1.wlj brachy 3.pdf

8/22/2019 1.WLJ Brachy 3.pdf

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Christie HospitalNHS Foundation Trust

NHS

Brachytherapy andRadioactive Sources

Lecture 3

Wendy Julian

Christie Physics and [email protected]


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Introduction

Lecture 3: Clinical applications of brachytherapy

Gynaecological treatments

Interstitial implant systems

Use of HDR micro-Selectron

Prostate treatments


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Gynaecological treatments


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Manchester System for Cervix


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Activity distribution rules 1955

Modified for Cs137 afterloading

Modified again for HDR/PDR

Prescription convention forCervix treatmentspoint A

Standard treatment times canbe used for standard geometry


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Point A

2cm along uterine canal

(from os) 2cm lateral

In paracervical triangle,

area of broad ligament

near to uterine vesselscrossing the ureter


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Non-Ideal Geometry

Point A 2cm along

uterine canal & 2cm

lateral Point B 5cm lateral to

patient mid-line

(Indicates dose at pelvic

wall/obturator node)


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Manchester System for Gynae

Intra-uterine tubes (various lengths)

Ovoids (various sizes)

Vaginal cylinders (if ovoids are not suitable)

Arrangements of (MDR) Selectron pellets were set to

replicate preloaded sources (Ra226

, Cs137

)

HDR or PDR dwells have replaced the Selectron


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Manchester System for Gynae

Dose rate at pt A

Manual sources ~ 55 cGy/hr

Selectron (MDR) ~ 160 cGy/hr

HDR ~ 400 cGy/hr


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Radiobiological Effects

Allowance must be made for the dose-rate

When moving to higher dose-rate, a dosereduction is usually required

Eg. When Selectron replaced manual Cssources, dose-rate ~ 3 x greater, leading to areduction of 10% for prescribed doses in

order to give same radiobiological effect

E.g. Moving from Selectron LDR treatmentsto fractionated HDR treatments


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Manchester System for Uterus

Longer intra-uterine tubes (up to 12cm)

Heavier weighting of activity at distal end of

the uterus

Nominal body dose average of dose at2cm lateral to the uterine tube


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Manchester System for Vagina

(post hysterectomy)

Transverse ovoids, or

Single applicator, cigar shaped treatment isodose

For disease extending further down the vagina

Sources spread out to reduce dose rate at vaginalmucosa

Prescribed at maximum vaginal mucosa dose point


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Brachytherapy ICRU 38 (1985)

The data recommended for reporting of

gynaecological brachytherapy are:

- Description of technique (source, applicator) - Total reference air-kerma (TRAK)

- Time-dose pattern

- Description of reference volume

- Dose at reference points(bladder, rectum,

lymphatic trapezoid, pelvic wall)


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Brachytherapy ICRU 38

Requirement to specify

the dimensions (width,

height, and thickness)

of the pear shapedisodose reference

volume


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Brachytherapy ICRU 38

Bladder on posterior surface of balloon catheter

Rectum from the centre of the vaginal sources, drop a verticaltowards the rectum (use gauze packing to identify vaginal posteriorwall)


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Brachytherapy GEC ESTROGroup Europen de Curiethrapie

European Society for Therapeutic Radiology and Oncology

2005 - European guidelines

recommend full image guided 3D

dosimetry and prescriptions, usingDVH analysis for target coverage

and organs at risk

2009 - RCR published guidelines

for implementation

CT imaging is widely used, but MR

is superior for tumour delineation


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Brachytherapy CT or MRI?

MR good soft tissue

detail

Conventional

CT


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Brachytherapy CT/MR fusion


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*** QUESTION! ***

The dose at 1cm depth in tissue from surface of an

ovoid, expressed as a percentage of the surface

dose, is:

A) Greater for large ovoids

B) Greater for small ovoids

C) Independent of ovoid size


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A set of rules to determine source arrangement andtreatment time

Manchester system Devised for Radium and Caesium needle implants

Tabulated data

Paris system Devised for Iridium wire implants (manual afterloading)

Widely used in Europe System introduced in 1960s with early planning computers


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An iridium wire implant

Requires manual afterloading


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Target length (l)

Needle Separation (s)

Between 5 and 20mm,

depending on target volume

Paris system rules

Overall source lengthshould exceed

target volume by

20% (long wires)

30% (short wires)


