Radiology

1. SEALED RADIONUCLIDE
Dr. Amrita Rakesh
DNB-RT
1

2. What is a sealed radioactive
source?
• Sealed radioactive sources are used widely in medicine, industry,
and agriculture. A sealed radioactive source is radioactive material
that is permanently sealed in a capsule or bonded and in a solid
form.
• In most practices, a sealed radioactive source is installed in a
device that is designed either to allow the source to move safely
out of the shielded device to where the radiation beam is used and
to return to the shielded device after the operation is complete.

3. • Brachytherapy is a method of treatment in which sealed
radioactive sources are used to deliver radiation at a
short distance by interstitial , intracavitary , or surface
application.
• In the past, brachytherapy was carried out mostly with
radium or radon surfaces. Currently, use of artificially
produced radionuclides such as Cs-137 , Ir-192 , Au-
198 , I-125 , Pd-103 is more common.

4. Sealed Sources
• Obsolete - Radium , Radon.
• Currently used sealed sources - Cesium , Iridium
,Cobalt , Iodine , Palladium , Gold ,Strontium.
• Developmental sealed sources - Americum ,
Ytterbium , Californium , Samarium

5. Basic Terminology
• Isotope - isotopes have the same number of protons and
electrons, but a different number of neutrons.
• Half-Life - the half-life of a radioactive substance is the
amount of time it takes for half of its atoms to decay.
• HVL - half-value layer (HVL) is the thickness of an
absorber of specified composition required to attenuate
the intensity of the beam to half its original value.

6. • Exposure- Rate Constant - the activity of a radionuclide
emitting photons is related to the exposure rate by the
exposure rate constant. In brachytherapy, this constant is
usually expressed as numerically equal to the exposure
rate in R/h at a point 1 cm from a 1-mCi point source. In
case of radium, it is expressed as milligrams of radium
instead of mCi.
• Specific Activity - amount of radioactivity or decay rate
of a radionuclide , per unit mass of the radionuclide.

7. • Low–dose-rate (LDR) implants deliver doses at the rate
of 40 to 200 cGy/hour (0.4 to 2 Gy/hour), requiring
treatment times of 24 to 144 hours, during which the
patient is confined to an inpatient treatment room.
• At the other extreme, high–dose-rate (HDR) BT uses
dose rates in excess of 0.2 Gy/minute (12 Gy/hour). In
fact, modern HDR remote afterloaders deliver
instantaneous dose rates as high as 0.12 Gy/second
(430 Gy/hour) at a distance of 1 cm, resulting in
treatment times of a few minutes.

8. • Medium dose-rate delivery, defined as the 2- to 12-
Gy/hour range, rarely is used.
• the ultra-low–dose-rate range (0.01 to 0.3 Gy/hour) is
of great importance; it is the dose-rate domain used
in permanent implants with 125I and 103Pd seeds.

10. Radium-226
• Radium 226 Sources - first radionuclide isolated.
• The unit of activity, the curie (Ci) , originally was defined as the
rate of disintegration within 1g of Ra-226.
• Ra-226 decays to gaseous Rn-222 with a half-life of 1,626 years.
• y-rays energy ranges between 0.05 to 2.4 MeV, with average
energy of about 0.8MeV.
• Specific Activity = 1

11. • Exposure weighted average energy of Ra-226 is 1.25 MeV when its
photon spectrum is filtered by 0.5 mm of platinum. It reduces the
surface dose contributed by b-particles to a negligible level.
• Source Construction - Radium supplied mostly in the form of
radium sulphate or radium chloride , mixed with inert filler and loaded
into cells about 1 cm long and 1 mm diameter. These cells are made
of 0.1 to 0.2 mm thick gold foil and sealed to prevent leakage of
radon gas.
• Radium sources are manufactured as needles or tubes in a variety of
lengths and activities.

12. • Source Specification
• Active Length - distance between ends of radioactive material ; 15
mm.
• Physical length - distance between the actual ends of the source
• strength of source, milligrams of radium content
• filtration, transverse thickness of the capsule wall, usually expressed
in terms of millimetres of platinum.
• Linear activity of source = activity / active length.

