Brachytherapy

1
Brachytherapy

• Elizabeth Small B.S.R.T.(T)

2
Brachytherapy

• What does the prefix brachy mean?
• How does brachytherapy compare to treatments with
  linear accelerators?
• What kinds of cancers are treated with
  brachytherapy?

3
Brachytherapy

• The use of brachytherapy is increasing
• Organ preservation
• Acceptable cosmetic results
• What are the major advantages of using
  brachytherapy?

4
Brachytherapy

• Brachytherapy is often used to supplement the
  dose given by external beam treatment
• Surrounding tissues are spared increased exposure
  because brachytherapy does not penetrate
  overlying tissues
• Treating from the inside out, not outside in as
  in external beam irradiation

5
Applications of Brachytherapy

• Interstitial
• Intracavitary
• Intraluminal
• Topical

6
Interstitial Brachytherapy

• The radioactive sources are implanted directly
  into the tumor or tumor bed
• Two kinds of interstitial implants
• permanent
• temporary
• Permanent implants- not removed!
• are used when the tumor is inaccessible making
  it difficult/impossible to remove the implant
• I and Au have short half lives

7
Interstitial Brachytherapy-Permanent implants

• A gun-type applicator with a long hollow
  insertion needle is used
• The needle is inserted into the deep tumor
  through the skin and the source is inserted into
  the tumor
• The needle is withdrawn 5 to 10 mm and another
  source is inserted
• Repeated until the number of desired sources are
  implanted
• Permanent implants are used for deep seated
  lesions in the pelvis, abdomen and lung

8
Interstitial brachytherapy-

• Temporary Implants
• The sources are placed directly into the tumor
  bed for a short period of time to deliver a high
  dose to the area
• Removable implants are used in areas where there
  is no body cavity or orifice to accept
  radioactive sources
• Temporary interstitial brachytherapy is often
  used for neck, breast, soft tissue sarcomas, and
  skin tumors
• Flexible tubes or rigid needles can be used

9
Administration of Brachytherapy

• Intracavitary brachytherapy
• radioactive sources are placed within the cavity
  for treatment
• Used for many years to treat cervical cancer/GYN
• Applicators- Tandem and Ovoids (p.330)
• central tandem- long narrow tube that is inserted
  through the cervical os and into the uterus
• Ovoids (colpostats)- are oval shaped and are
  inserted into the lateral fornices of the vagina

10
Intracavitary Applicators

• Tandem and Ovoids
• location of the applicator is verified with
  radiographs (x-rays)
• produces a pear-shaped isodose distribution
• Cervical and uterine, certain anatomical points
  are used to calculate dose
• points A and B
• Point A
• is located 2 cm superior and 2 cm lateral to the
  center of the cervical canal- at the cervical os
• Point B
• is located 3cm lateral to point A, also referred
  to 1 cm lateral from pelvic side wall
• Dose at point B 1/3 of point A

11
Intracavitary Applicators

• Heyman Capsules-
• stainless steel capsules that hold cesium sources
• pack the uterus
• Metal wires are attached to the capsule for
  removal, protrude through the vagina
• See page 331, figure 15-15

12
Intracavitary Applicators

• Vaginal cylinders-
• different lengths, diameters, and shielding
  designs
• Deliver a high dose to vagina, with minimal dose
  to bladder and rectum
• Can be used in conjunction with interstitial
  implants or tandem
• Cylinder can also provide shielding

13
Intraluminal brachytherapy

• Places sources within body tubes
• esophagus, uterus, trachea, bronchus, and rectum
• Similar applicators used for intracavitary
  procedures are used
• Pulsed high dose rate applications are done
  successfully- using cesium 137 and iridium 192

14
Administration of Brachytherapy

• Topical Brachytherapy
• radioactive sources are placed on top of the area
  to be treated
• Molds of the body part can be taken and prepared
  to place the sources in the treatment arrangement
  to deliver the prescribed amount
• Eye plaques are commonly used p. 326

15
Specification of Source Strength

• Source strength specification provides three
  functions
• 1. Standard means for describing quantities of
  emitted radiation
• 2. Allows for computational dosimetry
• to calculate the dose with the aid of a computer
• 3. Serves as a prescription parameter in
  brachytherapy

16
Specification of Source Strength

• The units used are Curie (Ci)
• Becquerel (Bq)
• 1 curie 3.7 x 10 dps
• 1 Bq 1 dps
• See table 15-1 for other common conversions

17
Radioactive decay

• The key relationship in understanding
  radioactivity is
• N number of atoms
• t the time
• The change in the number of atoms per change in
  the time is proportional to the number of atoms
  present

18
Radioactive Decay

• The proportion can be turned into an equation by
  adding the decay constant,
• The negative sign is added because there are
  fewer atoms present after a given amount of time

19
Radioactive Decay

• The equation can be rearranged to solve for the
  decay constant,
• N
• - N
• t

---------
20
Activity

• Activity, A, is the rate of decay of a
  radioactive material or the change in the number
  of atoms in a certain amount of time
• A
• The activity is directly proportional to the
  decay constant. As the decay constant increases
  the activity increases.

