All About Basics and physical properties and calculations related to brachytherapy sources and methods.

1. Brachytherapy

2. Brachytherapy Definitions
• Brachytherapy: Radiotherapy delivered using nuclides placed within or in
contact with the target volume.
• Sealed Source: Fully encapsulated.
• Low Dose Rate (LDR): < 2 Gy/h.
• Temporary
• Permanent
• Medium Dose Rate (MDR): 2–12 Gy/h.
• Almost never used for clinical treatment.
• High Dose Rate (HDR): > 12 Gy/h.
• Pulse Dose Rate (PDR): HDR treatment for a few minutes every hour, such that the
dose rate averaged over days is in the LDR range.
• Unsealed Source: Brachytherapy using freely floating radionuclides
(injected into a specific location, or administered systemically).

3. A Note on Brachytherapy
• Biologically speaking, there are several major differences between
brachytherapy and EBRT:
• Dose Rate: EBRT (excluding TBI) is usually performed at high dose rate.
Brachy may be HDR, LDR or PDR.
• Dose Gradient: Most EBRT plans attempt to achieve a uniform dose within
the target volume.
Brachy always produces steep dose gradients.
• Fractionation: Brachy is performed in far fewer fractions compared to EBRT.
• LDR implants may be performed in a single procedure (especially permanent implants).

4. Brachytherapy: Dose Rate Effects
To a first approximation, the LDR survival curve is equal to the survival curve of many small fractions of EBRT/HDR.
• This is the biological rationale for the use of PDR therapy.
Classic Dose-Rate Effect: LDR treatment results in decreased cell killing and no shoulder on the survival curve, compared to HDR/EBRT.
• The magnitude of this effect directly correlates with the amount of sublethal damage repair (SLDR) in that cell type.
• This is responsible for differential sparing of normal tissue with LDR, and is the biological rationale for the superiority of LDR.
• Intrafraction repair goes from 0 to 100 % between dose rates of 1 Gy/min and 0.01 Gy/min (60 cGy/h).
Inverse Dose-Rate Effect: In some rapidly cycling cells, cell killing actually increases between ~154 and ~37 cGy/h. This is a cell cycle effect.
• At 154 cGy/h the cell cycle is completely arrested, so radioresistant S-phase cells are radioresistant.
• At 37 cGy/h the cell cycle is allowed to progress into the radiosensitive G2/M, causing cell killing.
• This is another rationale for the superiority of LDR, proliferating cancer cells sensitize themselves but non-proliferating cells do not.
Very Low Dose Rate: Below the “critical dose rate”, fast-growing cells are able to repopulate faster than they are killed.
• For example, mouse jejunum treated at <0.54 cGy/min (32 cGy/h) shows very little killing.
• Permanent implants have an extremely low dose rate. Therefore they are ineffective on rapidly proliferating tumors.
• A typical I-125 prostate seed implant has a dose rate of ~7 cGy/h.
• Fortunately, prostate CA is a very slowly proliferating tumor.

5. Dose-rate to cell survival
curve that illustrates the
general improvement in
cell kill with increasing
dose rate with the
exception of the region
containing the inverse
dose-rate effect.

6. Dose Rate and Clinical Endpoints
• Mazeron did two studies on interstitial LDR implants: one on the oral cavity and one on the
breast.
• Oral Cavity: Dose rates <50 cGy/h were associated with less necrosis, with similar local control as long as total
dose was adequate.
• Breast: Between 30 and 90 cGy/h, higher dose rates were associated with improved local control.
• Typical temporary implant LDR dose rates are 50–60 cGy/h to the prescription point. However,
higher or lower dose rates may be used depending on clinical judgment and implant geometry.
• Higher dose rate = more efficacy, more toxicity.
• Permanent implant LDR dose rates are variable, and mostly depend upon which isotope is being
used.
• Shorter half-life = higher dose rate.
• Dose rate effects are largely irrelevant for HDR, as the dose rate is too high to allow intrafraction
repair or reassortment.

7. Brachytherapy: Choice of Nuclide and Implant
• Ra-226 was used for many decades but is almost never used anymore due to risk of radon
leakage.
• Permanent LDR implants generally use I-125, Pd-103, or less commonly Au-198.
• Temporary LDR implants may use Au-198, Ir-192, Cs-137, Co-60 or others.
• HDR implants almost always use Ir-192.
• Implants are classified as interstitial (such as prostate or breast brachy) or intracavitary (such as
GYN brachy).
Unsealed Sources
• I-131 is a beta-emitter that is taken up by thyroid tissue as well as differentiated thyroid cancers.
• Bone-seeking nuclides include Sr-89, Sm-153 and Ra-223 and are used to treat widespread bony
metastases.
• P-32 is a beta-emitter that can be used to treat the lining of a cyst, joint space, or body cavity.

