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α α α βγ βγ
Uranium Ra Rn RaA RaB RaC Pb
1620yrs 3.83D 3.05Min 26.8min 19.7min stable
Gamma rays of different energies are emmited – 0.6, 1.1 and 2.2
MeV , Average energy being – 1 MeV .
Ra B emits beta ray of 0.65 MeV .
Ra C emits beta ray of 3.17 MeV .
• AND SOON BECAME THERADIOACTIVE
• Few examples –
• Radiothor – salt containing radium endorsed
to have curative properties.
• Toothpastes contianing radium – to make the
teeth shiny .
• In bathing water , drinking water .
• Danlos and Bloc performed the first
radioactive implant in 1901.
• First schools of brachytherapy were at Holt
radium institute , Paris.
Brachytherapy consists of placing
sealed radioactive sources very close
to or in contact with the target tissue.
Types of brachytherapy
1. Intracavitary insertion consists of positioning
applicators (bearing RAS) into a body cavity in close
proximity to target tissue
2. Interstitial insertion consists of surgically implanting
small RAS directly into the target tissues
3. Surface dose ( mould ) applications consist of an
applicator containing an array of RAS usually designed
to deliver an uniform dose distribution to a
4. Transluminal application consists of inserting a single
line source into a body lumen to treat its surface &
Clinical advantages of brachytherapy -
• High biological efficacy
• Rapid dose fall off
• Tolerable acute reactions
• Decreased risk of tumor population
• High control rate
• Better cosmesis
Limitations of brachytherapy
• Difficult for inaccessible regions
• Limited for small tumor size
• Greater conformation
• Needs skilled personnel
• Radioactivity :The activity, A, of an amount of
radioactive nuclide, in a particular energy
state and at a given time,is defined by
equation: A= d N/ d T
• where dN is the expectation value of the
number of spontaneous nuclear transitions
from that energy state in the time interval dt.
• Problems with the radium
• Daughter element – radon (gas)- possibility of
leakage which is not detected by visual check .
• The practical maximum activity concentration
(the specific activity) of radium salt is low
(approximately 50 MBq mm~3 of active volume).
Sources of higher activity are therefore bulky and
not suitable for afterloading systems.
• Need for heavy fitration and shielding .
Properties of ideal brachytherapy
• Pure γ emitter – less α/β emission.
• Medium γ energy – high enough to target tumor with
homogenous dose & low enough to avoid normal
tissues & reduce shielding needs
• High specific activity – small size & suitability for HDR
• Stable ( not liquid/gaseous) daughter product
• Long t ½ /medium t ½ for permanent/ temporary
• Should be available in a form which doesn’t powder or
disperse if source is damaged or dispensed
NAME ORIGIN T1/2 γ
Rn 222 NATURAL 3.83
12 Pb 206
Cs 137 FISSION 30.1
- do - 6.5 87 Ba 137
Co 60 NEUTRON
0.38 - do- 11 1020 Ni 60
Ir 192 - do - 73.8
Platinum 4 7760 Pt 192
Tn 182 - do - 115
0.67 - Platinum 12 - -
Au 198 - do - 2.7
0.96 St. steel 3.3 - Hg 198
I 125 - do - 59.4
No Titanium 0.01
- Te 125
Pd 103 - do - 16.9 0.21 No Platinum 0.03 - Ru 103
Systems in Interstitial brachytherapy
• “System” denotes a set of rules which takes into
account the source types & strengths, geometry &
method of application to obtain suitable dose
distributions over the volume(s) to be treated.
• Implant should follow both source distribution rules
and method of dose prescription & specification of a
• Consist of
1. Distribution rules
2. Dose specification & implant optimization criteria
3. Dose calculation aids
Need for system
• Two questions
• How much radium will be required?
• and how must it be arranged?
How much of radium ?
• A unit of dosage rather than of intensity was
• Because -total dose is the most important single
factor in therapy (provided due consideration is
given to the duration of radiation).
• the clinician finds it very much easier to think in
• total dose delivered rather than in terms of
intensity and time, separately
How much of radium?
