Low Dose Rate Brachytherapy (LDR)

Low dose rate brachytherapy refers to permanent or temporary implants with an instantaneous dose rate of less than 20cGy/minute.

Interstitial Seed Placement Systems

Paterson-Parker System

The Parterson-Parker seed placement system was developed to deliver a uniform dose (+/- 10%) to a plane or volume. Tables were produced giving the dose distribution for given activity and geometries used in the Paterson-Parker system.

Paterson-Parker Planar Implant Rules

  1. Determine fraction of activity at the periphery of implant area.
    • Area < 25cm2: 2/3 of activity in periphery
    • Area 25 – 100cm2: 1/2 of activity in periphery
    • Area > 100cm2: 1/3 of activity in periphery
  2. Needle spacing <1cm
  3. If the ends of the distribution are uncrossed (needle rows are not capped with a perpendicular row) then the uniform area is reduced by 10% for each uncapped end.
  4. In the case of multiple planes obeying rules 1-3 and planes should be parallel.

Paterson Parker Volume Implant Rules

  1. Total activity is divided into 8 parts
    • The periphery uses 6 parts
    • The core uses 2 parts
  2. Needle spacing <1cm
  3. If ends are uncrossed (needle rows are not capped with a perpendicular row) then the uniform volume is reduced by 7.5% for each uncapped end.

Quimby System

In contrast to the Paterson-Parker system, the Quimby system uses uniform source activity to yield a non-uniform distribution of dose. Like the Paterson-Parker system, the Quimby system relies on dose distribution tables for dose calculation.

High Dose Rate Brachytherapy (HDR)

High dose rate (HDR) brachytherapy is defined as any brachytherapy procedure which uses a source with a dose rate of 20cGy/min (12 Gy/hour) or higher. In practice, the initial dose rate of a 10Ci Ir-192 source at 1cm is approximately 7.5Gy/minute.

Advantages and Disadvantages of HDR

Advantages

Optimization

Once the treatment catheters are in place, the patient may be imaged and a plan may be generated to optimize dose distribution. Inverse planning is fast due to the lower complexity of HDR plans, in which dwell times are the primary variable, compared to external beam treatments.

Immobilization

The shorter treatment times allow for patient immobilization which would be impractical or impossible with an LDR source. This decreases uncertainty in a dwell position.

Implant Position Stability

All implanted devices move as a result of internal anatomical motion. Because HDR catheters are in the patient’s body for less time, their dose distribution may be more localized.

Reduced Normal Tissue Dose

Organs at risk may be temporarily displaced during treatment. This is especially true for gynecologic and oral treatments but not usually for intraluminal treatments.

Outpatient Treatments

This improves patient comfort and health as the patient maintains a sense of freedom. Further, outpatient treatments are less costly to the patient.

Smaller Source, Less Discomfort

Typical HDR sources have a diameter of 1mm or less meaning compared with an LDR radium needle which may 1.65-2mm in diameter.

Fewer Delays

Treatments are readily adapted to circumstances and may be easily replanned without need to acquire new sources.

Intraoperative Procedures (IORT)

The high dose rate and low energy of Ir-192 makes it a good choice for IORT procedures.

Radiation Worker and Public Safety

Use of a remote afterloader means that staff is never inside the room while the source is unshielded. Further, since the treatments last from 5-30 minutes, the source never leaves the shielded treatment room.

Disadvantages

Reduced Therapeutic Ratios

Radiobiology predict that, for the same amount of damage to tumor cells, HDR will be more damaging to normal cells.

Increased Impact of Errors

Because of the reduced time of treatment, any error in delivery time will be more damaging than for LDR.

Afterloader Failures

The high dose rate means that a failure of the afterloader to retract the source will result in large doses to the patient and staff responsible for retrieving the source.

Require Greater Clinical Resources

The afterloader itself my cost as much as $1,000,000 and requires quarterly source changes. Greater staffing is also needed as the treatments progress from placement to planning to treatment in necessarily quick succession.

