Commissioning of a CT/MR compatible ring applicator was performed to: 1) Determine the reproducibility of source position offsets, which were found to have average values of 1.13 cm, 1.11 cm, and 1.13 cm for 20 mm, 40 mm, and 60 mm tandems respectively. 2) Establish a standard loading pattern of dwell positions for treatment planning. 3) Quantify the ring offset through various imaging modalities, with an average measured value of 11.6 mm, to properly account for the offset in treatment plans.

1. Commissioning of a Brachytherapy Applicator
CT/MR Compatible Ring Applicator
• Presented By: Priyanka Kundu Pijush Paul
• Resident Medical Physicist, TMH , Mumbai

2. Outline
 Introduction
 Need of Commissioning
 Objective
 Task Performed
 Results
 Discussions
2

3. Introduction
 The CT/MR ring applicator, enhanced with the addition of
nine guide holes in the ring tube.
 These guide holes allow placement of interstitial titanium
needles using the ring tube as a needle template.
3

4. Continue...
 The addition of interstitial needles makes it possible to achieve asymmetric
alteration of the dose distribution.
 The needles are inserted perpendicular to the ring and in parallel to the tandem.
 The applicator’s plastic and composite fiber materials minimize distortion on CT
or MR imaging.
4

5. Need of
Commissioning
5
 Source dwells within applicators can deviate
from intended positions by several millimetres.
 Quantifying this offset is an important part of
commissioning.
The maximum source distance can be
calculated as:
Lm = Lt + La – ( Lp + Ls + Lc)
Where, Lm = maximum source distance (mm)
Lt = transfer tube reference length (mm)
La = applicator length (mm)
Lp = applicator tip length (mm)
Ls = safety clearance between the source
tip and inner end of the applicator (mm)
Lc =center of the source length (mm)
(Lp + Ls + Lc ) is the offset that is not
reachable for the source within the
applicator.

6. Continue...
6
 Ring curvature introduces an uncertainty in
the determination of the actual dwell position
of the HDR source.
 The discrepancy arises because the source
wire is much smaller than the diameter of
the ring lumen through which it travels, leads
to multiple paths of source.
 Tight curvature of an applicator can cause the
source wire to take one of many possible
paths.
 It is difficult to predict if the source will tend
to follow a path that is shorter or longer than
intended

7. Objectives
7
 Determine compatibility of applicator with different imaging modality.
 Determine the reproducibility of the offset.
 Determine the Standard Loading Pattern.

8. Task Performed
8
 Offsets of 20mm,40mm,60mm tandem & Ring
were measured several times to determine the
reproducibility of source positioning.
 All measurements were done in different
modalities.
 Ring and ovoids both have similar dosimetric
characteristics, but the ring applicator is generally
smaller than a pair of ovoids, and the larger range
of dwell positions offers more flexibility for
treatment planning.

9. Continue...
9
 This flexibility for planning comes at a cost, however, as the tight curvature of the ring
can be difficult for an HDR source to navigate.
 If nothing is done to correct this discrepancy, there will always be an inherent
difference between the planned and delivered treatments.
 Once this systematic offset is determined, a correction can be applied to treatment plans
such that the delivered dwell positions will match the dwell positions used in the
treatment plan.

10. 10
The Project
Work Comprises
 Compatibility check of the applicator with different imaging modalities
 Estimation and verification of
 Length of the Intrauterine tube
 Diameter of the ring
 Angle of Intrauterine tube
 Distance between surface of the assembled ring and the source path
 Standard Loading Pattern
 Intrauterine tube & Ring offset

11. 01 CT Scan
02 MR Scan
03 Autoradiograph
04
C-Arm
Imaging
11
Different Methods Employed for
Commissioning Purpose

12. Accessories used
12
Thermocol Zig Water Dummy X-ray Dummy

13. CT Scan
Scans were taken in CT
Scanner GE Lightspeed
(120KV, 30mA of slice
thickness 0.625mm)
CT Scans were
imported to the
Oncentrabrachy
(Version-
4.5.2)treatment
planning system.
This confirm the
CT compatibility
of applicator
13

14. CT Images
14

15. Continue...
15

16. 6th position
Dummy sent to 1485.5 mm position
Symmetric
Position in Ring
7th position
Dummy sent to 1483 mm position
16
Use of source position simulator & Dummy source
For nth position, Distance = 1498- (2.5 x (n-1)) mm

17. 26th position
Dummy sent to 1435.5 mm position
Symmetric
Position in Ring
27th position
Dummy sent to 1438 mm position
17
Use of source position simulator & Dummy source
For nth position, Distance = 1498- (2.5 x (n-1)) mm

18.  If Dummy can go maximum 1402 mm distance, then source can go 1400 mm.
 As we know maximum distance travel by Dummy is 1500 mm, we desirably chosen 1st position
1498 mm, so that position of it can mimic position of actual source.
Source Wire
Simulator
18

19. Offset for Tandem
Offset
Determination
Offset for Ring
19
Use of water Dummy

20. The following Parameters
were Observed by Analysing
the CT Images
Length of the
Intrauterine tube :
60 mm, 40 mm, 20 mm.
Diameter of the ring:
30 mm
Angle of the
intrauterine tube:
600
Distance between
surface of the
assembled ring and
the source path:
6 mm
Tandem offset: 12 mm
Ring offset: 450
Symmetric dwell
position in Ring:
6 Or 7 & 26 Or 27
20

