IAEAInternational Atomic Energy Agency

Brachytherapy

Radiation Sources in Radiotherapy

Day 7 – Lecture 5

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Objective

To become familiar with the sealed sources, devices and ancillary equipment used in brachytherapy.

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Contents

• Radioactive sources;• Afterloading devices (HDR and LDR);• Implants (permanent and temporary);• Intravascular brachytherapy (IVB);• Other techniques (e.g. eye applicators);• Brachytherapy treatment planning.

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• Brachytherapy (also referred to as Curie therapy) is defined as a short-distance treatment of malignant disease with radiation emanating from small sealed (encapsulated) sources.

• The sources are placed directly into the treatment volume or near the treatment volume.

Introduction

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Brachytherapy compared to external beam therapy

Advantages of Brachytherapy•Improved localized dose delivery to the target•Sharp dose fall-off outside the target•Better conformal therapyDisadvantages of Brachytherapy•Only good for well localized tumors•Only good for small lesions•Very labor intense

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• very flexible radiotherapy delivery;

Brachytherapy

• the source position determines the success of the treatment;

• in principle, it is the ultimate ‘conformal’ radiotherapy;

Conformal radiotherapy describes the attempt to conform the treatment volume as closely as possible to the actual target volume thereby sparing the surrounding normal tissue as much as possible. In general, the normal radiation safety considerations will also apply to conformal radiotherapy.

• highly individualized;

• dependent on operator skill and experience.

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Brachytherapy Sources

Source

T1/2 γ (MeV) HVL (mm Pb) Forms

222Rn 3.83 d 0.047 - 2.45 (0.83 avg) 8.0 Seeds

60Co 5.26 y 1.17, 1.33 11.0 Tubes, needles, pellets

137Cs 30 y 0.662 5.5 Tubes, needles, pellets

192Ir 74.2 d 0.136 - 1.06 (0.38 avg) 2.5 Wires, ribbon etc.

198Au 2.7 d 0.412 2.5 Seeds

125I 60.2 d 0.028 avg 0.025 Seeds

103Pd 17 d 0.021 avg 0.008 Seeds

226Ra 1600 y 0.047 - 2.45 (0.83 avg) 8.0 Tubes, needles

90Sr 28 y 2.25 (beta) Curved applicator

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Brachtherapy Sources are commonly used as sealed sources, usually doubly encapsulated in order to:

Provide adequate shielding against alpha and beta radiation produced through source decay

Contain radioactive material

Prevent leakage of the radioactive material

Provides rigidity of the source

Practical Considerations

.

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• a pure gamma emitter with an energy suitable for the intended treatment site;

with a high specific activity; and suitable for high dose rate applications.

“Ideal” Brachytherapy Source

The ideal brachytherapy source would be:

• physically small;

• for temporary implants, a long half life; to allow economical re-use of sources

• for permanent implants, a medium half life.

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Types of equipment

Brachytherapy

• Manual brachytherapy;

• Afterloading devices.

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In manual brachytherapy several types of treatments are available, including:

Manual brachytherapy

• interstitial treatment of cancer;

• intra-cavity treatment of cancer;

• eye plaque implants; and

• topical application.

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• 137Cs and 60Co as sealed sources in needles and applicator cells;

Interstitial Treatments

The following sources are routinely used:

• 192Ir as seeds or wires encased in nylon ribbons;

• 198Au, 125I and 103Pd as sealed sources in seeds.

• 90Sr as a sealed source in an applicator for the treatment of superficial eye conditions.

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• The eye plaque consists of a curved, soft plastic insert that has a series of grooves molded into the rear convex surface to hold the radioactive seeds.

Eye plaques

• Because the plaque is placed in the orbit of the eye over the tumor site and sutured to the sclera, this is considered interstitial, not topical / surface treatment.

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The following sources are routinely used:

• 90Sr as a sealed source in an applicator for treatment of superficial eye conditions.

Topical (surface) treatments

• 137Cs and 60Co as sealed sources in needles and applicator cells.

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History

Surface applicator with irregular distribution of radium on the applicator surface.

(Murdoch, Brussels 1933)

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Treatment of superficial lesions with radioactive sources in close contact with the skin.

A mould for the back of a hand including shielding designed to protect the patient during treatment.

Surface Moulds

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Treatment of superficial lesions with radioactive sources in close contact with the skin.

Surface Moulds (cont)

Mould for the treatment of squamous cell carcinoma of the forehead.

Catheters for source placement.

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Fast dose fall-off in tissues due to the inverse square law. Can conform the activity to any surface.

Advantages

Surface Moulds (cont)

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Brachytherapy Applicators

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Storage for radioactive sources must:-• provide protection against

environmental conditions• be only for radioactive materials• provide sufficient shielding• be resistant to fire• be secure

Source storage

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Safe for 137Cs sources

Numbered and easily identifiable source drawers with color coding of sources

Source storage (cont)

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• be stored securely to prevent unauthorized access;

Source accountability

Sources must:

Source inventory records should identify:• the radionuclides and source activities;• the location and description of sources; • disposal details (including permanent implants in patients).