Remote Afterloaders

A remote afterloader (RAL), commonly referred to simply as an afterloader, is a motor driven system that transports a radioactive source for a shielded internal safe to a series of predetermined positions. Transfer tubes connect the afterloader to applicator catheters within the patient. The afterloader is then preprogrammed to send the source to specified locations for set amounts of time, referred to as dwell times.

Key Point: An HDR afterloader will typically have an initial source strength of around 10Ci which, for Ir-192 sources, equates to an Air Kerma Strength of approximately 40,820U. This means that, by the TG-43 formalism, the initial dose rate to water at 1cm is 7.6Gy/min!

Two HDR afterloaders. A: Nucletron MicroSelectron. B: Varian VariSource

Afterloader Design Features

Dummy Source/Check Cable

An non-radioactive duplicate of the source cable which is used to test channels, transfer tubes, and applicators for obstructions prior to sending the source cable. The check source will typically extend 2-5mm beyond the expected length of the source cable.

Illustration of an Ir-192 HDR source.

Emergency Retraction Motor

In fault conditions, a second source retraction motor may be engaged. This motor is typically a high-torque direct current motor. Emergency retraction motor should be tested as part of daily quality assurance.

Emergency Crank

An emergency crank allows for source retraction in event that both the primary and emergency motor fails. Since the emergency crank is on the RAL, its use requires staff to enter the treatment room with the source extended.

Source Cable

The source, typically Ir-192, is controlled by a thin metal cable. The source is typically 3-5mm long and <1mm thick. Note that the “cable length” is typically defined as the distance to the middle of the source rather than the end of the source!

HDR Sources

Iridium-192 is the most commonly used HDR source because of its high specific activity allowing for high dose rates from a small source. Further, the low energy of 192Ir (average 380keV) aids in rapid dose fall off and more conformal dose distributions. Because of its short half-life (73.83 days), Iridium-192 sources must be changed every 3-4 months and loose ~1% activity per day.

Cobalt-60 is less common but has the advantage of a longer half life (5.3 years) than 192Ir which extends the time between source changes to approximately every 5 years. Lower specific activity of Cobalt units necessitates larger source size for the same activity and dose rate.  60Co has an average energy of energy of 1.25Mev which is significantly greater than 192Ir making for a lower rate of dose fall off (note that dose fall off is still good as its dominated by the inverse square law).

Co-60 vs Ir-192 radial dose distribution.

Ytterbium-169 is a radionuclide that is gaining interest because of its very high specific activity and very low energy spectrum. It has a half life of 32.03 days. 169Yb decays via electron capture yielding  an average photon energy of 93keV. The low energy also introduces the possibility of directionally shielding sources or applicators.

Pulsed Dose Rate (PDR) Brachytherapy

Pulsed dose rate brachytherapy is a technique designed to produce therapeutic ratios similar to LDR treatments using HDR equipment and sources. In a PDR treatement, the applicator is installed in the patient for approximately 24 hours. During that time, the HDR source will periodically (every hour is common) extend into the treatment positions and deliver some dose. This technique was popular at the onset of high dose rate brachytherapy because of its similarity to low dose rate brachytherapy. The technique is not common today because of the logistical and safety issues of having an afterloader attached to a patient and in use for such a long time.

Breast (Accelerated Partial Breast Irradiation)

Treatment Regimen

Deliver 32-34Gy over 4-5 days BID (two treatments daily separated by at least 6 hours).

PTV is typically taken to a 1-2cm expansion of the lumpectomy cavity. Any portion of the PTV that extends to the ribs or skin are likely to be subtracted.

Intracavitary Breast Brachytherapy

Clinical Use

Intracavitary breast brachytherapy is commonly used to treat the surgical bed post resection of a hard tumor. Intracavitary applicators are best suited treatment of a tumor bed that is located at least 5mm (preferably deeper) from the skin surface.

The applicator will be inserted during surgery and left within the resection cavity for treatment. Once treatment is complete, the applicator is emptied of fluid (balloons) or adjusted to smallest diameter (SAVI) and removed from the breast.

