Brachytherapy pt 2
I¹²⁵ seeds are ordered for a prostate implant. The procedure is delayed by 100 hours, but the same I-125 seeds are implanted. What is the percent change in the actual delivered dose? (T½ = 60 days)
-5%
What is the minimum threshold dose rate defining high dose brachytherapy?
12 Gy/hr.
A patient receives a permanent prostate implant using ¹²⁵I. The intended dose is 144 Gy and uses 60 seeds. However, a TURP is performed 60 days post-implant, during which time 15 seeds are removed. The total delivered dose will be:
126 Gy The half-life of ¹²⁵I is approximately 60 days. The patient, therefore, has already received half the total dose, 72 Gy, prior to the TURP. During the TURP, a quarter of the seeds are removed. Therefore we can estimate the patient will subsequently receive only ¾ of the remaining intended dose. Dose = 72 + 0.75 x 72 = 126 Gy.
A ¹³⁷Cs seed has an air kerma strength of 5 μGy m² h⁻¹. What is the air kerma over the course of 2 hours at a distance 2 meters away from the source?
2.50 μGy
For an "ideal" Fletcher tandem and ovoids, with a total of 65 mg Ra eq, the typical dose rate at point A is _____ cGy/hr. Loading: Tandem: 15-10-10 mg Ra eq sources. Ovoids 15 mg Ra eq each.
55 Point A is defined as being 2 cm above the lateral vaginal fornix and 2 cm lateral to the cervical canal. A typical target dose rate to point A is roughly 50 cGy/hr.
An HDR treatment is planned to deliver 500 cGy to 0.5 cm into the vaginal tissue using a 3 cm cylinder. However, a 2.6 cm cylinder was used. What dose was delivered to the prescription depth in the middle of the cylinder length?
555 cGy For a line source, the dose falloff goes as 1/r. The distance from the source to the prescription point should have been 3 cm diameter/ 2 = 1.5 cm + 0.5 cm depth= 2 cm. Instead, it was 2.6 / 2 = 1.3 cm + 0.5 cm depth= 1.8 cm: 500 x (2.0 / 1.8) = 555 cGy. 1/r is valid in the middle of a long uniform cylindrical distribution. The dose at the cylinder axis in the superior direction is going to be the same.
The half-life of iodine-125 is
60 days
According to ICRU-38, how much radiopaque fluid should be inserted into the Foley bulb to calculate the dose to the bladder reference point during an intracavitary implant for the treatment of cervical cancer?
7 cc
¹³¹Cs has a T½ of 9.7 days, and a Γ = 0.64 R-cm² / mCi-hr. How much will a 10 gram source decay over 24 hours?
7% A = Ae^-(ln2/9.7)(1 day) = 93% remaining, so 7% decayed.
For a point 1 cm away from a ²²⁶Ra source, the exposure rate ratio of a point source to a linear source is:
> 1 Linear source suffers from oblique filtration and travel larger distance than point source. Hence the ratio of the exposure rate of a point to that of a linear source is > 1.
In brachytherapy treatment using tandem and ovoids, classical points A and B are defined as follows:
A is 2 cm lateral from the tandem, B is 5 cm lateral from the pelvic center
According to AAPM recommendations, quality assurance of multi-channel intracavitary HDR applicators for breast includes:
According to AAPM recommendations, quality assurance of multi-channel intracavitary HDR applicators for breast includes a daily CT scan, daily check of the balloon volume and consistency of the applicator position and check of each channel length prior to each treatment.
Rather than reporting the maximum point dose for an OAR, ICRU 83 recommends reporting the near maximum dose as which measure?
D2%
For a Fletcher-Suit applicator, what is the advantage of using a larger diameter ovoid?
Lower mucosal dose rate and a better depth dose distribution The inverse square law states that the intensity is inversely proportional to the square of the distance from the source of that physical quantity. Thus the larger ovoids decrease the dose to the mucosal surface but increase the depth dose. The dose to the bladder and rectum is not changed because ovoids have the same internal shielding.
