Brachytherapy Physics II
Yoichi Watanabe, Ph.D.
Office: Masonic Memorial Building M10-M
Telephone: (612)626-6708
E-mail: watan016@umn.edu
Spring Semester
Outline
1.
2.
3.
4.
5.
6.
7.
Radioactive Sources
Dose Calculation
Implant dosimetry systems
LDR Interstitial and Intracavitary
HDR/PDR
New Techniques
QA and radiation safety
Implant dosimetry systems
Manchester (or Paterson-Parker) system
(1934-1938)
 Quimby system (1935-1941)
 Memorial system, nomogram (1960-2000)
 Paris system (1960s)
 Computer aided system (1970s?-)
 Plan evaluation tool – DVH etc.

Ref: Brachytherapy Physics, AAPM Summer School 1994 and 2005
Common assumptions for
Manchester and Quimby systems




Both systems were originally developed for
Radium sources, which were unfiltered line or
point sources. There are tables for 0.5 mm and
1.0 mm Platinum filtered sources.
Oblique filtration was not taken into account.
Photon scattering and attenuation in tissue were
ignored.
Should be used for radioactive sources emitting
photons of energy greater than 200 keV.
Manchester System
(Patterson-Parker)



An implant planning system designed to deliver
uniform dose within ±10% to a plane or volume.
Crossing needles are used. The system gives the
total activity in mgRa-equivalent*hr required to
deliver 1000 R at the prescription point.
Planar implant: the uniform dose is achieved in
parallel planes at 0.5 cm from the implant plane
and within the area projection of the peripheral
needles on that plane. The stated dose is 10%
higher than the minimum dose.
Volume implant: Needles (or sources) are
implanted to cover a volume. The prescribed
dose is 10% higher than the minimum dose within
the implanted volume.
Manchester:
Planar Implant Table
Johns and Cunningham, Physics of Radiology 4th ed. (1983) Table 13-4.
Example:
Single plane implant - Manchester
•
4 cm
•
•
•
3 cm
•
a b
c d e
•
•
Source plane
The activities of a and e are 3 mgRa-eq.
The activities of b, c and d are 1 mgRa-eq.
The area of implant, A = 3 x 4 = 12 cm2.
No crossing source at both ends. Hence, the
area of implant must be reduced by 20 %. A =
12x0.8 = 9.6 cm2.
To give 1000 cGy at 0.5 cm from the source
plane, the total mgRa*hour = 244 mgRa*h.
Total loading time is 244/9 = 27.1 hours.
The exposure rate is 1000 /27.1 = 36.9 R/h.
Rule of Patterson-Parker system:
Treatment area
0.5 cm
Area
Peripheral
< 25 cm2
2/3
25 to 100 cm2
1/2
> 100 cm2
1/3
Patterson-Parker: Volume Implant Table
mg·hr to give 10Gy to volume implant: Ra-eq for 0.5-mm Pt filtration
Volume [cm3]
mg hr
Distribution Rules
5
106
Volume should be considered as a surface with 75% activity and core with 25%
10
168
15
220
20
267
30
350
Belt: 50% activity with minimum 8 needles
40
425
Ends: 12.5% of activity on each end
50
493
Core: 25% with minimum of 4 needles
60
556
For each uncrossed end, reduce volume by 7.5%
80
673
100
782
140
979
180
1156
220
1322
260
1479
300
1627
340
1768
380
1902
Rules for cylinders
Length/Diameter =
1.5
2.0
2.5
3.0
Increase mg hr =
3%
6%
10%
15%
Johns and Cunningham, Table 13.8 in page 477
Example:
Volume implant - Manchester

A dose of 7000 cGy is planned from a permanent implant of Au-198
seeds to a roughly spherical volume of 4 cm in diameter. What total
source activity of Au-198 will be needed?
1.8 cm
1.2 cm
2 cm
 The volume of implant V=4/3π23=33.5 cm3.
 From the table, the total activity A = 376.3 mgRaeq*hr for 1000 cGy dose.
 For 7000 cGy, the activity should be
376.3x7000/1000=2634 mgRa-eq*hr.
 The half-life of Au-198 is 2.7 days.
 The exposure rate constant of Au-198 is 2.35
Rcm2mCi-1h-1.
 The total activity in mCi*hr= 8.25/2.35*2634=9247
mCi*hr.
 The total activity needed is 9247/(1.44*2.7*24)=99.1
mCi.
 74.3 mCi is placed at 1.8 cm (rind) from the center of
the volume. The rest (24.8 mCi) is at 1.2 cm (core).
Quimby System





