Brachytherapy pt 1
Brachytherapy is defined as
(also referred to as Curietherapy) a short-distance treatment of malignant disease with radiation emanating from small sealed (encapsulated) sources.
Point A
- 2 cm up from os, 2 cm over from tandem - Where uterine vessels cross the ureter - Prescription Point Since point A relates to the position of the sources rather than to a specific anatomic structure, it may lie inside the tumor or outside the tumor. If the tandem displaces the central canal, point A moves with the canal, but point B remains fixed at 5 cm from the midline.
Types of brachy implants
- Interstitial - Intracavitary - Surface plaque - Intraluminal - Intraoperative - Intravascular
The two most commonly used systems for dose specification in treatment of the cervix are:
- Manchester system is characterized by doses to four points: point A, point B, bladder point, and rectum point. - ICRU system
The important aspects of any brachytherapy treatment are:
- Use of a suitable dosimetric model for the treatment time and dose distribution calculation. - Use of calibrated sources with the calibration traceable to a standards laboratory. - Accurate positioning of sources to prevent geographical misses.
Advantages of brachytherapy
-Improved localized dose delivery to the target -Sharp dose fall-off outside the target volume (Reduces dose to surrounding healthy tissue) -Better conformal therapy
A permanent seed implant with a 30 day half life will deliver 100 Gy. _____ Gy/hr is the initial dose rate.
0.1 For complete decay, the total dose is calculated as Dₜₒₜ = DR₀ x Tₘₑₐₙ, where DR₀ is the initial dose rate and Tₘₑₐₙ is the mean life. The mean life is calculated as Tₘₑₐₙ = T½ x 1.44 = 30 days x (24 hours / 1 day) x 1.44 = 1036.8 hrs. So, DR₀ = 100 Gy / 1036.8 hr= 0.1 Gy/h.
The gamma energy of cesium-137 is
0.66 MeV
A patient receives a permanent seed implant using an isotope with a T½ of 50 days. Post implant dosimetry shows that the patient has received 50 Gy after 50 days. What total dose will the patient receive from this implant?
100 Gy; This is a unique case where you can simply double the dose to get the answer. Half the total dose is delivered during the first half life. The remaining half, 50 Gy, will be delivered during the subsequent full decay. Mathematically, it may be calculated as: Dtotal = Initial Dose Rate x Mean life. Mean life = 1.44T½.
A patient will receive a series of identical HDR treatments. A plan is done for 5 Gy per fraction with a 10 Ci source. The time is computed to be 100 seconds. After a few treatments, the radiation oncologist decides to reduce the dose to 4 Gy/fraction, while keeping the relative distribution unchanged. The source has decayed to 8 Ci. What is the correct time for this new prescription.
100 seconds
Using the AAPM TG-43 formalism, which brachytherapy source would be expected to have the greatest anistropy correction?
11 mCi ¹⁰³Pd seed Since ¹⁰³Pd has the lowest photon energy (average energy = 21 keV), the dose rate as a function of angle will be most affected by absorption of the metallic seed casing.
The half-life of palladium-103 is
17 days
Typical storage time for unused ¹²⁵I seeds before disposal is approximately:
2 years Unused radioactive seeds should be kept in storage to decay until they are indistinguishable from background. The half-life of ¹²⁵I is 59.4 days.
An ¹⁹²Ir HDR plan uses a 4 Ci source, 90 dwell positions, with a sum of all dwell times of 600 sec. Consider what changes are necessary if a 10 Ci source is used instead. The expected sum of all dwell times is _____ seconds using the 10 Ci source for the same plan.
240 The cumulated activity, which is the product of source strength and dwell time, should be the same for each treatment. The original cumulated activity was 4 Ci x 600 sec= 2400 Ci-sec. With the new source we expect 2400 Ci-sec / 10 Ci = 240 sec.
A permanent implant is removed after 30 days. The radionuclide used for the implant has a half-life of 6 days. What percent of the original planned dose still needs to be delivered?
3% If the T½ is 6 days, then 30 days is 5 half lives, so, 1/(2⁵).
The energy of the most prevalent gamma ray emitted by ¹³¹I is _____ keV.
364 The ¹³¹I gamma rays have a range of energies. The most prevalent (82%) Is 364 keV.
The beta energy of iridium-192 is
380 KeV
The Patterson Parker tables, adapted for HDR, predict that 235 second are required to deliver 8 Gy to a treatment volume of 45 cm³, assuming a 10 Ci source. The treatment time required to deliver 10 Gy with a 7 Ci source is?