Applicators

Strut Adjusted Volume Implant (SAVI)

The SAVI applicator features multiple treatment channels and can be expanded to different diameters using a strut system. The number of channels allows for control over the dose distribution but the design places channels in direct contact with tissue increasing local hot spots.

Intracavitary balloons

Intracavitary balloons (MammoSite, Best Balloon, etc) feature a saline inflatable balloon with 1 or more channels available for treatment. Balloons are available in a number of sizes and can have their size adjusted by changing the volume of saline within the balloon.The multi-channel balloons allow for some tailoring of the dose distrubution around the balloon but care must be taken to avoid rotation of the applicator.

Strut Adjusted Volume Implant (SAVI) applicator.

Quality Assurance

CT, ultrasound, or external marking are commonly used to verify alignment and balloon size prior to treatment.

Air/seroma surrounding the applicator should be <10% of the balloon volume for balloon based treatments.

Multichannel intracavitary balloon applicator used in breast brachytherapy.

Interstitial Breast Brachytherapy

Interstitial breast treatments use needles inserted through the breast to treat non-resected or irregular volumes that could not be treated with an intracavitary approach.

Breast compression and a template are used to assist in needle placement with the patient under local or general anesthesia.

A common interstitial breast brachytherapy template.

Gynecological

Treatment Regimen

Low Dose Rate: The Manchester System

LDR alone (Stages IA1-IB1): 50-70Gy prescribed to point A
LDR as boost for external beam (Stage IB1 – IV): 30-40Gy to point A for a total including external beam of 85-90Gy

High Dose Rate Prescriptions

Cervical Cancer

Early stage:

20-45Gy external beam therapy followed by 30-45Gy in 5 to 8 fractions of brachytherapy.

Late stage:

45-50.4Gy external beam therapy followed by 28-35Gy in 4 to 6 fractions of brachytherapy.

Uterine Cancer
(Post Operative)

Brachytherapy Alone:

3 to 5 fractions of 7 to 4.7Gy prescribed to a depth of 5mm or 3 to 5 fractions of 10.5 to 7.5Gy prescribed to vaginal surface.

Brachytherapy following external beam: 

2 fractions of 5.5Gy prescribed to a depth of 5mm or 2 fractions of 8Gy prescribed to vaginal surface.

Uterine Cancer (Inoperable)

Brachytherapy Alone:

4 to 7 fractions of 8.5 to 5.7Gy prescribed to a depth of 2cm.

Brachytherapy following external beam:

45Gy external beam followed by 2 to 4 fractions of 8.5 to 5.2Gy brachytherapy prescribed to a depth of 2cm.

Tandem and Ovoid

Tandem and Ovoid (T&O) treatments are used to treat the cervix, uterus, and upper 1-2cm of the vagina. Tandem and Ovoid applicators, such as the Fletcher Suite applicator, consist of a central tandem inserted through the cervical os and into the uterus and two ovoids (colpostats) which are placed in the left and right vaginal fornices. A phlange prevents the over-insertion of the applicator which could otherwise result in perforation of the uterine wall.

Dose Reporting: The Manchester System

The manchester system is a system of dose reporting based on dose at positions relative to the applicator and surrounding anatomy.

Point A

This point is 2cm superior to the phlange (cervical os) and 2cm lateral to the tandem on either side.

Point A is a common prescription point corresponding, approximately, to the point where the uterine vessels cross the ureter. Radiation tolerance is of these structures are thought to be the main limiting factor in irradiating the uterine cervix.

Point B

Point B is another dose reporting point located 3cm lateral to point A.

Bladder Point

Bladder Point is the point of highest dose in the bladder.

Bladder dose should be  less than 80% of dose to point A.

Rectal Point

Rectal Point is the point of highest dose in the rectum.

Rectum dose should be less than 80% of dose to point A.

High Dose Rate Tandem and Ovoid

High dose rate tandem and ovoid treatment mimic the dose distribution of LDR treatments. However, incorporation of image guidance allows for better tailoring of exact dose distribution and more detailed dose reporting.

MR compatible HDR tandem and ovoid applicator.

