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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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Manchester System for Cervix
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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Manchester System for Cervix
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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Manchester System for Cervix
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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Interstitial implant systems
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Interstitial implant systems
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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Prostate Brachytherapy
Outlines
Dose distribution
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Prostate Brachytherapy
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!!!