420 seconds T(7 Ci, 10 Gy) = T(10 Ci, 8 Gy) ⋅ 10 Gy/8 Gy ⋅ 10Ci/7 Ci = 420 seconds
According to NRC 10 CFR 35, patients receiving therapeutic amounts of radioactive material may be released from the facility only if the total dose to a caregiver is not likely to exceed:
5 mSv According to NRC 10 CFR 35.75, a licensee may authorize the release from its control of any individual who has been administered unsealed byproduct material or implants containing byproduct material if the total effective dose equivalent to any other individual from exposure to the released individual is not likely to exceed 5 mSv. NUREG- 1556, vol. 9, describes methods for calculating doses to other individuals and contains tables of activities not likely to cause doses exceeding 5 mSv.
¹⁰³Pd seeds were ordered for prostate seed implants. Approximately how long would it take for the activities of the seeds to decay by 1%?
6 hours
¹³⁷Cs has a photon energy of
662 keV
What is the half-life of ¹⁹²Ir?
74 days
According to the ICRU, which of the following dose rates is most likely to be associated with HDR brachytherapy?
> 12 Gy/hr ICRU defines a very low dose rate (vLDR) as < 0.4 Gy/hr; LDR as 0.4 to 2 Gy/hr MDR as 2 to 12 Gy/hr; and HDR as > 12 Gy/hr.
Quality assurance tests following an HDR source change include _____ .
A wipe test of the package, source strength determination, source positioning accuracy, and dose linearity/timer accuracy are specified in AAPM TG-41, TG-56 and NRC 10 CFR part 35.
QA in source strength (activity)
Any discrepancy between the locally-determined and manufacturer's stated value exceeding 10 % should be investigated and the patient should not be treated until the discrepancy is explained and understood
What parameter is currently used to quantify brachytherapy source strength?
Brachytherapy source strengths are defined in terms of air kerma strength (cGy cm²/ hr) according to AAPM Task Group 43. Specific activity is the activity per unit mass and is NOT used to describe encapsulated brachytherapy sources. It is important to note that the SI unit of activity for brachytherapy sources is the Becquerel.
Common brachy source
Cs 131 has a half-life of 9.7 days and an energy of 364 KeV
Brachy photon sources
Emit gamma rays through gamma decay and possibly characteristic x rays through electron capture and internal conversion (examples: Co-60, Cs-137, Ir-192, I-125, Pd-103)
Brachy neutron sources
Emit neutrons following spontaneous nuclear fission (example: Cf-252)
Post-implant CT for dosimetry is obtained at approximately the same time for both ¹⁰³Pd and of ¹²⁵I seeds for permanent prostate implants. T/F?
False Current literature suggests that imaging studies for dosimetric evaluation are ideally obtained 2 - 3 weeks postimplantation for ¹⁰³Pd implants and approximately 4 weeks postimplantation for ¹²⁵I implants. The initial dose rate for ¹⁰³Pd implants is about three times that of ¹²⁵I implants. The average energies and half-lives are 20.8 keV and 17 days and 28.5 keV and 59.4 days for ¹⁰³Pd and ¹²⁵I, respectively.
For Permanent implants, cumulative dose is calculated by:
For permanent implants t = ∞ and the following relationship is used to determine the cumulative dose to complete source decay
An ¹²⁵I seed was accidentally dropped on the floor in the operating room during a prostate implant. What is the best detector to locate the missing seed?
Geiger-Muller Counter A Geiger-Muller counter is a very sensitive detector because of the avalanche of discharge (i.e., one hit will produce many clicks). This makes it particularly good as a survey meter.
According to the ICRU, what are the various brachy dose rates?
ICRU defines a very low dose rate (vLDR) as < 0.4 Gy/hr; LDR as 0.4 to 2 Gy/hr; MDR as 2 to 12 Gy/hr; and HDR as > 12 Gy/hr.
Gyn Rectal and bladder dose monitoring
In order to keep the dose to critical structures (rectum and bladder) as low as possible often surgical gauze is used to displace the sensitive structures away from the applicators
What is a limitation of the AAPM TG-43 brachytherapy dose calculation methodology?
It does not account for inhomogeneities
Brachytherapy treatment with eye plaques
Most commonly used seeds are iodine-125 seeds with typical activities of the order of 1 mCi.
The dose vs. distance within 5 cm from an ¹⁹²Ir source is very close to the inverse square law despite gamma ray absorption because:
Over the first few centimeters, increased scatter dose compensates for x-ray attenuation. Beyond about 5 cm from the source, however, beam attenuation dominates and the dose drops faster than the inverse square law predicts.