GYN GEC-ESTRO Recommendations

As gynecological brachytherapy transitioned from LDR to HDR, GEC-ESTRO wisely put together a working group to standardize dose reporting. This new system leverages improved use of imaging and computerized treatment planning.

Structures (GYN GEC-ESTRO Working Group I)

Read the full article here.

GTV (Gross Tumor Volume)

Includes macrscopic tumor extension as detected by clinical examiniation and visualized on MRI.

HR CTV (High Risk Clinical Target Volume)

Includes whole cervix and presumed extracervical tumor extent.

Additional disease extent may be determined by palpation, direct visualization, or on MRI/CT imaging.

HR CTV should receive full dose.

IR CTV (Intermediate Risk Clinical Target Volume)

Includes the GTV, HR CTV plus a safety margin of 5-15mm to account for significant microscopic residual tumor load.

IR CTV should receive at least 60Gy.

LR CTV (Low Risk Clinical Target Volume)

Subscripts

D = at time of diagnosis. (E.g. GTVD is the GTV at time of diagnosis)

B# = at time of brachytherapy treatment. (E.g. GTVB1 is the GTV at time of first brachytherapy treatment)

Image Credit: GYN GEC-ESTRO Working Group I
Image Credit: GYN GEC-ESTRO Working Group II

Dose Reporting (GYN GEC-ESTRO Working Group II)

Read the full article here.

The following items should be reported for a GYN brachytherapy case.

  • Prescribed dose
  • Total Reference Air Kerma (TRAK)
  • Dose to Point A (left, right, and mean)
  • D100% and D90% for GTV, HR CTV, and IR CTV
  • Dose to bladder and rectum ICRU 38 reference points
  • D0.1cc, D1cc, D2cc for organs at risk (rectum, sigmoid colon, bladder, and vagina)
  • D5cc, D10cc if contouring organ walls is performed
  • Biologically weighted doses (EQD2)

MR Guided Contours (GYN GEC-ESTRO Working Group IV)

Read the full article here.

  • A T2 weighted MRI is used in contouring as it allows for proper visualization of the tumor, cervix, bladder, vagina, rectum, and sigmoid colon.
    • A T1 weighted image may also be used but may be useful in determining the extent of nodal involvement.
  • The image should be oriented para-axial (orthogonal to the urterine axis) and para-sagital and para-coronal (parallel to the uterine axis).
  • 0.1T to 3T MR scanners are acceptable with 1.5T being the norm.
    • SNR increases with strength but spatial linearity and inductive heating become a limiting factor at high fields.
  • Consistent bladder filling is an important parameter in treatment planning.
    • A foley catheter is inserted and the bladder is first drained then filled with ~7cm3 of either diluded gadolinium contrast of saline solution.
  • MR compatible applicators may either be constructed of plastic or titanium.
    • Titanium applicators will induce susceptibility artifacts.
    • The low visibility of plastic on MR scans may require Gadolinium doped dummy wires to be inserted into the channels allowing for proper reconstruction.
  • Applicator should be fixed in place by packing the vagina with cotton gauze. This prevents applicator motion between simulation and treatment.
Desired MR slice orientation is para-axial and para-coronal. Image credit: GYN GEC-ESTRO Working Group IV

Tandem and Ring

Similar to T&O, the Tandem and Ring (T&R) applicator is able to deliver a wider and more Anterior/Posterior dose distribution compared. T&R may be appropriate for larger tumors, increased upper vaginal involvement, or pelvic side wall invasion.

Tandem and Ring Applicator.

Rotte Applicator

The Rotte Applicator is “Y” shaped but produces a “pear” shaped dose distribution useful for treating some cervical anatomies.

Rotte “Y” shaped applicator.

Vaginal Cylinder

Vaginal cylinders are cylindrically shaped applicators used in the treatment of vaginal disease. They may have a single central channel or their may be multiple channels nearer to applicator surface allowing for more control over the dose distribution.

Syed and Interstitial Needles

Interstitial needles are often used in gynecological brachytherapy as a supplement to one of the above applicators. They may be used to improve coverage to areas more distal to the vagina, uterus, or cervix.

