Transcript IAEA Training Material on Radiation Protection in Radiotherapy

IAEA Training Material on Radiation Protection in Radiotherapy
Radiation Protection in
Radiotherapy
Part 6
Brachytherapy
Lecture 2 (cont.): Brachytherapy Techniques
Brachytherapy
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Very flexible radiotherapy delivery
Allows a variety of different approaches,
creating the opportunity for special and highly
customized techniques
Not only used for malignant disease (=cancer)
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Special techniques
A. Prostate seed implants
B. Endovascular brachytherapy
C. Ophthalmic applicators
D. Other special techniques
Both point B and C are examples for the use
of brachytherapy for non-oncological purposes
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A. 125-I seeds for
prostate implants
Relatively new technique
 Indicated for localized early stage
prostate cancer
 Permanent implant
 Preferred by many patients as it only
requires one day in hospital

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Treatment Options for prostate
cancer
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Seed Implant Monotherapy (about 144Gy)
EBT (45Gy) + Implant Boost
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Seed Implant (108Gy)
HDR Implant (16.5Gy/3)
External Beam only (65-84Gy)
Surgery (Radical Prostatectomy)
 This
all could be combined with hormones
and/or chemotherapy
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Implant schematic
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A typical implant
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Deliver 144 Gy to entire prostate gland
Approximately 100 I-125 seeds (25 needles)
Needles are guided by ultrasound and a
template grid
Pre-planned needle positions to give even
dose but avoid pubic arch
Minimise rectal dose and avoid urethra
overdose
CT after 3 weeks for post-planning
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Isotopes in use
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Iodine 125 - 144Gy - I-125
Half Life = 60 days
 Energy = 28 keV
 TVL lead = 0.08mm

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Palladium 103 - 108Gy - Pd -103
Half Life = 17 days - dose rate about 2.5
times larger than for 125-I
 Energy = 22 keV
 TVL lead = 0.05mm

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Prostate Implant Process
Ultrasound Volume Study
 Pre-planning: what would be ideal
 Ordering I-125 seeds and calibration
 Needle loading
 Ultrasound guided Implantation
 CT post-planning a couple of weeks
after: what has been achieved?
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Pre-planning
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Several different systems
possible
Provides guidance for
approach, data on number
of sources required and
loading of needles
Avoid central column to
spare urethra
Cover target laterally
Conform to posterior border
(spare rectum)
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Preparation of seeds
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Ordering planned
number of seeds +
some spares
Checking seed
activity
Sorting and loading
seeds into needles
Seed alignment tray
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Implant needle
loaded with
seeds and
spacers
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Implant template
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Implant jig
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Ultrasound
Guided Implant
Procedure
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X-ray of implanted seed
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CT post-planning after 4 weeks
Swelling is gone - CT provides true three dimensional
information on the implant geometry
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Post CT planning = establishing the
actual dose distribution
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Quality of Implant
Depends on seed placement
 Seeds may migrate with time
 If large dose inhomogeneities exist, the
critical cold spots can be boosted by
either placing more seeds in the
prostate or using external beam
radiotherapy
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Notes on prostate seed implants

A similar technique is available using
103-Pd seeds
103-Pd has a shorter half life and therefore
a higher activity is implanted
 Otherwise the rules an considerations are
similar to 125-I seed implants

