Chapter 15: Brachytherapy
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What is brachytherapy? Pg. 315
Brachytherapy is a method of treatment in which sealed radioactive sources are used to deliver radiation at a short distance by interstitial, intracavitary, or surface application.
List five radionuclides that are used in brachytherapy? Pg. 315
Cs-137 (Cesium), Ir-191 (Iridium), Au-198 (Gold), I-125 (Iodine), and Pd-103 (Palladium).
What two radionuclides were used since 1898? Pg. 315.
Radium (Ra) and radon (Rn).
What is the half-life of Ra-226 and what type of decay does it go through? Pg. 315
~1,600 years forming Rn-222. It goes through Alpha-decay.
When treating a patient with Ra-226, there are at least 49 gamma rays from Ra to Pb, and there are also alpha and beta particles being emitted. What material and thickness of it is used to filter most charged particles? Pg. 315
At least 0.5 mm of platinum.
What is the chemical state of Rn-222 following a Ra-226 alpha decay? Pg. 315
Rn-226 is a heavy inert gas.
What four properties of radium sources used for brachytherapy are used for source specification? Pg. 316
- 1.) active length
- 2.) physical length
- 3.) active or strength of source; mg of radium content
- 4.) filtration, transverse thickness of the capsule wall
What are three types of of radium needles used for implants to treat brachytherapy? Pg. 316
- 1.) "Uniform" - uniform linear activity distribution
- 2.) "Indian Club" - higher activity at one end
- 3.) "DumbBell" - high activity at both ends
What two types of brachytherapy techniques are Cs-137 used as a radium substitute for? Pg. 318
- 1.) Interstitial
- 2.) Intracavitary
What are two important advantages that Cs-137 have over Ra-226? Pg. 318
- 1.) Requires less shielding
- 2.) Is less hazardous in the microsphere form
What is the approximate half-life of Cs-137? Pg. 318
Half-life of about 30 years. Due to this, Cs-137 sources can be used clinically for about seven years without replacement, although the treatment times have to be adjusted to allow for radioactive decay (2% per year).
What are the two types of decay of Cs-137 and what percentage of the time is a gamma-ray emitted? Pg. 318
Cs-137 first goes through a negatron decay. Only after one of the negatron decay does Cs-137 go into an isomer state hence emitting a gamma-ray. That happens approximately 94.6% of the time. 5.4% of the time the Cs-137 emits a negatron going straight to a stable nuclide.
What is the energy of the emitted gamma-ray of Cs-137? Pg. 318
0.662 Mev or 662 keV
Ra-226 either emits an alpha-particle going directly to a Rn-222, or it emits an alpha-particle of less energy hence ending up in an isomer state. What is the energy of that emitted gamma-ray and how often does that happen? Pg. 315
The energy is 0.186 MeV or 186 keV. Since Rn-222 is also unstable, more gamma-rays are emitted. The average energy of emitted gamma-rays from Ra-226 is approximately 0.830 MeV or 830 keV.
What are two differences between Cs-137 and Ra-226? Pg. 318
- 1.) Doses along oblique angles (near the longitudinal axis) significantly differ due to the "filtration effect".
- 2.) Despite the penetrating power of emitted gamma-rays from both radionuclides are approximately equivalent, the attenuation of the gamma-rays differ in steel and platinum, especially since Cs-137 emits monoenergetic photons while Ra-226 emits polyenergetic photons.
What is one radionuclide that has been used for brachytherapy but is rarely used now? What is the main advantage of this radionuclide? Pg. 318
That radionuclide rarely used today is the Co-60 and its main advantage is its high-specific activity, which allows fabrication of small sources required for some special applicators.
What are two properties of Co-60 that renders it inconvenient to use? Pg. 318
It is more expensive that Cs-137 and has a short half-life of 5.26 years, necessitating more frequent replacement and a complex inventory system.
How are Co-60 radionuclides fabricated and used? Pg. 318
Co-60 brachytherapy sources are usually fabricated in the form of a wire that is encapsulated in a sheath of platinum iridium or stainless steel. The sources can be used to replace Ra-226 in intracavitary applications. Curis-sized cobalt sources have also been used in a unit called the Cathetron. This is a remote-loading device and provides high dose rates for intracavitary therapy.