13. • Three types of Radium Needles
• UNIFORM - needles of uniform linear
activity.
• INDIAN CLUB - higher activity at one
end.
• DUMBBELL - high activity at both
ends
• Tube - furnished in multiples of 5 mg
of radium filtered by 1 mm platinum;
22 mm long; containing 5 to 30 mg of
radium.

14. • Hazards of Radium
• potential for damaged sources to leak radioactive
salts or emit radon gas (Rn-222).
• exposure hazard to interstitial BT practitioners.
• high cost for radium extraction.
• safe disposal is a significant financial liability.

15. CESIUM-137
• y-ray emitting radioisotope.
• used as radium substitute in both interstitial and
intracavitary brachytherapy.
• advantages over Ra- requires less shielding & less
hazardous the microsphere form
• half life of about 30 yrs ; can be used clinically for about 7
yrs without replacement.

16. • Cs-137 emits y-rays of energy 0.662 MeV.
(monoenergetic)
• Transformed to Ba-137 by b(-) decay which is a
metastable state and 93.5% of disintegration are from
Ba-137.
• exposure rate constant = 3.26 Rcm2 mCi-1 h-1.
• specific activity of 88 ci/g.

17. • supplied in the form of insoluble powders or ceramic
microspheres,doubly encapsulated in stainless steel
needles and tubes.
• wall thickness of 0.5 to 1.0 mm , active lengths of
13.5 to 15 mm , diameters of 2.6 to 3.1 mm and total
lengths of about 20 mm.

18. • Radium isodose curve exhibit significant retraction along the
longitudinal source axis due to oblique filtration through 1mm
thick platinum capsule.
• In contrast, lightly filtered Cs-137 tubes produce nearly
elliptical isodose curves.
• Consequently, vaginal applicator systems containing modern
Cs-137 sources with their axes positioned perpendicular to
the coronal plane always will give rise to higher bladder and
rectal doses than when loaded with Ra-226 tubes.

19. comparison of isodose
curves for a modern
steel-ciad Cs-137 source
(left)containing
radioactive ceramic
pellets and a Ra-226
(right) consisting of a
RaSO4 core
encapsulated in 1mm
thick platinum. Both sides
have an air-kerma
strength of 72uGy.m2.h-1
(10 mgRaEq)

20. Iridium-192
• Iridium-192 (alloy of 30% Ir and 70% Pt)
• Produced by bombarding nonradioactive Ir-191 with thermal neutrons in a nuclear
reactor.
• Ir-191 has an extremely large neutron-capture cross section, and produces no
significant contaminant radio-isotopes ; very high specific activity can be achieved.
• Fabricated in the form of thin flexible wires that can be cut to desired lengths.
• Nylon ribbons containing iridium seeds 3 mm long and 0.5 mm in diameter ,
spaced with their centres 1 cm apart, are also used.
• Both wires and ribbons suitable for after loading.

21. • Half-life = 73.8 days.
• Specific activity of 550-600 Ci/g.
• It has a complex decay scheme, dominated by b-decay to Pt-192, also including some
electron capture and b(+) decay.
• Photon spectrum includes characteristic X-rays and y-rays ranging from 63 KeV to 1.4
MeV.
• Ir-192 has complicated by y-ray spectrum with an average energy of 0.38 MeV.
• Exposure rate constant = 4.69 R-cm2/h/mCi.
• The thickness of lead and concrete shielding can be reduced by 33% and 20%
respectively as compared to Cs-137.
• Important advantage - compatibility with after-loading technique.

22. Iridium Seeds
• 0.5 mm in diameter and 3 mm
long, for LDR BT.
• Iridium seeds, encapsulated
in a 0.8 mm diameter nylon
ribbon are spaced at 1 to 0.5
cm centre-to-centre intervals,
are available in strength of 1
to 150 uGy.m2.h-1.

24. Iridium Wire
• 0.3 mm or 0.6 mm outer
diameter, consisting of an
iridium-platinum radioactive
core encased in a 0.1 mm
sheath of platinum.
• In addition to eliminating
radiation exposure hazards in
the operating room, Ir-192
ribbons and wires can be
trimmed to the appropriate
active length.
• Generally, used for one to three
patient procedures.