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2. Endovascular brachytherapy
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The issue: re-stenosis
After opening of a blocked blood vessel
there is a high (60%+) likelihood that
the vessel is blocked again: Re-stenosis
 Radiation is a proven agent to prevent
growth of cells
 Radiation has been shown to be
effective in preventing re-stenosis
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Dilation of blood vessels
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Mostly for cardiac
vessels but also
possible in some
extremities
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Endovascular irradiation
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Mostly for cardiac
vessels but also
possible in some
extremities
Many different
systems and
isotopes in use
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Isotopes for endovascular
brachytherapy
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Gamma sources: 192-Ir
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the first source which has been clinically
used (Terstein et al. N Eng J Med 1996)
Beta sources: 32-P, 90-Sr/Y, 188-Rh
(Rhenium)
 Activity around 1Ci
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Dose calculation
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Beta sources
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Most commercial systems use them
because:
finite range in tissues
 less radiation safety issues in the operating
theatre
 smaller, hand held units possible for use in
cardiac theatres
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Potential problem: may not reach all
cells of interest
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The Beta-Cath™ System (Novoste)
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Guidant system
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Employs centering
catheter to ensure
source is always in
the center of the
vessel
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Radiation safety in theatre
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Application of
radiation in theatre:
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time is of the
essence - planning
in situ
shielding would be
difficult
physicists must be
present
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Irradiation of extended lesions
Use “Radiation Source Train”
 Stepping source process to cover
desired length
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100 %
Longitudinal Dose
Distribution
50 %
0%
L/2
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L/2
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Angiographic Appearance of PDL
in Delivery Catheter
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Radiation Source Train:
Dose Profile at 2mm
Dose Profile Along Axis of 40 mm Sr-90 Source Train at 2 mm
Percent of Dose at Perpendicular Bisector
of Radiation Source Train
120
100
80
60
40
20
0
-40
-30
-20
-10
0
10
20
30
40
Distance along Radiation Source Train (mm)
40mm Radiation Source Train (RST)
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Radioactive stents
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Stents are used to
keep blood vessels
open
Can be impregnated
with radioactive
material (typically
32-P) to help
prevention of restenosis
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C. Ophthalmic applicators
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Treatment of pterigiums
and corneal
vasculations, a nononcological application
of radiotherapy
Use of beta sources mostly 90-Sr/Y
Typical activity 40 to
200MBq (10-50mCi)
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Ophthalmic applicators
Activity covered by thin plated gold or
platinum
 Curvature to fit the ball of the eye
 Diameter 12 to 18mm
 Activity may only be applied to parts of
the applicator
 Typical treatment time for several Gy
less than 1min
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Decay scheme of
90Sr
90Sr
/ 90Y
ß 0.54 MeV, T1/2 = 28.5 yrs
90Y
ß 2.25 MeV, T1/2 = 64 hrs
90Zr
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Dept Dose Curve of
90Sr
in H2O
Signal (a.u.)
1
0,9
0,8
Finite treatment depth
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Distance form RST axis (mm)
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Issues with ophthalmic
applicators - dosimetry
Dosimetry difficult due to short range of
particles
 Dose uncertainty > 10%
 Short treatment times taken from lookup tables - potential for mistakes
 Documentation often less than complete
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Other guidance and issues
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Never point source at someone - range in
tissue <1cm, but in air > 1m!!!
Radiation typically used by non radiotherapy
staff (eye specialists, nurses) - training
required
Sterilisation/cleaning - must not affect
integrity of the cover
Regular check of homogenous distribution of
activity required
Wipe tests required
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D. Other specialized
brachytherapy applications
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Intra-operative brachytherapy
Use of radiation in operating theatre
 Useful for incomplete surgical removal of
cancer
 Allows highly topical application of
radiation
 If surgery is followed by radiotherapy, one
is “10Gy ahead” in tumor dose

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Intra-operative brachytherapy
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In practice not often used because
not always possible to predict if radiation
will be needed during the operation
 requires radiation oncologist to be
available
 radiation safety issues
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 shielded theatre costly
 patient must be left alone during irradiation
 even if less than 5min this is a risk due to
anesthetics
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A note on radiation protection
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Many specialized brachytherapy applications
are performed outside of a conventional
radiotherapy department - this requires
consideration of:
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training
shielding
communication
Excellent planning and documentation is
required
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Intra-operative brachytherapy
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In principle
possible
Treatment units
(must be HDR)
available
Applicators are
available
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Summary I
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Brachytherapy is a highly customized and
flexible treatment modality
Quality of treatment depends on operator skills
From a radiation protection point of view
remote afterloading is most desirable: A variety
of equipment is available to deliver remote
afterloading brachytherapy
HDR brachytherapy is the most common
delivery mode nowadays.
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Summary II
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125-I seed implants are a alternative for
radiotherapy of early prostate cancer
Endovascular brachytherapy is one of an
increasing number of non-oncological
applications of brachytherapy
There may be radiation safety issues if
specialized brachytherapy procedures are
performed outside of a radiotherapy
department as staff not used to working with
ionizing radiation is using radioisotopes
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References
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Nath et al. Intravascular brachytherapy physics.
AAPM TG60 report. Med. Phys. 26 (1999) 119-152
Waksman R and Serray P: Handbook of vascular
brachytherapy (London: Martin Dunitz) 1998
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Any questions?
Question:
Please list some radiation safety issues when using
90-Sr/Y applicators for ophthalmic treatments - you
should consider the appendices of BSS to classify
them...
Radiation Safety Issues when using
90-Sr/Y applicators
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Occupational exposure:
cleaning
 sterilization
 contamination
 handling of sources by non-radiotherapy
staff
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Radiation Safety Issues when using
90-Sr/Y applicators

Medical exposure:
dosimetry difficult
 contamination from damaged applicator
 over/under exposure of the eye of the
patient
 irradiation of other areas of the patient

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Radiation Safety Issues when using
90-Sr/Y applicators

Public exposure:
transport of the sources
 security of sources
 storage and disposal

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Acknowledgement
Craig Lewis, London Regional Cancer
Centre
 Mamoon Haque, RPA Hospital

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