The system assumes a uniform distribution of
sources of equal linear activity.
This implant leads to a non-uniform dose
distribution.
The Quimby tables give the mgRa-equivalent*hr
to produce 1000 R (or 1000 cGy).
Planar implant: The stated dose is given in the
center of the treatment plane up to 3 cm from
the plane. The stated dose is the maximum dose
in the plane of treatment.
Volume implant: the stated dose is the minimum
in the implanted volume.
Quimby-like Implant

Ir-192 seeds with
equal activity were
implanted on a
plane. The figure
shows the dose
distribution
calculated by
computer.
Quimby Volume Implant Table
Anderson,LL, AAPM Summer School 1994, page 311
Example:
Volume implant - Quimby

A dose of 7000 cGy is planned from a permanent implant of Au-198
seeds to a roughly spherical volume of 4 cm in diameter. What total
source activity of Au-198 will be needed?
1.8 cm
1.2 cm
2 cm
 The volume of implant V=4/3π23=33.5 cm3.
 From the table, the total activity A = 607 mgRa-eq*hr for
1000 cGy dose.
 The Quimby table is for the minimum dose.
 Since 7000 cGy is 10% larger than the minimum dose for
the Manchester system, the prescription dose for Quimby
is 7000x0.9=6300 cGy.
 For 6300 cGy, the activity should be 607x6300/1000=3824
mgRa-eq*hr.
 The half-life of Au-198 is 2.7 days.
 The exposure rate constant of Au-198 is 2.35 R cm2 mCi-1
h-1.
 The total activity in mCi*hr= 8.25/2.35*3824=13425 mCi*hr.
 The total activity needed is 13425/(1.44*2.7*24)=143.9
mCi. (45% more activity than the Manchester system.)
Memorial System





An extension of Quimby system (uniform
loading).
The dose distributions around lattices of point
sources of uniform strength spaced 1-cm apart.
Planar implant: the prescription is given at the
minimum peripheral dose and reference
maximum dose points in the plane 0.5 cm from
the source plane.
Volume implant: additional points, “central line
peripheral dose points”, are chosen.
The Memorial system is available as a form of
Nomogram for specific implants, I-125/Pd-103
permanent and Ir-192 LDR temporary implants.
Dimension Averaging Technique




The average dimension (the average of three lengths) is
the “size”: <d> =(a+b+c)/3.
For permanent implants (I-125 or Pd-103), the total source
strength, A, may be proportional to the average dimension
of the target.
The technique leads to a lower dose to larger target
volume. (The larger target means more normal tissue
volume irradiated. Hence, the dose to target should be
smaller.)
In early 1990s, the formula was modified so that the total
source strength is a power function of AD.
A= K d
s
Note: K=5, s=1 for I-125 implants
Matched Peripheral Dose (MPD)





The volume of ellipsoid, V, is by measuring three
orthogonal dimensions a, b, and c of the target. (i.e.,
V=π/6·a·b·c)
For an implant, the dose-volume relationship is obtained.
On can use the calculated dose distributions to compute
the volume of each isodose level by measuring the three
dimensions of isodose surface or by computing DVH.
The matched peripheral dose (MPD) is the isodose
surface, which encloses the same volume as the
measured target volume, V.
MPD is not the prescribed dose..
MPD is an alternative concept to evaluate the coverage
of the target volume for brachytherapy.
MPD (cont)
120
%volume
100
V
80
60
40
20
MPD
0
0
50
100
150
Dose [Gy]
200
250
Nomogram



Nomograms are constructed for quick estimation
of the number of seeds and the spacing
between seeds for planar and volume implants.
The system takes into account the elongation
factor for the implant shape (cylinder and
spheroid).
There are nomograms for Ir-192 (temporary
LDR), I-125, and Pd-103 (permanent).
Nomogram: two-plane implant




Temporary Ir-192 implant.
Active length = 10 cm.
Width = 8 cm.
Treatment plane is 0.5 cm
from the source plane
 The peripheral dose rate
(at reference point on the
treatment plane) is 10
Gy/day
 The seed strength must be 0.5 mgRa-eq.
 The number of Ir-192 ribbons is six.
Nomogram: I-125 interstitial implant
 Nomograms are
used for permanent
prostate implants
using both I-125 and
Pd-103.
 3x3x1.5cm cylinder target.
 Elongation factor=2
 0.4 mCi/seed and 2.5 cm
average dimension.
 0.7 cm spacing between
seeds.
 32 seeds, 12.8 mCi
Ref: L.L.Anderson, Med Phys 3:48 (1976)
Paris System




