BrachytherapyBrachytherapy is a treatment method that uses sealedradiation sources placed close to tumor tissue. Takenfrom “brachys,” the Greek word for “near”. Brachytherapydelivers a dose of radiation usually to within a few inchesof a diseased area or tumor. It allows for a high dose ofradiation to reach the tumor while adjacent healthy tissuereceives low or reduced dose levels.
Ra-226• It was discovered in 1898. It is the sixth member of the uranium series, which starts with and ends with stable .
Ra-226 decay• Radium disintegrates with a half-life of about 1,600 years to form radon.• Ra(88) ----------> 222Rn(86) + 4He(2) 226• 49 γ rays - 0.184 ~ 2.45 MeV• Average energy - 0.83 MeV (filter by 0.5 mm platinum)
Ra-226 source construction• Radium sulfate or chloride is loaded into 1 cm long, 1 mm in diameter cell (gold foil) to prevent leakage of radon gas. The sealed cells are loaded into platinum sheath.
Ra-226 source specification• Active length• Physical length• Activity or strength of source• Exposure rate constant in filtration (mm of platinum)
Cs-137• The decay of Cs-137 transforms to Ba-137 by the process of β- decay and 95% of the disintegrations are followed byγrays from the Ba-137 metastable state.• Cs-137 emits γ rays of 0.662 MeV.
Cs-137• The advantages of Cs-137 over Ra-226 are that it requires less shielding and is less hazardous in the microsphere form.• With a long half-life of about 30 years.• Cs-137 is used as a radium substitute in both interstitial and intracavitary brachytherapy.
Co-60• The decay of Co-60 transforms to Ni-60 by the process of β-decay and 99% of the disintegrations are followed byγrays from the Ni-60 metastable state.• Co-60 emits γ rays of 1.17 & 1.33 MeV.
Co-60• The main advantages of is its high specific activity, which allows fabrication of small sources required for some special applicators.• It is more expensive than Cs-137 and has a short half-life (5.26 years), necessitating more frequent replacement and a complex inventory system.
Co-60 vs. Cs-137 & Ra-226• The variation in dose rate within 5cm depth range is about equal for source Ra-226,Cs-137 and Co-60.
Ir-192• Ir-192 decays through 4.7% electron capture and 95.3% β-1 transitions, followed by γ transitions and K- and L- shell x-rays.
Ir-192• 30% Ir and 70% Pt• Seeds 3 mm long, 0.5 mm in diameter• Wire or nylon ribbon∀ γ ray ~ 0.38 MeV average energy (less protection)• Half life = 74.2 days (disadvantage)
Ir-192 for HDR• The high specific activity of Ir-192 (~9000Ci/g) makes it an attractive source for use with high dose rates (HDR) are requried .
Au-198• For permanent interstitial use• A half-life of 2.7 days• Emits 0.412 MeV γ ray• 2.5 mm long, 0.8 mm diameter with 0.1 mm thick platinum wall
I-125• The decay of I-125 transforms toTe-125 by the process of electron capture and are followed byγrays from the Te-125 metastable state.• Low γ ray energy, 35.5 keV• half-life 59.4 days
I-125• Two most popular models: 6702 and 6711.• Dose distribution around the seed is highly anisotropic due to titanium end welds.