Ir-192 has a complicated decay scheme. Despite that, what are its two main decays and what is the average energy of all emitted gamma-rays? Pg. 318
The two main decays for Ir-192 are EC and negatron decay. It's half-life is 73.8 days. The average energy is approximately 0.38 MeV or 380 keV.
How is Ir-192 fabricated? Pg. 318
Ir-192 sources are fabricated in the form of thin flexible wires that can be cut to desired lengths. Nylon ribbons containing Ir-192 seeds 3mm long and 0.5mm in diameter, spaced with their centers 1cm apart, are also commonly used. Both the wires and the seed ribbons are quite suitable for the after loading technique.
What is the radionuclide Au-198 used for? Pg. 319
Seeds or "grains" consisting of a radioactive isotope of gold are used for interstitial implants.
What is the half-life of Au-198 and what is the energy of its emitted gamma-rays? Pg. 319
The half-life Au-198 is 2.7 days and emits a monoenergetic gamma-ray of 0.412 MeV or 412 keV. It goes through the beta decay prior to its gamma-ray emission.
What is the main use of I-125? Pg. 319
I-125 has gained a wide use for permanent implants in radiation therapy.
What are the two main advantages that I-125 have over Rn-222 and Au-198? Pg. 319
- 1.) Its long half-life of 59.4 days which renders it convenient for storage.
- 2.) The low energy of its emitted gamma-ray renders less necessary shielding.
Section 15.1 E was read and had only a couple flash cards made for it. Due to that, read this section again prior to next exam.
Name one radionuclide that has recently became available for use in brachytherapy. Pg. 320
What property of Pd-103 provides a biological advantage in permanent implants and why? Pg. 320
That property is its short half-life of 17 days. Pd-103 may provide a biological advantage in permanent implants because the dose is delivered at a much faster rate.
What type of decay does Pd-103 go through? Pg. 321
Pd-103 decays by electron capture with the emission of characteristic x-rays in the range of 20 to 23 keV (average enegy of 20.9 keV) and Auger electrons.
What are five different ways in which the strength of a brachytherapy source can be specified? Pg. 321
- 1.) Activity
- 2.) Exposure Rate at a Specified Distance
- 3.) Equivalent Mass of Radiation
- 4.) Apparent Activity
- 5.) Air Kerma Strength
Activity, which is a way in which the strength of a brachytherapy source can be specified, can be specified in terms of mCi and the exposure rate at any particular point is proportional to the product of activity and its exposure rate constant. What are some corrections that would need to be done to the approximated activity value? Pg. 321
Errors, however, may be introduced in this method from the fact that corrections must be applied for the source and wall filtration and that the exposure rate constant may not be known accurately. It should be recalled that the accuracy of the exposure rate constant depends critically on the accurate knowledge of the spectroscopic data and the relevant absorption coefficients.
What does the National Council on Radiation Protection and Measurements (NCRP) recommend on how any gamma emitter should be specified on its strength in brachytherapy? Pg. 321
The NCRP recommends that the strength of any gamma emitter should be specified directly in terms of exposure rate in air at a specified distance such as 1 m.
What are the benefits of calibrating the specified strength of brachytherapy source using the exposure rate at a long distance? Pg. 321
This specification can be carried out simply by measuring exposure rate in free air at a distance sufficiently large that the given source can be treated as a point. A long distance measurement geometry minimizes the dependence of the calibration upon the construction of the source and the detector because both can be treated as points.
There are historical reasons that make it convenient to specify brachytherapy sources in terms of the equivalent mass of radium. Why was it suggested that the exposure rate could be expressed in terms of "effective" equivalent mass of radium? Pg. 321
It was suggested because some users, especially the physicians who are accustomed to radium sources, continue to use g-Ra eq.
How do you convert the equivalent mass of radium (which is a way to specify a brachytherapy source) to the "effective" equivalent mass of radium? Pg. 322
This conversion is simply made by dividing the exposure rate at 1 m by the exposure rate constant of radium (point source filtered by 0.5 mm Pt) at 1 m.
What is the best way to calibrate and specify brachythrapy sources out of the five different ways of specification? Pg. 322
It should, however, be emphasized that the best way to calibrate and specify brachytherapy sources is still in terms of exposure rate or air kerma rate at a distance of 1 m. The effective mg-Ra eq should be used only to provide output comparison with radium sources.
When specifying the strength of a brachytherapy source, if the source is calibrated in terms of exposure rate at 1 m, what can the strength be specified as? Pg. 322
In that case, its strength may be specified as "apparent activity".
