18 chap 15 brachytherapy 2006-khan 葉
TRANSCRIPT
Brachytherapy
林口長庚醫院 放射腫瘤科
葉健一
Brachytherapy
Brachytherapy is a treatment method that uses sealed ra
diation sources placed close to tumor tissue. Taken from
“brachys,” the Greek word for “near”. Brachytherapy deli
vers a dose of radiation usually to within a few inches of
a diseased area or tumor. It allows for a high dose of radi
ation to reach the tumor while adjacent healthy tissue rec
eives low or reduced dose levels.
Radioactive Sources
Radioactive Sources
• Ra-226
• Cs-137
• Co-60
• Ir-192
• Au-198
• I-125
• Pd-103
Ra-226
• It was discovered in 1898. It is the sixth member of the uranium seri
es, which starts with and ends with stable .
Ra-226 decay
• Radium disintegrates with a half-life of about 1,600 years to form radon.
• 226Ra(88) ----------> 222Rn(86) + 4He(2)• 49 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-dec
ay 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 requir
es less shielding and is less hazardous in the microspher
e form.
• With a long half-life of about 30 years.
• Cs-137 is used as a radium substitute in both interstitial
and intracavitary brachytherapy.
Cs-137 vs. Ra-226
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
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 c
apture 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 titaniu
m end welds.
I-125
• For permanent interstitial implants
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 fa
ster dose rate
• 4.5 mm long and 0.8 mm in diameter
• Shorter half-life 17 days
Summary of source
Calibration of brachytherapy source
• Activity
mCi (1 Ci = 3.7 1010 disintegrations/sec)
A = ΔN / Δ t = -N
• 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 brac
hytherapy 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 dividin
g the measured exposure rate at 1 m with the exposure r
ate 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 squa
re of the distance between the calibration point and the center of so
urce. 2lKS lk
air kerma rate at distance l
-12hGym -1GyhDistance l
2m
air kerma strength.
Air kerma strength
2lKS lk
/
/
en
tr
e
WXK
Ave. mass transfer coeff
Ave. mass absorption coeff
2
e
WXS lk
= 1 because No bremsstralung,
Gy/R)m 10 x (8.76 (R/h) 23 XSk
Exposure rate measurement at 1 m
Gym2h-1
Air kerma strength
• Sk(U) = air Kerma source strength (U)
Conversion factors from mCi:
Ir-1921 mCi = 0.238 U
I-125 1 mCi = 0.787 U
Pd-103 1 mCi = 0.773 U
Exposure rate calibration
• Open-Air measurement – time-consuming measurement
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
Br = 1 + ka() Kb
Dr = Bre -
Calculation of dose distribution
Attenuation & scattering effect in water
1% per cm
Normalized at 1cm
SourceDose point
Water
Calculation of dose distribution
• Modular dose calculation models
D(r,) = A [G(r, )/G(r0,0)] g(r) F(r,) A = apparent activity
= dose rate constant (a unique value for each radioisotope)
G(r,) = 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,) = anisotropy factor
Isodose curve
lsodose 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
Paterson-Parker system
Quimby System
• The Quimby system of interstitial implantation is charact
erized by a uniform distribution of sources of equal linear
activity. Consequently, this arrangement of sources resul
ts in a non-uniform dose distribution, higher in the central
region of treatment.
Memorial system
• The Memorial system is an extension of the Quimby syst
em and is characterized by complete dose distributions a
round lattices of point sources of uniform strength space
d 1 cm apart.
Paris system
• The Paris system is intended primarily for removable imp
lants 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 cove
r 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 wit
h adequate coverage of the tumor. The dose is specified by the isod
ose 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
IMPLANTATION TECHNIQUES
• Surface Molds
• Interstitial Therapy
• Intracavitary Therapy
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 fo
rm of needles, wires, or seeds, which can be inserted directly into th
e tissue. There are basically two types of interstitial implants: tempor
ary 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 design
ed to hold the sources in a fixed configuration. A cervix applicator ba
sically consists of a central tube, called the tandem, and lateral caps
ules or "ovoids"
Dose specification• Cancer of the cervixPoint A• Defined to be 2 cm superior to the top of the lateral vaginal fornix an
d 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 t
he same level as point A.
Dose specification
Limitations 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
ICRU dose specification
• Prescription of the Technique
Minimum information should include the applicator type,
source type and loading and orthogonal radiographs of t
he 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.
4. In LDR remote afterloading, sources can be retracted into shielded
position to do better patient care under normal as well as emergenc
y 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 populatio
ns that would otherwise require prolonged hospitalization if treated b
y LDR brachytherapy.
REMOTE AFTERLOADING UNITS
REMOTE AFTERLOADING UNITS
Disadvantages
1. 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 shi
elding (if not located in an existing shielded facility) and installing an
cillary 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.
4. No significant improvements are expected in clinical outcome over st
ate-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 equipm
ent and frequent source changes.
REMOTE AFTERLOADING UNITS
Kerma, Exposure, and absorbed dose
A. Kerma: Kinetic energy released in the medium
e-h
h’
)(trtr
dm
dEk
: energy fluence
:tr Mass energy transfer
coefficient
h’’
K = Kcol + Krad
e-h
h’
h’’
e-h
h’
h’’
)(encollK
Exposure and Kerma
dm
dQX
)W
e(*)(KX air
col+
+
+
++
+
- - -
- - -
+
-
Calculation of absorbed dose from exposure
A. Absorbed dose to air under charged particle equilibrium
( limit to photon energy up tp C0-60)
)()(876.0)( RXR
rad
e
WXKD air
colair