Brachytherapy QA
brachy QA
- Acceptance testing - Tx QA - Tx Delivery Checks Includes: - Record and Verify - Imaging (Film, Electronic Portal) - Tumor localization - Positioning - Checklists - Diodes
strontium-90
- By-product of the fission of U and Pu in nuclear reactors and nuclear weapons - Half life = 28 years - Emits beta particle as it decays to yttrium-90 - Behaves chemically like Ca, and tends to concentrate in the bones and teeth - Differs from Stronium-89 used for metastatic bone lesions - Emits a beta particle as it decays to yttrium-90 - Max E = 0.5 MeV - Avg E = 0.93 MeV
conditions specific to remote afterloaders (source positioning: accuracy)
- Checked by taking radiographs of dummy sources in the applicator, with positions marked on ready pack film, combined with autoradiographs of the actual radioactive sources in the same applicators - The position of dummy sources and radioactive sources should correspond within + 1mm
QA
- Complete description of brachy sources should be kept on file - Physical inventory of all brachy sources must be completed quarterly - The uniformity of source material description should be checked by autoradiography -- Signature exposure of a radioactive source obtained by placing source on an unexposed film for a period of time long enough to darken the film -- Scan film for uniformity
radium-226
- Discovered by Marie and Pierre Curie in 1898 - Occurs naturally in very low concentrations - Decays by alpha particle radiation to an inert gas (radon-222) - decay product in the U-238 decay series - Half life = 1622 years - Energies 0.184 to 4.78 MeV • Alpha 4.78 MeV (94.5%), 4.61 MeV(5.55%) • Gamma 0.186 (3.5%) • Average energy is 0.83 MeV
brachy advantages
- Dose distribution around source - high concentration around source - Rapid fall off of dose around source (InvSq) - Accurate location of tumor - Good immobilization of tx area - Eliminates set up inaccuracies
cesium-131
- Extremely new isotope - First introduced at ASTRO in 2005 - Decays by electron capture to Xe-131 followed by characteristic x-ray emission - Photon energies - 4-34 keV photons - Average energy 30 keV - Half life = 9.7 days
brachy QA (radiation safety measures)
- Facility: inventory, storage and transporting - Maintenance: identification, cleaning, disposal - Clinical application: prep of sources, pt discharge, and personnel monitoring - Emergencies and special precautions - Contamination, lost source, death of pt, and education and training of all personnel
strontium-90 use
- Intravascular brachy (Plaque treatments) - tx of superficial lesions, ocular lesions and coronary vessels
endovascular stent applications
- Known as vascular brachytherapy or intravascular - The insertion of a brachytherapy source into vessel for tx of vascular restenosis - Gamma emitters (Ir-192) > Earliest source used for endovascular > Came as seeds (attached to a ribbon) or wire > Required manual afterloading > Remote afterloaded via a catheter
conditions specific to remote after loaders (source calibrations)
- LDR sources in remote afterloading units can be calibrated in a well ionization chamber. - For calibration of HDR sources, the well ionization chamber must be able to measure large currents associated with these sources - A cylindrical lead insert for a conventional well ionization chamber is used to calibrate HDR sources - Cylindrical ion chambers can be used to calibrate both LDR and HDR sources using a free air geometry - It is an interpolative method of obtaining exposure calibration factor
iridium-192 use
- LDR temporary implant or HDR source - Sealed in platinum or steel capsule - Can be used as a wire - Ir-192 implants used in head and breast - produced in wire form, introduced through catheter to target area - After in place for time required to deliver desired dose, implant wire is removed
phosphorus-32 use
- Liquid in a balloon - Wire in intravascular brachy delivery system - Infused by catheter into abdominal cavity or the linings around the lungs - Tx fluid accumulation in cavities, that occur in cancers that start in ovary or lung - Can be injected directly into certain tumors. - Limited depth of penetration
palladium-103
- Man made - May be created from pd-102 - Produced in nuclear reactor or cyclotron by proton bombardment - Half life = 17 days - Decays by electron capture to excited states of Rhodium-103 followed by characteristic x-ray emission - Multiple photon energies - 20-23 keV photons - Avg E = 0.021 MeV - HVL 0.004 mm Pb
iridium-192
- Manmade, formed from nonradioactive Ir metal in nuclear reactor - Half life = 74 days - Most decays (95%) by beta emission - Average energy is 0.38 MeV - HVL 2.5 mm Pb
cesium-137
- Most common radioactive form of cesium - By-product of nuclear fission - Naturally occurring isotope - Can be man made - Produced when U and Pu absorb neutrons and undergo fission - Examples of uses of this process are nuclear reactors and nuclear weapons - Half life = 30.23 years - Emits 0.662 MeV photons - HVL 5.5 mm Pb
preloaded needles
- Needles are filled with seeds and spacers - Accomplishes the same thing as a Mick applicator - Insertion of seeds into the prostate - Seeds are positioned inside needle in predetermined order - Seeds are kept in correct position with spacers - Spacers made of a material that will be absorbed by the body - Under US guidance a needle is inserted into prostate - Once in correct position (specifically depth), seeds are deposited while needle is removed - Slightly more accurate than the Mick applicator
remote after loading acceptance testing QA
- Operational performance of unit - Radiation safety check of facility - Checking of source calibration and transport - Checking of tx planning software Testing of functional performance and safety features - Activate various functions and observe results Accuracy of source localization and calibration - Use procedures that are similar to those used for conventional brachy
mick applicator
- Permanent seed implant - Device has magazine of radioactive seeds - Needle is inserted into the prostate - Once correct position is established a plunge releases a single seed into the prostate - An external ruler allows needle to be retracted a known distance before second seed is deposited into tissue. (Applicator allows practitioner to make on the fly adjustments to seed loading to achieve the best tx plan.)