The SI unit for brachytherapy source activity is _____ .
becquerel
AAPM TG-43 recommends incorporation of the anisotropy function when estimating dose for interstitial brachytherapy. Which of the following is primarily responsible for the anisotropic distribution of dose with brachytherapy seeds?
encapsulation material around the source
Which of the following brachytherapy applicators is most affected by the dwell time gradient:
endobronchial catheter The dwell time gradient is the difference between the dwell times at adjacent HDR source positions. Limiting the gradient value is much more important for thin catheters since the small diameter results in the smallest distance between the source and the surrounding tissue. Therefore, adjacent local high- and low-dose regions do not compensate each other as much as they do for thicker applicators with plastic surrounding the HDR channel (like vaginal cylinders). That results in high-dose regions in tissues proximal to the dwell locations with long dwell times. In the case of a vaginal cylinder, the same high-dose regions would be located in the plastic cylinders surrounding the applicator channel.
What action is required by the Nuclear Regulatory Commission (NRC) to confirm that no sources remain in the patient after treatment is completed?
post-treatment radiation survey of the patient The NRC required a post-treatment radiation survey after removal of the last temporary implant in order to ensure that no sources remain in the patient after treatment is completed.
The rapid dose falloff of a ¹³⁷Cs source in a 5 cm radius of tissue is primarily due to:
the inverse square law. The most significant factor of ¹³⁷Cs dosimetry implants is the dose falloff due to a distance change.
The isodose distribution around a typical ¹²⁵I brachytherapy seed is asymmetric because:
x-ray attenuation varies different parts of the metal casing X-rays are emitted isotropically, but attenuation by the metal casing varies with angle of incidence. Also, the thickness of the metal casing is variable.
The beta energy of palladium-103 is
21 kV
A patient receives a permanent implant using an isotope with a 10-day half-life. The initial dose rate of the implant is 0.1 Gy/hr (2.4 Gy/day). Assuming full decay, what will the total dose ( in Gy) be to the target?
34.6 Gy The total dose= (initial dose rate) x (average life). In this case, total dose= 0.1 Gy/hr x 10 days x 24 hr/day x 1.44 = 34.6 Gy.
A patient receives a permanent seed implant using an isotope with a half-life of 50 days. Post implant dosimetry shows that the patient has received 50 Gy after 50 days. What dose will the patient have received 100 days post implant.
75 Gy
Which brachytherapy dose calculation factors is often approximated by either a point or line source?
Geometry factor Geometry factor accounts for dose variation due to spatial distribution within the source (ignoring scatter and attenuation in the source). It is often approximated by either a point or line source.
An ¹⁹²Ir HDR unit has an activity of 8.6 Ci on May 2. The treatment time for a vaginal cylinder is 300 seconds. If the treatment time for a similar treatment is not to exceed 10 minutes, the source must be changed on:
July 15 The treatment time will double with the activity has dropped to one-half, i.e., after one half life, or 74 days.
Patterson Parker (PP) rules cannot be used for calculation of dose distributions for a ¹⁰³Pd brachytherapy implant because:
PP was designed for radium, which has much higher energy than ¹⁰³Pd ¹⁰³Pd has a much lower energy than does radium, which means that its absorption characteristics and scatter dose are much different.
_____ is 𝗡𝗢𝗧 required after an HDR source exchange.
Percent depth dose PDD is a difficult and unnecessary measurement. The depth dose characteristics are determined by the isotope and its encapsulation.
According to the NRC, at which point during an HDR treatment can the physician designated as the authorized user (AU) leave the treatment area so long as he or she is still "on site" in the hospital?
The AU can leave after observing the initiation of treatment, as long as both a qualified medical physicist and another trained physician remains physically present. According to NRC information notice 2012-08 both the authorized medical physicist (AMP) and the AU must be present and within "hearing distance of normal voice" during the initiation of treatment. An AMP and an AU or a non-AU physician under the supervision of the AU and trained in the emergency procedures are required to be present for the remainder of the treatment.
When using the MammoSite, which of the following parameters are evaluated to determine whether the patient can be treated?