21
Activity Equation

• AA e
• This formula is commonly used to calculate
  activity of a radioisotope after some length of
  time has passed

22
Half Life

• The half-life is the amount of time it takes for
  the activity to decay to one half its original
  value
• Half-life is related to the decay constant by
  this formula
• The relationship between the half-life and
  activity is inversely proportional
• As half-life increases the activity decreases

23
Activity and Half-Life

• Depending on what info you have will dictate
  which formula you use
• or if you do not know the decay constant

24
Example Problem

• Cesium 137
• The original activity on September 3, 1995 was
  69.5 mCi. What would be the activity for this
  source 365 days later? The half-life of cesium
  137 is 30 years.
• We are not given the decay constant so we will
  use the formula with the half life
• A.0695Ci e
• A.06791Ci or 67.91 mCi

25
Mean Life

• Mean life is the average lifetime for the decay
  of radioactive atoms.
• Commonly used to determine a final dose time for
  permanent implants such as I and Au
• Mean life is related to half-life
• Mean life T X 1.44

1/2
26
Mean Life Problem

• 106 mCi of Au is implanted into a pelvic mass.
  Determine the emitted radiation.
• Au 2.7 days
• Mean life of Au 1.44 X 2.7 3.89 days
• 106 mCi X 3.89 days
• 412.34 mCi -days

27
Average energy

• The photons emitted from the radioisotopes have
  different energies
• The average energy is derived from the decay
  schemes of each isotope
• See Table 15-3 Average energies of radioisotopes
• Beta emissions are filtered by the encapsulation
  of the sealed sources- both ends are welded

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Half Life and Average Energy

• Isotope (MeV)
• I 60.2 days 0.03
• Ir 74.1 days 0.38
• Au 2.7 days 0.41
• Cs 30.0 yrs 0.662
• Ra 1622 yrs 0.83
• Rn 3.82 days 0.83
• Co 5.26 yrs 1.25
• Cu 12.8 hrs -

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Sealed Radioactive Sources

• Source is encapsulated by welded ends
• The metal casings around the radioactive sources
  serve two functions
• Prevent escape of radioactivity
• Absorb beta particles
• The International Organization of Standardization
  (ISO)
• Classifies sources based on safety requirements
• Manufacturer and User Tests
• Leak tests and Wipe tests

30
Ra

• Most brachytherapy procedures were developed
  using Ra
• Ra was the first radioisotope to be isolated
  and identified
• Most of the isotopes have their dosimetry based
  on the original work done with Ra and are
  referred to as Ra substitutes

31
Radium

• Ra decays by alpha emission
• It is part of a long decay chain that begins with
  U and ends with Pb
• Half-life of Ra ???
• Radium decays to Radon
• then decays to Pb
• Safety Hazard
• Where do you find Radon gas??? Is it bad for
  you??

32
Specific Activity

• Radium has a high specific activity which made it
  very practical to use
• Specific Activity
• is the activity per unit mass of a radioactive
  material (Ci/g)
• The specific activity dictates the total activity
  a small source can have
• A small size and high specific activity are
  necessary for practical purposes in brachytherapy
  use

33
Review Questions

• Name 4 different types of applications for
  brachytherapy
• What are tandem and ovoids?
• What shape do the isodose lines typically make
  when a tandem and ovoids are used?
• What are the half lifes of
• Ra Co Cs Ir I Au Rn Cu

34
Radium Sources

• A typical radium source consists of
• Radium salts placed inside .1-.2 mm thick gold
  foil cells
• about 1 cm long and 1 mm in diameter
• Cells are sealed to prevent radon gas leakage
• The sealed cells are in a sealed metal (platinum)
  sheath
• One radium source contains
• 1 to 3 cells depending on the source length
• Variety of lengths and activities
• Radium sources are available in needle or tube
  form
• Khan pg 355

