ch8. brachytherapy 近距离照射剂量学. why we use brachytherapy ？

Ch8. Brachytherapy

近距离照射剂量学近距离照射剂量学

Why we use Brachytherapy？

Brachytherapy is a method of treatment in which Brachytherapy is a method of treatment in which

sealed radioactive sources are used to deliver sealed radioactive sources are used to deliver

radiation at a short distanceradiation at a short distance ，， high radiation high radiation

dose dose ，， locally delivered to the tumor with locally delivered to the tumor with

rapid dose fall-off in the surrounding normal rapid dose fall-off in the surrounding normal

tissue.tissue.

What is Brachytherapy


There are mainly two types of brachytherapy

treatments:

Intracavitary, where the sources are placed in body

cavities close to the tumour volume.

Interstitial, where the sources are implanted within

the tumour volume.

Superficial brachytherapy gradually replaced by EB

Carcinoma treatment with brachyterapy

Endocavitary brachytherapy

Superficial brachytherapy

Interstitial brachytherapy

Uterus, vagina, lung

Skin

Prostate

远距离照射，其放射线的能量大部分被准直器、限束器等屏蔽，只有少部分能达到组织，近距离照射则相反，大部分能量被组织吸收

近距离照射，其放射源活度较小，治疗距离较短，约在5mm~5cm之间

远距离照射与近距离照射的基本区别远距离照射与近距离照射的基本区别

远距离照射，其放射线必须经过皮肤和正常组织才能到达肿瘤，肿瘤剂量受到皮肤和正常组织耐受量的限制，为得到高的均匀的肿瘤剂量，需要选择不同能量的射线和采用多视野照射技术

由于距离平方反比定律的影响，在腔内组织间近距离照射中，离放射源近的组织剂量相当高，距放射源远的组织剂量较低，靶区剂量分布的均匀性远比远距离照射的差，应注意靶区部分组织剂量过高或部分组织剂量过低的情况发生

远距离照射与近距离照射的基本区别远距离照射与近距离照射的基本区别

近距离治疗发展历史近距离治疗发展历史

Brachytherapy History• Early 1900’s

– radium molds for skin cancer

• Recently– Iridium 192 is used for removable prostheses in head and

neck cancers

• Old method:– “hot” radium seeds placed by hand, personnel exposed

• Current method:– afterloading technique where tubes are placed in tumor and

source loaded remotely


Brachy: Greek word for “short”→Fast (targeted) radiation treatment→Catheter-driven insertion system→breast, head, neck, prostate, esophagus, etc.

Typical sources Gamma & beta emitters Small: few mm3 Geometry: cylindrical (Iridium-192,

Palladium-103)


The physical advantage improved localized delivery of dose to the target volume of

interest.

The disadvantage can only be used in cases where the tumour is well localized

and is relatively small.

In a typical radiotherapy department about 10 to 20% of

all radiotherapy patients are treated with brachytherapy.

Brachytherapy applicationBrachytherapy application


Ultrasound Guided Implant Procedure

ChCh88. Brachytherapy. Brachytherapy

8.1 Radioactive Sources

8.2 Calibration of Brachytherapy Sources

8.3 Calculation of Dose Distributions

8.4 System of Implant Dosimetry

8.5 Computer Dosimetry

8.6 Implantation Techniques

8.7 Dose Specification: Cancer of The Cervix

8.8 Remote Afterloading Units

8.1 Radioactive Sources

3.5 mm

5 mm

1.1 mm

2 m

0.6 mm

88.1.1.1.1 Radioactive Sources Radioactive Sources

Brachytherapy sources are usually encapsulated and the capsule serves multiple purposes:

containing the radioactivity, providing source rigidity, and absorbing any α and, for photon-emitting sources, ra

diation produced through the source decay.

General considerationGeneral consideration


The useful radiation fluence from a brachytherapy source generally consists of:

- Gamma rays form the most important component of the emitted radiation,

- Characteristic x rays emitted incidentally through electron capture or internal conversion that occurs in the source, and

- Characteristic x rays and bremsstrahlung that originate in the source capsule.



The choice of an appropriate photon emitting radionuclide for a specific brachytherapy treatment depends on several relevant physical and dosimetric characteristics:

- Photon energies and photon beam penetration in tissue and in shielding materials.