One common applicator template is the Syed/Neblett GYN template. This design features a central cylinder (obturator) and lateral needle holes.

Syed GYN applicator

Hepatic Brachytherapy (Y90 Microspheres)

Selected Readings

AAPM TG-144: Dosimetry, Imaging, and Quality Assurance Procedures for 90Y Microsphere Brachytherapy (link)

Overview

Ytterium-90 microsphere’s are used for the treatment of Hepatocelular Carcinoma and metastatic tumors of the liver.

90Y incorparated into glass or resin sphere approximately 20-60μm in diameter. The spheres are injected into the hepatic artery, often accessed through the femoral artery. This approach to delivery is very effective because the hepatic artery supplies 80-100% of the blood flow to these tumors. In contrast, the unaffected liver will only receive 20-30% of its blood from the hepatic artery and the remaining 70-80% from the portal vein. The use of Y-90 microspheres allows for greater tumor dosing compared to external beam which may be limited by tissue toxicity to non-tumor destroying levels.

There are currently two types of Yttrium-90 microspheres available in the US. SIR-Spheres uses a resin based material and is available in 3GBq vials. TheraSphere uses a glass based material and is available in 3, 5, 7, 10, 15, and 20GBq vials. Theraspheres are slightly denser (3.29 vs 1.6 g/cm3) and has a higher activity per sphere (2500 vs 50 Bq).

Prescriptions

Prescriptions may be written either in either terms of administered activity or mean dose to the target liver lobes. Mean target lobe doses are commonly in the range of 120-150Gy!

Yttrium-90

Y-90 decay scheme

Yttrium-90 (90Y) is a beta (electron) emitter with an average energy of 0.9267 MeV and a half-life of 2.67 days. Over 90% of emitted energy is absorbed within 5.3mm and the maximum range of emitted electons is 11mm. Y-90 is most commonly used in radiation therapy as the source in microsphere liver treatments.

Quick facts

  • Half life: 2.67 days
    • Decays approximately 1% per hour
  • Decay mode: β
  • Beta energy: 0.9267 MeV (mean)

Determining Patient Eligibility

Imaging Studies

Triple-phase contrast enhanced CT is common to determine liver volume and extent of tumor burden.

MRI, especially with Gadolinium contrast, may also be used to determine volumes.

PET imaging using 18F-FDG is commonly used to assess regions of metabolically active tumor.

SPECT/Gamma Camera imaging is useful in determining the expected distributions of microspheres. This is accomplished using 2-4mCi of 99mTc microaggregated albumin (MAA) infused into the liver.

Key Point: SPECT imaging of the Y-90 bremsstrahlung is also used post procedure to verify microsphere deposition.

An angiographic procedure is performed prior to treatment to asses blood flow and to guide the catheter through the femoral artery to the hepatic artery and into treatment position

Hepatic PET/MRI showing microsphere location within liver.

Key Point: Lung Shunting
The 99mTc microaggregated albumin (MAA) imaging is used to asses the fraction of microspheres expected to enter (be shunted to) the lungs. If lung shunting is >20% of administered activity, the patient is a poor candidate for microsphere therapy.

Indications and Contraindications

Data from Table II TG-144

IndicationsContraindications
Unresectable hepatic primary or metastatic diseaseLimited hepatic reserve
Liver dominant tumorEstimated lung dose greater than 30Gy
Life expectancy greater than 3 monthsUncorrectable extrahepatic deposition

Dosimetry

Medical Internal Radiation Dose (MIRD) Committee of the Society of Nuclear Medicine Standard

  • k is a constant yielding dose rate in the desired units
  • E¯ is the average energy emitted per nuclear transition
  • A is the activity
  • is the mass of tissue absorbing the radiation
    • For the target, this is often taken as the mass of the affected lobes of the liver.
    • The the lungs, this is often taken as the total lung mass.
  • Important: Remember to subtract the fraction of activity shunted to the lungs from the liver activity and vice-versa during calculation.

