IAEA Training Material on Radiation Protection in Radiotherapy
Radiation Protection in
Radiotherapy
Part 5
External Beam Radiotherapy
Lecture 2: Equipment and safe design
Objectives
• To review physics and technology of external
beam radiotherapy equipment
• To understand the design and functionality of
the equipment including auxiliary equipment
• To appreciate the role of international
standards such as IEC 601-2-1 for
equipment design
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Part 5, lecture 2: Equipment - superficial, telecurie
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Contents
1. Superficial/orthovoltage equipment
2. Telecurie treatment units
3. Linear accelerators (linacs)
4. Other accelerator types
5. Associated equipment
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1. Superficial and Orthovoltage
• “conventional” X Ray tube with
electrons accelerated by an electric field
• stationary anode (in contrast to
diagnostic tubes which have a rotating
anode to allow for a smaller focal spot)
• filtration important
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Photon percentage depth dose comparison
Superficial beam
Orthovoltage
beam
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Superficial and orthovoltage
Superficial
Orthovoltage
• 40 to 120kVp
• small skin lesions
• maximum applicator
size typically < 7cm
• typical FSD < 30cm
• beam quality measured
in HVL aluminium (0.5
to 8mm)
Radiation Protection in Radiotherapy
• 150 to 400kVp
• skin lesions, bone
metastases
• applicators or
diaphragm
• FSD 30 to 60cm
• beam quality in HVL
copper (0.2 to 5mm)
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Superficial X Ray tube (Philips RT 100)
• Manufacturers picture...
X Ray tube
Cooling
water
Target
Applicator/
collimator
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Use of cones essential
• Large focal spot and close treatment
distance (Focus to skin distance FSD often
10cm or less) means the beam MUST be
collimated on the skin
• Cones are highly suitable to do this.
Additional shielding can be achieved using
lead cutouts on the skin as detailed in part
10 of the course.
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Output in superficial beam depends on:
• On/off effect
• Strong dependence on
FSD --> applicator
length significantly
affects output
• Electron contamination
from the applicator
(significant for skin
dose around 100kVp)
Radiation Protection in Radiotherapy
Inverse
Square Law
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On/off effect
on
off
output
time
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Kilovoltage Equipment (10 - 150 kVp)
• Filters are used to remove unwanted low
energy X Rays (which only contribute to skin
dose)
Interlocks must
ensure that the
correct filter is
in place
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Kilovoltage Equipment (10 - 150 kVp)
• Dose rate is approx. proportional to kVpn
where 2 < n < 3
• Dose rate is approx. proportional to electron
current (mA)
• Therefore it is important that kVp and mA
are stable.
• It is also obviously important that the timer is
accurate and stable - and that the on/off
effect is accounted for.
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Kilovoltage Equipment (10 - 150 kVp)
• Dose control is achieved by a dual timer
system - one should count time up, one
should count time down from a pre-set
treatment time
• Interlocks must be present to prevent
incorrect combinations of kVp, mA, and
filtration
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Operator control
Radiation on
indicator
kV and mA
indicator
Dual timer
Emergency
off button
Selection
of filter
Key for
lock-up
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Beam Half Value Layer (HVL)
• Possibly the most important test to
characterize beam quality
• Checks whether there is sufficient filtration in
the X Ray beam to remove damaging low
energy radiation
• Need not only a radiation detector, but also
high purity (1100 grade) aluminium - most Al
has high levels of high atomic number
impurities e.g. Cu
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HVL Measurement
• Be careful of beam
hardening (semi-log plot is
not a straight line)
• The second HVL is typically
larger than the first
• Use points either side of
half initial value
• Calculate HVL :
Relative response
10
(initial value = 9
50% of this = 4.5,
thus HVL = 2.6 mm Al)
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1
0
1
2
3
4
mm Al
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Orthovoltage units
• 120 to 400kVp
• conventional X
Ray tube
• Applications:
• deeper skin
lesions
• bone metastasis
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Orthovoltage Equipment (150 kVp)
400
• Different applicators and filters
filters
Applicators for
different field
sizes and
distances
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Orthovoltage units
• Uses mostly cones
• More recently also a
diaphragm with light
field has been
introduced. Care must
be taken to:
• ensure correct
distance
• account for wide
penumbra due to large
focal spot
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Orthovoltage Equipment (150 - 400
kVp)
• The Inverse
Square Law is
important
• Depth dose
dramatically
affected by
FSD
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FSD 6cm,
HVL 6.8mm Cu
FSD 30cm,
HVL 4.4mm Cu
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Orthovoltage Equipment (150 kVp)
400
• Control console
mA and kV control
Dual timer
On and emergency
off button
Filter and kV selection
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Orthovoltage Equipment (150 - 400
kVp)
• It is possible to use a transmission ionization
chamber as the primary dose control system
instead of treatment time
• The backup (secondary) dose control
system can be either an independent
integrating dosimeter or a timer
• Alternatively, two independent timers are
used - this is the most common scenario
