Chapter 22: High-Dose-Rate Brachytherapy
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What is the threshold for high-dose rate brachytherapy commissioned by ICRU? Pg. 466
The International Commission on Radiation Units and Measurements (ICRU) Report 38 classifies high dose rate (HDR) as 20cGy/min (1200cGy/hr, 12Gy/hr).
What is one benefit that HDR has over LDR while using remote afterloading? Pg. 466
With the introduction of remote afterloading technology, it is possible to deliver HDR brachytherapy safely and more precisely than possible with the classical LDR brachytherapy. Although LDR brachytherapy can also be delivered using remote afterloading devices, logistic problems of prolonged treatment and patient hospitalization make LDR less attractive.
In other words, using HDR instead of LDR remote afterloading would significantly speed up the treatment process (shorten treatment time) hence not having to deal with patient hospitalization; more patients can be treated within a shorter period.
What is the activity of a single source of an HDR remote afterloading unit? Pg. 466
An HDR remote afterloading unit contains a single source of high activity: ~10 Ci or 370 GBq.
What three radioactive sources have been used for HDR? Which is the most commonly used radioisotope? Pg. 466
Although Co-60 and Cs-137 have been used in the past, Ir-192 is the most most commonly used radioisotope in HDR.
- 1.) Co-60 (T_1/2 = 5.26 years)
- 2.) Cs-137 (T_1/2 = 30 years)
- 3.) Ir-192 (T_1/2 = 73.8 days)
For HDR brachytherapy, Ir-192 is the best choice because of its higher specific activity (allows for smaller source for the same activity) and lower photon energy (requires less shielding).
Knowing that Ir-192 is the most commonly used radioisotope for HDR brachytherapy, what is one significant drawback with it? Pg. 466
A disadvantage, on the other hand, is its shorter half-life (73.8 days, less than 5.26 years or 30 years), necessitating source replacement every three to four months.
When dealing with any HDR radioisotope, what are the NRC regulations regarding leakage radiation from the safe in which the radioisotope is placed? Pg. 466
In compliance with the NRC regulations, the leakage radiation elvels outside the unit do not exceed 1 mR/h at a distance of 10 cm from the nearest accessible surface surrounding the safe with the source in the shielded position.
How are the HDR radioisotopes controlled when being placed in the patient? And what is the positioning accuracy of source? Pg. 467
The source wire (or dummy wire) can be advanced or retracted through individual channels, transfer tubes, and applicators by a remote computer-controlled drive mechanism consisting of step-per motors. The positioning of the source at the programmed dwell positions in the applicators is accomplished in precise increments by the stepper motors.
The positioning accuracy of the source is specified at + or - 1 mm. The dose control precision is provided by a 0.1 second dwell time resolution.
What happens if the control system detects blockage or excessive friction during the radioisotope source transit? Pg. 467
The treatment is aborted.
Can brachytherapy applicators used for LDR implants also be used for HDR? Pg. 467
Yes, brachytherapy applicators used for LDR implants can also be used for HDR. For example, some of the most commonly used applicators, for a variety of HDR aplications, are Fletcher-Suit or Flecter-Suit-Delclos. These applicators are used for the treatment of gynecologic malignancies of the uterus, cervix, and pelvic side walls.
List five HDR/LDR brachytherapy applicators and briefly explain each. Pg. 467
1.) Vaginal Cylinder: These are acrylic cylinders having a variety of diameters and axially drilled holes to accommodate a stainless steel tandem. Coupling catheters for attachment to the transfer tubes and marker wires to fit the length of the tandem are provided in the set. The applicator is suitable for treating tumors in the vaginal wall.
2.) Rectal Applicator: Acrylic cylinders of different diameters are designed to treat superficial tumors of the rectum. Selective shielding is incorporated to spare normal tissue. Coupling catheters and marker wires are provided in the HDR set.
3.) Intraluminal Catheter: Suitable diameter catheters of various lengths are available for treating intraluminal disease such as endobronchial carcinoma.
4.) Nasopharyngeal Applicators: These applicators are used for treating nasopharyngeal tumors with HDR. THe applicator set includes a tracheal tube, catheter, and nasopharyngeal connector. Besides the above examples, HDR applicators and catheters are available for virtually every type of application deemed suitable for intracavitary brachytherapy.