Image Based Dosimetry

The 99mTc MAA study data may be used prospectively to determine the expected dose distribution to the patient. This is accomplished using convolution of the activity distribution and a Monte Carlo generated dose deposition kernel. The dose deposition kernel is spatially invariant meaning.

Activity Calibration

Key Point: The is no current standard method of determining the source activity independent of the manufacturers measurements. Both SIR-spheres and TheraSpheres provide end-users with calibration samples with which they can determine their own calibration coefficients for their equipment.

NIST uses liquid scintillation to characterize Y-90 in solution. This procedure is very accurate and is a primary measurement (i.e. relies on fundamental principles rather than calibration factors).

TheraSphere calibration is NIST traceable through the NIST Radioactivity Measurement Assurance Program (NRMAP).

SIR-spheres are not NIST traceable. Instead they are calibrated against the Australian Nuclear Science and Technology Organization (ANSTO) and the Australian Radiopharmaceuticals and Industrials (ARI) standards. These methods dissolve the SIR-spheres into a solution and measure the activity using an ionization chamber. This procedure induces additional error (relative to direct liquid scintillation) as it is dependent upon the volume and homogeneity of the sample. Users are supplied with calibration samples which allow them to determine calibration factors for their own equipment.

Many end-user choose to trust the manufacturer’s calibration value of treatment sources but it remains the responsibility of the institution to verify this calibration. Agreement to within 5% is considered acceptable. Deviations exceeding 10% should be reported to the manufacturer.

Regulatory Requirements

The US Nuclear Regulator Commission (NRC) regulated 90Y microspheres as medical devices under 10 CFR part 35 code.

Intravascular Brachytherapy

Selected Readings

AAPM TG-60: Intervascular Brachytherapy Physics (external link)

AAPM TG-149: Dose Calculation Formalisms and Consensus Dosimetry Parameters for Intravascular Brachytherapy Dosimetry (external link)

Overview

Intravascular brachytherapy is used as a treatment for restenosis and recurrent arterial blockage following angioplasty or stenting of a blocked artery. This technique is effective for blockages that are resistant to drug infuse stents or anti-coagulant (anti-clotting) drugs.

Treatment Regimen

  • Prescription: 15-20Gy typically prescribed at 2mm from the source (intracoronary) or 2mm beyond lumen radius (peripheral arteries).
  • Treated volume: a 2-5cm length of artery to a depth of between 0.5 and 2mm beyond the lumen.
  • Dose rate:
    • 1-5cGy/hour for radioactive stents
    • >5Gy/minute for HDR afterloader type deliveries
  • Dosimetry: follows the conventions of TG-43 (internal link)
    • This is the recommendation of TG-149.

Balloon Angioplasty Procedure

Balloon angioplasty is a treatment of stenosis or narrowing of the blood vessels, especially those supplying the heart. During the treatment, a catheter is inserted into the femoral (at groin) or radial artery (at wrist) and guided to the affected artery. Once in location, a balloon is repeatedly filled and drained unclogging the artery. A stent (mesh reinforcement) is inserted to the artery to prevent future narrowing and support weakened arteries.

Restenosis may occur after an angioplasty procedure due either to blood clotting or neointimal (scar tissue) growth within the vessel. Anticlotting drugs can be used to prevent clots and drug eluding stents (DES) prevents some neointimal growth. However, restenosis may still occur due to neointimal growth and intravascular brachytherapy can be an effective treatment for such cases.

Key Point: Radioactive devices compete with Drug Eluding Stents (DES) to prevent restenosis. DES are popular as they are both effective and simple to use clinically. However, DES have difficulty delivering drugs in areas of repeat stenting. Both Beta-Cath and activated stents are regaining popularity for repeat treatments as they overcome this limitation.

Delivery Devices

Intravascular brachytherapy is either administered using a LDR implanted mesh stent or using an HDR remote afterloader device.

Low Dose Rate Mesh Stents

Radioactive mesh stents typically use either P-32 or Y-90, both of which are beta emitters.