- Half-life. - Half-value-layer in shielding materials such as lead. - Specific activity. - Source strength. - Inverse square fall-off of dose with distance from the source (this is the

dominant dosimetric effect because of very short treatment distances used in brachytherapy)



A. Radium-226

B. Cesium-137

C. Cobalt-60

D. Iridium-192

E. Gold-198

F. Iodine-125

G. Palladium-103

He Rn Ra 42

22286years 1600~

22688


3 m m ste e l c a b le

5.0 m m

0.6 m m

3.5 m m

1.1 m m

Ac tive Ir-192 C o re

8.1.1 Radioactive Sources

8.1.1 Radioactive Sources

“ Tiny” radioactive seeds, containing either 125I or 103Pd, are injected into the prostate with hollow needles.

The seeds deliver high doses of radiation within the prostate, resulting in less damage to surrounding healthy cells

The seeds are tiny canisters of titanium (4.5mm long x 0.8mm

diameter) that contain the radioactive isotope.

The canister prevents the radioactive iodine diffusing though the body and so affecting other organs.

產生方式：Radium-226 ：同位素衰變Cesium-137 ：核分裂Cobalt-60 ：中子活化 Cobalt-59

Iridium-192 ：中子活化 Iridium-191

Gold-198 ：中子活化 Gold-197

Iodine-125 ：中子活化 Xe-124 Xe-125 ， decay I-125

Palladium-103 ：中子活化、同位素衰變

88.1.1.2 Production of .2 Production of Radioactive Sources Radioactive Sources

88.1.1.3.3 Application of Application of Radioactive SourcesRadioactive Sources

使用方式：暫時性植入Radium-226 、 Cesium-137 、 Cobalt-60 、 Iridium-192

( 使用最廣泛者： Iridium-192 )

對於 Co-60 用於遠隔治療時， 10cm10cm 照野之回散射因子(BSF) 約為 1.036 。

永久性植入Gold-198 、 Iodine-125 、 Palladium-103

( 使用最廣泛者： Iodine-125 )


A. Specification of Source Strength1. Activity Definition ： A=-dN/dt

Unit: Bq, Ci 1 Ci = 3.7×1010 s-1


A.A.Specification of Source StrengthSpecification of Source Strength2. Exposure rate at a specified distance

8.2 Calibration of Brachytherapy Sources A.A.Specification of Source StrengthSpecification of Source Strength

2. Exposure rate at a specified distance


A. Specification of Source StrengthA. Specification of Source Strength


A. Specification of Source Strength3. Equivalent (effective) mass of radium

This conversion is simply made by dividing the exposure rate at 1 m by the exposure rate constant of radium (point source filtered by o.5 mm Pt) at 1 m.


A. Specification of Source StrengthA. Specification of Source Strength

4.Apparent activity

for a given brachytherapy source is defined as the activity of

a hypothetical unfiltered point source of the same

radionuclide that would give the same air-kerma rate in air

at a reference distance (typically 1 m) along the

perpendicular bisector of the source.

The SI unit of apparent activity is the becquerel (1 Bq = 1 s-

1), the old unit is the curie Ci, (1 Ci = 3.7×1010 s-1). The

apparent activity is sometimes called the equivalent activity.


放射源衰变率

(a)Cs-137 的年衰變率為？ ((t1/2=30y 2%/ 年 ) 經一年衰減後剩下的比例為則每年衰減掉的百分比為

(b)Ir-192 每天約衰減掉多少百分比？ (t1/2=74.2d) 經一天衰減後剩下的比例為則每天衰減掉的百分比為

%93.00093.0N

N1

9907.0N

N

0

1d 74.2d

0.693

0

e

tλ

0N

N e

%28.20228.0N

N1

9772.0N

N

0

1y 30y

0.693

0

e


Accurate measurements of radiation intensity

(energy fluence rate) at a specified point are p

ossible, and hence the reference air-kerma rat

e in air and the air-kerma strength （ AAP

M ） are now the recommended quantities for

specifying source strength.


reference air- kerma rate defined by the ICRU as the air-kerma rate in air, at a reference distance of one meter, corrected for air attenuation and scattering.


B. Exposure Rate Calibration


B. Exposure Rate CalibrationB.1 Open-air measurements


B.2 Well-type

ion chambers


• • Exposure rateExposure rate

– – Effect of inverse square lawEffect of inverse square law

• • Absorbed Dose in TissueAbsorbed Dose in Tissue

• • Modular Dose Calculation ModelsModular Dose Calculation Models

• • Isodose CurvesIsodose Curves


1.dosecalculation1.dosecalculation

1).1).点源周围剂量分布计算点源周围剂量分布计算

近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近

近近近近近近近近近近近近近近近近近近近近

2r

AX


2

sec

r

edx

L

AdI

t

deyL

AdII t

2

1

2

1

sec

2).2).空气中线源周围剂量计算空气中线源周围剂量计算


近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近 MeisbergerMeisberger 近近近近近近近近近近近近