• be audited regularly to confirm the source inventory in compliance with the requirements of the Regulatory Body.

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• Afterloading techniques are those in which non-radioactive applicators or guide tubes are initially placed in the patient and the radioactive sources are later loaded into these applicators.

Afterloading devices (HDR and LDR)

• Depending on the manufacturer and model, remote afterloaders typically employ 192Ir, 60Co or 137Cs sources.

HDR – high dose rate / LDR – low dose rate

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• HDR units typically have a single source with activities ranging from ~180 GBq to ~740 GBq.

HDR and LDR devices

• LDR units may have single or multiple sources with activities of ~370 MBq to ~550 MBq.

• Because of the higher activities involved, the shielding requirements for HDR are greater than LDR units.

• HDR devices typically step the single source through a series of dwell positions while a LDR device does not reposition its multiple source string.

• HDR employs shorter irradiation time at higher dose rates than LDR treatment.

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• Treatments are usually fractionated (e.g. 6 fractions of 6 Gy each);

HDR brachytherapy

• Either the patient has a new implant each time or stays in hospital for bi-daily treatments;

• The time between treatments should be >6 hours to allow normal tissue to undertake some recovery.

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Catheters are indexed to avoid mixing them up.

Transfer catheters are locked into place during treatment - green light indicates the catheters that are in use.

HDR brachytherapy (cont)

Varian

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Portable HDR Unit

HDR brachytherapy (cont)

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LDR (137Cs) Cervix insertion• 10 pellets each 550 MBq = 5.5 GBq total• ~ 0.2 mSv/h at 1 m• ~ 5 days for 1 mSv

HDR and LDR radiation levels near patients

HDR (192Ir) • 370 GBq source• ~ 47 mSv/h at 1 m• ~1.3 minutes for 1 mSv

The treatment room door should be interlocked

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• This technology uses radioactive catheters, pellets, and stents to treat coronary and peripheral vascular problems.

Intravascular brachytherapy

• The radioactive source can be ion implanted, plated, or encapsulated in a sealed source device attached to a guide wire used in the angioplasty procedure.

• The radioactive device can be either permanently implanted or removed via the guide wire following treatment of the affected vessel wall.

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• After opening of a blocked blood vessel by angioplasty there is a high likelihood (60%+) that the vessel will again block. i.e. restenosis will occur.

Intravascular brachytherapy (cont)

Purpose of treatment

• Gamma sources – 192Ir; Beta sources - 32P, 90Sr/Y, 188Re

• Radiation is a proven agent to prevent growth of cells and has been shown to be effective in preventing restenosis

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• have a finite range in tissue;• have fewer radiation safety

issues• physically smaller size

Intravascular brachytherapy (cont)

Beta sources

Beta-Cath™ System (Novoste)

Delivery catheter

Hydraulic delivery

Source train (90Sr)

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Pre-PTCA

Post-PTCA

6 Months later

Intravascular brachytherapy (cont)

During treatment

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• Optimization procedure needs extensive checking.• How are dwell times transferred to the treatment unit?• Where is the source strength correction applied?• How is transfer time modeled?

HDR Planning

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Dose calculation algorithm

Typically, there is no user control for these features. However it is essential that the user:-

Software coding and implementation

• is familiar with the physics algorithm;

• is aware of its implementation and possible software shortcuts;

• has tested the algorithm for most possible treatment scenarios.

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The following should be done on receipt of sources and documented.

Tests for brachytherapy sources

• Radionuclide distribution and uniformity:

autoradiograph;

uniformity of activity amongst seeds;

visual inspection of seeds in ribbons.

• Physical / chemical form;

• Source encapsulation, wipe test;

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• Ideally, the calibration of all sources should be checked on receipt.

Calibration

Tests for brachytherapy sources (cont)

• However, for a large number of short lived sources a sample check (~10%) may be satisfactory.

• Document results.• Suggested calibration tolerances:

mean of batch (3%) deviation from mean (5%)

• Review manufacturer’s documentation for tolerances.

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•Source calibrators are radioisotope and source design specific and should be calibrated by an organization recognized by the Regulatory Authority, noting:-

Brachytherapy source calibrators

• Precision

• Scale factors and linearity

• Ion collection efficiency

• Geometry and length dependence

• Energy dependence

• Dependence on the wall of the source

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• Use a long half-life encapsulated source with:-• reliable mechanical integrity• well known decay constant• a manufacturer’s calibration certificate

Brachytherapy source calibrators (cont)

Quality Assurance

Consider cross checking with another calibrator

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• Johns H E and Cunningham J R 1983 The Physics of Radiology, 4th edition (Springfield: C Thomas).

• Khan F M 1994 The Physics of Radiation Therapy, 2nd edition (Williams & Wilkins, Baltimore).

• Williams J R and Thwaites D I 1993 Radiotherapy Physics in Practice (Oxford: Oxford University Press).

• International Commission on Radiation Units and Measurements. Determination of dose equivalent resulting from external sources, ICRU report 39, Bethesda: ICRU; 1985.

• International Commission on Radiation Units and Measurements ICRU 55, Bethesda. Prescribing, recording, and reporting interstitial brachytherapy, 1993.

References