BrachytherapyRTT Review Course Laura Doyle, PhD

Application of lecture to the exam• Applicable section of ARRT Examination Content to Brachytherapy• Safety: 1.A.1. Sources of Radiation, radioactive material• Safety: 2.C.1. Measurement of Radiation

– Units of measurement, Instrumentation• Safety: 2.D.2. Fundamental principles, Basic methods of radiation

protection• Safety: 1.E. Quality Control Procedures• Procedures: 2.A.1. Radiation Therapy Techniques• Procedures: 3.A. Treatment prescription• Procedures: 4.A.3.b. Brachytherapy

– Only basic concepts related to common uses of brachytherapy are covered on the exam, including dose to surrounding tissue and radiation protection issues. Specific procedures and isotope characteristics are not covered.

• https://www.arrt.org/docs/default-source/discipline-documents/radiation-therapy/thr-content-specifications.pdf?sfvrsn=88de01fc_18

Brachytherapy - Definition

• Refers to the treatment of cancer with radiation at close distances

• Uses small, sealed sources of radiation

Brachytherapy Applications• Intracavitary- sources placed into body cavity close to target volume

– Vaginal cylinder• Interstitial- sources implanted within tissue/tumor

– Head and Neck• Surface- sources placed on top/over the tissue

– Eye plaque, skin• Intraluminal- sources placed inside a lumen

– Lung• Intraoperative- sources implanted during a surgery

– Lung mesh• Intravascular

– Beta-Cath

Brachytherapy Classifications

• Temporary– Dose delivered over a short time and sources are

removed once prescribed dose has been fulfilled

• Permanent– Dose is delivered over the radioactive life of the

source until complete decayed

Brachytherapy Classifications

• Low Dose Rate- 0.4-2 Gy/hr• High Dose Rate- >12 Gy/hr

Definitions from ICRU report 38

Brachytherapy Isotopes

• Selecting the most appropriate isotope depends on a number of factors:– Energy– Half life– Shielding considerations– Source strength– Dose rate – Economic considerations

Brachytherapy Isotopes

• Ra-226*• Cs-137*• Ir-192• I-125• Ra-223

• Pd-103• Au-198• Sr-90/Y-90• Cs-131

Types of Radiation

• Gamma • Characteristic x-rays• Alpha• Beta

Review of decay equation

teAA λ−= 0

where: A0 = Initial activity; A= activity after time t; t = time;λ= decay constant

λ =0.693

T12

Half life

• Half Life (T1/2): The time required to reduce its activity to half of its original value.

Example of half life

• 10 Ci Ir-192 Source on 1/1/15• What is the activity of the source on 4/14/15?• Where do we start…

– We know half life = 74 days– We need to calculate the time from original

activity…74 days

–STOP! Think before you calculate

Example of half life

• 10 Ci Ir-192 Source on January 1st• What is the activity of the source on April

15th?

daysdays

eA 7474*693.0

10−

=

Approximation of source decay

Isotope Half Life Approximate time for 1% decay

Co-60 5.26 yr 1 month

Cs-137 30 yr 5 months

Ir-192 73.8 day 1 day

I-125 59.4 day 1 day

Ra-226 1622 yr 23 years

Units of activity/source strength

• Apparent activity: mCi, Ci, Bq• Activity (A): Number of disintegrations per

second(dps)• 1 Ci = 3.7 x 1010 dps• 1 dps = 1 Becquerel (Bq)

• Equivalent mass of Radium: mgRaEq• Air Kerma Strength: U = cGycm2h-1

Remote afterloading systems

• Most commonly Ir-192• Usually single source, maximum 12 Ci• Used for intracavitary, interstitial, surface,

intraluminal applications

Ir-192

• 73.8 day half life• Usually replaced quarterly in remote

afterloading units• Mean energy- 380 keV

Attenuation Ir-192

• Half value layer – Lead: 6mm; Concrete: 43mm

• Tenth value layer – Lead: 16mm; Concrete: 152mm

– Remember that you can’t TURN OFF a radioactive source, so you must rely on sufficient shielding

IAEA Report 47 p 109

Dwell time/Dwell position

• Dwell position- location where the source stops

• Dwell time- time the source spends at each location

• Step size- distance between dwell positions

• As the source decays, dwell time must increase to achieve the same dose distribution

Dwell time/source activity

Dwell time Sourceactivity

Treatment with Remote Afterloader

• Patient is positioned on table/bed/stretcher• Afterloader is secured close to the patient• Applicator or needles are connected to HDR

afterloader through specified channels• Personnel exit the room and the treatment is

delivered• Patient is monitored via audio and visual means• The patient and room are surveyed after the

conclusion of the treatment

Advantages of remote afterloading brachytherapy

• Decreased radiation exposure to personnel• Shorter treatment times• Outpatient procedure• Decreased size of applicators

Brachytherapy Written Directive

• Must Specify:– Applicator– Isotope– Total Dose– Dose/Fraction– Point/Volume

HDR Documentation

• Written Directive• Source Activity• Total Dwell time• Dwell positions• Number of channels/catheters• Room/Patient survey

Emergency Personnel/Equipment

• Must have on hand:– Lead pig– Wire cutters– Large Forceps– Syringe (for mammosite/balloon applicators)

• Most states require at least an AMP and AU present throughout the entire treatment. Know your state regulations!!!

