18 chap 15 brachytherapy 2006-khan 葉

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


• 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


- 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.


• 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


• Exposure rate


• Absorbed dose in tissue

Br = 1 + ka() Kb

Dr = Bre -


Attenuation & scattering effect in water

1% per cm

Normalized at 1cm

SourceDose point

Water


• 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


• If the ends of the implant are uncrossed, the area should reduced by 10 % for each uncrossed end.


• 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 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


• Prescription of the Technique

Minimum information should include the applicator type,

source type and loading and orthogonal radiographs of t

he application.


• Reference Volume:

The reference volume is the volume

of the isodose surface that just

surrounds the target volume.


• 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.


• 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.



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.


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

Technology

18 chap 15 brachytherapy 2006-khan 葉