What is the definition of "apparent activity" which is the specification for the strength of a brachytherapy source when its calibrated in terms of exposure rate at 1 m? Pg. 322
It is defined as the activity of a "bare point" source of the same nuclide that produces the same exposure rate at 1 m as the source to be specified.
How is the specification "apparent activity" determined? Pg. 322
The apparent activity of a brachytherapy source is determined by dividing the measured exposure rate at 1 m with the exposure rate constant of the unfiltered source at 1 m.
What quantity involved with the specification of the strength of a brachytherapy source is in the process of being phased out, or in other words, not going to be used any more? Pg. 322
Although exposure rate at a specified distance is the method of choice in designating source strength, the quantity that is being phased out is EXPOSURE. Most of the standards laboratories have already replaced exposure by the quantity AIR KERMA.
Due to the trend of most laboratories using AIR KERMA instead of EXPOSURE, what has AAPM recommended with regards to the specification of the strength of a brachytherapy source? Pg. 322
In keeping with these trends, the AAPM recommended the quantity AIR KERMA STRENGTH for the specification of brachytherapy sources.
How is AIR KERMA STRENGTH defined mathematically? Remember that AIR KERMA STRENGTH is a way in which the strength of a brachytherapy source is specified. Pg. 322
The AIR KERMA STRENGTH is defined as the product of air kerma rate in "free space" and the square of the distance of the calibration point from the source center along the perpendicular bisector.
When performing "open-air" measurements for the calibration of brachytherapy sources knowing that the dose rate (output) from the source is low at large distances, what is one important property of the ionization chamber that should be of concern? Pg. 323
The chamber volume should be large enough to include more photons within the measurement and to render the signal-to-noise ratio to at least 100.
What is one significant hindrance when performing "open-air" measurements in the calibration of brachythrapy sources? Pg. 323
The one significant drawback is the difficulty in obtaining "good geometry" hence rendering the calibration process long enough to cause problems.
Instead of performing "open-air" measurements when calibrating a brachytherapy source, there is an alternative method used. What is it? Pg. 324
That alternative, routine method of brachytherapy source calibration is carried out by using well-type ionization chambers where the walls of the chamber are surrounding the source rather than leaving the source in "open-air".
When performing brachytherapy source calculation with the "well-type" ion chamber method, what are some corrections that need to be done? Pg. 325
The response of well chambers is known to depend on the source position in the well and on the length of the source. Correction factors must be determined for these effects for a given instrument and the type of source to be calibrated.
What is one mathematical equation is used to calculate the exposure rate at a specific point around a brachytherapy source? Pg. 325
The Sievert Integral introduced by Sievert in 1921.
The Sievert Integral is used to calculate the exposure rate around a brachytherapy source at a point in order to calculate the dose delivered to that point, but what is one significant drawback with the Sievert Integral? Pg. 326
Because the Sievert integral uses the energy absorption coefficient, the underlying assumption is taht the emitted energy fluence is exponentially attenuated by the filter thickness traversed by the photons. This is an approximation that has been shown to work well for Ra-226 and Ir-192 seeds in the region bounded by the active source ends. However, Monte Carlo simulations have shown that beyond the end of the active source region, the Sievert approach introduces significant errors and practically breaks down in the extreme oblique directions.
When comparing the exposure dose rate at a certain point from both point source and linear source, what are two important facts derived from the comparison? Pg. 327
When measuring the dose rate at points close to the linear source, due to the oblique angles the photons travel causing greater attenuation than exiting the source wall in a perpendicular direction, the exposure dose rate closer to the linear source doesn't follow the inverse square law. But as the point gets further from the linear source, the exposure dose rate gradually begins to follow the inverse square law.
The Sievert Integral is used to measure the exposure dose rate at a point from a linear source. If you want to measure the absorbed dose in tissue at the same point, what are some drawbacks with the Sievert Integral? Pg. 327
The Sievert Integral gives the exposure rate distribution in air and considers only the inverse square law and filtration effects. When a source is implanted in the tissue, one needs to consider, in addition, attenuation as well as scattering in the surrounding tissue. The exposure rate calculated at a point in tissue can then be converted into absorbed dose rate by using the appropriate roentgen-to-rad factor.
