8 Brachytherapy
What instrument would you use to locate a missing source?
A Geiger-Muller (GM) counter.
What instrument would you use to survey a patient before release?
A sensitive, calibrated ion chamber (e.g. one with a large, pressurized gas-filled chamber) is typically used.
What are the techniques and applicators used in brachytherapy treatment of cervical cancer?
An intracavitary approach using a tandem and cylinder, tandem and ovoids, or tandem and ring can be used, or interstitial needles and template for bulky disease.
What is anisotropy with respect to radioactive sources?
Anisotropy refers to the directional dependence of the fluence from a source due to the location of the radioactive material within the source and differences in wall thickness and construction.
What is brachytherapy?
Brachytherapy is a special procedure in radiation oncology that uses radioactive sources placed at short distances (hence brachy) from the target. Brachytherapy generates highly conformal dose distributions in a target volume because radioactive seeds (or sources) are placed directly within or in the vicinity of the target tissue.
What radionuclides are currently most used for radiotherapy treatments?
Gamma emitters - Cs-137 and Ir-192 for high energy gammas, I-125 and Pd-103 for low energy gammas Beta emitters - P-32, Ru-106, Sr-90, Y-90
What are the tolerance doses to the bladder and rectum for combined external beam and HDR treatment of endometrial or cervical cancer?
In equivalent dose of 2 Gy/fx (EQD2): Target coverage: D90 > 100% Rx dose Sigmoid: D2cc < 75 Gy Rectum: D2cc < 75 Gy Bladder: D2cc < 90 Gy
What are different types of brachytherapy loading systems?
Manual "hot" loading - used for LDR seeds such as prostate or eye plaque Manual afterloading - not frequently used anymore Remote afterloading - most frequently used for HDR treatment
What HDR brachytherapy option is available for inoperable endometrial cancer?
Medically inoperable endometrial cancer may have a double dander placed for a boost component of therapy or for palliative intent. The treatment isodose distribution should cover the outer surface of the uterus and give an adequate dose to the gross disease.
What is the release criterion for a prostate seed implant patient?
Per Nuclear Regulatory Commission (NRC) regulations (hence also agreement states), "a licensee may authorize the release from its control of any individual who has been administered unsealed radioactive material or implants containing radioactive material if the total effective dose equivalent to any other individual from exposure to the released individual is not likely to exceed 5 mSv (0.5 rem)." For prostate patients, this is satisfied if the exposure rate measured at 1 m from the patient is <1 mR/h.
What are the simplifications in the TG-43 calculation used for prostate implants?
Per Radiation Therapy Oncology Group (RTOG) protocols, the source is modeled as a point and the anisotropy as a function of distance (r) only with no directional information about seed orientation within the implant.
You have a brachytherapy source with a half-value layer (HVL) of 2 cm of lead. You are doubling the usage of this source. On one side of the HDR suite is a bathroom and on the other side a hallway (occupancy factor 1/4) that is being converted into a work area (occupancy factor 1). To maintain safe shielding levels, how many cm of lead if any should be added to the wall facing the bathroom and to the wall facing the new work area?
Since the usage of the source is set to double one HVL (2 cm of lead) should be applied to all walls to maintain current shielding levels. In addition, for the wall facing the new work area where the occupancy factor has increased fourfold, two additional HVL (4 cm) of shielding should be added to this wall. Therefore, 2 cm on the bathroom side and 6 cm on the work area site are needed.
What are the physical states of radionuclides used in brachytherapy?
Solids, liquids, and gases (Xe-133). Solid sources are sealed (encapsulated in a metal shell).
A patient undergoes a LDR prostate brachytherapy implant with Pd-103 to a prescription dose of 125 Gy. What is the initial dose rate (cGy/h) of the implant?
The initial prescription dose will be equal to the total dose (125 Gy) divided by the average life of the source. The average life is 1.44 times the half life, so for Pd-103: Tavg = (17 d) x (24 h/d) x (1.44) = 587.5 h Therefore, dose rate is (125 Gy)/(587.5 h) = 21.3 cGy/h. By comparison, the initial dose rate for an I-125 implant of 145 Gy is ~7 cGy/h, which is lower due to its longer half life.