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Paris System rules

Uniform activity per unit length

Same activity and length

Straight and parallel

Equal spacing


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Paris System rules

Single plane implant - cross-

section view

A thicker target needs a multi-plane implant

Cross-section must be a

series of equilateral trianglesor squares


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Paris system rules

Treatment volume

Defined by minimum dimensions of reference isodose

Length, l

Thickness, t


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Wires Paris System

Basal dose points =local dose minima in theimplanted volume

Average of basal dosepoint dose rates is theBasal Dose Rate

Reference Dose Rateis 85% of BDR usethis to calculatetreatment time


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Wires Paris System calculation

Calculate

Dose rate at each basal dose point

Mean dose rate for basal dose points (mean BDR) x 0.85 to give reference dose rate

dose_ratereference_

_doseprescribed

timetreatment_


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Wires Paris System example

A single plane iridium wire implant has

4 wires, each 40mm long, separation

12mm. From tables, dose rate ateach basal dose point is 0.415, 0.438

and 0.415 Gyh-1. Calculate the time

to give a treatment of 60Gy


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Wires Paris System example

Mean basal dose rate = 0.423 Gyh-1

Reference dose rate = 0.423 x 0.85= 0.359 Gyh-1

Treatment time = 60/0.359

= 167 hours


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Brachytherapy ICRU 58(1997)

The dosimetry information recommended for reporting ofinterstitial implant treatments consists of:

Description of clinical target volumes.

Sources, technique and implant time.

Total reference air-kerma (TRAK)

Description of dose: prescription point/surface, prescription dose,reference doses

Greater detail is recommended only 3D computer calculation,including dose volume histograms, can fully comply


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*** QUESTION! ***

Which of these are features of Paris system??

1) Crossed ends?2) All wires parallel?3) Higher activity around periphery?4) Wires longer than treatment length?

5) Dose prescribed to Basal Dose point?6) Dose prescribed to reference isodose?


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Question

Does this implant comply with Paris rules?


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Use of HDR micro-Selectron


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HDR micro-Selectron

Single 10 Ci Iridium-192

source welded to a steel

transfer cable

Machine has shielding

built-in

Machine can be portable


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Dose distributions HDR source

Dose distribution around the

HDR source is approximately

spherical

(almost a point source)

e.g. For MammoSite breast

treatments, a single dwell

position is used to treat aspherical shell around a

balloon inside a lumpectomy

site


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HDR micro-Selectron

For most treatments, a different shape of dose

distribution is required


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HDR micro-Selectron

The single is source driven along by a cable

Dose distribution made up by adding contribution from

several dwell positions


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HDR micro-Selectron

A series of HDR dwell

positions can be used to

simulate a line source

Used for intraluminal lung

treatment


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HDR micro-Selectron

Marker wire shows

position of catheter for

ILT treatment

Radio-opaque markersat 1cm intervals

Use markers to select

dwell positions for

HDR treatment


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HDR micro-Selectron

HDR dwell positions can be combined to treat a skin

lesion, using a superficial mould


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HDR micro-Selectron Surface Mould

Catheters held in surface applicator Dwell positions chosen to provide desired dose

distribution


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HDR micro-Selectron Surface Mould

Catheters are connected to HDR unit

Depth of treatment depends upon height of dwells above skin

1cm dwell height treats depth 2-3mm, depending on area


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HDR & PDR QC

QC checks following quarterly source

replacement

Source calibration Positional checks with autoradiograph

Timer checks

Power failure checks Interlock checks

Leak test


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Brachytherapy - HDR Calibration

Autoradiograph

To check accuracy ofsource dwell positions

Catheter taped to film

Marker wire inserted intocatheter (as for lungtreatments)

Pin pricks/marks made onfilm

Treat between the marks& check coincidence


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Prostate Brachytherapy


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Prostate Brachytherapy - HDR


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Prostate brachytherapy - I125 seeds


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Prostate brachytherapy

Trans-rectal ultrasound is used to image

the prostate


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Prostate brachytherapy

The planning process

Outline prostate, bladder,rectum and urethra

Locate possible needlepositions

Plan final sourcepositions

Calculate dosedistribution


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Outlines

Dose distribution


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Sagittal views


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Post-implant dosimetry

for prostate seed implants

Take images 6 weeksafter treatment

Reconstruct actual sourcepositions in 3D

Calculate actual dosedelivered or generateactual dose distribution


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The End!!!

1.wlj brachy 3.pdf

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