Intended for temporary implants of long line
sources such as Ir-192 wires.
Assumes a uniform linear activity and an implant
in parallel lines.
The dose is specified on “reference isodose”
surface, i.e., 85% of the basal dose. The basal
dose is defined as the average of the minimum
dose between sources.
There is no table for Paris system, but dose
calculation formulas are available.
Ref: Piergquin, B and G.Marinello, A practical manual of Brachytherapy,
Medical Phys Pub., (1997).
Paris System: single plane
 Treated volume = 1.2x0.5x2 cm3.
 The length of line source = 2.8 cm
(=1.4 x 2 cm).
 Source spacing = 2 x 0.5 = 1 cm.
 Lateral margin = 0.37 x 1 = 0.37 cm.
 Basal dose is the dose at “X”.
 The “reference dose rate” is 85% of
the basal dose rate.
 To obtain the 60 cGy/h dose rate on
the reference dose surface or over
the target volume, a source of 5.25
U/cm (or 1.25 mCi/cm) is required.
x
Brachytherapy Physics 2005, pp.351-372, AAPM
Computer aided systems
Brachytherapy treatment planning
software
Varian BrachyVision
 Varian VariSeed (for prostate implant)
 Philips Medical Systems Pinnacle
 CMS Interplant (for prostate implant)
 Nucletron PLATO Brachytherapy/Oncentra
 Prowess Panther 3D Brachy Pro
 Rosses Medical Systems Strata Suite

Brachytherapy Treatment Planning
1)
2)
3)
4)
5)
Simulation – takes films or CT images.
Define spatial coordinates of potential
source locations.
Specify points-of-interest.
Determine the source locations and their
source strength to meet the treatment
prescription.
Prepare treatment report for loading.
Source Localization
Two films (orthogonal, arbitrary angle,
stereo-shift)
 Three films
 Ultrasound
 CT (kVp)
 MVCT
 No-film

Two film localization technique
Orthogonal films
 Simple and
accurate.
 Sensitive to the
patient
movement.
 May misidentify
source locations
(2 circles in fig.)
Orthogonal Films: Interstitial
Orthogonal Film Localization:
Interacavitary
Two film localization technique
Stereoshift method
 Move the source by
distance s.
 Easy to identify
sources.
 The accuracy is
lower than the
orthogonal method.
Two-films: arbitrary angle
S2
S1
A target point (T) at R1 for source S1
I
z’
T
R1
θ1
R1 = S1T + OS1 = t ⋅ S1 P1 + OS1 = t (OP1 − OS1 ) + OS1
z
x’
O
 − I sin θ1 


OS1 = 
0

 I cos θ 
1 

x
f
Ref: Brachytherapy 1994 page 249





 x1   x1′ cos θ1 + F sin θ1   − I sin θ1 

 
  
+
0
R1 =  y1  = t 
y1′

 

 
  
 z1   x1′ sin θ1 − F cos θ1   I cos θ1 
O’
F=I+ f
 cos θ1 0 − sin θ1  x1′


1
0  y1′
OP1 =  0
 sin θ 0 cos θ  − f
1
1 

P1(x’1,y’1)
For the source S2, the coordinate of T is R2.
 x2 
 x2′ cos θ 2 + F sin θ 2   − I sin θ 2 
 

 

+
0
R2 =  y2  = u 
y2′
 

z 
 x′ sin θ − F cos θ   I cos θ 
2
2 
2 
 2
 2
Since R1=R2, solve the (3) equations for t and u.
Then, obtain (x,y,z) of T.
CT Source Localization
Reconstruction of CT data
Treatment planning software
Eclipse-BrachyVision




Localization can be done with both two
orthogonal films or CT images.
The dosimetry data format complies TG-43
formalism.
Both LDR and HDR planning can be performed.
For HDR, dwell time optimization can be done
with geometric and volume optimization
methods to conform the target volume and
minimize the dose to normal organs.
Plan Evaluation
Dose volume histogram (DVH)
 Differential dose volume histogram (DDVH)
 Natural dose volume histogram (NDVH)
 Coverage quantifier – V100 or D100
 Homogeneity index
 Conformity index
 Tumor control probability