What structures should be considered for planning balloon-based accelerated partial-breast irradiation (APBI)?
1. Applicator - balloon surface 2. External - skin surface 3. Lung or the closest ribs 4. Air or Serena within 1 cm from the applicator
What are the recommendations for image-based volumetric approach to cervical brachytherapy?
1. Image-based volumetric information shall consist of CT or MRI using contiguous slice acquisition with slice thickness of <3 mm 2. Organs at risk shall be contoured (including bladder, rectum, and sigmoid) 3. Dose-volume histogram (DVH) information should be used for assessment of coverage of the target and dose to organs at risk 4. Reporting: standard parameters reported for consistency and comparison should be the D2cc for the organs at risk, D90 and V100 for the tumor.
What are the available balloon sizes in the balloon-based accelerated partial-breast irradiation (APBI)?
1. Single lumen 4-5 cm, filled with 34-70 cc of contrast liquid 5-6 cm, filled with 70-125 cc of contrast liquid 2. Ellipsoidal 4x6 cm, filled with a maximum of 65 cc of contrast liquid 3. Multilumen 3.5-5 cm, filled with 23-61 cc of contrast liquid
What are the geometric and dosimetric parameters relevant to accelerate partial-breast irradiation (APBI)?
1. The conformity between the tissue and balloon 2. The symmetry of the balloon within 2 mm of expected dimensions 3. The volume of trapped air/fluid inside Serena <10% of the PTV_EVAL volume 4. The minimum balloon to skin distance >7 mm and max skin dose <145% of Rx dose 5. The minimum balloon to chest wall distance >7 mm and max chest wall dose <145% of Rx dose 6. The physical geometry of the single-lumen balloon should not deviate 2 mm from the expected dimensions
What is the dose prescription and treatment delivery timeline for accelerated partial-breast irradiation (APBI) HDR brachytherapy?
1. Treatment begins within 9 weeks of lumpectomy or re-excision of margins 2. Prescription dose of 34 Gy, with at least 90% of the prescription dose covering 90% of the PTV_EVAL 3. Two fractions per day, each of 3.4 Gy, separated by at least 6 hours 4. Five treatment days (over a period of 5-10 days), a total of 10 fractions and 34 Gy
What are the safety features and operational interlocks of the HDR afterloader?
Audio/visual system Radiation monitors and treatment on indicator Door interlock Emergency shut-offs Emergency crank Backup battery
What are the organs at risk for HDR treatment of endometrial cancer?
Bladder, rectum, and sigmoid colon
What sources can be employed with the Patterson-Parker and Quimby systems?
Both systems were designed for Radium implants. Isotopes similar to Radium (Radium equivalents) Ir-192 and Cs-137 can be calculated, but these systems are not compatible with lower energy brachytherapy sources such as I-125.
What is the purpose of postimplant dosimetry (PSI)?
CT-based postimplant dosimetry is usually conducted roughly 30 days after a prostate seed implant procedure. This allows for gland swelling due to edema to subside. The postimplant dosimetry is primarily performed as a quality assurance measure for the implant procedure. It has also been used for regulatory purposes (i.e. defining a "medical event").
Describe the eye plaque in current clinical use.
Collaborative Ocular Melanoma Study (COMS) plaques consisting of a gold shield with a silastic insert containing the I-125 seeds as well as Eye Physics gold plaques with embedded seeds are used. There are COMS plaques available with a notch to facilitate placement near the optic nerve.
What has largely supplanted systems such as Patterson-Parker in modern interstitial brachytherapy planning?
Computerized calculation of doses delivered with summation of doses to each point from all contributing sources. Furthermore use of afterloading systems allow for evaluation of the plan with dummy sources in place and active source placement after the plan has been evaluated.
What are the advantages and disadvantages of Cs-137 as an HDR brachytherapy sources relative to Ir-192?