5.) Interstitial Implants: Hollow, stainless steel needles are implanted into the tumor following standard brachytherapy rules of implant (see Chapter 15) and closed-ended catheters are inserted to accommodate the HDR source wire. Marker wires are used to plan the dwell positions of the source as with the other HDR applicators. Examples of interstitial implants are prostate gland, breast, and some head and neck tumors.
Who mandates the shielding requirements for the room in which HDR will be performed? Pg. 468
The Nuclear Regulatory Commission (NRC). The HDR treatment room can be designed as a dedicated facility (e.g., "HDR Suite") or adopted from an existing Co-60 or linac room. In either case, the shielding must satisfy or exceed the NRC requirements.
The shielding calculations are based on the dose limits specified by the NRC in 10 CFR 20.1301 (for individual members of the public) and 10 CFR 20.1201 (for occupational personnel).
The NRC annual effective dose equivalent limits follow the National Council on Radiation Protection and Measurements (NCRP) guidelines (see Table 16.5). Summarize it. Pg. 468
Public: 0.1 rem (1 mSv) in 1 year for continuous or frequent exposures; or 0.5 rem (5 mSv) in 1 year for infrequent exposure. In the case of HDR, the limit for infrequent exposure, namely 0.5 rem in 1 year, is more relevant.
Occupational: 5 rems (50 mSv) in 1 year.
In addition to the annual limits, the NRC requires that the dose in any unrestricted area must not exceed 2 mrem (0.02 mSv) in any 1 hour.
The methods of calculating primary and secondary barriers are the same as discussed for megavoltage beams in Chapter 16. Eqns 16.4, 16.6, 16.10 are valid also for HDR room design, provided appropriate factors related to Ir-192 source are used.
- 1.) Average photon energy: 380 keV.
- 2.) Tenth-Value Layer (TVL): 5.8 inches of concrete (density 2.35 g/cm^3).
- 3.) Exposure rate constant: 4.69 (R*cm^2/mCi-hr).
Pages 468 and 469 shows examples in calculating barrier thickness.
Ir-192 requires less shielding than a megavoltage therapy unit and is assumed to be isotropic (same intensity in all directions) in the context of shielding design. In this case, is it reasonable to construct all barriers of the same thickness? Pg. 468
Are rooms designed with adequate shielding for megavoltage therapy units more than adequate for HDR shielding? Pg. 469
Calculations have shown that rooms designed with adequate shielding for megavoltage teletherapy units are more than adequate for HDR shielding. Whether it is a dedicated HDR suite or an existing teletherapy vault, the shielding adequacy of the facility must be documented before applying for an HDR license. Since most institutions use existing teletherapy rooms to house HDR units, a shielding evaluation report must be submitted with the license application as required by the NRC.
Reminder: No flashcards will be made for the introduction in section 22.3 because that seems to be more important for actual medical physicists who are interested in beginning an HDR treatment center.
Also, just read through this entire section since this is maybe a bit too specific for the qualifying exam. Carlos did say it was going to be a general exam rather than specific on the duties of a medical physicist.
How does the AAPM recommend in defining the strength of a radionuclide? Pg. 474
The AAPM recommends air kerma strength when quantifying the strength of a brachytherapy source.
How is the air kerma strength calculated/measured? Pg. 474
In practice, the air kerma strength, is determined from exposure rate measured in free air at a distance of 1 m from the source. The relationship between air kerma strength and the exposure rate has been derived in section 15.2.
Is calibrating Ir-192 HDR sources with a thimble chamber using open-air geometry suitable for routine calibrations? Pg. 475
Calibration of Ir-192 HDR sources with a thimble chamber using open-air geometry is a time-consuming procedure and is not suitable for routine calibrations.
What type of chamber has been created specifically for Ir-192 HDR? Pg. 475
A well-type re-entrant chamber of smaller volume has been designed specifically for Ir-192 HDR sources.