Dose Volume Histogram
Prostate
Volume
V ( D)
y = 100
V ( 0)
Rectum
Urethra
Dose [Gy]
W.S.Bice, Brachytherapy Physics, pp.604-640 (2005)
Differential Dose Volume
Histogram
dV ( D)
dD
Entire
calculation
volume
Prostate
Natural Dose Volume Histogram



The dose (x-axis) is scaled for NDVH so that the low dose volume is
less emphasized than the standard differential dose volume
histogram.
For an ideal point source, NDVH is a flat line as function of dose.
The peak should appear at near the prescription dose for good
implant.
Calculation
volume
%Volume
Prostate
Dose
Matched Contiguous Dose (MCD)
The minimum dose that encloses all
sources in an implant.
 MCD can be estimated by plotting isodose
surfaces for various dose levels. The
isodose surface just covers all sources
indicates MCD.
 MCD should cover the treatment target.

Uniformity Indices



Uniformity qualifier, Q2 (J.M.Paul, 1986): The ratio of the
average dose over the treatment volume to the
prescribed dose.
Dose homogeneity index, DHI (A.Wu, 1988): The
fraction of the treatment volume that receives a dose
between the prescribed dose and a dose 1.5 times the
prescribed dose.
Dose-nonuniformity ratio, DNR (C.B.Saw, 1991): The
ratio of the high-dose volume to the reference-dose
volume. The high-dose volume is that irradiated to a
minimum dose higher than the reference dose by a
specified factor (i.e., 1.5). The reference-dose volume is
the volume covered by the prescription isodose surface.
Uniformity index: formulas
Dt
Q2 =
D0
V100
V150
Target, Vt
(V 100 − V 150) ∩ Vt
DHI =
Vt
V ( D ≥ 150)
DNR =
V 100
Coverage (Conformity) Indices



Conformity qualifier, Q1 (J.M.Paul, 1986), or
Coverage index: The ratio of the treatment
volume to the (idealized) target volume.
Conformity index, CI: The ratio of the target
volume covered by the prescription dose to the
volume covered by the prescription dose.
V(100): The percentage volume of the target
volume covered by the prescription isodose
surface.
Conformity index: formulas
V 100
Q1 =
Vt
V100
Target, Vt
V (100) = V 100 ∩ Vt
V (100)
CI =
V 100
Outline
1.
2.
3.
4.
5.
6.
7.
Radioactive Sources
Dose Calculation
Implant dosimetry systems
LDR Interstitial and Intracavitary
HDR/PDR
New Techniques
QA and radiation safety
Interstitial Implants
Temporary or permanent
 Cs-137 needle, Ir-192, I-125, and Pd-103
 Templates

Treatment sites:
 Brain, eye
 Head and neck: nasopharynx, base-oftongue, floor of mouth, etc.
 Sarcoma: thigh, extremities (legs or arms)
 Breast
 Prostate or Cervix
Catheter implant technique
a) A hollow steel needle is inserted through the target.
b) The thin header end of a nylon catheter is threaded through the needle. The
catheter is then pulled through the target together with the steel needle.
c) Both ends of the catheter is sealed with buttons and fixed to the treatment site.
d) Source ribbons (Ir-192) are afterloaded by inserting the ribbons through the
catheters.
Interstitial implant: head&neck




Head and neck cancers comprise about 2% of
all cancer incidences in USA.
Approximately 30,000 new cases per year.
Head and neck cancers are treated using
brachytherapy, either alone or in combination
with EBRT.
Treatments sites include oral cavity (base of
tongue, oral tongue, floor of month, lip, buccal
mucosa), orophanrynx, nasopharynx, nose
(nasal vestibule).
Head and neck anatomy
Non-looping technique
for base of tongue implant
Permanent prostate implant




A radiation therapy option for treatment of prostate
cancer in Stage I-III.
Prostate implants were performed using retropubic
technique in 1970 and 80’s. An introduction of
transperineal technique lead to an explosion of the
implant practices. The procedure gained wide
acceptance in mid 1990s.
I-125 or Pd-103 seeds are implanted permanently
in the prostate grand.
Dose is 100 Gy to 150 Gy to the periphery of the
treatment volume.
Prostate cancer mortality rate
 30 death/100,000 male per year. 30x130,000,000/100,000=39,000
per year.
Prostate cancer clinical staging
Biochemical
progression free
survival
Prostate implant outcome
Intermediate risk patients: ≥T2b or PSA > 10, or Gleason score ≥ 7.
Treatment-related morbidity
or Quality-of-life outcome (QOL)