The balloon diameter, symmetry, balloon to skin distance and the tissue-balloon conformance are all assessed to determine whether the patient can be treated with mammosite.
The HDR source is changed between a patient's first and second vaginal cylinder treatment. What parameters remains the same?
The total treatment time will decrease after the source change, but the treatment dose, cure-seconds (cumulated activity), and movement of the source from one dwell position to the next will all remain the same.
The person preparing a shipment of a used HDR source should be _____
a specially trained authorized shipper of radioactive materials
What gas is responsible for the high internal pressure in a double sealed radium tube?
helium
Two patients are being treated with a high-dose-rate afterloader using vaginal cylinders. The prescribed dose, treatment length (i.e., the number of dwell positions), and target depth of 0.5 cm from the cylinder surface are all the same for both. Patient A is treated using a 2.5 cm diameter cylinder, and patient B is treated using a 3.5 cm diameter cylinder. When compared to patient A, the treatment time for patient B will be _____ and the dose at the cylinder surface will be _____.
longer; lower
The dose rate at the isocenter of a megavoltage linear accelerator is 4 Gy/min. The dose rate 1 cm from a typical HDR Ir-192 source is about:
the same. The activity of a typical high-dose-rate (HDR) Ir-192 source is 5 to 10 Ci, and the dose rate is about 3 to 8 Gy/min.
What is the purpose of the dummy wire in an ¹⁹²Ir HDR remote afterloader unit?
to confirm that the path the radioactive source travels is clear of obstruction
Which of the following sealed radioactive sources has the most anisotropic dose distribution?
¹²⁵I ¹²⁵I has a much lower energy, and thus, the source radiation is absorbed in the source capsule, leading to anisotropy (less dose along the source axis).
For the first 5 cm, the dose falloff with distance in water from a point source follows the inverse square law (l/r²) for all of the following isotopes except _____.
¹⁰³Pd, and ¹²⁵I For ¹³⁷Cs, ¹⁹²Ir, ¹⁹⁸Au, and ⁶⁰Co, over a distance of about 5 cm from the source, the attenuation of the primary photons is very much compensated for by the contribution of scattered photons. For ¹⁰³Pd, and ¹²⁵I, the attenuation component is larger than the scattering due to their low photon energies.
Isotopes that could be used in eye plaques for treatment of uveal melanomas include:
¹⁰⁶Ru and ¹²⁵I ¹⁰⁶Ru is a beta emitter with a half-life of 373.6 days. ¹²⁵I emits low-energy gammas as part of its decay chain with a half-life of 59.4 days. While ¹⁰³Pd has also been used, ¹⁹²Ir would not be appropriate due to the average photon energy of 0.38 MeV.
Yttrium-90 microspheres are used in the treatment of hepatocellular carcinoma. Which particle is responsible for delivering almost all of the dose?
β⁻ ⁹⁰Y undergoes β⁻ decay with a decay energy of 2.28 MeV. 0.01% of the decays produce 1.7 MeV photons.
How many mCi of radium are contained in a 1 mg mass of radium? (Γ - 8.25 R-cm² / mCi-hr).
1 Since radium is the standard to measure activity, a 1 mg mass quantity of radium = 1 mCi of activity. Hence the units mg and mCi are equivalent for radium.
A patient is to receive a ²²³Ra injection (T½ = 11.4 days) at 10:00 AM. What is the reduction in activity if the patient does not receive the injection until 2:00 PM?
1% A = A₀e^-λT.
What is the average energy of a Cobalt 60 source?
1.25 MeV The quality of a Cobalt 60 beam has the same energy of gamma rays of 1.17 and 1.33 MeV. The average of these gamma rays are 1.25 MeV.
A MammoSite HDR treatment, with a single dwell position at the center of the spherical balloon, is planned from orthogonal x-rays. By mistake, a magnification factor of 1.4 is used instead of the correct value (1.29). The delivered dose will then be:
15% too low Since the wrong magnification is used, the balloon radius will be assumed to be too small, and a lower dwell time will be prescribed, resulting in an underdosing equal to 1 - (1.29/1.4)².