近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近近

32 DrCrBrA 空气中吸收剂量

水中吸收量

2.2.组织中剂量计算组织中剂量计算


11 ）） . Exposure rate (. Exposure rate ( Effect of inverse square law) Effect of inverse square law)


22 ）） . Exposure rate (. Exposure rate ( Effect of inverse square law) Effect of inverse square law)

近近近近近 cm內的劑量能符合距離平方反比關係的點射源： Ir-192, Au-198


3.Modular dose calculation models3.Modular dose calculation models


4.Isodose4.Isodose curve curve


组织间治疗亦称为插植治疗，是根据靶区的形状和范围，将一定规格的多个放射源，按特定的排列法则，直接插植入肿瘤部位，以期在肿瘤部位产生高剂量照射。为了使治疗部位获得满意的剂量，必须根据放射源周围的剂量分布特点，按一定的规则排列放射源，多年来许多物理学家致力于这方面的研究，建立了一些为临床所能接受的剂量学系统和治疗法则。


组织间插植治疗的剂量学体系，包括：组织间插植治疗的剂量学体系，包括：布源规则布源规则剂量学参数描述剂量学参数描述参考点剂量计算参考点剂量计算


A. The Paterson-Parker System

The Paterson-Parker system was developed to deliver uniform dose (within ±10%) to a plane or volume.

For plane implant: definition of treatment plane: the enclosed area of ulterior sources.

the arrangement of sources depend on how much area (volume) we wanted to treat




For plane implant:

平面插植治疗时，周边源与中心区源的活度比例随辐射平面的面积而定。面积小于 25 平方厘米时，边源为总源量的 2/3 ； 25-100

平方厘米，周边源占总源量的 1/2 ；治疗面积大于 100 平方厘米时，

周边源占总源量的 1/3 。放射源相互平行，间距不宜大于 1 厘米。在源两端与其它源的活性

区间隔也应在 1 厘米之内，源彼此间在插植平面上封闭。




For volume implant: 治疗厚度和靶区较大的体积治疗厚度和靶区较大的体积

布源规则：柱状、球状、多面体

源量的分配、源间距及源的数量、体积大小、剂量计算


B. The Quimby System

The Quimby system is characterized by a uniform distribution of source of equal linear activity.


C. The 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.

D. The Paris System

The Paris system is intended primarily for removable implants of long line source. The system prescribes wider spacing for longer sources or larger treatment volumes.


D. The Paris System

布源规范：布源规范：

放射源为直线源，相互平行，各源相互等间隔，排列成正方形或等腰三角形，源的活度均匀等值，线源与过中心点的平面垂直。


表 7-3 巴黎系统的组织间插植布源方式

布源间距：单平面布源 2 列源情况，间距 E = 2t

3 列源情况，间距 E = 3t

双平面布源正方形排布情况，间距 E = 0.92t ～ 0.64t

等边三角形排布情况，间距 E = 0.25t ～ 0.29t

不论采用何种布源方案，允许的最小源间距为 5mm ，最大源间距为 20mm 。

靶区宽度：单平面插植时，计划靶区比最外缘铱线源之间宽出 0.3E 。

线源长度选取：线源长度尺寸应比靶区长度放宽 20% -- 30% ，即等于 1.33 ～ 1.54 倍的计划靶区长度。


D. The Paris System

布源规范：布源规范：

放射源为直线源，相互平行，各源相互等间隔，排列成正方形或等腰三角形，源的活度均匀等值，线源与过中心点的平面垂直。

源尺寸与布局随靶区大小的对应关系：源间距、源尺源尺寸与布局随靶区大小的对应关系：源间距、源尺寸取决于靶区厚度，体积。寸取决于靶区厚度，体积。


D. The Paris System

基准剂量（基准剂量（ BDBD ）和参考剂量（）和参考剂量（ RDRD ））：：

基准剂量率：中心平面各源之间的剂量均值。单平面插植基点选择在两源连线中点。正方形布源，选择在四边形对角线交点。三角形布源选择在各边中垂线交点。

参考剂量率 RD ＝ 0.85BD ，总照射时间：

T ＝ Dt/RD×k 衰变 ×k 空气 / 组织剂量校正因子

N

1

/BDi

i NBD＝

理想的布源条件：理想的布源条件： 0.9BD≤BDi≤1.1BD0.9BD≤BDi≤1.1BD

88..55 Intracavitary therapyIntracavitary therapy

8.5 Intracavitary therapyIntracavitary therapy

8.5 Intracavitary therapyIntracavitary therapy Dose Specification: Cancer of The Cervix

The Manchester System A 點之位置是在子宮頸 (cervix) 口往上 2 公分、往兩側 2 公分的地方。通

常用 A 點的劑量來表示 brachytherapy 所給的劑量。

B 點之位置是在 cervical o.s. 往上 2 公分、往兩側 5 公分的地方，但較少用。 Bladder point and Rectum point.