HDR applications• Cervical• Endometrial• Vaginal• Breast• Endobronchial• Intravascular• Prostate• Surface/skin

Intracavitary GYN Brachytherapy

• Cervix• Tandem and ovoids• Tandem and ring• LDR or HDR techniques• Source distribution or dwell times based on

dose to Point A

Tandem, ovoids, and ring

Nucletron product catalog

Ring and Tandem/ovoids dose distribution

• “Pear distribution”• Normalized to point A

Dose Specification – Point A

• Uterine vessels cross ureters• 2 cm superior to external os• 2 cm lateral to cervical canal• Relative to tandem position

Dose specification- Point B

• Point B – parametrium/nodes• 2 cm superior to os and 5 cm lateral from mid-

pelvis• Does not move with shift of tandem or uterus• Typically 25% of prescription dose

Points A & B

AB

ICRU points

• Bladder and Rectum• Maximum dose approximately 80% of Point A

or prescription dose• Can be shielded by ovoid shields or cap on

ring

Vaginal cylinder

Surface dose and cylinder

• Dose prescribed to surface or 0.5 cm from the surface

• Treatment length• Vaginal cancers• Vaginal cuff/apex for

endometrial cancer

r+0.5r

Dose Specification – Cylinder Diameter

• As cylinder diameter increases– Treatment time to

achieve same dose increases

– Mucosal dose compared to dose at 5 mm depth decreases

Dose Specification – Cylinder Diameter

• WHY??• Remember ISL• Choose the largest

cylinder

0

100

200

300

400

500

600

1 2 3 4 5

Dose

(cG

y)

Distance from source (cm)

Inverse Square Law

Bladder/Rectum points

• Rectum- 5mm from posterior vaginal wall• Prescription point generally 5mm from

cylinder surface• Bladder- identified with Foley balloon and

contrast

Interstitial GYN

• SYED implant• Either LDR or HDR• Involves overnight stay in hospital and surgical

placement of device

Accelerated Partial Breast Irradiation

• 3.4 Gy/fx to 1 cm from surface of balloon/applicator

• 10 fractions, 2 fractions per day• Variable fill volume based on cavity size• Single or multiple lumens, multiple dwells

Multi-lumen Breast Applicators

Mammosite Training Guide

Multi-lumen Breast Applicators

Image from: http://www.ciannamedical.com/savi/ & http://www.hologic.com

Prostate• Monotherapy or combined with EBRT• TJU protocol: 15 Gy followed by external beam boost• Plastic needles, similar procedure to permanent LDR implant

Leipzig/Valencia Applicators

• Tungsten shielded• Superficial skin, intra-oral and

vaginal cuff• Alternative to orthovoltage or

electrons

HDR for Skin Malignancies

http://www.nucletron.com/en/ProductsAndSolutions/Pages/LeipzigApplicatorSet.aspx

Flap Applicators

• Adapts to any shape or location on body (great for extremities and curved surfaces

• Ensures fixed skin to catheter distance of 5 mm

HDR for Skin Malignancies

Beta-Cath system

• Sr-90• 20, 40, 60 mm source train• Prescription based on vessel

diameter and injury length (between 18-23 Gy)

• Treatment indication: prevention of in-stent restenosis

Intravascular brachytherapy

Advances in HDR Brachytherapy• Imaging

– CT planning has allowed for better assessment of OAR doses as well as target coverage (points vs DVH)

– Useful when combining with external beam or in situations of retreatment

– Fusion capability allows for more accurate assessment of applicator placement in subsequent fractions and delineation of target and critical structures

• Dosimetry– Optimization– Isodose manipulation– Dwell time and position flexibility

Low Dose Rate Brachytherapy

LDR Applications

• Prostate• Cervix• Vaginal• Breast• Head and neck• Surface

Prostate Seed Implant

• Most common indication – early stage prostate cancer

• Permanent placement of radioactive sources• Total dose (I125:145 Gy; Pd103: 110Gy)• Varies based on technique

– Pre-planning– Real-time planning

• Inverse optimization

Prostate Seed Implant - Procedure

• Volume Study – TRUS or CT• Pre-plan/nomogram• Seed order• Implant date

– US– Needle placement– Seed insertion

• Imaging• Post-implant dosimetry

Common LDR Sources

I-125• Half life- 60 days• Mean Energy- 28 keV• HVL- 0.025 mm Pb

Pd-103• Half life- 17 days• Mean Energy- 21 keV• HVL- 0.008 mm Pb

Cs-131• Half life- 9.7 days• Mean Energy- 30 keV• HVL- 0.025 mm Pb

LDR Source Composition

• Physical length• Active length• Contain radio-opaque

markers

SIR and Thera Spheres• SIRT

– Selective Internal Radiation Therapy

• Y-90 microspheres inserted into hepatic artery become lodged in tumor/normal liver vasculature (β emitter)

• Goal– To deliver a high dose to liver tumor while sparing

normal liver (achievable due to preferential blood flow and spheres uptake in tumors)

Radioembolization• Y-90

– Beta emitter– Average beta emission energy = 0.93

MeV– Average penetration range in tissue

~ 2.5mm – Physical half life = 64.1 hr– Sir-spheres (resin) for hepatic

metastases– Theraspheres (glass) for

unresectable hepatocellular carcinoma (HCC)

– 20-40 µmCourtesy of Sirtex

Radiation Safety and QA

HDR Brachytherapy Quality Assurance

• Daily checks• Quarterly Source Exchanges• Annual checks

LDR Brachytherapy Quality Assurance

• Per patient source assay• Inventory• Storage and decay• Annual checks

Brachytherapy vs. External Beam

• Distance from source of radiation to treatment site

• Duration of treatment• Total dose and fractionation schemes• Heterogeneity corrections*

Inverse Square Law

• Main component of dose reduction for higher energy brachytherapy sources

• Remember the equation:

I2I1

D12

D22

Handling of Radioactive Sealed Sources

• Remember the basics of Radiation Safety:– Time– Distance– Shielding

• Always practice ALARA

Radiation Detectors

• Ionization Chamber

• GM Meter

• Scintillation Detector

Thank you!

Good Luck!