Absolute dosimetry

1. ABSOLUTE DOSIMETRY
Presented
By
Md. Ashikur Rahman
Scientific Officer
INMP, AERE
Savar, Dhaka.
12. 03. 2019

2. 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.

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. Medical dosimetry is the calculation of
absorbed dose and optimization of dose
delivery in radiation therapy. It is often
performed by a professional health physicist
with specialized training in that field.
Absolute dosimeter produces a signal from which the dose in its
sensitive volume can be determined without requiring calibration in
a known radiation field.

5. Radiation dose delivered to the target and
surrounding tissues is one of the major
predictors of radiotherapy treatment outcome .
It is generally assumed that the dose must be
accurately delivered within +/-5% of the
prescribed dose to ensure the treatment aims
are met.
Minimum dose to tumour = 95%
Maximum dose to tumour = 107%

6. ABSOLUTE Dosimetry is a direct measure of
ionization or absorbed dose under standard
conditions, which are things like calorimetry
[measure energy deposited which eventually appears
as heat], electrons released (in an ionization
chamber where electronic charge is measured), or
ion formation where the number of valence changes
in a known amount of ions is directly related to the
number of electrons (chemical dosimeter). Because
of the need for accuracy, absolute dosimetry
informs a standard and is usually tied to a single
government based agency responsible for 'the
standard'.
ABSOLUTE DOSIMETRY

7. Absolute dosimetry is a technique that
yields information directly on absorbed
absorbed dose in Gy. This absolute
absolute dosimetric measurement is also
is also referred to as calibration. All
calibration. All further measurements
measurements are then compared to this
compared to this known dose under
under reference conditions.

8. DOSIMETRY CONCEPTS
AAPM (American Association of Physicists in
Medicine)
Task Group-21 (1983)
TG-51 (1999)
IAEA (International Atomic Energy Agency)
Technical Report Series 277 (1997)
TRS-398 (2000)

9. Farmer Chamber FC65-P
Dose-1 Electrometer

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

11. Table 1: Properties of 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

12. 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

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