When measuring the absorbed dose at a point from the linear brachytherapy source, there are two apparent facts that stem from the curve of the ratio of Dr/D1cm. What are those two apparent facts? Pg. 328
When the point is near to the linear source, the ratio is close to 1 since the water attenuated photons are compensated by the nearby water photon scattering. As the point gets further from the linear source, the scattering doesn't make up for the attenuation hence the ratio decreases as the point gets further from the source. This ratio can be perceived as a "attenuation correction factor" that makes up for those two facts.
For Section 15.3.C, re-read that before the exam and study TG-43 and TG-43U1 for more clarification on the equations.
What are the two objectives of brachytherapy treatment planning? Pg. 332
The two objectives of treatment planning are
- 1.) to determine the distribution and type of radiation sources to provide optimum dose distribution.
- 2.) to provide a complete dose distribution in the irradiated volume.
OVer the past 50+ years, there have been numerous systems of dosimetric planning for brachytherapy. Out of those, which two have received the most widespread use? Pg. 332
- 1.) The Paterson-Parker system
- 2.) The Quimby system.
What was the Paterson-Parker or Machester brachytherapy dosimetric planning systems developed to deliver? Pg. 333
The Paterson-Parker or Manchester system was developed to deliver uniform dose (with plus or minus 10 percent) to a plane or volume.
For the Paterson-Parker brachytherapy dosimetric system, there are planar and volume implants. When dealing with the volume implants, what three-dimensional shapes would be easy to use? Pg. 334
Cylinders, spheres, and cuboids.
In the Paterson-Parker brachytherapy dosimetric planning system, what are the prescribed doses for both the planar and volume implants? Pg. 334
Planar system: The "stated" dose, I'm assuming Kahn means prescribed dose, is determined fom the Paterson-Parker tables, which is 10% higher than the minimum dose. The maximum dose should not exceed 10% above the stated dose to satisfy the uniformity criterion.
Volume Implants: For a volume implant, the prescribed dose is stated 10% higher than the minimum dose within the implanted volume.
What corrections need to be done to the values in the Paterson-Parker table in order to convert the Roentgens to cGy in tissue? Pg. 335
- 1.) Exposure rate constant: The tables assume that it equals 8.4 Rcm^2/mg-h instead of the current value of 8.25 Rcm^2/mg-h.
- 2.) A roentgen:cGy factor of 0.957 should be used to convert exposure into dose in muscle.
- 3.) Oblique filtration: Paterson-Parker tables do not take into account increased attenuation by oblique filtration by the platinum capsule, giving rise to a 2% to 4% error for typical implants.
- 4.) Paterson-Parker tables are based on exposure in air. Corrections are needed for tissue attenuation and scattering.
How can you characterize the Quimby system and how does the result of system affect surrounding dose distribution? Pg. 336
The Quimby system of interstitial implantation is characterized by a uniform distribution of source of equal linear activity. Consequently, this arrangement of sources results in a nonuniform dove distribution, higher in the central region of treatment.
What two systems have a similar problem with their tables like the Paterson-Parker tables? Pg. 336
The original Quimby tables, like the Manchester tables, are based on an out-dated exposure rate constant which is 8.4 Rcm^2/mg-h instead of the current accepted value of 8.25 Rcm^2/mg-h.
Do corrections, similar to those applied to the Paterson-Parker tables have to be applied to the Quimby and Manchester tables? Pg. 336
What system is an extension of the Quimby system? Pg. 336
The Memorial system, as described by Laughlin et al. in 1963, is an extension of the Quimby system and is characterized by complete dose distributions around latices of point sources of uniform strength spaced 1 cm apart.
Does the Memorial system need corrections to its tables like the Paterson-Parker, Quimby, and Manchester systems? Pg. 336
What is the primary intention of the Paris system? Pg. 337
The Paris system of dosimetry is intended imarily for removable implants of long line sources, such as Ir-192 wires. The system prescribes wider spacing for longer sources or larger treatment volumes.
For the Paris system, what is the value of the reference isodose fixed at? Pg. 337
In the Paris system the dose specification is based on an isodose surface, called the "reference isodose. However, in practice, the value of the reference isodose is fixed at 85% of the "basal dose", whicch is defined as the average of the minimum dose between sources.
An implant system tha thas evolved through the use of computers but bears no formal name is used in many institutions in the United States. What does Kahn call it and how is it implemented? Pg. 337
He calls it the "Computer System". The implantation rules are very simple: The sources of uniform strength are implanted, spaced uniformly, and cover the entire volume.
