Paraffin wax phantom for absolute dosimetry

1. A STUDYOF THEABSORBEDDOSESIN DIFFERENTPHANTOMMATERIALS
ANDFABRICATIONOF A SUITABLEPHANTOM
Presented
By
Md. Ashikur Rahman
Scientific Officer
DRiCM, BCSIR
Dhaka
M. A. Rahman1, M. M. Rahman2, M. A. Hai2, M. A. Islam1
1Dept.of Physics, Rajshahi University;
2Cancer Centre, KYAMCH, Sirajgang

2. What is CANCER ?
 Surgery
 Chemotherapy
 Radiation therapy  Teletherapy
 Brachytherapy
Cancer is a large group of diseases (over 200)
characterized by uncontrolled growth and spread
of abnormal cells.
TEREATMENT OPTIONS

3. AIM OF RADIATION DOSIMETRY
 The prescribed dose in radiation therapy has to be converted
into machine monitor units for patient treatment.
 It is necessary to plan how we deliver the prescribed dose to a
patient.
 The dose distribution inside the patient cannot be measured
in the body of the patient himself.
 Hence, the patient needs to be replaced by a tissue-equivalent
material.

4. Patient is replaced by
tissue-equivalent
material to calculate
the dose inside the
body
Tissue-equivalent
material or Phantom

5. RADIATION DOSIMETRY
 Radiation dosimetry is the measurement and calculation
of the absorbed dose in matter and tissue
 Scientific determination of amount, rate, and distribution
of radiation emitted from a source of ionizing radiation
 Measuring the radiation-induced changes in a body or
organism and
 Measuring the levels of radiation directly with
instruments.

6. A phantom is a substance which is made of tissue
equivalent materials.
A phantom represents the radiation properties of the
patient and allows the introduction of a radiation detector
into this environment, a task that would be difficult in a real
patient.
A very important example is the scanning water phantom.
Alternatively, the phantom can be made of slabs of tissue
mimicking material or even shaped as a human body
(anthropomorphic).

7. Fig. Image of water phantom Fig. Image of PMMA phantom
Fig. Image of solid water phantom

8. Suitability of different Phantoms
Water phantom is widely used for dosimetry purposes and it is
recommended as the reference medium for absorbed dose
measurements for both photon and electron beam by many
international institute.
Although water phantoms are standard for absolute dosimetry
but those are inconvenient for both linac and cobult-60 machine.
Due to the problems of water phantom they are replaced by
solid phantoms which have the same properties as like as water
with slight variation.

9. Solid phantoms are expensive and procured with
difficulty from abroad.
It would be advantageous if an inexpensive and locally
available tissue equivalent material is available.
This is particularly relevant for developing countries like
ours.
Paraffin wax is found to have density and electron
density similar to that of water with a slight variation and
we decided to fabricate a phantom using paraffin wax.

10. Fig. Paraffin wax phantom
without bubbles
Fig. Paraffin wax phantom
with bubbles
CT scan image of phantom-1
(without bubbles)
CT scan image of phantom-2
(with bubbles)

11. FABRICATION OF PARAFFIN WAX PHANTOMS
Two paraffin wax phantoms were made one
was with bubbles and another without
bubbles.
In this study our object is to justify the suitability of
the paraffin wax phantoms and find out a scaling
factor between those readily available solid
phantoms with water phantom

12. Table 1: Properties of used phantom materials
Material name Chemical
Composition
Mass density
(gm/cm3)
Number of
Electrons/g
(×1023)
Water H2O 1.00 3.34
Solid water
Epoxy resin-
based mixture
1.00 3.34
PMMA (C5O2H8)n 1.16 - 1.20 3.24
Paraffin wax
CnH2n+2 ,
20 ≤ n ≤ 40
0.88 - 0.92 3.44

13. After making proper
dimension, two paraffin
wax phantoms were drilled
with a d.c drill machine and
the holes were similar to
the chamber dimension
The phantoms were
then placed on a CT
scanner for checking
internal integrity

14. Temperatures inside
phantoms were collected
by digital thermometer
and room pressures were
taken with the help of a
barometer.
Then the phantoms were placed on the couch of
the linear accelerator. The phantoms surface
center were aligned with the central axis of the
beam from the gantry at zero degree angle. The
distance between the phantoms surfaces to the
source were made 100 cm with the help of
optical mark reader.