Choice of radionuclide for prostate implant
Palladium-103, which has a shorter half-life (17 d) than iodine-125 (60 d) delivers a higher initial dose rate and is thus useful in treating fast growing high grade tumors.
Intravascular brachytherapy
Places a catheter with a sealed source inside a blood vessel (usually an artery); Application of radiation (temporary or permanent implant) after treatment of arterial stenosis with angioplasty and stent placement has been proven useful in preventing restenosis. Restenosis within 6 months following angioplasty occurs in about 40 % of angioplasty patients.
For HDR brachytherapy, what is required by the NRC's 10CFR Part 35 at the end of every treatment after the source retracts?
Radiation survey of the patient This item requires knowledge of required safety measures to minimize unintentional irradiation of the patient. Out of the provided options, 10CFR35 specifically requires that post-treatment radiation survey of the patient be performed.
Uniformity of activity within each seed is established by?
Radiography and autoradiography using a single film exposure with a simulator can be used to check the uniformity of activity.
What is the MOST appropriate response to an HDR source that is "stuck" in an applicator inside a patient?
Remove the applicator from the patient and place it in a shielded container If a source is "stuck" inside a patient, the applicator should be removed from the patient and placed in a shielded container.
Specific activity
SA is independent of the mass of the radionuclide and has a fixed value independent of the time units of SA are Ci/g
Brachytherapy classification with respect to treatment duration
Temporary implant • Dose is delivered over a period of time that is short in comparison with the half-life of the sources. • Sources are removed when the prescribed dose has been reached. Permanent implant • Dose is delivered over the lifetime of the sources. • The sources undergo complete radioactive decay.
Dose distribution calculation around brachy sources are based on:
The AAPM TG 43 established the 2-D dose distribution around cylindrically symmetric brachytherapy sources such as palladium-103, iodine-125 and iridium-192.; assuming only photon interactions, and are influenced by: • Emitted radiation • Surrounding media
During an emergency HDR source retraction, a person entered the room for 1 min while standing approximately 1 meter away from the patient and manually retracted the source. What is the estimated dose (cGy) to the person?
The approximate dose rate at 1 m from a 10 Ci source is 0.08 cGy/min.
valid reason(s) for not directly summing brachytherapy (BT) and external beam radiotherapy (EBRT) dose distributions so as to provide a "composite" dose distribution?
The biological meaning of summed dose is difficult to evaluate. Due to the higher dose gradients of BT and the very different dose rates, there is a large disparity in the biologically effective dose for the two techniques.
The exposure rate at 1 meter from a point source of 10 mCi of ¹³⁷Cs is _____
The exposure rate constant of ¹³⁷Cs is 3.3 R-cm²/mCi-h. Exposure rate = Exposure rate constant ⋅ Activity ⋅ 1/d² = 3.3 R-cm²/mCi-h ⋅ 10 mCi ⋅ (1/100cm)² = 3.3 mR/hr
The exposure rate at 1 meter from a point source of 10 mCi of ¹³⁷Cs is _____ .
The exposure rate constant of ¹³⁷Cs is 3.3 R-cm²/mCi-h. Exposure rate = Exposure rate constant ⋅ Activity ⋅ 1/d² = 3.3 R-cm²/mCi-h ⋅ 10 mCi ⋅ (1/100cm)² = 3.3 mR/hr
Pre-planning dosimetry systems include
The most commonly used systems are: • Patterson-Parker (Manchester) system. • Quimby (Memorial) system. • Paris system.
A breast HDR treatment is carried out using a spherical balloon of 3 cm diameter and a single dwell position. The prescription point is 1 cm from the applicator surface, and the chest wall is 1.5 cm from the applicator surface. What is the percentage dose to the chest wall?
Use 1/R²: distance from source to Rx point = 2.5cm; distance from source to CW = 3 cm. Therefore (2.5/3)² = 0.69 or 69%.
A cervical applicator consists of
a central tube (tandem) and lateral capsules (ovoids or colpostats). The most commonly used applicator in the treatment of cervical cancer is the Fletcher-Suit-Delcos system
Pulsed Dose Rate (PDR)
a continuous low dose rate (LDR) treatment is simulated by a series of short duration "dose pulses" of the order of 30 minutes separated by intervals of 1 to several hours of no dose given.
Radioactive seeds for a prostate implant should be calibrated using _____.
a well chamber Well chambers calibrated by an Accredited Dosimetry Calibration Laboratory (ADCL) can be used to measure LDR, HDR, and beta sources.