A brachytherapy balloon is inflated to a radius of 2 cm and an ¹⁹²Ir source is positioned at the center. 340 cGy is prescribed to 1 cm depth beyond the surface of the balloon. A second fraction is given without recognizing that the balloon leaked and has a new diameter of 3 cm. What percent of the first fraction dose to the surface of the balloon is received at the balloon surface with the second fraction?
178% Dose fall off for an iridium source is governed by the inverse square law. If the surface of the balloon is 2 cm from the source and is reduced to 1.5 cm from the surface, the dose will increase by a factor of (2 / 1.5)² = 1.78 = 178%.
What is the total dose delivered by a permanent ¹²⁵I seed implant with an initial dose rate of 0.1 Gy/hr?
207 Gy Total Dose = initial dose rate x 1.44 x T½. = 0.1 Gy/hr x 1.44 x 60 days x 24hr/day = 207 Gy
If a balloon breast brachytherapy treatment using an ¹⁹²Ir source is delivered with a 189 second exposure time, how long will the same treatment with the same source take 2 weeks later?
215 seconds T½ = ¹⁹²Ir = 74 days. time = 189 seconds ⋅ e^-λT = 189 seconds ⋅ e^+(.693/74)(14 days) = 215 seconds.
According to the ACR-AAPM Technical Standard for the Performance of High-Dose-Rate Brachytherapy Physics, after an HDR source exchange, the activity of the source must be verified to fall within _____ of the manufacturer's certificate.
3% According to the ACR-AAPM technical standard for the performance of high dose rate brachytherapy physics, the qualified medical physicist must determine that the measured source strength is accurate to within ± 3% of that reported by the manufacturer and establish a course of action if the source strength does not fall within the limits.
The half-life of cesium-137 is
30 years
A permanent seed implant will deliver 100 Gy with an isotope having a 20-day half-life. The patient will have already received _____ by the time he returns for follow-up, 40 days post-implant?
75 Gy Forty days is two half-lives, which means the activity is at 25% of its original strength. This implies that 75% of the radioactive material has decayed, and so the patient has received 75% of the final dose or 75 Gy.
What is the total dose delivered by a ¹³¹Cs permanent prostate implant if the initial dose rate is 0.25 Gy/hr? (Note: T½ = 9.7 days)
83.8 Gy
AAPM recommendations for quality assurance for HDR specify all of the following as daily checks
AAPM Task Group 56 specifies safety checks such as door interlock and radiation monitor functionality, dose delivery accuracy checks such as verifying the source strength on the HDR unit and TPS, and system function tests, such as source retraction on a daily basis. Source calibration is a quarterly check.
How often should the activity of a brachytherapy source be corrected for physical decay before use?
At intervals consistent with 1% physical decay
An ¹⁹²Ir seed of 1 mCi is equivalent to _____ mg-Ra eq. (Note: exposure rate constant: ²²⁶Ra 8.25 R-cm²/mg-h; ¹⁹²Ir 4.69 R-cm²/mCi-h)
By definition, 1 mg-Ra eq gives 8.25 R/h at 1 cm. Based on the above exposure rate constant, 1 mCi ¹⁹²Ir gives 4.69 R/h at 1 cm, therefore: equivalent mass of radium = 4.69/8.25 mg-Ra eq = 0.568 mg-Ra eq.
The percent depth dose for a vaginal cylinder is given by the equation
Dose at a distance d from the cylinder / Dose at the surface of the cylinder As the vaginal cylinder radius ↓, the surface dose ↑. The percent depth dose (PDD) for a vaginal cylinder = Dose at a distance d from the cylinder / Dose at the surface of the cylinder. The PDD increases as the cylinder radius increases.
Regarding high-dose-rate (HDR) brachytherapy, daily QA procedures should be performed every single day T/F?
False According to AAPM Task Group 56 recommendations, it is sufficient to perform "daily QA" tests only on days when HDR patients are treated.