35
Radium Sources

• The entire length of the source is longer than
  the area of radioactivity
• Active length and physical length are different
• Active Length
• the length of the area which is _______________
• Filtration (shell thickness) provided by the
  metal casing (.5 mm platinum) is sufficient to
  absorb the low energy gamma rays and alpha
  particles produced in the decay process
• X Y Q

36
Gamma Factor

• Various filtration thicknesses affect the
  exposure rate
• for every .1 mm added to the .5 mm Platinum 2
  incr filtration
• What is the gamma factor? Exposure rate _at_ 1m
  from the source of activity
• As filtration is increased by each additional mm
  of platinum what happens to the gamma factor?
• Does it increase or decrease?
• The Gamma factor for Ra 226 is 8.25 Rcm/mCih

37
Radioactivity of Radium

• Amount of radioactivity is expressed in mg
• 1 mg of radium 1 mCi (approximately)
• 1Ci 3.7 X 10 dps
• Radium needle strength expressed
• Full strength
• .66mg/cm activity
• Half strength
• .33 mg/cm activity
• Example problem- A full strength radium source
  has an active length of 3 cm. What is the
  activity of this source?
• .66 mg/cm X 3 cm 2.0 mg

38
Sources have different Arrangements of Activity

• Uniform Source-equal concentration active length
• Nonuniform Distributions
• Indian Club Source- more activity on one end
• Dumbbell Source- heavier on the ends, and less
  activity in the center
• These sources are in the needle form
• Sharp points on one end
• Eye on the other
• Secure
• Ease in removing

39
Problems with Needles

• Personnel exposure
• Stiff- so body area must be thin enough to accept
  them without bending
• Breaking is a possibility
• Led to the development of applicators
• Hollow tubes inserted first
• Afterloading technique
• Sources are put in later when pt is in room
• Higher activities- up to 50 mg per tube

40
Seeds

• Gold 198
• Radon 222- not used much anymore-dangerous
• 3-5 mm long
• Diameter of a pencil lead
• Used permanent implants
• A gun-like applicator is used to place them in
  the treatment area

41
Radium Substitutes

• Refers to any isotope whose dosimetry is based on
  radium
• Cesium 137
• Iridium 192
• Gold 198
• Iodine 125
• List is not inclusive- there are others
• Unit to equate an isotope to radium
• Radium equivalent

42
Radium Equivalent

• mg Ra eq (A , mCi) ( )
• the activity of the source in
  mCi
• Gamma Factor of the isotope
• is found in a chart
• See table 15-4

A
43
Example Radium Eq Problem

• Example
• What is the radium equivalence of a 25.0 mCi
  source of Cesium 137?
• Cs 137 3.226Rcm /mCi-h
• mg Ra eq (25.0mCi)( 3.226/8.25)
• 9.78 mg Ra eq

44
Cesium 137

• Widely used
• Uterus and cervix
• Primary photon energy of 662 keV
• Very similar to Radium average energy of 830 keV
  (47-2450 keV) gt2 MeV safety hazard
• Advantages of Cesium over Radium
• Reduced radiation hazards
• Also has a long half life
• 30 years
• Produced in a nuclear reactor, as a natural
  by-product of nuclear fission
• widely available
• Wide variety of tube/needle strengths
• Very popular in hospitals/privately owned
  facilities

45
Iridium 192

• Iridium wires- provide flexibility and strength
  or small seeds attached to nylon ribbons with a
  spacing of 1 cm between seeds
• Beta decay
• X Y
• Average energy of 370 keV (.37 MeV)
• Wires provide filtration of beta particles
• Half life
• 74.2 days
• Used for temporary implants
• Easily reached sites- breast, tongue
• Made in a nuclear reactor

46
Cobalt 60

• Not commonly used in brachytherapy, more often
  EBT
• Some countries
• needles and tubes
• eye tx
• Two tiered beta decay that produces a ____MeV and
  a ______MeV gamma rays
• Average energy _________
• Half life___________

47
Gold 198

• Popular for permanent implants
• Short half life 2.7 days
• Monoenergetic (means ?)
• 412 keV (.412 MeV)
• Form Grains or seeds encapsulated in platinum
• Because of short half life shipped very high
  activities
• At time of use- 5 mCi/seed (HDR therapy)
• What does HDR stand for and what does it mean
  compared to LDR?
• prostate

48
Iodine 125

• Popular in interstitial seed implants
• Produces low energy gamma rays
• 35.5 keV
• Less shielding required
• Half life
• 60.2 days
• LDR- dose is deposited over a longer period of
  time

49
Radon Seeds

• I 125 and Au 198 have replaced Rn 222
• Why?