11. Definitions
• AAPM TG-43: Task Group Report 00 of the American Association for Physics in Medicine.
• Activity (A): Amount of radioactive material.
• Units: 1 Curie (Ci) = 3.7 x 1010 Becquerel (Bq)
• Radius (r): aka distance, depth.
• Dose Rate (D ): not to be confused with total dose (D)
• Initial Dose Rate (D0)
• Exposure Rate Constant (Γ), aka Gamma Constant: Exposure rate per millicurie of isotope at 1 cm distance.
• Exposure Rate (X ) = ΓA
• Milligrams Radium Equivalent (mgRaEq):
• 1 mgRaEq = 8.25 R -cm2 -h-1 -mg-1 exposure rate
• Air kerma strength (SK): Kerma (kinetic energy released in matter) measured in air @ 1 m.
• 1 U = 1 cGy -cm-2 -h-1 = 1 μGy -m-2 -h-1
• Proportional to Activity.
• Dose rate constant (Λ): Dose rate to water for 1U air kerma strength at 1 cm (cGy -cm-2 -h-1 /U).
• Low Dose Rate (LDR): 0.4–2 Gy/h
• Medium Dose Rate (MDR): 2–12 Gy/h
• High Dose Rate (HDR): >12 Gy/h
• Pulse Dose Rate (PDR): HDR fractionated over time to approximate LDR dose rates.
Details of the units and activities along with dosimetry can be found in AAPM TG-43.

12. The Historical Role of Radium
• 226Ra brachytherapy was used for many decades prior to 60Co, 137Cs, 192Ir, or megavoltage X-
rays.
• Radium sources consist of radium chloride powder placed within a doublesealed platinum tube.
• 226Ra comes to a secular equilibrium with 222Rn and its decay products by emitting alpha rays.
• This results in accumulation of multiple radioactive daughter nuclides emitting alphas, betas and gammas.
• The encapsulation is designed to absorb everything except for the gammas.
• Average photon energy 0.83 MeV (range 0.18–2.29 MeV).
• 226Ra is no longer used because of the risk of radon gas leakage and other safety concerns.
• Many LDR brachytherapy systems are based on “milligrams radium equivalent” (mgRaEq).
• For a source of activity A and gamma constant Γ:
Radium Equivalent (mCi) = ΓA x mg x Ra x Eq / 8.25 R/cm
2
/hr

13. Production of Radionuclides
• Naturally Occurring: Byproducts of uranium decay, these nuclides can be mined from the Earth.
• 226-Ra, 223-Ra, 222-Rn among others.
• Fission Byproduct: Obtained from nuclear reactors.
• 137-Cs, 131-I, 90-Sr among others.
• Neutron Bombardment: Creates beta-minus emitters. Cyclotrons can produce high intensity
proton and neutron flux. Nuclear reactors can produce very high intensity neutron flux.
• 198-Au, 192-Ir, 153-Sm, 125-I, 103-Pd, 89-Sr, 60-Co, 32-P among others.
• Proton Bombardment: Creates beta-plus emitters, often used for PET imaging. Protons are
accelerated by a cyclotron.
• 123-I, 18-F, 15-O, 11-C, 3-H among others.
• Daughter Elution: A longer-lived mother nuclide (“cow”) decays into a shorter-lived daughter
nuclide (“milk”) that can be repeatedly eluted for clinical use. This is an example of transient
equilibrium.
• 90-Y, 99m-Tc among others.

14. Sealed Source Properties
• Classically, source strength is measured as activity (Ci or Bq) or milligrams radium equivalent
(mgRaEq).
• Two sources with the same Activity (Ci) may emit very different amounts, energies and types of radiation due
to encapsulation and filtration. Hence, their dose rate may be different.
• Source strength is specified as air kerma rate at a distance of 1 m as mentioned above. (1 U = 1
μGy/h/m2).
Unsealed Source Properties
• Unsealed sources do not have to worry about encapsulation so they simply are specified as
nuclide, activity, and chemical formulation. (ie elemental vs. colloidal vs. antibody-bound).
• An unsealed source will have separate physical and biological half-lives.
• Effective half-life equation:

15. Implant Instrumentation and Technique (Ircu-38 and 58)
An intracavitary implant is placed within an applicator such that the sources do not directly contact tissue.
• Tandem and ovoids (ie, Fletcher-Suit)
• Ring and tandem
• Vaginal cylinder
• Partial breast balloon brachytherapy
• Endobronchial
An interstitial implant is inserted into tissue.
• Template-based catheters
• Free-hand catheters
• Permanent seeds
Other types
• Surface applicator (eye plaque, intraoral, skin)
• Intravascular
• Intraoperative
Unsealed sources may be given systemically (oral, intravenous) or injected in a specific location (intracystic, intra-articular).