brachy disadvantages
- Placement of the source usually requires surgery - Requires skilled rad oncologist to place sources - Handling of radioactive sources
loop
- Primarily used to treat oral cavity tumors - Particularly base of the tongue - Two large bore needles are inserted through neck into oral cavity on opposite sides of the tumor - Single catheter is threaded through one needle and redirected down second catheter - Needles are removed leaving the catheter lopped around the tumor - Catheters typically spaced 1 to 1.5 cm apart - Both ends of catheter are stabilized with metal and plastic buttons - Ends of catheter are cut off with at least 4 cm extending beyond the metal button
gold-198 (+use)
- Produced by neutron activation in a reactor - Half life = 2.7 days Use - Not commonly used in US as a brachy source - Small seeds or gold "grains" - Suitable for permanent implants (prostate) - Can be used for HDR procedures - Near monoenergetic photon emission - 0.412 MeV - HVL 2.5 mm Pb
iodine-125
- Produced in a nuclear reactor - Half life = 60 days - Electron capture to excited state of Te-125 followed by characteristic x-ray emission - Characteristic photon emission - Average energy of 0.028 MeV - HVL 0.025 mm Pb
brachy tx planning
- Source arrangement/parameter changes - Source properties - Source input and display, plan optimization and evaluation tests - Clinical system/process checks
surface applicators (sources)
- Sr-90 (Plaque) - Pd-103 (Plaque) - Ruthenium-106 (eye plaque txs) - Ir-192 (Leipzig applicator) - Co-60 > eye plaque tx (Primarily retinoblastoma) > Very easy to use and prepare > Doesn't allow customization of dose distribution > Cannot shield optic nerve - I-125 (Plaque) > Created on-site for each pt (customized) > seeds are placed into the plaque in specific positions and then sealed
iodine-125 use
- Temporary or permanent implants - Seeds encapsulated in 0.05 mm of titanium - Liquid form = tracer in thyroid studies and therapy in hyperthyroidism - Used in bioassays
through and through
- Used when both sides of tumor are accessible - Tumors of the skin, breast, or neck - Large bore needle is inserted through skin into tumor and exits through skin on opposite side - Catheter is then passed through the needle and needle is removed - Process is repeated until tumor is adequately covered - Both ends of catheter are stabilized with metal and plastic buttons
palladium-103 use
- Widely used for permanent implants - Can replace I-125 for prostate brachytherapy
plaque
- hollow, thin silver casing that encloses radioactive powdered salt or seeds - Sometimes made of gold or steel - Come in various shapes and sizes (10-25 mm) in diameter. - Circular - Semicircular - Used to treat tumors around the optic nerve
phosphorus-32
- known as chromic phosphate P 32 - One of most commonly used radioactive substance used in biomedical research - Half life= 14.3 days - P-32 decays to S-32 (which is stable) by beta emission - Max E = 1.7 MeV - Avg E = 0.69 MeV
miniature x-ray emitter
- not a radiation source - oncologist or physicist to be present or involved in tx - The x-ray tube is 1.25-5 mm in diameter - Attached to coaxial cable that's placed inside a delivery sheath - tx time for doses such as 15 Gy for lesion length of 30 mm are planned to be less than or equal to 10 mins - advantages: doesn't require use of isotopes, it emits radiation only when activated by operator
Freiburg Flap applicator
- series of parallel catheters connected to a square sheet - used either intraoperatively or as surface applicator - Used in conjunction with an HDR afterloading - pliable sheet easily conforms to various skin contours - Very time intensive in set-up and planning
decay formula
A = (Ao e ^ -[0.693/t ½] t) • Ao - original known activity • A - current activity • t ½ - half life • t - length of time passed since time of known activity • e - base of natural log
surface applicators (external applicators)
A custom mold of a body part treated may be made to be able to place sources in definite arrangements to deliver the dose - Commonly used for non-melanoma skin cancers - Used for tx of oral cavity, nasal cavity, hard palate, orbital cavity - Molds can be designed to incorporate shielding to reduce dose to surrounding critical structures
tumor sites
Any tumor where a source can be placed near or in the tumor: Body cavities • Uterus • Vagina • Esophagus • Bronchus Tumor accessible via needles or catheters: • Breast • Head and neck • Prostate • Skin
cesium-137 use