50
Exposure Rate from a Radioactive Source

• Gamma Factor See table 15-4
• Used to calculate the absorbed dose from
  radioactive sources (One method)
• Gamma factor ( factor) the exposure rate at
  1 m from a radioactive source of known activity
• Units of factor are
• Roentgen X cm /mCi X hr
• Units of exposure
• Roentgen/hr
• Only useful with point sources

51
Calculating Exposure Rate

• X ( ) (A) (1/d)
• X exposure rate ( dot indicates rate)
• d distance from the source to the point of calc
  in cm
• A mCi

52
Calculating Exposure Rate

• Calculate the exposure rate at 10 cm from a
  cesium 137 source with an activity of 10 mCi
• Exposure rate (3.226) (10 mCi) (1/10)
• Exposure rate.3226 roentgen/hr (322.6mR/hr)

X ( ) (A) (1/d)
53
Using Exposure Rate to Calculate Activity

• Example
• At 15 cm from an iridium 192 source the exposure
  rate is 305 mR/hr (.305 R/hr). What is the
  activity of this source?
• Gamma factor ( )for iridium 192
  4.57 R x cm / mCi x hr
• X ( ) (A) (1/d)
• Activity X/ ( ) (1/d)
• .305 R/hr / (4.57) (1/15)
• 15.02 mCi

54
Brachytherapy Dosimetry and Dose Distribution

• Manual Systems used to calculate dose
  distribution
• Do you think dosimetrists use these manual
  systems now?
• Paterson-Parker (Manchester) System
• Planar Implants
• Volume Implants
• Quimby/Memorial Dosimetry System
• Paris System
• Computer Calculation Methods
• Sievert Integral

55
PatersonParker or Manchester System

• Guidelines/dosimetry method developed in the
  1930s
• Prior to this method pt were implanted prior to
  planning
• If followed, a dose of /- 10 will be provided
  within the implanted area
• Implant procedure strives to deliver a uniform
  dose to a plane or volume
• This system uses a nonuniform distribution of
  radioactive material to get a uniform dose
  distribution
• Assumes linear sources

56
PatersonParker (Manchester) System

• Planar Implants- Square and rectangular implants
• Spacing sourcesSee page 332, Table 15-6
• Based on the size of the area to be treated
• 2/3, 1/2, or 1/3 or source will be in periphery-
  depends on size of area
• See different arrangements- crossed/uncrossed
  ends
• Uncrossed ends result in a reduction of 10 of
  the area for table reading purposes
• Sources should be spaced no gt than 1 cm apart
• See pg 332

57
PatersonParker (Manchester) System

• Volume Implants
• Usually for seed implants
• If an end is uncrossed, reduce area by 7.5
• Three dimensional areas
• Cylinders
• Ellipsoids
• Spheres
• Cubes
• Others
• See pg 333

58
Quimby/Memorial Dosimetry System

• Similar to Paterson/Parker System
• Tables used to calc dose
• given a number of parameters- area, volume, total
  activity
• Quimby method the implant is uniform source
  distribution which results in a nonuniform
  distribution of dose
• Quimby method less often used than the
  Paterson-Parker method, but has been adapted into
  a system called the Memorial system and the
  tables are based on computer calculations
  accounting for filtration, modern units of
  activity and dose

59
Paris System

• Developed in the 1920s
• Uniform distribution of sources as in the Quimby
  method
• Based on three principles
• The sources must be rectilinear and arranged so
  that their centers are in the same plane, which
  is perpendicular to the direction of the sources
  and is called the central plane
• Dose is defined and calculated in this plane but
  not restricted to this plane
• Sources have uniform linear activity
• Sources must be uniformly spaced, even if more
  than one plane
• Iridium 192 wires

60
Computer Calculation Methods

• 1970s programs were developed based on manual
  methods
• Method of calc- Sievert Integral
• A linear source is broken into tiny components,
  dose is calculated at every point in the patient
  from every component, and adds the values to get
  the final result
• Exact isodose distributions
• Most departments use computers to calc dose
• Good idea to have manual calc to verify computer

61
Radiation Safety and QA

• Complete description of all sources in use on
  file
• Uniformity of source material should be verified
  by an autograph (film is exposed by the source)
  Film can be scanned to check dose uniformity
• Stored by source type/strength
• Documentation to verify control- initial receipt,
  calibration, inventory, and disposal
• Double checking of dose calculations
• Good QA program can prevent disaster