Intravascular Afterloaders

Beta-Cath

  • Sr-90/Y-90 (beta emitter)
    • Sr-90 Half-life = 28.8 years
    • Y-90 Half-life = 64.1 hours
  • Handheld hydraulically activated afterloader

RADIANT (liquid filled balloons)

  • Re-188 (beta emitter) liquid source injected into angioplasty balloon.

Additionally, several historical products are no longer available in the US market. These include:

Cordis CHECKMATE (No Longer Marketed, 2003)

  • Ir-192 (gamma emitter) ribbon
    • 3mm by 0.5mm seeds separated by 1mm
    • Variable source length 19-80mm
  • Cart mounted mechanical hand crank afterloader

Guidant GALILEO (No Longer Marketed, 2004)

  • P-32 (beta emitter) enclosed in a spiral balloon around the central wire
    • Spiral design improves line flexibility
  • Remotely controlled by motorized afterloader device
Guidant GALILEO Intravascular Brachytherapy Afterloader
Beta-Cath Intravascular Brachytherapy Afterloader
Cordis CHECKMATE Intravascular Brachytherapy Afterloader

Additional Considerations

Beta Vs Gamma Emitters

Beta emitting devices have the advantage of a higher dose rate and more localized dose distributions relative to gamma emitters but produce a less uniform dose distribution. The dose distribution within the treated area (~2mm) is essentially the same for both gamma and beta sources because of the dominance of inverse square law. Both beta and gamma emitting isotopes are considered adequate for intravascular brachytherapy although Khan notes a preference for Beta emitters.

LDR (Radioactive Stents) Vs HDR

LDR intravascular brachytherapy is a relatively simple procedure in that it entails essentially the same steps as a standard stent placement. Further, the low dose rate of the stent makes the procedure safer for radiation workers than many of the HDR approaches. However, use of radioactive stents has fallen out of favor due to “candy wrapper effect” in which restenosis forms at the ends of a LDR stent.

Ocular Brachytherapy

Selected Readings

AAPM TG-129: Dosimetry of  125I and 103Pd COMS Eye Plaques for Intraocular Tumors (external link)

Collaborative Ocular Melanoma Study (COMS) (external link)

Overview

The Collaborative Ocular Melanoma Study (COMS) set the standard for I-125 eye plaque brachytehrapy.

Image of Ocular Melanoma (COMS) plaque with I-125 seeds.
Alternative eye plaque design. Credit: Eye Physics LLC.

Treatment Regimen

Disease: Ocular Melanoma

Prescriptions

  • COMS: 85Gy to water at a depth of 5mm (for tumors with apex height of 5mm or less) or to apex depth (if apex is greater than 5mm).
    • Max dose is may be as much as 10 times greater than prescription dose (1000Gy)!
  • Implant duration: 5-12 days
  • Radionuclide: 125I, 103Pd
  • Seed activity: 0.5 – 5 mCi
  • Dose rate: 0.5 – 1.25 Gy/hr
  • Plaque size
    • Chosen to cover radial extent of tumor with 2-3mm margin.
    • Available plaque sizes: 10mm to 22mm in 2mm increments

Alternative Treatments

  • External beam proton therapy.
  • Enucleation (removal of eye).

COMS Planning

  1. Tumor is localized using Fundoscopy, Ultrasound, CT, or MRI to generate a Fundus diagram.
  2. Physician determines radial and apical dimensions of tumor using MRI and Fundus diagrams.
  3. A plaque is chosen exceeding the radial dimensions of the tumor.
  4. I-125 seeds are ordered in a quantity determined by plaque size and activity determined to yield full dose at tumor apex.
  5. Sources are assayed upon arrival and entered into inventory.
  6. Plaque and seeds are assembled and sterilized prior to insertion.
  7. During the insertion operation, the rectus muscles attaching the eye are cut. The eye is rolled to access tumor and the plaque is stitched in place. The eye is then rolled back to position.
  8. After 5-12 days the plaque is removed, rector muscles are attached, and patient is released (after survey to verify source removal).
Fundus diagram showing lesion in left eye.