Biological behavior similar to K - After entering organism, all Cs gets more or less uniformly distributed through the body, with higher concentration in muscle tissue and lower in bones Biological half-life of Cs isotopes is short, so they get excreted in urine fairly quickly Small amounts of Cs-134 and Cs-137 were released into environment during nuclear weapon tests and some nuclear accidents - Most notably the Chernobyl disaster 2005 - principal source of radiation in the exclusion zone around the Chernobyl power plant
SI units
Bq = one disintegration per second
traditional units
Ci = 3.7 x 10^10 disintegration/second Measured from 1 gram of radium
californium-252 use
Determine the health of the bone Tx ovarian and cervical cancer
tx team
Interventional cardiologist - Places the catheter Radiation oncologist - Places and removes the source Medical physicist - Radiation safety issues
interstitial brachy (base of tongue)
Loop technique • Necessary to keep the catheters as far from the mandible as possible
californium-252
Manmade isotope produced in nuclear reactors Decay of Californium-252 (Not a pure neutron emitting source) • Alpha decay (97%) • Fission (3%) • Half life = 2.6 years • Avg neutron E = 2.15 MeV • Avg photon E = 0.8 MeV
Advantage of HDR over LDR
More convenient • Less tx time (5-10 min), pt stay is less • Less space required to house unit, don't need special hospital room for overnight stays • Less expensive • Similar outcomes than LDR • Can be given on a fractionated outpatient basis • Less radiation dose to medical personnel • Tx techniques more consistent
HDR (iridium-192, Co-60)
Most use Ir-192 or Co-60 sources (because of high specific activity and ability to produce a gamma ray with sufficient penetration) Disadvantage of Ir-192 - T½ = 74 days (has to be replaced every 3-4 mo) Ir-192 preferred over Cs and Co-60 due to small diameter of 1mm (Co60 and Cesium twice this size and limits application of HDR to intracavitary-pelvis and intraluminal-esophagus)
temporary brachy
Placement of radioactive sources in or next to a target volume for a limited amount of time before they are removed - Usually administered through afterloading technique - Applicators or catheters are placed in pt, tx plan devised sources are readied and checked and tx is delivered - Sources are placed manually into applicators or catheters by physician, after tx is given the sources and applicators are removed
interstitial
Places radioactive sources directly into the tumor bed (permanent or temporary) - Usually requires needles - H&N, breast, skin, sarcomas > I-125, Pd-103, Au-198 (permanent implants) - pelvis, abdomen or lung > Ir-192 (temporary) - used where there is no body cavity or orifice - breast or chest wall irradiation
intraluminal
Places sources inside body tubes (esop, uterus, trachea)
intracavitary
Places the sources inside the body - cervical tx
Free Hand
Sealed end • Used when tumor is only accessible from one side • Tumors in thoracic or abdominal cavities • Catheters placed during surgery • Sewn in place with chromic catgut sutures • Exact number of catheters is determined by tumor size • Catheters placed 1 to 2 cm apart • Can be placed in multiple planes or single plane • Metal and plastic buttons are placed on the catheters on the skin surface
permanent brachy
Seed implants - placement of small radioactive sources directly in the targeted tissue where they are left to decay Sources are left in to deliver a dose to tumor over a period of several weeks or months (i.e. prostate)
procedures to evaluate intracavitary sources and after loading applicators (4)
Source identity - Physical length, diameter, serial No., source color-coding Source uniformity and symmetry - Superposition of autoradiograph and transmission radiograph Source calibration - Well ionization chamber - ±5% between vendor and user calibration Intracavitary Applicator Evaluation - Source placement, shielding and mechanical function - Orthogonal radiographs to examine internal structure - Ease of source loading and removal
topical
Sources are placed on top of the area to be treated - skin, oral cavity, nasal cavity, hard palate, orbital cavity Requires creation of body molds - Eye plaque - Iodine-125 used for melanoma of eye
activity
The intensity of a radioactive source The rate of decay or the change in the number of atoms in a certain material over a given amount of time
half life (t½)
Time period in which the activity decays to one half its original value
interstitial brachy (base of skull)
Used for recurrent or unresectable tumors Used in combination with external beam or alone Advent of SRS has limited usefulness of interstitial txs of the skull base
interstitial brachy (rectum)
Used in combination with external beam for medically inoperable pts Sometimes delivered intro-operatively in order to displace the normal bowel