• For dosage unit of exposure – Roentgen (r) is
• Roentgen is ‘the amount of Xray or gamma
radiation such that the associated corpuscular
emmision , per 0.001293 gramme of air,
produces in air ions carrying 1 electrostatic
unit charge of either sign ‘
• To determine exposure rate ‘R’ at a given point from
a point source ‘A’ mCi whose specific gamma ray
constant is ‘Γ’ at a distance ‘d’
• R = Γ x A (Roentgen/mCi hr.)
• Thus exposure rate of 1 mg of Ra at 1 cm with
standard 0.5mm filtration thru Pt is found to be
8.4 R/ hr.
• Line source :
We divide the line into multiple point sources and the exposure rate simply added.
> 1 cm
> 1 cm
1. Amount of Ra depends on
1. Value of desired exposure
2. Area being treated
3. Treating distance
2. Clinical working unit was 1000 Roentgens
3. Exposure rate was in R/hr
4. Exposure Rate constant Γ= 8.4 R/mg hr
5. Table compiled for Mg hr/1000R for a given
area/actual implant volume at a given distance
6. Filtered by 0.5mm of platinum (2% filtration
correction factor for every 1mm)
7. A radiation field could be described as ‘uniform’, if
variation is not more than +/-10 %.
8. R to cGy factor = 0.957
• The amount of radium needed for is
determined – for a perticular area , treating
distance , total exposure and treatment time
for the designed dosage chart.
• If an area of 20 cm2 is to receive 6000R over 50 hrs
from a mould for which h=1.0 cm , taking the values
from dosage chart-
• The milgramme hours(mgh)per 1000Rfor 20 cm2 and
h=1 cm is 641 .
• Therefore , 6000R: 641x6=3846mgh will be needed .
• And radium required for 50 hr treatment will be
3846/50=76.9 mg .
The distribution-governed by the
inverse square law .
• “for a point source , the radiation intensity at
any place varies inversely as the square of the
distance from the source to the place at which
intensity is being considered”.
• Dose varies more rapidly in 1st cm of distance
Geometrical terms :
• The plane formed by
the parallel sources -
• The centre of the
radioactive plane is
called the centre of
• The central plane
through this point is
located at right angles
to the needles and
thus at right angles to
the radioactive plane.
A third plane parallel to the sources and at
right angles to the radioactive plane, will
be referred to as the coronal plane
• The length of the implant is the length of the radioactive
plane in the direction of the sources .
• The width of the implant is the dimension at right angles
to the length, in the radioactive plane or parallel to it.
• The thickness of the implant is the dimension at right
angles to the length and to the radioactive plane, in the
central plane or parallel to the central plane.
• The word ‘planar’implies that the radium
sources are mounted on a flat or curved
surface parallel to the area being treated ,the
curved surface being less than a semicircle .
• Mould arrangement-Circular/square/rectagle
• Treatment surface-Flat/convex/concave
The term mould is used to describe the situation in which the
radioactive sources are positioned external to the patient, usually at a
distance from the patient's skin known as the treatment distance and
represented by the letter d.
The treatment dose is prescribed to the plane which is at distance d
from the sources and the dose in this plane will be delivered with a 10%
accuracy if the rules are followed.
For circular mould
• The distribution of radium depends upon the
D/d. (D- diameter of the treatment area , d-
treatment distance )
• For ‘ideal circle’ central and periferal doses are
equal and the variation elsewhere across the
circle is minimal.
• If D/d is less than 3 , a single circle of radium will
produce uniform irradiation over the treatment
area , within the defination of ‘uniformity’
• Most uniform distribution is achived with D/d
<3 3- <6 6-
100 95 80 75 70
- - 17 22 27
- 5 3 3 3
For square mould :
• Source is devided in to periphery and in lines,
called as bars.
• The number of bars is such that area is
devided in to strips of width not greater than
2 d .
the amount of radiumdistribution depends upon
the number of bars .
For square moulds
• If one bar suffices , its linear density
(miligrammes per cm) should be half of that in
• If two or more bars are needed , linear density
should be 2/3rd of that of the periphery.
• Attempt is made to maintain the space
between two active ends of the sources less
than the treating distance (d)
For rectangular moulds
• The bars are taken in a line parallel to the
longer side .
• To get the vallue of mgh per 1000R ,
elongation correction factor is applied
1.5:1 2:1 3:1 4:1
1.025 1.05 1.09 1.12
For curved surface
The smaller area is chosen.