Radiopharmaceuticals are required to exceed a certain level of radiochemical purity prior to administration to human subjects. What is meant by radiochemical purity?
The fraction of the stated isotope present in the stated chemical form Radiochemical purity refers to the fraction of radioactivity associated with the desired product. It is determined by chromatography and is the fraction of the total counts in the peak corresponding to the desired radiopharmaceutical (the rest being usually metabolites or unconjugated radionuclide that was not removed by purification). It is usually required to be greater than 90 - 95%.
The dose rate at 1 m from a brachytherapy safe is 64 mR/hr. How many TVLs of shielding are required to reduce this to 2 mR/hr.
To reduce 64 to 2 requires a reduction factor of 2/64 = 1/32. This is achieved with n TVLs, where ⅒ⁿ = 1/32. Thus, n = 1.5.
The typical method for routine calibration of brachytherapy sources is:
a well ionization chamber calibrated by the Accredited Dose Calibration Laboratories (ADCLs) for the given type of source Routine calibration of brachytherapy sources is carried out by the well ionization or re-entrant chamber. The chamber response depends significantly on the energy of radiation, source construction, and source position in the chamber. It is essential that the well chamber has a NIST-traceable calibration for the type of source to be calibrated.
Over the initial 5 cm distance from a ¹⁹²Ir point source embedded in water, the percent dose rate will decrease _____ the inverse square law due to _____ .
according to, attenuation and scatter compensating for one another When a point source is implanted in tissue, typically the inverse square law and attenuation contribute to decrease the dose rate. However, for an ¹⁹²Ir source, the attenuation of the primary photons is very much compensated for by the contribution of scattered photons. The percent dose rate as a function of distance in water is practically indistinguishable from the inverse square law curve up to 5 cm.
An HDR vaginal cylinder plan is optimized to deliver a uniform doe 0.5 cm beyond the cylinder surface. Which of the following is TRUE regarding the dose distribution at different distances?
at shorter distances, the ends will be hotter than the center, whereas at longer distances the center will be hotter For an HDR vaginal cylinder plan optimized to deliver a uniform dose 0.5 cm beyond the cylinder surface, at shorter distances, the ends will be hotter than the center, whereas at longer distances the center will be hotter.
The anisotropy function in the brachytherapy dose calculation for a line source accounts for the:
difference in the dose around the source due to self-absorption. Anistropy is the property of being directionally dependent, which implies different properties in different directions, as opposed to isotropy. According to AAPM TG report 43, the anisotropy function, F(r, θ), accounts for the variation in dose as a function of polar angle relative to the transverse plane of the source.
How often do the sources need to be changed in a Gamma Knife unit?
every 5 years The T½ of ⁶⁰Co is 5.27 years, thus the sources are changed approximately every 5 years in order to maintain reasonable treatment times.
Compared to gamma sources, _____ is a disadvantage of beta sources used for brachytherapy.
extremely rapid radial dose fall-off within the target In general, the beta sources for brachytherapy have several advantages over gamma sources: higher specific activity, higher dose rate, longer half-life, and greater radiation safety for the patient as well as personnel. The major disadvantage of beta sources is the extremely rapid radial dose falloff within the target or nonuniform target dose.
When patients are treated with ¹³⁷Cs brachytherapy sources, the dose deposited at 1 cm in tissue is from:
gamma only ¹³⁷Cs decays by beta minus with subsequent gamma emission, but the beta emissions are absorbed in the source and its encapsulation.
A planar implant is planned using a uniform distribution of sources. The implant consists of a 4 x 6 cm plane of sources, with all sources 1 cm apart, and all sources having the same air-kerma strength. For the same initial dose rate, the required total activity of ¹²⁵I will be _____ that needed if ¹⁹²Ir were to be used.
greater than A greater activity of ¹²⁵I is required because it has a lower exposure rate constant than ¹⁹²Ir.
A vaginal cylinder used in brachytherapy is designed to _____ .
improve dose homogeneity over the treated volume
For ¹⁹²Ir HDR prostate brachytherapy, the size, and shape of the dose distribution are dominated by _____.
inverse square dose fall-off Due to the very short treatment distances used in brachytherapy, inverse square dose fall-off dominates the size and shape of the dose distribution.