Cs-137 has a higher average energy than Ir-192 that could be beneficial for improved percent depth dose, but higher energies are accompanied by higher shielding requirements. Cs-137 has a longer half life, requiring less frequency replacement than Ir-192. However, Cs-137 has lower specific activity, requiring a larger source and hence larger catheters.
How many dwell positions are used for a multilumen accelerated partial-breast irradiation (APBI) treatement plan and how is the plan prescribed?
Depending on balloon volume, there are typically 7-9 dwell points available in each lumen. The APBI plan is normalized to the target points on the surface of the planning target volume (PTV). With inverse planning, objectives for conformity, coverage and homogeneity, as well as skin and chest wall sparing can be optimized.
Can HDR brachytherapy be used as a boost for treatment of endometrial cancer?
Depending on the extent of myometrium invasion, radiation therapy may include external beam and brachytherapy boost (often considered for more than 50% invasion) or brachytherapy alone (less than 50% invasion).
A brachytherapy balloon is inflated to a radius of 2 cm and an Ir-192 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 first fraction dose to the surface of the balloon is received at the balloon surface with the second fraction?
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 source, the dose will increase by a factor of (2^2)/(1.5^2) = 1.78 = 178%.
What is a traditional dose specification point for cervical cancer?
Dose is prescribed to point A, which is the major critical point for dose specification of intracavitary brachytherapy. It represents the crossing of uterine artery and ureter forming the paracervical triangle.
When is interstitial brachytherapy used for GYN patients?
Endometrial and cervical cancers that relapse after surgery in and around the vaginal cuff area with a depth greater than 0.5 mm can be treated with interstitial brachytherapy. Since there is no uterus, a tandem can't be used and the vaginal cylinder is the only way to treat up to 5 mm depth. 3D dimensional treatment planning is recommended with CT scan and/or MRI. The treatment plan should be optimized to conform to the clinical target volume and should reduce the dose to critical organs, including the rectum, bladder, urethra, and sigmoid colon.
What radiation safety precautions are taken for prostate implant patients?
Following the operating room implant procedure, the patient is taken to a recovery area. He is provided with a urinal for his use. After discharge (typically a few hours), the urinal and patient bedding are checked for radioactive sources. If any sources are found, they are stored in a locked cabinet until removal by Radiation Safety. Patients with permanent implants are provided with a small card indicating the date and nature (isotope, total activity) of the implant and a document of precautions to be followed to minimize exposure to others.
What is the prescription dose most commonly used for treatment of endometrial cancer?
For HDR brachytherapy, a regimen of 3 fractions of 7 Gy (21 Gy total dose) to 0.5 cm depth is commonly used. The dose for the brachytherapy may vary when combined with external beam dose.
What are the typical dosimetric parameters for a prostate implant?
For I-125, the prescription dose is 145 Gy to cover the prostate delineated on the ultrasound images. V150 (volume of prostate receiving 150% of Rx) values ranging from 40%-50% of the prostate and V200 from 10%-20% of the prostate are acceptable. Although there may be no explicit margin contoured, the implant volume receiving 145 Gy is approximately twice that of the prostate itself. For Pd-103, the prescription dose is 125 Gy.
How many dwell positions and prescription points are used for a single-lumen accelerated partial-breast irradiation (APBI) treatment plan?
For a single lumen, one dwell point in the center of the balloon and two prescription points at the distance from the center of the radius of the balloon plus 1 cm. For better coverage and conformity, more dwell points can be used, which can create a nonsymmetric dose distribution, based on prescription points on a surface 1 cm beyond the balloon.
What is the main determinant of the dose rate from a source?
For distances greater than a few mm, it is the inverse square factor.
How is radiation source strength typically specified in clinic?
For photon emitters, air kerma strength (U) is commonly used. Ci is an older unit that is still commonly used as well. For electron emitters, Bq is typically used.
What are the corrections applied to the electrometer readings when performing a calibration check?
If the well chamber is open to the atmosphere, as most are, a temperature and pressure correction must be applied. In addition, there may be an electrometer scale reading correction.
