TISSUE PHANTOM RATIO - THE PHOTON BEAM QUALITY INDEX

TISSUE – PHANTOM RATIO :
THE PHOTON BEAM QUALITY INDEX
Victor EKPO
Medical Physicist
ASI Ukpo Cancer Centre,
Calabar – NG
August 2021

WHAT IT IS
Tissue Phantom Ratio (TPR) is the ratio of the
dose (or dose rate) at a given depth in phantom to
the dose at the same source-point distance, but at
a reference depth.
2

where,
z is the reference depth in the phantom (also zref)
A is the field size
hv is the energy
D and D are the dose and dose rate respectively
Q is an arbitrary point on the central axis
3
.

GEOMETRY FOR MEASUREMENT OF TPR
(a) The geometry for the measurement of DQ at depth z in a phantom;
(b) the geometry for the measurement of DQref at depth zref in a phantom.
4

EXPERIMENTAL SETUP
TPR is defined at a constant source-point distance,
referring to the Source to Axis Distance (SAD) , also
known as the Source to Chamber Distance (SCD) or
Source to Detector Distance (SDD).
For a linear accelerator (linac), this distance is
measured from the linac head to the point of insert of
the ionization chamber inside the phantom.
For many linac measurements, SAD is kept at 100 cm.
5

EXPERIMENTAL SETUP (2)
TPR is also defined at a reference depth (z).
This refers to the depth of the ionization chamber
from the phantom surface (e.g. 10cm).
If two dose measurements are taken at 20cm and
reference depth 10cm, that TPR formulation is referred
to as TPR20,10.
6

WHY TPR?
TPR was necessary because of the limitations of TAR
(tissue-air ratio).
TAR concept uses two measurements – one in air, and
another in phantom. Whereas TPR takes both
measurements in phantom.
The TAR concept works well in isocentric set-ups for
photon energies of 60Co and below.
7

WHY TPR?
TAR does not work for megavoltage beams because of
difficulties in measuring the dose to small mass of
water in air at high energies.
This occurs because the required size of the buildup
cap of the ion chamber becomes excessively large, and
impracticable.
8

WHY TPR?
Just like TAR, TPR depends on three parameters:
z (depth), A (field size) and hv (energy), but do not
depend on SAD or SSD (source-to-surface distance).
TPR is independent of electron contamination of
the incident photon beam.
It does not require the use of displacement correction
factors at two depths when cylindrical chambers are
used.
9

WHAT IS TMR?
Tissue Maximum Ratio (TMR) is a type of TPR.
TPR becomes TMR when reference depth zref equals
depth of dose maximum (zmax or dmax).
10

RELATIONSHIP BETWEEN TMR AND PDD
where,
P is percentage depth dose
f is SSD
Sp is phantom scatter factor
to is reference depth of maximum dose
d is depth
r is the field size at the surface
rd = r(f + d) / f ; rto = r(f + to) / f
11

RELATIONSHIP BETWEEN TPR AND PDD
where,
PN is normalized percentage depth dose
f is SSD
Sp is phantom scatter factor
to is reference depth of maximum dose
do is reference depth
r is the field size at the surface
12

PHOTON BEAM QUALITY INDEX
✘ The quality of a beam is its penetrating ability.
✘ For kilovoltage x-ray beams, it is defined by the Half
Value Layer (HVL) - - the thickness of an absorber
of specified composition required to attenuate the
intensity of the beam to half its original value
(e.g. 2.0 mm Al HVL) .
13

✘ In the diagnostic orthovoltage range, beam quality
can be affected by adding filters.
✘ The filter attenuates the low energy photons, so the
beam becomes increasingly “harder”, that is,
contains a greater proportion of higher-energy
photons.
✘ HVL increases with increasing filter thickness.
14

✘ HVL is related to linear attenuation coefficient ( µ ):
✘ HVL is usually defined for a specified energy.
15

TYPICAL HVL VALUES FOR PHOTON BEAMS
16
Beam quality
index for
megavoltage
electron beams
is R50, the range
at 50% depth
dose.

✘ The beam quality of a gamma ray beam (e.g. Co-60)
is usually stated in terms of the energy of the rays,
since the source decays with a specific, known and
almost mono-energetic energy.
✘ Radionuclides have homogenous energy spectrums.
✘ For Co-60, beam quality is expressed as 1.17 and 1.33
MeV (avg. 1.25 MeV) or simply Co-60 beam.
17

For megavoltage photons, HVLs vary little in that photon energy
range, making HVLs as a beam quality index undesirable.
Several beam quality indices have been proposed:
✘ TPR20,10 (Tissue-Phantom Ratio; IAEA TRS-398);
✘ PDD(10) [Percentage Depth Dose at 10 cm depth; AAPM TG-51
& TG-142];
✘ NAP [Nominal Accelerating Potential; NACP]
✘ d80 [depth of the 80% depth dose; BJR S.17 & 25]
18

For megavoltage photon linac beams, several beam
quality indices have been proposed:
✘ TPR20,10 (Tissue-Phantom Ratio; IAEA TRS-398);
✘ PDD(10) [Percentage Depth Dose at 10 cm depth;
AAPM TG-51 & TG-142];
✘ NAP [Nominal Accelerating Potential; NACP]
✘ d80 [depth of the 80% depth dose; BJR S.17 & 25]
19

ADVANTAGES & DISADVANTAGES OF TPR20,10
ADVANTAGES
1. TPR is not affected by the
electron contamination at depth
2. Stopping-power ratios and
TPR20,10 are very well correlated
and lie on an almost universal curve
(±0.5%).
3. On the practical side, TPR20,10 is
very simple to measure in a
clinical beam.
4. It distinguishes clinical beams from
non-clinical beams produced by lab
accelerators which use a bigger
target and/or filter.
20
DISADVANTAGES
1. TPR20,10 can be meaningless if the
accelerator potential and the target and
filter combinations used to derive
stopping-power data are completely
ignored.
2. Not suitable for non-clinical accelerators.
TPR20,10 cannot select, with an accuracy
better than 0.5%, stopping-power ratios for
the very high energy photon beams
produced by non-conventional clinical
accelerators.