When selecting ovoids for a tandem and ovoid application, the optimal choice is the _____ diameter, since this will _____ .
largest, spare the vaginal mucosa The largest diameter ovoids will decrease the mucosal surface dose, but increase the depth dose below the surface, due to the inverse square law.
A Fletcher applicator is loaded with ¹³⁷Cs sources. The activity reported to the Radiation Safety Officer is 160 mg Ra equ instead of the true value of 160 mCi. The dose rate they measure at 1 meter from the patient will be _____ expected.
lower than The dose rate will be lower by the ratio of the exposure rate constants (the exposure rate constant for Ra = 8.25 R·cm²/mg·hr; for ¹³⁷Cs = 3.26 R·cm²/mg·hr) i.e., 3.26 / 8.25 = 0.40. Also, whereas 160 mCi (64 mg Ra equ) is a reasonable loading dose for a Fletcher type applicator, 160 mg Ra equ would be unusually high.
Currently, most commercial HDR brachytherapy treatment planning systems take into account the following parameters:
only source anisotropy Some commercial planning systems are working on developing inhomogeneity corrections, but at present it is not commonly performed for HDR brachytherapy.
A 5 x 7 two plane implant of uniform activity ¹⁹²Ir seeds is proposed. To calculate the required activity to deliver 1000 cGy/day at 0.5cm from the upper surface, which of the following calculation systems could be used?
quimby table and a commercial computer treatment planning system Paterson-parker tables cannot be used for seeds of equal activity. Paterson-parker tables were designed for Ra needles, with lower activity at the center of the plane to give a more uniform dose at the treating distance. Commercial computer treatment planning systems can calculate the 3D dose distribution for any seed arrangement.
An HDR source cable breaks and the source is "stuck" inside the patient. The _____ is responsible for removing the source and placing it in a lead "pig."
radiation oncologist Removing the source or catheter from a patient is clearly a medical procedure that might even entail removing sutures and other surgically implanted devices, thus it must be performed by a physician. The medical physicist, however must be present to assist and also to ensure that once removed, the source is properly placed in a shielded container.
The dose distribution in tissue up to 5 cm from an ¹⁹²Ir seed closely follows the inverse square law because:
scatter dose buildup and attenuation approximately cancel out each other The dose distribution in tissue up to 5 cm from an ¹⁹²Ir seed closely follows the inverse square law because scatter dose buildup and attenuation approximately cancel out each other.
A radionuclide with a high specific activity is preferred for brachytherapy because it has the advantage of _____ .
small source size Specific activity is activity per unit mass (mCi/g). A high specific activity allows for the fabrication of small sources required for some special applicators. The higher the specific activity, the smaller the source size for the same activity.
Brachytherapy dose calculations based on AAPM Task Group 43 include corrections for:
source anisotropy but not tissue inhomogeneity
The purpose of a multi-lumen balloon for breast brachytherapy is _____ .
to provide dose shaping by using different loading patterns in different channels Compared to a single-lumen balloon, which will deposit dose isotopically from the single source, a multi-lumen balloon can be used to conform the dose to irregularly sized cavities.
What factor is used to calculate the administered activity for ²²³Ra treatment of castration-resistant prostate cancer metastatic to bone?
weight In the ALSYMPCA trial, individuals received six injections of radium-223 at a dose of 50 kBq per kilogram of body weight intravenously.
Patterson Parker (PP) rules can be used to calculate dose distributions for the following brachytherapy sources:
²²⁶Ra, ¹⁹²Ir, and ¹³⁷Cs In the PP (or Manchester) system, a non-uniform distribution of radium needles is used to produce a reasonably uniform dose distribution, usually not exceeding ± 10 %. PP rules were originally designed for radium and radon only, but any source with energy greater than a few hundred keV has similar dosimetric characteristics, and PP tables are still reasonably accurate. ¹²⁵I photons are less than 35 keV and do not meet this criterion.