What are the types of brachytherapy?
Interstitial - radioactive sources are placed in the target tissue directly either permanently or temporarily. Intracavitary - radioactive sources are contained in an applicator that is inserted into body cavities such as the vagina or uterus. Intraluminal - subclass of intracavitary brachytherapy in which the radioactive sources are inserted in the lumen of the patient such as the blood vessel, bronchus, esophagus, or bile duct. Surface - radioactive sources (or seeds) are placed in the surface plaques or molds, which are then placed on the treatment area such as the eye or skin.
What are the disadvantages of HDR brachytherapy compared with LDR brachytherapy?
Investment - machine cost can be anywhere between $1M and $2M Radiobiology - as the dose rate increases, the radiosensitivity (damage per unit dose) increases for both normal tissues and tumors, the radiosensitivity for the normal tissue increases faster, increasing the likelihood of injuring the patient while controlling the tumor. Overcoming this requires the use of the advantages of optimization, geometry, stability, and dose reduction to normal tissues, and multiple fractionation treatment. Safety - if the HDR machine has a malfunction or if a patient has an emergency situation, the risk of accidental radiation exposure to the patient and the staff is much higher in HDR than in LDR.
What is the most common HDR brachytherapy source, its half life, dimensions, and typical activity?
Iridium-192 Half life = 73.8 days Mean energy = 380 keV Typical source is 0.3-0.6 mm in diameter and 3.5-10 mm in length Typical initial activity ~10 Ci
What are the dose rate ranges for low, medium, and high dose rate brachytherapy according to International Commission for Radiation Units and Measurements (ICRU) Report No. 38?
LDR: 0.4-2.0 Gy/hr MDR: 2-12 Gy/hr HDR: >12 Gy/hr - usual dose rate is 100-300 Gy/hr
What is the Nuclear Regulatory Commission (NRC) limit on leakage levels for HDR units?
Leakage may not exceed 1 mR/h at a distance of 10 cm from the nearest accessible surface of the safe when the source is in the shielded position.
What are the requirements regarding calibration of new sources?
New sources must be calibrated before treating patients using a dosimetry system that has a National Institute of Standards and Technology (NIST) traceable calibration. The dosimetry system typically comprises a well-chamber and an electrometer capable of reading in the current mode. For the source activity in the mCi range, the typical current readings from the electrometer are on the order of E-11 A (ampere). For an HDR source, the current is E-7 A. This current is then converted into activity of the source.
What are the advantages of HDR brachytherapy compared with LDR brachytherapy?
Outpatient procedure Safety - reduction or elimination of radiation exposure to the radiation therapy staff Optimization - moving source allows optimization of dose distribution by adjustment of the dwell times for each dwell position in each channel (catheter or needle), permitting very fine control of the dose distribution Stability - HDR intracavitary treatments take less time (usually under an hour), and the movement of the applicators during treatment is minimized Dose reduction to normal tissue - shorter duration of HDR treatments allows for physical displacement of normal tissue structure during treatment Applicator size - the small size of the HDR source permits the use of smaller applicators
What radiation safety precautions are taken regarding eye plaque patients?
Patients can remain in the hospital for the duration of the therapy. The room in which they are housed requires a radiation sign posted on the outside of the room. The patients wear special leaded glasses that reduce radiation exposure levels by a factor of 5-6 when visitors or nursing staff are present. Nursing staff require annual radiation safety training. Following patient discharge, the room and contents are surveyed for radioactive material.
What types of radiation are used in brachytherapy?
Photons, electrons, and rarely neutrons (Cf-252) and alpha particles are used.
What is a definition of point B?
Point B represents the pelvic side wall/obturator nodes. Point B is located 5 cm lateral to midline at the same level as point A. It shows the lateral spread of the radiation dose. It's dose is usually 25% of the point A dose.
How is the dose specified and calculated for eye plaque treatments?