TPR20,10
TPR20,10 is defined as the ratio of water absorbed doses
on the beam axis at the depths of 20 cm and 10 cm in a
water phantom, obtained with a constant source
chamber distance (SCD) of 100 cm and a 10 cm × 10 cm
field size at the position of the detector.
21

TPR20,10
The parameter TPR20,10 is a measure of the effective
attenuation coefficient describing the approximately
exponential decrease of a photon depth dose curve
beyond the depth of maximum dose.
22

EXPERIMENTAL SETUP FOR TPR20,10
TPR experiments are conducted under reference conditions:
 Constant field size (A), e.g. 10 x 10 cm
 Constant energy (hv), e.g. 6 MV
 Constant SCD, e.g. 100 cm
 Constant phantom (e.g. water)
 Constant source (same linac)
 Measurement depths (20g/cm2 and 10g/cm2)
 Phantom material: Water
 Chamber type: Cylindrical (recomm.) or Plane-parallel
23

24
Fig: 1D Water Phantom and cylindrical ionization
chamber used for measurement of TPR20,10

There are three (3) ways TPR20,10 experiment can be set
up. When measuring at 20cm depth, one can:
Method 1: Fill the phantom with more water.
Method 2: Move chamber deeper and couch higher.
Method 3: Move chamber deeper only (PDD method).
25

26
METHOD 1: VARYING WATER VOLUME
M10 M20
Measurement at 10 cm depth Measurement at 20 cm depth
where,
M is electrometer readings, in nC
D is dose

27
METHOD 2: SAD METHOD
M10 M20
= 90 cm
= 80 cm
where,
D is dose

28
METHOD 3: PDD METHOD
M10 M20
where,
PDD is percentage depth dose

CALCULATING TPR20,10 FROM PDD(10)
TPR20,10 can be estimated from a fit to the data for the
percentage depth dose at 10 cm depth, PDD(10),
measured for a 10 × 10-cm field size at an SSD of 100 cm.
29
TPR20,10 = –0.7898 + 0.0329 PDD(10) – 0.000166 PDD(10)2

WHEN IS TPR USED?
✘ TPR20,10 is required for determination of correction
factors during absolute photon dosimetry (TRS-398).
✘ During commissioning, TPR tables should be prepared
for all energies.
✘ Commissioning requires direct measurement of TPRs for
all photon energies and selected field sizes (e.g., 5×5, 10 ×
10, 40 × 40 cm) and depths (5, 10, 30 cm) for verification
of values calculated from percent depth doses.
30

WHEN IS TPR USED?
✘ The measured and the computer-generated values for
all clinically used depths and field sizes should agree
within ±2% (preferably ±1%).
✘ During annual QA, TPR20,10 should be checked, and
should not be more than ±1% from baseline.
✘ TPR values should be between 0.5 to 0.84, usually
around 0.67 for 6 MV photon, 0.73 for 10 MV, and 0.76
for 15 MV.
31

SUMMARY
✘ TPR is important for quality assurance of the linear
accelerator.
✘ Maintaining TPR value to within ±1% of baseline ensures
that the photon beam quality remains good and stable.
✘ The ultimate goal is to ensure the linac produces a
precise dose to treat the tumour volume at the right
depth (as prescribed), thus maintaining high quality of
patient care for cancer patients.
32

33
REFERENCES
khan FM. The Physics of Radiation Therapy, Lippincott, Williams and Wilkins, 5th ed. Baltimore, MD (2014).
International Atomic Energy Agency. Podgorsak EB (ed.) Radiation Oncology Physics : A Handbook for Teachers and
Students. Vienna (2005).
International Atomic Energy Agency. Absorbed Dose Determination in External Beam Radiotherapy. Technical Report
Series No. 398. Vienna (2000).
International Commission On Radiation Units And Measurements, Determination of Absorbed Dose in a Patient Irradiated
by Beams of X or Gamma rays in Radiotherapy Procedures, Rep. 24, ICRU, Bethesda, MD (1976).
Klein, EK, Hanley J, Bayouth J, TG-142: Quality Assurance of Medical Accelerators. American Association of Physicists in
Medicine Task Group 142 Report, DOI: 10.1118/1.3190392. Med. Phys. 36(9) (2009).
Gerbi BJ, Antolak JA, Deibel FC, et al. TG-70: Recommendations for clinical electron beam dosimetry: Supplement to the
recommendations of Task Group 25. American Association of Physicists in Medicine Task Group 70 Report . DOI:
10.1118/1.3125820 Med. Phys. 36(7) (2009).

34
THANK
YOU!
Any questions?
You can find me at
✘ ekpovictortoday@gmail.com
✘ about.me/overjoy

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