Urinary morbidity: Approximately 12 months is
required for the mean International Prostate
Score (IPSS) to return to the pre-implant
baseline. Dose of 100% to 140% of the minimum
prescribed dose (mPD) is well tolerated.
Rectal morbidity: 4 – 10% incident of mild
proctitis. The volume of rectum exposes high
dose affects the morbidity.
Erectile dysfunction (ED): 6-90% of patients
experience ED.
Prostate anatomy
Prostate implant techniques
A.
B.
C.
D.
Imaging tools – CT, Transrectal
Ultrasound (TRUS), X-ray fluoroscopy
Treatment planning – nomogram,
computer optimization of seed locations
Source loading – Mick applicator,
needles, and robotic.
Pre-implant planning vs. real time
planning&implant
Prostate implant equipment
Transperineal implant
Image guided real time prostate
implant procedure
1)
2)
3)
4)
5)
6)
7)
Pretreatment transrectal ultrasound (TRUS) is taken to
determine the volume of the prostate.
The seeds are ordered for the implant.
On the date of implant, the patient are anesthetized on a
treatment table.
TRUS images are taken and imported into a treatment
planning software.
Treatment planning is performed to determine the seed
loading pattern. The locations are optimized to achieve
the best target coverage and to minimize the dose to
urethra and the rectum.
Seeds are loaded into the prostate under US guidance
according to the plan.
After the implant, CT is taken for post-implant evaluation.
Typical dose distribution
prostate implant
Selection of Isotope
Radiobiologically, Pd-103 is favorable over
I-125 for faster growing, more aggressive
tumors because of its shorter half-life.
 Clinical results including a clinical trial
suggest slight advantage of Pd-103 over I125.
 The effect of post-implant edema of
prostate is more pronounced with Pd-103
than I-125.

Source strength of prostate implant




The cost of seeds is proportional to the number
of seeds used for an implant. => higher strength
of seeds is economically favorable.
The dose is scaled with the total source strength
(not the source strength of individual seed.)
The implant with high strength seeds were more
robust and produced higher values of
postoperative dosimetry parameters than the low
strength implants when D80 and V100, V150, V200
are compared. (V.S.Narayana et al, IJOBP
61:44-41,2005).
Typical source strength:
I-125 (0.4-1U), Pd-103 (1.4-3.5U)
Selection of seed model
TG43 update (2004)
Prescription dose
Primary
Boost
I-125
145 Gy
115 Gy
Pd-130
135 Gy
105 Gy
Typical source strength: I-125 (0.4-1U), Pd-103 (1.4-3.5U)
Plan evaluation parameters
Planning volume coverage => V100.
 Urethral dose => urethral V125 and V150.
 Urethral dose => mean dose to urethra.
 Homogeneity => V150.
 High dose volume => V200.
 Number of needles => minimize.
 Number of seeds => minimize.

Post-implant evaluation

Postoperative post-implant dosimetry
ensures that the dose is delivered as it is
planned.
1) The patient comes back for a CT scan (two
weeks) after the implant.
2) Target contours are drawn on the CT images.
3) By digitizing the seeds visible on the CT images
and using the source strength used for the
implant, dose calculations are performed.
Gynecological interstitial implants
Syed-Neblett template
Intracavitary Implant


An applicator or mold made of tissue-like
material is placed in a cavity around which tumor
is expected.
Radioactive source(s) is placed inside the
applicator for temporary irradiation.
Treatment sites:
 Brain
 Head and neck - nasopharynx
 Cervix/vagina
 Breast – MammoSite
Gynecological malignancies
Cancer of cervix




Louis Wickham (Paris) treated cervical cancer
with radium as early as 1906 and reported
results for 1000 patients by 1913.
Intracavitary brachytherapy with MV external
beam therapy is a standard treatment.
Five-year disease free survival: 70%-90% for
FIGO stages I&II, 25% to 48% for stage III, and
5% to 34% for stage IV.
In the 1970s, Cs-137 was widely adopted as a
radium substitute.
Uterine Cervix Anatomy
Fundus
Uterine
cavity
Endometrium
Uterus
Corpus
Myometrium
Cervix
External
cervical os
Fornices
Vagina
Applicators for Cervix Implants
Fletcher-Suit Delclos
 Henschke
 Variations of those

