2. Source position accuracy :
Verification of positional accuracy requires that one con firms that the
intended sequence of active sources or dwell positions is delivered to the
correct position in the correct applicator.
METHODS :
Source positioning accuracy can be determined by directly viewing, by
CCTV, the source moving through a transparent scaled applicator so that
source position can be determined visually with millimeter accuracy .
3.
4. • Source position accuracy can be checked by taking radiographs of dummy
sources in the applicators with their positions marked on a ready pack film and
combining with autoradiographs of the radioactive sources in the same
applicators. The position of dummy sources and radioactive sources should
correspond within ±1 mm.
• Special test devices have been designed to test source positioning by
autoradiography .The system consists of an acrylic plate with channels drilled
to hold the treatment catheters. Lead sheets of dimensions 20 × 20 × 0.8 mm3
are accurately set into one surface at a regular spacing of 20 mm. The phantom
is placed on a ready pack XV2 film with lead sheets facing the film. The sources
are programmed to stop every 10 mm for intervals suitable for
autoradiography. The autoradiograph thus obtained shows source positions
together with fiducial markers provided by the edges of lead sheets . An
accuracy of ±1 mm in the source positions is acceptable.
5. Positioning Accuracy and Uniformity (Tolerance: ± 1 mm)
(use autoradiograph / radiograph wherever required)
1. Coincidence between dummy : ------------- mm
and active source positions
2. Accuracy of source position within the : -------------- mm
applicator
3. Reproducibility of dummy source position : -------------- mm
6. Source strength :
The source strength for any radionucleide may be specified in terms of
milicuries(mCi).
The exposure rate at any point is proportional to the product of activity and its
exposure rate constant.
Measurement of source strength should be measured in term of “Reference air
karma rate” as per ICRU-38.
Reference air karma rate is the kerma rate in air at a reference distance of 1m,
corrected for air attenuation and scattering. Unit is micro Gy m sq per hour.
The AAPM recommend air karma strength and Ref. air karma rate numerically
both are the same when reference distance measurement is taken as 1m.
7. The different methods for source calibration-
A.Open-Air Measurements.
B.Well-Type Ion Chambers.
Open-Air Measurements-
The arrangement consists of a large source to ion chamber distance
relative to source and detector dimensions. The apparatus is set up
as far away as possible potential scattering surfaces. Because the
output from brachytherapy sources is low at large distance, the
chamber volume should be large, For Example, 100 mL or larger, A
signal-to-noise ratio greater than 100:1 should be achievable.
8.
9. Disadvantage
Because of difficulty in obtaining “good geometry”
conditions, the open-air-method is a time-consuming
measurement.
It is not suitable for routine calibration checks required in a
busy department.
10. Well-Type Ion Chambers :
Routine calibration of brachytherapy sources is usually carried
out with “re-entrant” type ion chamber surrounds the source,
approximately 4 pi Geometry. A source holder is devised to re-
product the source geometry in radiation to the surrounding
chamber walls. As the detection volume is large, it produces
with high ionization current that can be measured with well-
type ion chamber fill with argon gas under high pressure.
11.
12. Source Strength Verification
1. Quoted source strength (as on ……………) : ----------- mGy. h-1 at 1m
( GBq / Ci )
2. Measured source strength (as on …………) : ---------- mGy. h-1 at 1m
( GBq / Ci )
3 .Deviation between specified : ----------- %
and measured source strength
(Tolerance: ± 3%)
4 .Deviation between measured : ------------- %
mean and individual source strength
(Tolerance: ± 5%)
13. Source strength verification
• Chamber used: Well type (SourceCheck 4π)
• Electrometer used: PTW Unidos E
• Voltage applied: +400V
• Range: High
Quoted Source Strength = 22.013 mGym2/hr as on 01/07/2022
Measured Source Strength = 21.67 mGym2/hr as on 01/07/2022
Deviation between specified and measured source strength = 1.5%
(Quoted Activity-Measured Activity) *100/Quoted activity
14.
15. Temporal accuracy :
A treatment system achieves temporal accuracy if each
source sequence or single source dwell position
remains at its intended position for the length of time
specified by the treatment program.
For manually afterloaded temporary implants, the goal is to develop a
procedure to ensure that the radioactive sources are removed upon
completion of dose delivery.
16. Contd.
Remotely afterloaded brachytherapy places treatment duration under
control of an electronic timer. Tests of absolute timer accuracy are required
whenever source calibration is based on an external time standard,
whereas relative tests suffice when the machine timer is used both to
control treatment delivery duration and to integrate charge measurements
during source-strength calibration.
An accuracy criterion of 62% seems easily achievable, both by manual
afterloading techniques and commercially available remote afterloading
systems.
In addition, the influence of transit dose on dose delivery accuracy must
be evaluated and corrected for, if necessary. Transit dose is the additional
dose delivered while the source is in motion.
18. Timer Accuracy
• Chamber used: Well type (Source Check 4π)
• Electrometer used: PTW Unidos E
• Voltage applied: +400V
• Range: High
• Set time: 60 Sec
Timer Accuracy = {(Measured - Set)/Set} X 100
Measured time = 59.41 Sec
Timer Accuracy = {(Measured - Set)/Set} X 100
% Deviation Timer Accuracy between specified and measured time = -0.99 %
19. Timer linearity
• Chamber used: Well type (Source Check 4π)
• Electrometer used: PTW Unidos E
• Voltage applied: +400V
• Range: High
• Set time: 60 Sec; Dwell position: 51
A graph has been plotted with Tset in X-axis and Tmeas in Y-axis.
From the graph the slope has been found using the most deviated readings on both sides.
Timer Linearity = (1 - Slope) * 100 = 0.17%
End Error = 0.2466 Sec
20. Timer Linearity and End Error
y = 0.9983x + 0.2466
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
180.00
200.00
0 50 100 150 200
Tmeas
in
seconds
Tset in seconds
Timer Linearity and End Error
21. Timer error
• Chamber used: Well type (Source Check 4π)
• Electrometer used: PTW Unidos E
• Voltage applied: +400V
• Range: High; Set time: 60 Sec
• Programmed length: 1400 mm
With interruption at 30Sec = 6.563 nC
Without interruption = 6.536 nC
Timer error = (R2-R1)t/(2R1-R2) = 0.24889 Sec
% Timer error = (timer error in sec * 100)/60
= 0.41%
22. REFERENCE:
AAPM TG-40
AAPM TG-41
AAPM TG-56
KHAN’S The Physics of Radiation Therapy
PROFORMA For ACCEPTANCE/QUALITY ASSURANCE TETS O
REMOTE AFTER LOADING BRACHYTHERAPY UNIT.