25. GOLD-198
• Seed or “grains” consisting of radioactive isotope of gold , Au-198 , have been used in
the past for interstitial implants.
• Au-198 seeds also been used in eye plaques for treating intraocular tumors such as
choroidal melanoma.
• Currently I-125 seeds are most commonly used for eye plaques.
• Au-198 has a half-life = 2.7 days
• Monoenergetic y-ray energy 0.412 MeV.
• b-rays of maximum energy 0.96 MeV are also emitted but are absorbed by the 0.1 mm
platinum wall surrounding the seed.
• Gold seeds replaced Radon seeds as Radon was associated with chronic irradiation.

26. COBALT-60
• Main advantage - high specific activity , which allows fabrication of
small sources required for some special applicators.
• half life - 5.26 yrs.
• more expensive than Cs-137.
• cobalt brachytherapy sources fabricated as wires encapsulated in a
sheath of platinum iridium or stainless steel.
• specific activity = 1132 Ci/g.

27. Iodine-125
• Used as low-energy sources for temporary interstitial brachytherapy
and ultra-low-dose rate permanent implant.
• I-125 seeds now are used routinely as temporary interstitial sources
for episcleral plaque treatment of intraocular choroidal melanoma.
• 0.5 mm thick gold shield over episcleral plaque, tissue posterior to
the eye are shielded.
• A disadvantage of high-intensity I-125 seed therapy is its high relative
to Ir-192 seeds.

28. • half-life of 59.4 days.
• Convenient for storage.
• Due to it’s lower photon energy , it requires less shielding.
• Three I-125 seed models , designated 6701 , 6702 and 6711 , have been
manufactured, which are identical in size and encapsulation but differ in
active source design.
• The earlier model 6701 and 6702 are now obsolete.
• Model 6702 contained ion-exchange resin beds, impregnated with I-125 in
the form of iodide ion.

29. • I-125 is produced by neutron activation in a specially equipped
reactor designed to minimise activation of contaminant radioisotope,
I-126.
• It produces a single 35-keV y-rays.
• the captured K-shell electron produces a cascade of 27- and 32-keV
characteristics X-rays.
• 93% of y-rays are internally converted to characteristic X-ray.
• Thus I-125 is an “x-ray emitter” because 95% of the useful primary
photons are characteristic x-rays of atomic rather than nuclear origin.

30. • The widely used Model 6711 seed,
the only I-125 source available from
1983-1998 , contains a 3 mm long
silver rod on which radioactive
iodine is absorbed and is available
in strengths of 0.5 to 7 uGy.m2.h-1.
• The silver rod is radio-opaque, so
that the seeds can be visualised on
orthogonal radiographs.
• Titanium encapsulation serves to
absorb electrons and X-rays with
energies less than 5 KeV.

31. • The Bebig model illustrates
the more recent I-seed I-125
source product, which
consists of radioactive iodine
distributed in a low density
cylindrical annulus that fits
over a gold rod used for
radiographic localisation.

32. • Because of the presence of
titanium welds , the dose
distribution around iodine
seeds is highly anisotropic ;
can pose problems of creating
cold spots near the source
ends.
• Exposure rate constant = 1.464
R-cm2/mCi/h for an unfiltered
point source.

33. Palladium - 103
• 17-day half life.
• Decays by K-electron capture; energy range of 20 to 23 KeV (average energy 20.9
KeV).
• The low energy photons emitted dramatically reduces external exposure hazards : an
8 cm thickness of tissue reduces radiation by 10 fold.
• 0.2 mm lead foils also produce complete shielding.
• Provides a biologic advantage in permanent implants because the dose is delivered at
a much faster rate.
• Used for transperineal ultrasound (TRUS) - guided permanent implant for definitive
treatment of low- and intermediate-risk prostate cancer.

34. • The theragenics model 200
for palladium-103 - is the only
commercially available Pd-
103 seed from 1988-1999.
• The radioactive palladium is
distributed within a thin
palladium metal coating of two
graphite pellets, encapsulated
in Titanium tubing of the same
dimensions.

35. • Photon fluence distribution in
air for palladium-103 seed.
• The distribution around the
source is anisotropic due to
self-absorption by the source
pallets , the welds , and the
lead x-ray markers.

36. Thank You