Radiation Protection in Radiotherapy

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Radiation Protection inRadiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 6
• Brachytherapy
• Lecture 2 (cont.) Brachytherapy Techniques

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Brachytherapy

• 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

• Seed Implant Monotherapy (about 144Gy)
• EBT (45Gy) Implant Boost
• 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

• 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

• Iodine 125 - 144Gy - I-125
• Half Life 60 days
• Energy 28 keV
• TVL lead 0.08mm

• 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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Patient flow in brachytherapy
Treatment decision
Ideal plan - determines source number and location
Implant of sources or applicators in theatre
Localization of sources or applicators (typically
using X Rays)
Treatment plan
Commence treatment
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Pre-planning

• 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

• 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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Patient flow in brachytherapy
Treatment decision
Ideal plan - determines source number and location
Implant of sources or applicators in theatre
Localization of sources or applicators (typically
using X Rays)
Treatment plan
Commence treatment
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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

• Mostly for cardiac vessels but also possible in
  some extremities

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Endovascular irradiation

• 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

• Gamma sources 192-Ir
• 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

Dose calculation
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Beta sources

• 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
• Potential problem may not reach all cells of
  interest

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The Beta-Cath System (Novoste)
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Guidant system

• Employs centering catheter to ensure source is
  always in the center of the vessel

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Radiation safety in theatre

• Application of radiation in theatre
• 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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Longitudinal Dose Distribution
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0
L/2
L/2
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Angiographic Appearance of PDL in Delivery
Catheter
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Radiation Source Train Dose Profile at 2mm
40mm Radiation Source Train (RST)
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Radioactive stents

• Stents are used to keep blood vessels open
• Can be impregnated with radioactive material
  (typically 32-P) to help prevention of re-stenosis

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C. Ophthalmic applicators

• Treatment of pterigiums and corneal vasculations,
  a non-oncological 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 / 90Y
90Sr
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
Finite treatment depth
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Issues with ophthalmic applicators - dosimetry

• Dosimetry difficult due to short range of
  particles
• Dose uncertainty gt 10
• Short treatment times taken from look-up tables -
  potential for mistakes
• Documentation often less than complete

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Other guidance and issues

• Never point source at someone - range in tissue
  lt1cm, but in air gt 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

• 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

• 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
• 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

• Many specialized brachytherapy applications are
  performed outside of a conventional radiotherapy
  department - this requires consideration of
• training
• shielding
• communication
• Excellent planning and documentation is required

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Intra-operative brachytherapy

• In principle possible
• Treatment units (must be HDR) available
• Applicators are available

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Summary I

• 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

• 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

• 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?
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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...

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Radiation Safety Issues when using 90-Sr/Y
applicators

• 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