15. The monitor unit of the linear accelerator was made 100,
SSD = 100 cm, Field size = 10 × 10 cm2
The readings were taken three times for
each of 6 MV and 15 MV photon beam
with the help of an electrometer in every
phantom.
Fig. Optical mark reader indicate 100 cm SSD
Fig. Linear accelerator

16. Two types of ionization chamber were used in the
experiment one was Farmer and another was Semi-Flex.
Absolute dosimetry protocols TG-51 and IAEA TRS 398
was followed.
Fig. Farmer type ionization chamber Fig. Semi-Flex type ionization chamber

17. Correction factor
The ratios between the doses in the water phantom and
those in other phantoms are scaling factors. Naturally this
scaling factor is unity for water.
A number close to 1 would be a good ratio.
Although slight variation also acceptable.
Various correction factor was calculated for the
determination of absorbed dose in different phantoms.

18. Absorbed dose to water at the reference depth, zref,
in a water phantom irradiated by a beam of quality
Q is
DW, Q = MQ ×ND, W × kTP × kS × kpol × kQ, Q0
MQ → Monitor reading
ND,W → Calibration factor in terms of absorbed
dose to water
kTP → Temperature pressure correction factor
kS → Ion recombination correction factor
kpol → Polarity correction factor
kQ,Q → Chamber specification factor
0

19. Correction for temperature and pressure
Polarity correction factor
Ion-recombination correction factor
Chamber specification factor
And,

20. Table 2: Scaling factor for different phantom materials for 6 MVphoton beam
Name of
phantom
Absorbed
dose in
water
phantom
Dw, Q (Gy/C)
Absorbed
dose in
phantom
Dph,Q (Gy/C)
Scaling
factor (S.F)
Dw, Q/ Dph,Q
Deviation
from
water
phantom
Paraffin wax-1
(without bubbles)
0.6892 0.6948 0.992 -0.8%
Paraffin wax-2
(with bubbles)
0.6892 0.7051 0.977 -2.3%
Solid water 0.6892 0.6758 1.020 +2.0%
PMMA 0.7062 0.6691 1.055 +5.5%

21. Table 3: Scaling factor for different phantom materials for 15 MVphoton beam
Name of
phantom
Absorbed
dose in
water
phantom
Dw, Q (Gy/C)
Absorbed
dose in
phantom
Dph,Q (Gy/C)
Scaling
factor (S.F)
Dw, Q/ Dph,Q
Deviation
from
water
phantom
Paraffin wax-1
(without bubbles)
0.8142 0.8042 1.012 +1.2%
Paraffin wax-2
(with bubbles)
0.8142 0.8350 0.975 -2.5%
Solid water 0.8142 0.7688 1.059 +5.9%
PMMA 0.8633 0.7897 1.093 +9.3%

22. Research findings
Deviation from water phantom is minimum for Paraffin
wax phantom and maximum for PMMA phantom. Although
in the case of solid water phantom this deviation should be
least but it varies about 6% , where the deviation of Paraffin
wax phantom is around 1%
The cost of solid water phantom is at least 3000
dollars, where the cost of Paraffin wax phantom is
ONLY 35 DOLLARS

23. Conclusion
Now it is seen that a Paraffin wax phantom is much more
suitable than other solid phantoms due to their availability,
price and process of fabrication.
 The scaling factor is a multiplication factor it will help us to
use any solid phantom and also convert the absorbed dose in
solid phantoms equivalent to water phantom.

24. Thank
You all