因為每個人的腫瘤、照射劑量均不同，在討論 brachytherapy 給的劑量時，會取基準點 A 、 B 作參考。

The maximum dose to bladder and rectum should be less than the dose to point A.

Dose Specification: Cancer of The Cervix


Cervical Ca TargetsCervical Ca Targets


Determine dose and fractionation Determine applicator Determine dwell positions Determine optimization scheme Establish quality management


The treatment usually has external beam treatments to about 44 - 50 Gy at 1.7 - 2.0 Gy/fraction.

Total treatment to about 100 - 110 Gy10.Typical HDR regimen is 5 fractions of 5.5 Gy.Chemotherapy strongly affects both normal

tissue and tumor reaction.


Determine dose and fractionation Determine applicator Determine dwell positions Determine optimization scheme Establish quality management


Cervical Ca TargetsCervical Ca Targets

Tandem and Cylinders

Because of the nature of Because of the nature of the anisotropy, this the anisotropy, this maximizes the relative maximizes the relative contribution to the bladder contribution to the bladder and rectum per dose to and rectum per dose to cervix, and usually cervix, and usually prevents adding distance prevents adding distance to those organs.to those organs.

ShortShortdistancedistance

0.5cm0.5cm

PoorPoorDepth doseDepth dose

PoorPoorContributionContribution

Tandem & Ring Geometry

Simple but complex geometry Ring diameter

Ring + Cap diameter

36mm, 40mm, 44mm

constant 6mm source to surface

Tandem Angle 30°, 45°, 60°

2cm, 4cm, 6cm, 8cm

Tandem & Ring Geometry

Fixed geometry - tandem fixed in center of ring

Choose combination according to anatomy

Dosimetry needed only for 1st fraction?

Adapt fraction to fraction if needed

Dosimetry Methods-Tandem

Dose optimization points are tapered along the tandem axis

12mm, 14mm, 16mm, 18mm,20mm down to level of Point A

Dwell locations down to ring

Dosimetry Methods-Tandem

Tandem length will affect the dose around Point A more tandem dwells,

less relative contribution from ring dwells

goal percentage 100%, optimized 90-110%

Dosimetry Methods-Ring

Dwell locations are specified as part the prescription 4, 5, or 6 dwells to a side

Dose optimization points are placed radially at 6mm non radial placement means

different depths and not on ring surface

Ao

Af

XX

X

Ao

Af

2 cm2 cm

2 cm2 cm

X

raduis

VdVs

0.5 cm

Tandem and Ovoids

1. Determine dose and fractionation.

2. Determine applicator

3. Determine dwell positions

4. Determine optimization scheme

5. Establish quality management

HDR and LDR T&O

LDR HDR

Duplicate the LDR Source Distribution with HDR Dwell Weights?Duplicate the LDR Source Distribution with HDR Dwell Weights?

Can we? Certainly, and a lot of work was done to do this well in the late 1980s.

Should we? Absolutely not! Duplicating the physical distribution does not

duplicate the biological distribution because BED depends on dose/fraction.

Fails to give the patient the benefit of optimization.

Selecting Dwell PositionsUnused (Spacing)

Add spacing in tip to protect bowel. Load tandem to about mid-ovoid. Ovoid use dwells 2-8.

Dwell 1 irradiates rectum. Dwell 9 irradiates bladder.

A Sample of Optimization

X

2 cm

X

1.8 cmX

X

XX

XX

Unused (Spacing)

1 cm

2 cm Pt. A

Tip dwells to variable for optimization

Tandem dwell inferior to Pt. A hard to specify

Need to place points for ovoids

A Sample of Optimization

Optimization SchemeOptimization Scheme

Specify relative doses to the optimization points (e.g., 100% tandem points, 125% ovoid points with chemo - depends on Pt A Dose)

Use optimization on dose points,Distance optimization.Minimize the dwell gradient weighting factor.