Pd-103• Shorter half-life 17 days• Pd-103 decays by electron capture with emission of 20 ~ 23 keV characteristic x rays• Due to self absorption, it is highly anisotropic .in photon fluence
Pd-103• Recently become available for prostate permanent implants• Provide a biologic advantage in permanent implants due to much faster dose rate• 4.5 mm long and 0.8 mm in diameter• Shorter half-life 17 days
Calibration of brachytherapy source• Activity mCi (1 Ci = 3.7 × 1010 disintegrations/sec) A = ΔN / Δ t = -λN• Specific exposure rate constant Exposure rate at 1 meter in R-m2/Ci-h to eliminate the dependence on the construction of source and the detector
Calibration of brachytherapy source• Equivalent mass of radium There are historical reasons that make it convenient to specify brachytherapy sources in terms of the equivalent mass of radium. mg-Ra Eq = (R-cm2/mCi-h) / ( 8.25 R-cm2/mgh)Example: An iridium-192 source has been calibrated and its strength is specified as 0.495 mR/h at 1 m. What is the strength of this source in terms of effective mg-Ra eq?Ans: Exposure rate constant of radium filtered by 0.5 mm Pt = 0.825mR-m2/mgh Effective mg-Ra eq =0.495/0.825=0.6mg
Calibration of brachytherapy source- Apparent activity Activity of bare point source and is determined by dividing the measured exposure rate at 1 m with the exposure rate constant of the unfiltered source at 1 m.
Calibration of brachytherapy source• Air kerma strength defined as the product of air kerma rate in “free space” and the square of the distance between the calibration point and the center of source. ⋅l2 Sk = Kl air kerma strength. air kerma rate at distance l Distance l µGym2 h -1 µGyh -1 m2
Air kerma strength Ave. mass transfer W µ / ρ coeff Sk = Kl ⋅ l 2 K = X tr e µ /ρ en = 1 because No Ave. mass bremsstralung, absorption coeff W Sk = X l 2 e S k = X (R/h) (8.76 x 103 m 2 µGy/R) µGym2h-1 Exposure rate measurement at 1 m
Air kerma strength• Sk(U) = air Kerma source strength (U) Conversion factors from mCi: Ir-192 1 mCi = 0.238 U I-125 1 mCi = 0.787 U Pd-103 1 mCi = 0.773 U
Exposure rate calibration• Well-type ion chamber – for radio-pharmaceuticals – inaccurate (fixed energy, filtration, source position)
Calculation of dose distribution• Exposure rate
Calculation of dose distribution• Exposure rate
Calculation of dose distribution• Absorbed dose in tissue -µγ Dr = Bre Br = 1 + ka(µγ) Kb
Calculation of dose distribution Attenuation & scattering effect in water Dose point Source Water1% per cm Normalized at 1cm
Calculation of dose distribution• Modular dose calculation models D(r,θ) = A ) [G(r,θ )/G(r0,θ0)] g(r) F(r,θ)A = apparent activityA = dose rate constant (a unique value for each radioisotope)G(r,θ) = geometry factor (accounts for absorption and scatter along the transverse axis)g(r) = radial dose function (accounts for the variation from 1/r2 due to the distribution of the radioactive material in the source)F(r,θ) = anisotropy factor
Isodose curvelsodose curves in terms of radlh around a 1-mg radium source. Active length = 3.0 cm; filtration = 0.5 mm Pt.
System of implant dosimetry• Paterson-Parker system• Quimby system• Memorial system• Paris system• Computer system
Paterson-Parker system• Manchester system• deliver a uniform dose (± 10%) to a plane or volume• The system specified rules of source distribution Spacing of needle - not more than 1 cm One end uncrossed - area -10% Multiple implant planes - parallel to each other
Paterson-Parker system• If the ends of the implant are uncrossed, the area should reduced by 10 % for each uncrossed end.
Paterson-Parker system• Cylinder, cuboid, sphere• Uncrossed end - -7.5%• Cylinder – belt 4 parts – core 2 parts – each end 1 part
Quimby System• The Quimby system of interstitial implantation is characterized by a uniform distribution of sources of equal linear activity. Consequently, this arrangement of sources results in a non-uniform dose distribution, higher in the central region of treatment.
Memorial system• The Memorial system is an extension of the Quimby system and is characterized by complete dose distributions around lattices of point sources of uniform strength spaced 1 cm apart.
Paris system• The Paris system is intended primarily for removable implants of long line sources, such as Ir-192 wires.• The system prescribes wider spacing for longer sources or larger treatment volumes.
