SitesInterstitial brachytherapy can be performed

nearly anywhere•Breast tumours•Brain tumours•Head and neck tumours•Gynecological implants

Intravascular brachytherapy (restenosis)

Bile duct

Nasopharynx

Partial Breast Treatment

brachytherapy

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