endovascular
Used to treat blood vessels - Use of coated stents has reduced the use
template (def)
Used when the accuracy of seed placements is extremely important • Typically used for permanent seed implants • Used for prostate, vagina, and rectum cancers • Preferred to use rigid stainless steel needles rather than flexible catheters
low dose rate (LDR)
brachy delivered in a conventional low dose rate regimen that lasts several days and requires a hospital stay Visitors of pts undergoing LDR brachy should only remain in the pt's room for 20 mins per visit per day
high dose rate (HDR)
delivering brachy on an outpatient basis - Actual tx delivery lasts about 5-10 mins - > 1200 cGy/hr or more than 20 cGy/minute
interstitial brachy
implantation of radioactive sources, usually in the form rigid needles or tubes, directly into a tumor or tumor bed Used in tx of the breast, neck, prostate or soft tissue It may be used as sole form of radiotherapy or combined with external radiation Temporary vs. permanent o Temporary implants can be afterloaded o Permanent can not and must be hot loaded
radium-226 use
o 1920's - IV injected for variety of ills, many pts later develop bone cancer or other malignant diseases. o Chemically similar to Ca, when ingested, small fx is transferred across SI and most is deposited in bone, which contains 70-95% of the total ingested body radium o Mixed with fluorescent zinc sulfide to make a luminous paint • Aplastic anemia, leukemia and bone cancers developed in workers who repeatedly used their lips to make a point out of the paintbrush bristles
Disadvantages of HDR
o Afterloaders are expensive o Increased maintenance cost o Increased room shielding may be needed o Increased staffing required
cesium-131 use
prostate cancer (2004)
brachy QA (Acceptance testing)
purpose is to ensure that the sources and the associated equipment meet the vendor's specifications - These test results must be evaluated and documented
brachytherapy
radiotherapy where the source of radiation is placed close to the body surface or within a body cavity Refers to therapeutic use of encapsulated radionuclide's within or close to a tumor
afterloading
source is placed in the applicator after the applicator is placed in or on the pt Manual - sources are manually inserted and removed from the applicators Remote afterloading - sources are inserted and removed from the applicator through some type of source transportation device
preloading
source is placed in the applicator before the applicator is placed in or on the pt - Ra-226 needles - Cs-137 needles - I-125 eye plaques - Sr-90 eye applicators
brachy goal
to conform the radiation dose to the size and shape of the target, and limit side effects by sparing the surrounding healthy anatomy technique takes advantage of the inverse square law (the intensity of radiation is reduced by the square of the distance from the source) to achieve the goal
gynecologic template
• Cervical and vaginal cancers • Ultrasound can be used for guidance • 1 cm gap between needles
template types
• General • Most templates fall into one of two brands - Syed-Neblett - Universal Martinez • Each has a number of variations • Variations depend on cancer being treated
yttrium-90 (+use)
• Half life = 64.2 hours (2.67 days). • Pure beta emitter, decays to stable zirconium 90 • Avg E = 0.9367 MeV. • Max E = 2.27 MeV Use • Used in tx of metastatic liver cancer • TheraSphere® consists of insoluble glass microspheres where yttrium-90 is an integral constituent of the glass
placement of template
• Inserted in OR under general anesthesia • Temporary implants require the template be sewn to the perineum • Template remains in place until txs are completed • Typically 3-5 days
interstitial brachy (applicators + sources)
• No specific applicator • Common materials - Large bore needles - Nylon afterloading catheter - Metal buttons (prevent catheter from slipping) - Plastic buttons > Used to separate the skin from the metal button > Prevents e- scatter from metal button from reaching the skin Source = Iridium-192 - Most commonly used source for temporary implants
commonly used isotopes (8)
• Radium-226 (1622 yrs) • Cobalt-60 (5.27 yrs) • Cesium-137 (30 yrs) • Iridium-192 (73.83 days) • Iodine-125 (59.4 days) • Palladium-103 (16.99 days) • Gold-198 (2.7 days) • Radon-222 (3.82 days)
rectal template
• Similar to the gynecologic template • Full circle arrangement • Accommodates 36 needles
prostate template
• Temporary implant vs permanent • Multiple varieties • The plate has two circles of holes • Can accommodate 18 needles