Radial dose rate function, g(r)
accounts for the effects of attenuation and scatter in water on the transverse plane of the source (θ = π/2) , excluding falloff which is included by the geometry function G (r, θ). - It may also be influenced by filtration of photons by the encapsulation and source materials.
Geometry factor, G (r, θ)
accounts for the geometric falloff of the photon fluence with distance r from the source and also depends on the spatial distribution of activity within the source.
Brachytherapy sources are often encapsulated in another material. This encapsulation absorbs _____.
alpha and beta radiation The encapsulation of brachytherapy sources serves a few purposes including to contain the radioactivity, provide source rigidity and to absorb both the alpha and beta radiation produced through the source decay.
²²³Ra dichloride treats metastatic disease in bone. The vast majority of the dose to the targeted cells comes from _____.
alpha particles ²²³Ra goes through a decay chain yielding 4 alphas, 3 betas, and numerous gammas. About 95% of the dose to the target cells is from alpha particles.
Post-implant CT for dosimetry for ¹⁰³Pd seeds and ¹²⁵I seeds is obtained at approximately
are ideally obtained 2 - 3 weeks postimplantation for ¹⁰³Pd implants and approximately 4 weeks postimplantation for ¹²⁵I implants to allow for seed migration and volume reduction resulting from edema. The initial dose rate for ¹⁰³Pd implants is about three times that of ¹²⁵I implants. Dose calculations are performed and compared with prterm-75e-implant dose distributions.
According to NRC Part 35, what personnel are required to be present during a HDR brachytherapy treatment?
authorized medical physicist/therapy physicist and the authorized user/radiation oncologist
According to NRC Part 35, which personnel are required to be present during a high-dose-rate brachytherapy treatment?
authorized medical physicist/therapy physicist and the authorized user/radiation oncologist The main reason for this regulation is for situations where the source cannot be retracted. The MD must remove the applicator and source from the patient, and the physicist must do a radiation survey to ensure that the source has been safely stored. Usually a radiation therapist should also be present, but legally an MD can turn the HDR beam off or on without a therapist being present.
In AAPM Task Group 43, the anisotropy factor, F(r, θ), accounts for the angular dependence of _____.
both photon absorption and scattering in the source encapsulation and the medium In TG-43, the anisotropy factor accounts for both photon absorption and scattering in the source encapsulation and the medium, with the inverse square correction factored out.
A brachytherapy source which emits a single gamma ray per decay has a half-life (T½), average gamma energy (E), and activity (A). The dose rate in air at 5 cm distance from this source depends on _____.
both the activity and the energy, but not the half-life The half-life is irrelevant. The activity specifies how many gammas are emitted per second, and the energy allows calculation of the dose per decay.
Brachytherapy sources calibration
by Accredited Dosimetry Calibration Laboratory (ADCL); and are best done in Well type (re-entrant) ionization chambers which are suitable for calibration of both high and low strength (activity) sources (can be used to measure LDR, HDR, and beta sources.) For high strength sources, the source strength (activity) measurements may also be carried out with calibrated stem type ionization chambers
A titanium applicator _____ be used for tandem and ovoid HDR planning based on MR imaging. A CT scan _____ also necessary if perfoming dose calculations based on TG-43
can; is not Even though they are metallic, titanium applicators are not magnetic and, therefore, are compatible for both MR and CT scans. Inhomogeneity corrections are not currently done using the TG-43 protocol, so planning can be done using MR only, without the need for an additional CT scan.
Prior to the second fraction of a vaginal cylinder case, the HDR source was changed to a higher activity. When delivering the second fraction:
dose stays the same, time decreases because of the higher activity.
An HDR treatment takes 6 minutes with a ¹⁹²Ir source. Using the same source, the same treatment 100 days later would take _____
e =2.718
Cumulative dose Dcum delivered during time t is
e^1 = 2.718 e^-1 = 0.368
The air kerma rate in air of a point source in free space (no absorption or scattering effects are present.):
follows an inverse square law function with radial distance r from the source:
Brachy source capsule serves multiple purposes:
including it absorbs alpha and beta radiation produced through source decay in addition to it contains the radioactivity and provides source rigidity
For the same total activity, the initial dose rate of a brachytherapy source increases with:
increasing dose rate constant (Λ) Dose rate is linearly proportional to the dose rate constant. For a permanent implant, higher half life means higher total dose but not higher initial dose. Decreasing the anistropy factor will decrease the average dose rate (integrated over all angles) not increase it.