Computer system• The implantation rules are very simple the sources of uniform strength are implanted, spaced uniformly (e.g., 1.0 to 1.5 cm, with larger spacing for larger size implants), and cover the entire target volume.• the target volume is designed with sufficient safety margins so that the peripheral sources can be placed at the target boundary with adequate coverage of the tumor. The dose is specified by the isodose surface that just surrounds the target or the implant.
Localization of Sources• Orthogonal imaging method
Localization of Sources• Stereo-shift method A tabletop fiducial marker is used to serve as origin at 0 . Because the x, y coordinates of a point source or a source end can be obtained from either of the films, the z coordinates can be derived as following equation: Table
Surface Molds• Plastic molds are prepared to conform to the surface to be treated and the sources re-securely positioned on the outer surface of the mold. The distance between the plane of the sources to the skin surface is chosen to give a treatment distance of usually 0.5 to 1.0 cm.
Interstitial Therapy• In interstitial therapy, the radioactive sources are fabricated in the form of needles, wires, or seeds, which can be inserted directly into the tissue. There are basically two types of interstitial implants: temporary and permanent.
Intracavitary Therapy• Intracavitary therapy is mostly used for cancers of the uterine cervix, uterine body, and vagina. A variety of applicators have been designed to hold the sources in a fixed configuration. A cervix applicator basically consists of a central tube, called the tandem, and lateral capsules or "ovoids"
Dose specification• Cancer of the cervixPoint A• Defined to be 2 cm superior to the top of the lateral vaginal fornix and 2 cm lateral to the middle of the cervical canal.Point B• Defined to be 5 cm from the mid-line of the pelvic bony structure at the same level as point A.
Dose specificationLimitations of point A• Pt A relates to the position of the sources and not to a specific anatomical structure.• Dose to point A is very sensitive to the position of the ovoid sources relative to the tandem sources, which should not be the determining factor in deciding on implant duration.• Depending upon the size of the cervix, Point A may lie inside the tumor or outside the tumor.
ICRU dose specification• Prescription of the Technique Minimum information should include the applicator type, source type and loading and orthogonal radiographs of the application.
ICRU dose specification• Reference Volume: The reference volume is the volume of the isodose surface that just surrounds the target volume.
ICRU dose specification• Absorbed Dose at Reference Points
REMOTE AFTERLOADING UNITS• Ir-192 is the most commonly used radioisotope in remote afterloaders, although Cs-137 or Co-60 sources also are used in some units.
REMOTE AFTERLOADING UNITS• Advantages 1. The major advantage of the remote afterloaders is the elimination or reduction of exposure to medical personnel. 2. Well-designed systems can provide the capability of optimizing dose distributions beyond what is possible with manual afterloading 3. Treatment techniques can be made more consistent and reproducible.
REMOTE AFTERLOADING UNITS4. In LDR remote afterloading, sources can be retracted into shielded position to do better patient care under normal as well as emergency conditions.5. HDR remote afterloading permits treatment on an outpatient basis, using multiple fraction regimens.6. HDR remote afterloading is suited for treating large patient populations that would otherwise require prolonged hospitalization if treated by LDR brachytherapy.
REMOTE AFTERLOADING UNITSDisadvantages1. Remote afterloading devices are expensive and require a substantial capital expenditure for equipment acquisition.2. In the case of HDR, additional costs must be considered for room shielding (if not located in an existing shielded facility) and installing ancillary imaging equipment.3. Locating HDR in an existing radiation therapy room compounds the problem of patient scheduling unless the room is dedicated to HDR brachytherapy.
REMOTE AFTERLOADING UNITS4. No significant improvements are expected in clinical outcome over state-of-the-art conventional LDR brachytherapy, although the issue is still controversial and needs further investigation.5. Quality assurance requirements for remote afterloading devices are significantly greater because of the greater complexity of the equipment and frequent source changes.
Kerma, Exposure, and absorbed dose• Kerma: Kinetic energy released in the medium dEtr µ tr hν’’ k= = Ψ( ) dm ρ µtr Mass energy transfer :hν ρ coefficient e- Ψ: energy fluence hν’