Đ. Milković, M. Ranogajec-Komor, S. Miljanić, Ž. Knežević and K. Krpan

Children Hospital Srebrnjak Zagreb, Croatia

Ruđer Bošković Institute, Zagreb, Croatia

COMPARISON OF DIFFERENT DOSIMETRY SYSTEMS FOR DOSE MEASUREMENTS IN

DIAGNOSTIC RADIOLOGY

Our wish is that all children are safe and protected in radiology department!

INTRODUCTION

Pulmonary X-rays are essential in the diagnostics of lung diseases of children and youth.

Chest radiography represents the majority of radiological examinations.

The starting basis for radiation protection is the exact determination of doses.

FEATURES:

very low doses at low and variable energies have to be measured

there exists a considerable variation in radiation doses delivered to patients (different X-ray equipment, different staff, etc.).

AIM

to test a new Shimadzu X-ray unit used for thorax examination of children

to compare a thermoluminescence (TL) dosimetry system based on LiF: Mg,Cu,P with the radiophotoluminescent (RPL) glass dosimetry system (FGD-200).

MATERIALS AND METHODSIrradiations:

137Cs gamma rays in air – for calibration

ISOVOLT 420 X Ray Unit (40-300 kV, 1-20 mA) at the SSDL in air and on the water phantom (plastic bottle, Φ=11 cm, V=2.5 l) – energy dependence

Energies: 33, 48, 65 keV

Dnom= 2 mGy (air kerma)

150 kV Shimadzu CH-200M unit in air

Phantoms:

water phantom (plastic bottle)

doll phantom

BABY PHANTOM

Voltage: 70 kVQuantity of charge: 1.6 mAsTime of irradiation: 5 msSize of the focus: 0.6 mmDistance: 150 cm

BABY FIX

Dosimetry systems

RESULTS

Energy dependence in SSDL

Doses in diagnostic X-ray unit

Energy dependence in SSDL

Relative dose in air: the mean values of doses measured (Dmeasured) “in air” relative to delivered doses specified as “air-kerma free-in-air” (Ka).On phantom: the mean values of the doses measured on the phantom relative to delivered doses specified as air kerma free-in-air

The energy dependence of TL and RPL dosimeters in SSDL

0.00.2

0.40.6

0.81.0

1.21.4

0 20 40 60 80

Mean photon energy (keV)

Re

lativ

e d

ose

0

0.5

1

1.5

2

0 20 40 60 80

Mean photon energy (keV)

Re

lativ

e d

ose

In air On phantom

: TLD : RPLD : TLD : RPLD

● : Calculated values of Hp(10)/Ka

Mean value and standard deviation (SD) of doses measured on “phantoms “ in

diagnostic unitPhantom Doll (unknown plastic) Bottle (water)

Dosimeter RPL TL RPL TL

Place of dosimeter

Back Sternum Back Sternum Entr. Exit Entr. Exit

Mean dose (mGy) 0.040 0.019 0.049 0.022 0.041 0.004 0.030 0.002

SD (mGy) 0.002 0.006 0.008 0.007 0.002 0.001 0.006 0.004

SD(%) 4.3 30.6 15.5 30.0 3.9 28.2 19.5 154.9

Dentrance/Dexit 2.5 2.3 11.3 13.0

Entr.: Entrance

Doses in diagnostic X-ray unitOn the doll: agreement of the dose values of RPL and TL dosimeters in entrance and exit beams On the water phantom: difference between the mean values measured in the ingoing beam with the two dosimeters

Reason: ▪ different materials of phantoms ▪ different energy absorption characteristics of the two dosimeters (below 50 keV) on water phantom

CONCLUSION

TLD (LiF:Mg, Cu, P )(termoluminiscent dosimeter)

RPL(radiophotoluminiscent glass dosimeter)

TLD (LiF: Mg, Cu, P ) (termoluminiscent dosimeter)

High sensitivity

In spite of its anomalous energy dependence nearly tissue- equivalence

Agreement with earlier results

RPL (radiophotoluminiscent glass dosimeter)

Higher sensitivityEnergy dependance “in air” is better than for LiF:Mg, Cu, P (33-65 keV mean energies)Energy dependance curve on the water phantom changes in opposite direction than the calculated Hp(10) values

The absolute difference from Hp(10) is not larger than for LiF:Mg, Cu, P

CONCLUSION

The measured dose values in X-ray diagnostic unit are in accordance with the characteristics found in SSDL for both dosimeters.

The RPL system seems to be suitable for dosimetric measurements in X- ray diagnostics.

ZAGREB

Thank you for your

attention!

The authors are grateful to Chiyoda Technol Corporation, Japan for the support of this work.