For the same total activity, the initial dose rate of a brachytherapy source increases with
increasing dose rate constant (Λ); Dose rate is linearly proportional to the dose rate constant. For a permanent implant, higher half life means higher total dose but not higher initial dose. Decreasing the anistropy factor will decrease the average dose rate (integrated over all angles) not increase it.
Dose rate constant, ʌ per AAPM TG 43 algorithm
is defined as the dose rate to water at a distance of 1 cm on the transverse axis (reference point) per unit air kerma strength in water
The benefit of using an in vivo kV x-ray source rather than ¹⁹²Ir for balloon-based partial breast irradiation is _____.
kV x-ray tubes for brachytherapy do not use "live" sources. Therefore, there is no need for a radioactive materials license and no need to perform a post-treatment radiation survey. The average energy of Ir-192 x-rays is 380 keV, but the energy of the x-rays typically used in electronic brachytherapy is <100 keV, which requires minimal room shielding.
Radiation monitoring around patients with permanent implants
may be discharged from the hospital if at the time of discharge the radiation level at 1 m from the implant is less than 0.5 mSv/h
In addition to a course of HDR treatment using tandem and ovoid applicators, radiation oncologists often prescribe _____ to boost radiation dose to the peripheral aspects of the treatment site.
parametrial external x-ray treatment
Usually a clinic will change the ¹⁹²Ir source in an HDR afterloader when the treatment times start to becomes twice as long as they would with a fresh source. Given this, which of the following source replacement intervals is the most reasonable?
quarterly (every three months) The half-life of ¹⁹²Ir is 74 days, so after that length of time, the treatment time will be twice as long. This is because treatment time is inversely proportional to source activity.
Specification of beta ray sources
ref absorbed dose rate in water at a reference distance from the source, generally between 0.5 mm and 2 mm from the source (because reference distance differs from one type of source to another)
ICRU dosimetry system (Report 38)
relates the dose distribution to the target volume rather than to a specific point. Report identifies a dose level of 60 Gy as the appropriate reference dose level for LDR treatments.
unit of exposure
roentgen (R): 1 R = 2.58 × 10−4 C/kg of air.
Reference air kerma rate in air, defined by the ICRU as:
the air kerma rate in air at a reference distance dref of 1 m, corrected for air attenuation and scattering (unit: µGy/h). SI unit of the reference air kerma rate is Gy/s, but for purposes of source specification it is more convenient to use µGy/h for LDR sources and µGy/s for HDR sources
¹³⁷Cs sources used for brachytherapy have a similar distribution to radium sources because:
the inverse square law dominates the dose distribution
For a vaginal cylinder HDR plan entered manually at the console, a step size of 1.0 cm is entered instead of the planned 0.5 cm. How does this affect the dose?
the shape, dose, and location of the distribution are affected
Radiation monitoring around patients after a permanent or temporary implantation of radioactive sources
via a radiation survey, which must be carried out in areas within and around the patient and the patient's room. Radiation levels in adjoining patients' rooms should be low so that no individual will be exposed to an equivalent dose exceeding 0.2 mSv in any one hour.
According to NRC 10 CFR Part 20 and 10 CFR Part 35, how often does a radiation shielding survey of adjacent areas need to be conducted for an ¹⁹²Ir HDR source?
whenever the source is changed in the HDR unit
¹²⁵I has an average photon energy of
¹²⁵I has an average photon energy of 28 KeV.
Why is the prescribed dose for a ¹⁰³Pd permanent brachytherapy implant usually lower than that for ¹²⁵I?
¹⁰³Pd has a shorter half-life and, therefore, the time-dose factor is larger. The half-life of ¹⁰³Pd is only 17 days, whereas the half-life of ¹²⁵I is 60 days. Thus, the total dose for ¹⁰³Pd is delivered over a shorter period of time, meaning less tissue repair and, therefore, higher effectiveness, even for a lower prescribed dose.
Specification for brachytherapy seed strength is given in terms of air kerma strength. The correct units are _____.
μGy m²/h
Useful radiation produced in brachy sources
• Gamma rays resulting from gamma decay. • Characteristic x rays resulting from electron capture. • Characteristic x rays resulting from internal conversion. • Characteristic x rays and bremsstrahlung originating in the source capsule.
Disadvantages of brachytherapy
• Only good for well localized tumors • Only good for small lesions • Very labor intensive
Advantages of HDR machines are:
• Optimization of dose distribution. • Treatment on outpatient basis. • Elimination of staff radiation exposure.
Brachytherapy treatment outcome is affected by
• Particular model used for source distribution in the target volume. • Algorithm used for calculation of dose distribution. • Methods used for the determination of the source strength. • Dose rate and prescribed dose used in treatment.