Radium milligram-hour approach


Stockholm system: three fractions over one
month, 20-30 hours/fraction. 30 mg to 60 mg (in
linear tube) in uterus, 60 mg to 80 mg (in
shielded silver or lead boxes) in vagina. Total
mg*h was 6500 to 7100.
Paris system: equal amount in uterus and in
vagina. One single application of 7200 to 8000
mg*h. The intrauterine tube contained 3 sources
(1:1:0.5). The intravaginal cylinders (colpostats)
contained one source each of the same source
strength.
Manchester System - Cervix

8000 R to point A in two sessions of about
72 hours each with a 4-to-7 day interval
between. (a dose rate of 55 R/h).

Point A : 2 cm lateral to the uterine canal and 2
cm from the mucous membrane of the superior
fornix of the vagina in the plane of the uterus.
Point B : 5 cm from the midline and 2 cm up
from the mucus membrane of the lateral fornix. It
represents the dose to the vicinity of pelvic wall
near the obturator nodes and a good measure of
the lateral spread of the effective dose.

Dose Specification of Cervix Implant
Manchester
Original point A and B
U3
U2
U1
Modified point A and B
Manchester: Dose Rate
 For U1=10 mg, U2=10 mg, U3=15 mg, and two 20 mg medium ovoids with
spacer => Point A dose rate=35+19=54cGy/h.
Source loading
Point A
[cGy/h]
Point B
[cGy/h]
1)
U1=10 mg, U2=10 mg, U3=15 mg
35
8.0
2)
U1=15 mg, U2=10 mg
35
7.0
3)
Large ovoids with spacer: 2x22.5 mg
19
9.0
4)
Medium ovoids with spacer: 2x20 mg
19
8.2
5)
Small ovoids with space: 2x17.5 mg
19
7.4
6)
Special loading: U1=20 mg
28
5.7
Johns and Cunningham Table 13-10
ICRU System
International Committee on Radiation
Units and Measurement (ICRU) published
Report No. 38 “Dose and Volume
Specification for Reporting Intracavitary
Therapy in Gynecology” in 1985.
 The report recommends a system of dose
specification that relates the dose
distribution to the target volume.

Data for reporting (ICRU)
Description of techniques
 Total reference air-kerma
 Description of reference volume
Dose level if not 60 Gy
Dimension of reference volume
 Absorbed dose at reference points
Bladder, rectal, Lymphatic, and pelvic wall
 Time-dose pattern

ICRU System: reference volume
ICRU bladder and rectum points
Inf
Sup
ICRU system – Reference points
Paraaortic and iliac nodes
ICRU system – Reference points
pelvic wall

On the AP film, the pelvic walls are located at the intersection of a
horizontal tangent to superior aspect of the acetabulum and a
vertical line touching the medical aspect of acetabulum. On the
lateral film, the points are marked as the highest middistance points
of the right and left acetabulum.
RPW
LPW
Reference Points: example
Dose distribution of Cervix implant
Intraluminal brachytherapy

Radioactive sources are inserted in the
lumen of a vessel such as the blood
vessel, bronchus, esophagus, or bile duct.
Surface molds and plaques
Radioactive sources are placed in customdesigned molds or plaques, which are
then placed on the surface of the target
tissue.
 Eye plaque

90Sr




eye applicator
Pterygium is a raised wedgeshaped growth of the
conjunctiva. It is surgically
removed.
Post-operative radiation
therapy is delivered with a Sr90 applicator.
The dose is prescribed to the
surface; i.e., 10Gy x 6 weeks
or 25Gy.
The dose rate is 100 to 300
cGy/s.
90
38
β − ( 0.5 MeV )
β − ( 2.27 MeV )
90
Sr (28.9 y )   
Zr
→ Y (64h)   
→36
90
37
Episcleral Eye Plaque
Intraocular malignancies – melanoma,
retinoblastoma
 Benign disease – macula degeneration

Episcleral Eye Plaque (COMS)



Collaborative Ocular Melanoma Study (COMS)
Group set guidelines.
Patients with medium size tumors between 2.5
and 10 mm in apical height and 16 mm or less in
basal diameter are randomized between
enucleation and I-125 eye plaque treatment.
85Gy delivered in 5 to 12 consecutive days
(dose rate = 0.5 to 1.25Gy/h) to 5mm from tumor
base center for height smaller than 5 mm;
otherwise, tumor apex.
Eye Plaque Dosimetry
 I-125 (6711)
 Dashed line =Dose
w/o plaque
 Solid line = dose w/
Eye plaque
Brachytherapy Physics 2005, pp673-705 (AAPM)