Activity/Dose Calculation

The required activity per seed varies based on the seed design, radionuclide, implant duration, and plaque size. Additionally, some users may asymmetrically load the plaque to yield a tailored dose distribution (e.g. to reduce optic nerve dose). For standard (uniform) seed loading the dose distribution for a reference activity (1Ci) and duration is typically mapped for each plaque size. This may be accomplished either in a treatment planning system, physically using film, or by referencing the table in AAPM TG-129. This dose distribution map can then be by activity to yield the prescription dose at a given depth.

Dose map for a single seed loaded COMS eye plaque.

Prostate Brachytherapy

Selected Readings

AAPM TG-64: Permanent Prostate Seed Implant Brachytherapy (external link)

Overview

Prostate cancer has been found to have a low alpha/beta ratio (~1.5) making it a good candidate for low fraction treatments. Modern prostate treatments involve 15-20 perineal needles guided by transrectal ultrasound.

Treatment Regimen

LDR Prescriptions

The prescription varies significantly based on the radionuclide used. This is because differences in half life influence integral dose deposition and the radiobiology.

  • I-125: 145Gy
  • Pd-103: 125Gy

HDR Prescriptions

HDR alone: 13.5Gy x 2 fractions = 27Gy

HDR boost to external beam: 15Gy single fraction

LDR Prostate Brachytherapy

Treatment Process

  1. A volume study is conducted prior to treatment to assess the number of seeds required.
    • Volume commonly assessed via CT or transverse ultrasound imaging.
  2. A treatment plan is generated either by computer or standard template.
    • Seed spacing ~1cm
    • 80-120 seeds are commonly used!
    • Sources
      • I-125 with an activity of around 0.3mCi
      • Pd-103 with an activity of around 1.7mCi
  3. Implantation is performed under anesthesia and guided with a transrectal ultrasound probe.
    • A Mick Applicator is commonly used to place correctly spaced seeds.
      • Applicator uses a seed magazine and spacing indexer to simplify placing process.
    • 21cm long 18 gauge needles are used to place the seeds.
    • A template may be used to assist in uniform needle placement.
  4. A cystroscopy is performed at the end of the procedure removing any seeds that may be been placed in the bladder of the urethra.
I-125 seeds

Key Point: The urethra runs centrally through the prostate with a slight curve in the anterior-posterior direction. To prevent blockage or urethral penetration, the mid line of the prostate may not be seeded.

HDR Prostate Brachytherapy

Prostate HDR brachytherapy is typically accomplished using 15-25 interstitial needles inserted through the perineum. Needles are indexed for planning using a template and are guided by rectal ultrasound imaging. Once the needles are inserted, CT imaging is most often used to produce a 3D planned dose distribution.

Prostate interstitial needle placement guided by ultrasound
SpaceOAR is a useful tool in the treatment of prostate cancer using high dose rate brachytherapy.

SpaceOAR

Space OAR is a gel that may be inserted via needle between the prostate and rectum. It is useful in HDR prostate planning as a method of reducing rectal dose by moving the rectum away from the treated volume.

Superficial Brachytherapy

Brachytherapy may be used for superficial lesions especially for uneven treatment sites of those site which are otherwise difficult to target with external beam electron therapy. Further, Ir-192 brachytherapy may be preferable to electron therapy for skin lesions because of improved superficial skin dose (145% is typical) a high degree of dose conformality.

Treatment Regimen

Skin Cancer is typically treated with 40Gy in 10 fractions (4Gy/fx) every other day prescribed to a depth of 1cm.

Skin keloid (tissue overgrowth) is treated with 12Gy in 3 fractions (4Gy/fx) prescribed to a depth of 0.5cm.

Skin Keloid is a scar like overgrowth of skin tissue.

Applicators

Freiburg Flap

The Freigurg flap is a skin applicator consisting of plastic catheters threaded through a series of 1cm silicone spheres. There spheres position the catheters 0.5cm above the skin surface aiding in dose uniformity. Rows of catheters are attached together forming the flap. A flap of this design allows treatment of a large area with the flexibility to conform to the skin surface.

Freinburg Flap used for superficial brachytherapy HDR treatments.

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