16. Brachytherapy Dose Rate
• LDR implants deliver dose over days (temporary) to months (permanent).
• Temporary LDR implants: Typical dose rates are approx. ~60 cGy/h or 1 cGy/min.
• Permanent implant dose rates are much lower, but total dose is very high such as in prostate seed implants
(120–145 Gy).
• Normal tissue sparing effect due to sublethal damage repair (SLDR).
• HDR implants typically deliver dose over a few minutes, with typical dose rates >50 cGy/min
(>3,000 cGy/h).
• Like external beam RT, fractions are given over a time scale shorter than that of DNA repair.
• Computer-controlled HDR afterloaders allow for detailed optimization of dwell positions and times.
• Geometric normal tissue sparing is used to make up for loss of biological normal tissue sparing.
• PDR is a method that uses an HDR afterloader to deliver fractions every hour or so, to
approximate LDR dose rates.

23. Loading Patterns: Basic Principles
• In a uniformly loaded catheter, the
center will receive more dose than
the ends.
• Therefore if you want a
homogenous dose, you need
peripheral loading – more source
strength at the ends.
• This is true for both LDR and HDR.

24. Classical Dose Systems (Interstitial)
• Prior to computer planning era, pre-calculated tables were used to calculate how much radium was needed to load
an implant. These are of mainly historical interest.
Paterson-Parker (Manchester):
• Different dose-loading tables for single plane, two-plane, and volume implants.
• Peripherally loaded – non-uniform loading.
• Uniform dose within implanted volume.
• Crossed ends – needles/catheters run perpendicular to each other.
Quimby
• Different dose-loading tables for single plane, two-plane, and volume implants.
• Uniform loading.
• Central hot spot within implanted volume.
• Crossed ends – needles/catheters run perpendicular to each other.
Paris
• Volume implants with multiple parallel needles or catheters.
• Uniform loading, identical for all needles.
• Uniform spacing of all needles.
• Central hot spot within implanted volume.
• Parallel ends – no crossing of needles.
Other
• Prostate – computer planning is preferred over fixed systems.

25. Classical Dose Systems (Intracavitary)
Fletcher-Suit (named after Gilbert Fletcher and Herman Suit)
• Dose is prescribed to Point A:
• 2 cm superior to the top of the ovoids as seen on a lateral film, and
• 2 cm lateral to the tandem, in a direction perpendicular to the tandem as seen on an AP film.
This is supposed to represent the paracervical triangle where the uterine vessels cross the ureter.
• Revised Point A is 2 cm superior to the flange:
• Unlike classical Point A, this point can be visualized on AP film alone (no need for laterals).
• Point H is the prescription point used by the American Brachytherapy Society.
• Find the intersection between the tandem and a line drawn between the mid-dwell positions of both ovoids.
• Move cephalad along the tandem by 2 cm plus the radius of the ovoids.
• Then, move lateral by 2 cm.
• This is intended to be the same point as classical Point A, but with more reproducible delineation.
• However, it is a bit lower than classical Point A.
• Typical LDR dose rate is 50–60 cGy/h to Point A.
• Additional dose measurements at:
• Point B is 3 cm lateral to Point A (5 cm from midline), represents the obturator nodes.
• Point P is the bony pelvic sidewall, either at the level of Point A or at the top of the acetabulum.
• Bladder Point is defined by the posterior extent of the bladder directly behind the Foley catheter.
• Vaginal Point is defined by the posterior extent of the vaginal packing, at the level of the midpoint of both ovoids.
• Rectal Point is defined as 5 mm posterior to the vaginal point.

26. Definitions of Point A:
Point A is the typical prescription point for cervical brachytherapy.
• The original definition is 2 cm lateral to the tandem and 2 cm above the top
of the ovoids.
• The revised definition is 2 cm lateral to the tandem and 2 cm above the top
of the flange.

27. Rules of Thumb

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