Briefly describe the physical properties of the re-entrant chamber used for Ir-192 HDR sources? Pg. 475
The University of Wisconsin re-entrant ion chamber is filled with air and communicates to the outside air through a vent hole. The active volume of the chamber is approximately 245 cm3, which for HDR measurements is just large enough to give an optimum ionization current to be measured accurately with most clinical electrometers. A thin-walled aluminum tube is fitted on the axis of the chamber, which allows the insertion of the HDR source catheter until the end of it touches the bottom. The thickness of aluminum between the source and the ion-collecting volume of the chamber exceeds 0.3 g/cm^2, as required for attaining electronic equilibrium with Ir-192 gamma rays. The bias voltage applied to the chamber is about 300V, which gives ionic collection efficiency of better than 99.96% for measuring a 6.5 Ci Ir-192 source.
How is the air kerma strength determined for Ir-192 HDR sources? Pg. 475
It is determined measuring a current due to ionization in a re-entrant chamber. The current measured is multiplied by five correction factors:
- 1.) temperature and pressure
- 2.) electrometer calibration factor
- 3.) chamber calibration factor
- 4.) ion recombination factor at the time of chamber calibration
- 5.) ion recombination correction at the time of source calibration.
For a routine calibration of the HDR source, what type of measurement is preferred and why? Pg. 475
For a routine calibration of the HDR source, it is preferable to use the current mode of measurement because it is free of the source transit effect.
What does the HDR treatment-planning process start with? Pg. 475
The HDR treatment-planning process starts with patient preparation and placement of applicators, catheters, or needles, depending on the procedure.
The physician places the implant devices in the treatment area, normally under local anesthesia. Give three examples. Pg. 475
- 1.) Gynecologic applicaotrs with palpation and visual inspection.
- 2.) Prostate template with ultrasound.
- 3.) Endobronchial tube with brochoscopy guidance.
What happens after the the physician completes his/her task of placing the implant devices in the treatment area? Pg. 475
The patient is then siimulated using an isocentric x-ray unit sch as a C-arm or a simulator. Marker wires are inserted into the applicators all the way to the closed ends. Orthogonal radiographs are obtained to localize the applicators and the marker wires. These radiographs allow the radiation oncologist to plan the treatment segment and dwell locations in relation to the distal end of the applicator.
Why are orthogonal radiographs used during treatment planning simulation? Pg. 475
Orthogonal radiographs are obtained to localize the applicators and the marker wires. These radiographs allow the radiation oncologist to plan the treatment segment and dwell locations in relation to the distal end of the applicator.
Following the acquisition of orthogonal radiographs, how is the total length of the catheter required to travel determined? Pg. 476
This is accomplished by connecting the transfer guide tube to the applicator and passing a measurement wire through the catheter to the distal end. A measurement clip is attached to the wire at the point where it exits the free end of the guide tube. The wire is then removed and inserted into a calibration ruler until the measurement clip is at the zero end of the ruler. The catheter length is determined by reading the tip of the measurement wire within the catheter against the ruler graduation.
What are the two types of imaging modalities used to plan the dose distributions within the patient? Pg. 476
- 1.) Orthogonal Radiography Based.
- 2.) Three-Dimensional Image Based.
What three methods can be used to calculate the dose distribution around a linear Ir-192 HDR source afterloader? Pg. 476
- 1.) Sievert integral.
- 2.) TG-43 formalism.
- 3.) Monte Carlo.
It's been said that TG-43 and Monte Carlo are better than the Sievert Integral, but despite that, there are clinics that don't include certain conditions in the calculations and there are other inconsistencies with how these three different calculation methods are used.
What is an essential part of the HDR quality assurance? Pg. 477
Independent verification of a computer plan is an essential part of HDR quality assurance. Some of these checks consist of verifying the accuracy of input data such as,
- 1.) dose prescription.
- 2.) catheter lengths.
- 3.) dwell times.
- 4.) current source strength.
- 5.) etc.
Due to the severe dose gradients encountered in brachytherapy, what is a reasonable percentage range for the verification of a dose at a prescription point? Pg. 477
Verification of the dose at the prescription point (or another suitable point) within plus or minus 5% is considered reasonable.
Where can you find a number of manual methods of checking HDR computer calculations? Pg. 477
AAPM TG-59. The reader is referred to these for review and possible adoption in the quality assurance program. One of the simplest methods consists of using inverse square law.
What is a quality assurance (QA) program? Pg. 477
A quality assurance (QA) program is a set of policies and procedures to maintain the quality of patient care. It's main purpose is to minimize the occurrence of treatment mistakes caused by equipment malfunction or human error.
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