21. MR Scan
Agar gel (3% Agarose + 1%
Cupper Sulphate) taken to
simulate biological tissue in
which applicator was
embedded.
Images were
acquired with
water dummy in
1.5 T magnetic
field in GE –
SignaHDxt
scanner
T1 , T2 weighted
FRFSE with band
width of 125 bps
& of equal matrix
of 256 X 256
pixels
21

22. Continue...
 Phantom size 30cmX20cmX15cm in which applicator can be positioned in a reproducible
geometry.
22

23. MR Images
23
T1 Weighted
images
T2 Weighted
images
Use of Water Dummy

24.  Proper perpendicular lines should be
drawn for proper analysis.
 Ensured the center of the intersecting
lines was at the center of ring.
Autoradiograph
 Tip of the Tandem was marked with a
marker.
24
Use of Gafchromic Film

25. Continue...
 Setup for Autoradiograph
25
Afterloader Details:
Version: V3 ( 18 channels)
Sr. No. : 20034
Indexer Length: 1500 mm
Dwell Time: 15 S

26. Continue...
 Average of all these exposed films taken to determine the Offset.
26

27. Continue...
 Average of all these exposed films taken to determine the Offset.
27

28. Continue...
Films were scanned using a
Epson Scanner and
imported to IMRT-
OmniPro(Version-1.6.0009)
for analysis.
From position of
center of the dose
cloud(done by
averaging out) on
the film, distance
& angle was
obtained.
The arc length for
Ring calculated
based on the θ
value:
28

29. Continue...
 Centre of the dose cloud taken by averaging out in both X and Y direction
 Offset distance is measured from tip(marked) to the centre of the dose cloud.
29

30. Continue...
30
Tandem
length (mm)
Value1 Value2 Value3 Value4 Average
(cm)
Standard
Deviation
(cm)
20 1.10 1.2 1.13 1.10 1.13 ±0.05
40 1.06 1.13 1.13 1.12 1.11 ±0.03
60 1.15 1.15 1.1 1.10 1.13 ±0.03

31. Continue...
 Angle was measured w.r.t. to Y-axis
31

32. Scanned image for source position at 6 & 26
Symmetric
Position in Ring
Scanned Butterfly image for source position
at 3,5,7,9 & 23,25,27,29
32

33. Scanned image for source position at 7 & 26
Symmetric
Position in Ring
Scanned Butterfly image for source position
at 4,6,8,910 & 23,25,27,29
33

34. C-Arm
34
C-Arm Details:
Model no: MEXSICOM-2000
Sr. No.:1054
 Cu wires were taken to get
perpendicular lines & ensured
that center of the intersecting
lines was at the centre of the
ring.
 Images were taken with 40 kV
and 2 mA.

35. Image for Source position 7
C-Arm Imaging
Image for Source position 26
35
Image for Source position 27
Use of source position simulator & Dummy source
For nth posion, Distance = 1500- (2.5 x (n-1)) mm
 Lowest position of dummy source indicates the actual source position
Cu Wire

36. The following Parameters
were Observed by Analysing
the Autoradiograph & C-Arm
Images
Offset for Tandem:
11mm
Offset for Ring:
 Values for offset 44.40, 44.40,
44.50.
 Mean value 44.40.
 Standard deviation: ±0.5°
 i.e. offset in term of Arc Length
=(44.50/180*π*15mm)=11.6mm
Symmetric position in
Ring:
7&26 dwell position
36

37. Standard Loading
Pattern
37
RING 4 6 8 10 23 25 27 29
TANDEM
2cm** 1 4
4cm 1 3 5 7 10 13 16
6cm 1 3 5 7 10 13 16 20

38. Using TPS (9 mm offset)
Using 9 mm offset in TPS , better placement of symmetric dwell position was found by reconstructing the
Ring with 7 & 26 as symmetric dwell position
38

39. Using TPS (10 mm offset)
Using 10 mm offset in TPS , placement of symmetric dwell position was found to be degraded than that
with 9 mm offset by reconstructing the Ring with 7 & 26 as symmetric dwell position
39

40. Result
For Current Practice we
have chosen Ring Offset
as 10 mm to
intermediate the two
aspects.
40
Ring offset coming
from
CT Image &
Autoradiograph: 11.6
mm
Ring offset coming
from TPS by
reconstruction: 9 mm.

41. Treated Plan (10 mm offset)
41

42. References
1. Waid D, Morrison J, Glennie G. Commissioning the ring & tandem combination applicator set — CT & MR compatible, for
clinical use with the VariSource HDR unit [abstract]. Med Phys. 2009;36:2525.
2. Stern R and Liu T. Dwell position inaccuracy in the Varian GammaMed HDR ring applicator. J Appl Clin Med Phys.
2010;11(4):291–96.
3. Varian Medical Systems. Product Notification Letter: CT/MR ring and tandem combination applicator set [PNL-GM-CR
30271]. Palo Alto, CA: Varian Medical Systems; 2008.
4. Varian Medical Systems. Product Notification Letter: CT/MR ring and tandem combination applicator set [PNL-VS/GM-CR-
30271]. Palo Alto, CA: Varian Medical Systems; 2008.
5. Varian Medical Systems, Treatment planning for the Varian ring and tandem applicators using a GammaMed afterloader and the
Brachyvision treatment planning software [Customer Technical Bulletin CTB-BV-345A]. Palo Alto, CA: Varian Medical
Systems; 2003.
6. Commissioning of Varian ring & tandem HDR applicators: reproducibility and interobserver variability of dwell position offsets
Ryan McMahon, Tingliang Zhuang, Beverly A. Steffey, Haijun Song, Oana I. Craciunescua
42

43. 43
Discussion

44. THANK YOU
44