The Collaborative Ocular Melanoma Study (COMS) dose is 85 Gy to 5 mm or the tumor apex, whichever is greater. Treatment times are typically around 100 hours. For COMS, the dose was calculated using a point model for the seed and no anisotropy or heterogeneity correction. Current AAPM recommendations are to use a line-source approximation and calculate for homogeneous water media. Included in the TG-129 report are data for heterogeneous corrections. Software such as Plaque Simulator modifies the calculation by accounting for silastic carrier attenuation and changes in scatter due to the gold shield.
What are the regulations regarding seed activity assays?
The Nuclear Regulatory Commission (NRC) and hence agreement states require a 10% independent assay to confirm the accuracy of the specified seed activity. The assay may be performed by a third party, but the AAPM recommends that a check be done at the local institution.
What are the rules regarding a radioactive materials inventory?
The Nuclear Regulatory Commission (NRC) requires a licensee to maintain an inventory log for all radioactive materials. The log must contain the type of source (isotope), source strength, and its location. Permanent sources implanted in a patient are not subject to this inventory once the patient has been released from the facility.
What is the goal of the Paris system and its key features?
The Paris system prescribes dose to an isodose surface (reference isodose). Like the Quimby system, uniform sources placed in parallel lines are employed.
What are the acceptable dose volume endpoints for the breast tissue in accelerated partial-breast irradiation (APBI) plans?
The actual volume of tissue receiving 150% (V150) and 200% (V200) of the prescribed dose should be limited to 50 mL and 10 mL, respectively.
Where is the bladder and rectum dose traditionally determined in GYN HDR brachytherapy?
The bladder dose point is the most posterior part of an antero-posterior line drawn through the center of the bladder catheter balloon. The rectal dose point is located 5 mm posterior to the posterior vaginal wall, along a line perpendicular to the midpoint of the ring or the ovoids.
How is the point A defined in the planning system?
The definition is 2 cm superior along the tandem from the external cervical os, and 2 cm lateral to the tandem. Point A is recorded for all tandem-based treatments, even when using 3D image-based techniques.
What are the steps for initiating an HDR procedure?
The following steps must be completed to be compliant with the Nuclear Regulatory Commission (NRC): 1. Authorized physician user must sign the complete written directive. 2. Patient is identified and verified by two methods. 3. Treatment plan must be checked by a medical physicist. 4. Pretreatment HDR unit safety checks must be completed. 5. Prior to treatment delivery, the authorized user must verify patient and treatment detail. 6. Treatment must be supervised from start to end by an authorized user and treatment records must be recorded. 7. A post delivery survey must be completed. 8. Medical events must be recorded and reported. 9. Periodic reviews at least yearly are performed and source and calibration is performed and recorded.
How does the Quimby system differ in their goal and source loading from the Patterson-Parker system?
The goal of the Quimby system is to calculate the maximum dose in the center of the treatment plane at a distance of up to 3 cm. The system relies on a uniform dose distribution of sources of equal linear activity. By comparison, the Patterson-Parker system uses nonuniform distributions and may employ varied source activities to achieve a uniform dose to a plane(s).
What isotopes are used for permanent implants and what are the reasons for choosing them?
The isotopes most commonly used are I-125, Pd-103, and Cs-131. They are used because of their low average photon energies (0.028 MeV, 0.021 MeV, and 0.029 MeV) so that they will only treat the tumor, and have relatively short half lives (59.4 d, 17 d, 9.7 d).
In vaginal cuff brachytherapy treatment for endometrial cancer, what is the preferred vaginal cylinder diameter?
The largest diameter cylinder that can reasonable fit for the patient should be used. The larger cylinder provides a more uniform dose distribution as the diameter of the cylinder increases.
How can a multichannel vaginal cylinder help in reducing dose to the bladder or rectum?
The multichannel vaginal cylinder is a variation of the standard vaginal cylinder. The applicator contains a central channel and six to eight peripheral channels along the surface of the cylinder. The multichannel cylinder increases the dosimetric control with differential loading of the channels, resulting in an iOS dose distribution that reduces the dose to the bladder and rectum while maintaining the target dose. A standard cylinder with one central catheter provides only a circular isodose distribution. Another variant is a shielded cylinder, which allows for the insertion of shields in ant of the four quadrants of the cylinder to reduce dose to that quadrant.