Dose below the treated surface
• As tumors treated by moulds are superficial
and thin , the dose at the distance d’ below
the surface can be calculated by adding d’ and
d, and looking for the value of mghper 1000R
for treatment distance (d’+d) in the dosage
Planar interstitial implant
• Considered as direct development of planar
mould with d=0.5 cm , inside the tissue .
• They are implanted inside the tissue to get the
uniform dose distribution on either side
within distance of 0.5 cm .
• For the values of mgh per 1000R , values from
the charts for mould can be used .
Distribution rule for single planar
• The proportion of the radium used on
periphery depends upon the area of the
treatment itself .
Area Under 25
2/3 1/2 1/3
Importance of Differential Loading
1. All sources equal
2. Weaker sources in interior
prescribed dose does
not conform to target
Dose conforms to
2. The needles should be arranged in parallel rows 1
cm apart with the ends crossed(active ends < 1 cm
from crossed needles)
Areas of under dosage
End source fills in under dosed areas
Shape of isodose line
around a linear source
3.. If the ends of the implant are uncrossed, the area should
reduced by 10 % for each uncrossed end for table reading
A. Both ends crossed. The area treated is (a x b)
B. One end uncrossed. The area treated should be considered as 0.90 (a x b)
C. Both ends uncrossed. The area treated should be considered as 0.80 (a x b)
4. If two planes are to be used, the separate planes should
be arranged as for single planes, parallel to each other, and
if they differ in area, then the average area is used to
determine the mg-hrs and the activity is proportioned to
Volume interstitial implants
• To calculate the amount of radium required , it
follows the various set of charts .
• M=34.1 V2/3 x f
• M- miligramme per 1000R
• V- implanted volume in cubic cm
• f – correction factor
Rules for distribution in a
1. Any volume can be visualized as having two
components : outer surface(RIND) and CORE.
Total amt. of Radium is divided into 8 parts and
distributed for the various shapes:
If these rules followed exposure throughout treatment volume
will not drop by > 10% or rise by > 15% above stated exposure
2. The sources on each surface should be spaced as
evenly as possible, with 1 -1.5 cm separation
3. For the core, the sources should be spread as
evenly as possible throughout the volume and not all
of it at the center.
4. In cylinders, belt should consist of not less than 8
needles, the core not less than 4.
5. If crossing not possible, volume of implant must be
reduced by 7.5% for each uncrossed end. Crossing
needles inserted at active ends, not at tips.
• Introduced the points at which dosage can be
• Such a point must be anatomically
comparable in the patients and should be at
which dosage is not highly sensitive to small ,
clinically unimportant alteration in the source
geometry and should be in the region of
• Introduced – ovoids and intrauterine tubes.
Point A- a point 2 cm latersl to the centre of uterine canal and 2 cm
superior to the mucous membrane of lateral fornix , in the plane of uteru
Point B - 5 cm laterally
to midline at the same level as the A points
• Point A - The dose at point A is representative
of the dose throughout much of the malignant tissue.
Corresponds to the the point where uterine artery
crosses the uterus .
Point B – anatomically proximal to the obturator glands .
Represents the lateral fall off of the dose .
• The dose at point B is approximately 20-25% of the
dose at point A and is of importance when calculating
the total dose when brachytherapy is combined with
• Not more than about a third of total exposure
rate at Pint A should be delivered from the
Showed that biological changes in tumor as well as normal tissue
not only depend upon the dose of radiation but also on the
duration over which this dose is spread.
• The dose required to produce lethal change rises as the total
period of radiation increases.
• “It has been found that, subject to the proviso regarding
duration of exposure already discussed, a dose of 6,000 ‘r,’ if
delivered to the whole of tumour and tumour-bearing zone,
causes permanent resolution of the great majority of
epitheliomata. This figure may, therefore, be taken as a
serviceable in vivo ‘tumour-lethal’ dose”.
• Data regarding the other tumors is not extensive.
Suggested readings :
• 1. Patterson, R. and Parker, H.M. (1934) A dosage
system for gamma ray therapy. Br.J. Radiol., 7, 592.
• Fundamental Physics of Radiology by W.J. Meredith
& J.B. Massey (dosage calculation for plesiotherapy)
• Principles and practice of brachytherapy –CAF Joslin