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2. Telecurie units
• Very high activity
source (>1000Ci)
• Virtually all 60-Co
• Some older units
using 137-Cs
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Stamp to celebrate
the 50th anniversary
of 60-Co external
beam radiotherapy
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Telecurie units
• 137-Cs
• Photon energy 0.66MeV
• Relatively large source to relatively low specific
activity
• Medium FSD (around 60cm)
• No isocentric mounting - similar to orthovoltage
equipment in set-up
• Not sold anymore and should not be in use
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Cobalt - 60
• Photon energy around
1.25MeV
• 2 lines at 1.17MeV and
1.33MeV
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Cobalt - 60
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Cobalt - 60
• Photon energy around
1.25MeV
• Specific activity large
enough for FSD of
80cm or even 100cm
• Therefore, isocentric
set-up possible
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Cobalt - 60 equipment
• Isocentric set-up allows movement of all
components around the same centre
• collimator
• gantry
• couch
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Photon percentage depth dose comparison
60-Co
beam
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Control area of a 60-Co unit
• Dual timer control
• Patient monitoring
• lead glass
• video system
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On/off effect
on
off
output
Shutter opens
Shutter
closes
time
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Gamma-ray equipment
• A more recent Cobalt - 60 unit
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Gamma-ray equipment
• Source head and transfer mechanism
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Gamma-ray equipment
• Other source drawer transfer mechanisms
Moving jaws
Rotating source draw
Mercury shutter
(employed in the first
60-Co unit in 1951)
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Gamma-ray equipment
Source assembly:
• The source must be
sealed so that it can
withstand
temperatures likely
to be obtained in
building-fires
• Dual encapsulation
is recommended to
avoid leakage
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Cobalt source design
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Source
assembly
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Gamma-ray equipment
• Limited half life: 60Co 5.26years
• Source change
recommended
every 5 years to
maintain output
Source transport
container
Treatment
unit head
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Picture of a Co source change
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Gamma-ray equipment
• Mechanical source position indicator
Essential to:
• indicate if source
is out of safe
• often coupled with
mechanical device
to push source back
if stuck
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Cobalt unit head for a source in a
rotating source draw
Beam on
indicator
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Gamma-ray equipment
• The beam control mechanism shall be a ‘fail
to safety’ type.This means the source will
return to the Off position in the event of:
• end of normal exposure
• any breakdown situation
• interruption of the force holding the beam control
mechanism in the On position, for example
failure of electrical power or compressed air
supply
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Gamma-ray equipment
• Geometric
penumbra
typically wide
because
source
diameter is
large (>2cm)
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Gamma-ray equipment
• Penumbra
trimmer bars
may be
employed to
reduce
penumbra
width
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Gamma-ray equipment
• Gantry rotation
There should be
two independent
read-outs for all
mechanical
movements:
1. Electronic at
console and or
monitor in the
treatment room
2. Mechanical
Second non-isocentrical
rotational axis for the
60-Co source
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Gamma-ray equipment
• Leakage from the head with the
source in the Off position
• max. 10 Gy h-1 at 1 meter from source
• max. 200 Gy h-1 at 5 cm from housing
• This can contribute a significant
proportion of the maximum permissible
dose to staff
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Quick question
Please estimate the dose to a staff member
setting up patients at a 60-Co unit.
Annual dose to staff
• Assume:
• 200 days, 8hours per day working time
per year
• 10% of this time in treatment room
• 3 Gy h-1 typical dose averaged over all
locations of the staff member in the
treatment room
• Dose = 200 x 8 x 0.1 x 3 Gy 
0.5mGy/year (half of dose limit for
general public)
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Gamma-ray equipment
• At commissioning, drawings
of the head should be
examined to identify
locations where radiation
leakage could be a problem.
• Accurate ionization chamber
readings should be made at
the location of any hot spots
and also in a regular pattern
around the head.
• Film wrap techniques can be
used to identify positions of
‘hot’ spots.
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Film wrapping technique
• Here shown with
only a single film
on a linac
• In practice film can
be wrapped around
all the treatment
head
• This technique is
useful also for
other treatment
units such as
superficial and
orthovoltage.
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Never forget to mark and label
the film
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Gamma-ray equipment
• Wipe tests should be carried out initially at
installation and at regular intervals to check
for surface contamination. This test need not
be carried out directly on the source surface
and can be carried out on a surface which
comes into contact with the source during
normal operation of the equipment.
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