Knowing that the Computer System implants sources of uniform strength, what is one unique property of that which is similar to the Quimby and Paris systems? Pg. 339
It is realized that the implantation of uniform activity sources gives rise to an implant that is "hotter" in the middle than in the periphery, as is the case with the Quimby and the Paris systems. However, this dose inhomogeneity is accepted with the belief that the central part of the target would need higher doses to sterilize than the periphery.
Dose calculation algorithms require spatial coordinates for each radioactive source. What are two methods in which a three-dimensional reconstruction of the source geometry is usually accomplished? Pg. 339
- 1. Orthogonal Imaging Method.
- 2. Stereo-Shift Method.
Give a brief summary on the Orthogonal Imaging Method. Pg. 340
The Orthogonal Imaging Method basically uses an anterior-posterior (AP) image and a lateral image 90-degrees from the AP image. The two planes, x-y and y-z, are used to correlate the positions of each source with respect to both planes. Once those are accomplished, corrections for magnification are implemented.
Give a brief summary on the Stereo-shift method. Pg. 340
This method uses two AP images where the patients is in different positions in each. Once the geometry of the target with respect to the point of localization and origin, geometry is used to derive equations for the coordinates of the target-source.
Which of the two imaging methods used to derive spatial coordinates (three-dimensionally) for the target and source is more accurate and when would one be used over the other? Pg. 341
The Orthogonal Imaging Method is more accurate than the Stereo-shift method but the latter would be more effective in situations where the sources cannot be easily identified by orthogonal films.
What are three ways in which brachytherapy sources are applied? Pg. 341
- 1. Surface Molds.
- 2. Interstitial Therapy
- 3. Intracavity Therapy
Briefly summarize how brachytherapy using surface molds are executed? Pg. 341
Plastic molds are prepared to conform to the surface to be treated and the sources are securely positioned on the outer surface of the mold. The distance between the plane of the sources to the skin surface is chosen to give a treatment distance of usually 0.5 to 1.0 cm. The dosimetry and source distribution rules are the same for external molds as for interstitial sources.
In interstitial therapy, what are three ways in which the radioactive sources are fabricated? Pg. 341
In interstitial therapy, the radioactive sources are fabricated in the form of needles, wires, or seeds, which can be inserted directly into the tissue.
What are the two types of interstitial implants? Pg. 342
In general, which implant, temporary and permanent, provides better control of source distribution and dosimetry? Pg. 342
In general, a temporary implant provides better control of source distribution and dosimetry than a permanent implant. However, the permanent implant is a one-time procedure and is a preferred method for some tumors such as those in the abdominal and thoracic cavities.
What three areas of the body is usually treated with intracavitary therapy? Pg. 343
Intracavitary therapy is mostly used for cancers of the uterine cervix, uterine body, and vagina.
- 1. uterine cervix.
- 2. uterine body.
- 3. vagina.
What are the six advantages of "remote afterloading units"? Pg. 349
- 1. The major advantage of the remote afterloaders is the elimination or reduction of exposure to medical personnel.
- 2. Well-designed systems can provide the capability of optimizing dose distributions beyond what is possible with manual afterloading.
- 3. Treatment techniques can be made more consistent and reproducible.
- 4. In LDR remote afterloading, sources can be retracted into shielded position to allow better patient care under normal as well as emergency conditions.
- 5. HDR remote afterloading permits treatment on an outpatient basis, using multiple fraction regimens.
- 6. HDR remote afterloading is suited for treating large patient populations that would otherwise require prolonged hospitalization if treated by LDR brachytherapy.
What are five disadvantages of the "remote aftrerloading units"? Pg. 350
- 1. Remote afterloading devices are expensive and require a substantial capital expenditure for equipment acquisition.
- 2. In the case of HDR, additional costs must be considered for room shielding (if not located in an existing shielded facility) and installing ancillary imaging equipment.
- 3. Locating HDR in an existing radiation therapy room compounds the problem of patient scheduling unless the room is dedicated to HDR brachytherapy.
- 4. No significant improvements are expected in clinical outcome over state-of-the-art conventional LDR brachytherapy, although the issue is still controversial and needs further investigation.
- 5. Quality assurance requirements for remote afterloading devices are significantly greater because of the greater complexity of the equipment and frequent source changes.
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