Quality Management

Things to check:

1. Dose specification (right dose - right point)

2. Applicator (right geometry)

3. Dose distribution (right doses - right places)

4. Normal Tissue doses (in tolerance)

5. Correct programming (right source movement - right catheter)

Physicist’s Worksheet for Tandem and Ovoids

HDR DOSIMETRY CHECKTreatments Using Tandem and Ovoids

Check in the box indicates parameter is correct.

Date: MR#: Patient: Fraction No. of Disease & Stage: Dose/Fraction from protocol:

1. Location and Dose Checks____ a. Dose for this fraction on Rx __________ Gy Average dose to applicator points __________ Gy____ b Difference between right and left A and prescribed dose is less than 5%____ c Distance of Point A (Starting from midovoid line)

Distance cephalad as defined in Rx ________ mm Distance cephalad on films ________ mmDistance lateral as defined in Rx ________ mm Distance lateral on printout _______ mm

Distance lateral on coronal plane _______ mm____ d Ovoid cap sizes

Rt Visible marker ______ Rt size ________ mm Rt Distance to vaginal dose points ________ mmLt Visible marker ______ Lt size ________ mm Lt Distance to vaginal dose points ________ mm

____ e Dose percentile to vaginal surface ___________ % of Rx dose = __________ Gy and isodose lineson the plan fall on the vaginal surface

____ f Starting dwell for tandem on plan corresponds to start indicated on film Dwell #:_________________ g Bladder ____________ Gy ( ____________%) Rectum _____________ Gy ( ____________%)

__________ Physician alerted if > 70%2. Time Checks:____ a Time index for dwell 1 cm from first dwell Index ________ Posted range ______ to __________ b Time index for total time Index ________ Posted range ______ to __________ c Total Time Index from previous treatmentIndex ________ Agree within 5%3. Program Transfer Check____ a Morning QA length

Rt. ovoid programmed to channel 1 Length for this channel Lt. ovoid programmed to channel 2 Length for this channel Tandem programmed to channel 3 Length for this channel

____ b Step size ________ 2.5mm ________ 5.0mm____ c Patient’s file has been saved.4. Programming of the HDR Unit____ Dwell times, positions, length and step size on print out match that from the computer planningAll the appropriate checks above prove satisfactory

Checking Physicist Time Date


The ICRU System Bladder Point. Localized by using a Foley catheter, which the balloon filled

with a contrast material.

Rectal Point. Identified on the frontal radiograph at the midpoint of the ovoid sources.

Lymphatic Trapezoid of Fletcher. These points correspond to the paraaortic and iliac nodes and are show in Figure 15.28.

ICRU在其 38号报告中力图使宫颈癌的放射治疗规范化，以便不同的放射治疗中心对宫颈癌的腔内放射治疗具有统一、规范的、准确的剂量学描述。 ICRU在其 38号报告中定义了参考体积，即参考等剂量面包罗的体积。参考剂量对低剂量率（ 0.4-2Gy/h）治疗为 60Gy；对高剂量率（＞ 12Gy/

小时）为相应的（ <60Gy）等效值。参考体积的长（ dl）、宽（ dw）、高（ dh）可以由模拟定位正侧位片确定。



相关的重要器官的参考点，主要有膀胱和直肠的剂量参考点。沿膀胱中心与阴道容器连线，过膀胱后表面一点为膀胱受量的参考点。宫腔源后端点（或阴道源中心）与阴后壁的垂直线，距阴道后壁0.5cm的位置为直肠受量参考点。

淋巴引流区和盆壁剂量参考点，Fletcher梯形平面，用以确定右、左腹主动脉旁（R.Lpar

a ）、骶髂联合旁（R.Lcom）、髂外（R.Lext）的淋巴引流区和左右盆壁的剂量参考点（R.Lprw）。


Preloading( 前荷治療 ):以人工方式將核種置入病患體內。

Afterloading( 後荷治療 ):先用人工方式將導管置入病患體內，再將核種經導管置入病患體內。Manually afterloading: 手動式後荷治療Remotely afterloading (RAL): 遠隔式後荷治療

( 以機器控制射源 )


Low dose rate (LDR):Dose rate 0.4~2 Gy/hr 。

Ex: A single Ir-192 source of activity in the range of 0.5 to 1Ci is used.

Medium dose rate (MDR):Dose rate 2~12 Gy/hr 以上。

High dose rate (HDR):Dose rate 12 Gy/hr 以上。

Ex: HDR remote afterloading implants are achieved by moving a single high strength (e.g. 10 Ci) Ir-192 source.

88..66 Remote Afterloading Units Remote Afterloading Units

ch8. brachytherapy 近距离照射剂量学. why we use brachytherapy ？

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