Describe a prostate seed implant procedure.
The patient is placed in the dorsal lithotomy position under general anesthesia. Radioactive sources are implanted through the perineum using preloaded needles containing either loose seeds or seeds in a strand. Radioactive seeds are spaced at least 1 cm apart in the needle. The number of needles needed for an implant depends on the volume of the prostate. The needles are typically arranged to provide a modified peripheral loading, resulting in fewer sources in the center of the prostate near the urethra. The needles are inserted through a special template that is attached to a transrectal ultrasound probe. Seeds may also be implanted using a Mick applicator.
What is the primary goal and key features of the Patterson-Parker (Manchester) system of interstitial implants?
The primary goal is to deliver a uniform dose to a plane or volume in parallel planes 0.5 cm from the implanted plans(s) (within +/- 10% of the prescribed dose). To achieve this goal, the system uses specified rules for distribution of sources and activity. For a planar implant, the smaller the treated area, the greater the fraction of the amount of Radium (or equivalent) to be placed at the periphery. The implant volume is determined and tables are used to determine the milligram hours of Radium per 1,000 roentgens. Note that the tables employed the historical exposure rate constant (Γ) for Radium (8.4 R*cm2/mg/h) instead of the current value for Radium Γ=8.25 R*cm2/mg/h and thus the tables truly reflect milligram hours of Radium per 900 cGy.
What is the typical target length for the treatment of endometrial cancer?
The target is typically defined to encompass the dome of the vagina. Most patients have the upper one-half of the vagina treated.
What brachytherapy treatment techniques can be used for accelerated partial-breast irradiation (APBI)?
The typical brachytherapy treatment techniques for accelerated partial-breast irradiation include multi planar interstitial catheter implant and balloon-based treatment.
What is the most commonly used applicator for treatment of endometrial cancer?
The vaginal cylinder applicator is most commonly used. Cylinders come in a variety of diameters (2-4 cm) and lengths to better conform to the patient's anatomy. The cylinder should closely fit to the superior aspect of the vaginal cuff and contact the lateral surfaces of the vagina.
Why is HDR brachytherapy used for the treatment of endometrial cancer?
The vast majority of patients undergo curative hysterectomies. However, with no further therapy, approximately 12% of the patients are likely to suffer recurrences. Patients who received prophylactic radiation to the vaginal cuff had much reduced recurrences.
How is balloon-based accelerated partial-breast irradiation (APBI) treatment verified?
To assure integrity of the balloon throughout treatment, an ultrasound or projection X-ray is acquired prior to each fraction and evaluated for any change in the balloon diameter or orientation. These parameters should be compared with those obtained from the treatment plan. For multilumen treatment, additional care must be taken to check the rotation of the balloon.
What is the International Commission for Radiation Units and Measurements (ICRU) recommendation for specifying dose to the bladder and rectum?
To provide a bladder calculation point, a Foley balloon is placed in the bladder and filled with ~7 cc of contrast. The bladder dose point is located at the posterior aspect of the balloon. The rectal dose point is located 0.5 cm beyond the posterior vaginal wall.
What brachytherapy procedure is used for treating ocular leasions?
Uveal melanoma and retinoblastoma may be treated using a surface plaque containing radioactive material (typically I-125, Ru-106, Pd-103, or Cs-131). Melanomas are treated with a prescription dose of 85 Gy in 3-7 days. The plaque also shields the eye cavity from radiation.
what is the advantage of using volumetric imaging such as CT or MRI compared with the use of conventional radiography?
While localizing and reconstructing source positions based on radio graphic markers is simple and accurate, radiographs cannot show soft tissue such as the tumor, the rectum, or the bladder. Only volumetric imaging modalities can visualize soft tissues and provide 3D characterization of the dose distribution with respect to targets and normal structures.
