1. Radiation Sources in Radiotherapy
Brachytherapy
Day 7 – Lecture 5

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

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

4. Introduction
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.

5. 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 therapy
Disadvantages of Brachytherapy
Only good for well localized tumors
Only good for small lesions
Very labor intense

6. Brachytherapy very flexible radiotherapy delivery;
the source position determines the success of the treatment;
in principle, it is the ultimate ‘conformal’ radiotherapy;
highly individualized;
dependent on operator skill and experience.
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.
The lecturer can cross reference brachytherapy to the modern external beam attempts of conformal RT and even intensity modulated radiotherapy (IMRT). Both of these techniques (conforming the high dose region to the target volume and optimizing the dose distribution in target and normal tissues) have been implemented in brachytherapy well before they were technologically possible in EBT. This has been possible as the brachytherapy oncologist is literally in touch with the tumour region and more recently has acquired the potential for optimization due to stepping source high dose rate brachytherapy.

7. Brachytherapy Sources
γ (MeV)
HVL (mm Pb)
Forms
222Rn
3.83 d
(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
192Ir
74.2 d
(0.38 avg)
2.5
Wires, ribbon etc.
198Au
2.7 d
0.412
125I
60.2 d
0.028 avg
0.025
103Pd
17 d
0.021 avg
0.008
226Ra
1600 y
Tubes, needles
90Sr
28 y
2.25 (beta)
Curved applicator

8. Practical Considerations
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 .

9. “Ideal” Brachytherapy Source
The ideal brachytherapy source would be:
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.
physically small;
Lecture notes:The requirements of long half life and high specific activity are working against each other.
Instructions for the lecturer/trainer
for temporary implants, a long half life;
to allow economical re-use of sources
for permanent implants, a medium half life.

10. Types of equipment Brachytherapy Manual brachytherapy;
Afterloading devices.

11. Manual brachytherapy
In manual brachytherapy several types of treatments are available, including:
interstitial treatment of cancer;
intra-cavity treatment of cancer;
eye plaque implants; and
topical application.

12. Interstitial Treatments
The following sources are routinely used:
137Cs and 60Co as sealed sources in needles and applicator cells;
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.

13. Eye plaques
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.
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.

14. Topical (surface) treatments
The following sources are routinely used:
90Sr as a sealed source in an applicator for treatment of superficial eye conditions.
137Cs and 60Co as sealed sources in needles and applicator cells.

15. History
Surface applicator with irregular distribution of radium on the applicator surface.
(Murdoch, Brussels 1933)

16. Surface Moulds
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.

17. Surface Moulds (cont)
Treatment of superficial lesions with radioactive sources in close contact with the skin.
Mould for the treatment of squamous cell carcinoma of the forehead.
Catheters for source placement.

18. Surface Moulds (cont) Advantages
Fast dose fall-off in tissues due to the inverse square law. Can conform the activity to any surface.

19. Brachytherapy Applicators

20. Source storage Storage for radioactive sources must:-
provide protection against environmental conditions
be only for radioactive materials
provide sufficient shielding
be resistant to fire
be secure

21. Source storage (cont) Safe for 137Cs sources
Numbered and easily identifiable source drawers with color coding of sources

22. Source accountability
Sources must:
be stored securely to prevent unauthorized access;
be audited regularly to confirm the source inventory in compliance with the requirements of the Regulatory Body.
Source inventory records should identify:
the radionuclides and source activities;
the location and description of sources;
disposal details (including permanent implants in patients).
MODIFIED

23. Afterloading devices (HDR and LDR)
HDR – high dose rate / LDR – low dose rate
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.
Depending on the manufacturer and model, remote afterloaders typically employ 192Ir, 60Co or 137Cs sources.

24. HDR and LDR devices
HDR units typically have a single source with activities ranging from ~180 GBq to ~740 GBq.
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.
Lecture notes: ( about 100 words)
Instructions for the lecturer/trainer

25. HDR brachytherapy
Treatments are usually fractionated (e.g. 6 fractions of 6 Gy each);
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.

26. Catheters are indexed to avoid mixing them up.
HDR brachytherapy (cont)
Catheters are indexed to avoid mixing them up.
Varian
Transfer catheters are locked into place during treatment - green light indicates the catheters that are in use.

27. HDR brachytherapy (cont)
Portable HDR Unit

28. HDR and LDR radiation levels near patients
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 (192Ir)
370 GBq source
~ 47 mSv/h at 1 m
~1.3 minutes for 1 mSv
The treatment room door should be interlocked

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

30. Intravascular brachytherapy (cont)
Purpose of treatment
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.
Radiation is a proven agent to prevent growth of cells and has been shown to be effective in preventing restenosis
Gamma sources – 192Ir; Beta sources - 32P, 90Sr/Y, 188Re

31. Intravascular brachytherapy (cont)
Hydraulic delivery
Beta sources
have a finite range in tissue;
have fewer radiation safety issues
physically smaller size
Source train (90Sr)
Delivery catheter
Beta-Cath™ System (Novoste)

32. Intravascular brachytherapy (cont)
During treatment
6 Months later
Pre-PTCA
Post-PTCA

33. HDR Planning 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?

34. Dose calculation algorithm
Software coding and implementation
Typically, there is no user control for these features. However it is essential that the user:-
is familiar with the physics algorithm;
is aware of its implementation and possible software shortcuts;
has tested the algorithm for most possible treatment scenarios.

35. Tests for brachytherapy sources
The following should be done on receipt of sources and documented.
Physical / chemical form;
Source encapsulation, wipe test;
Radionuclide distribution and uniformity:
autoradiograph;
uniformity of activity amongst seeds;
visual inspection of seeds in ribbons.

36. Tests for brachytherapy sources (cont)
Calibration
Ideally, the calibration of all sources should be checked on receipt.
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.

37. Brachytherapy source calibrators
Source calibrators are radioisotope and source design specific and should be calibrated by an organization recognized by the Regulatory Authority, noting:-
Precision
Scale factors and linearity
Ion collection efficiency
Geometry and length dependence
Energy dependence
Dependence on the wall of the source

38. Brachytherapy source calibrators (cont)
Quality Assurance
Use a long half-life encapsulated source with:-
reliable mechanical integrity
well known decay constant
a manufacturer’s calibration certificate
Consider cross checking with another calibrator

39. References
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.