Comparison of dosimeter response of TLD 100 and ionization chamber for high energy photon beams at KIRAN karachi in pakistan

Comparison of dosimeter response of TLD 100 and ionization chamber for high energy photon beams at KIRAN Karachi in Pakistan The Egyptian Journal of Radiology and Nuclear Medicine xxx (2017) xxx–xxx C[.]

Original Article

Comparison of dosimeter response of TLD-100 and ionization chamber

for high energy photon beams at KIRAN Karachi in Pakistan

Muhammad Waqara,d,⇑, Asdar Ul-Haqb, Syed Bilald, M Masoodc

a Nuclear Medicine Oncology and Radiotherapy Institute Nawabshah (NORIN), Pakistan

b

Karachi Institute of Radiotherapy and Nuclear Medicine (KIRAN), Pakistan

c

Pakistan Institute of Nuclear Science and Technology (PINSTECH), Pakistan

d

Pakistan Institute of Engineering and Applied Sciences (PIEAS), Pakistan

a r t i c l e i n f o

Article history:

Received 15 October 2016

Accepted 21 January 2017

Available online xxxx

Keywords:

TLD

Dosimeter

Ionization chamber

Phantom

Relative variation

a b s t r a c t

This study was conducted by measuring point dose at different depths of water phantom by using two types of dosimeters (PTW N30013 ionization chamber and TLD-100 chips) at Karachi institute of radio-therapy and nuclear medicine (KIRAN), Pakistan Two different TLD chips were used (circular pallets and square chips) The main aim of this study was to compare the responses of two different types of dosime-ter irradiated with 6 MV X-rays using same paramedosime-ters Both types of dosimedosime-ter were irradiated with dif-ferent dose value ranging from 25 cGy to 500 cGy The deviation between difdif-ferent shapes of TLD and ionization chamber remained within 5% limit Maximum deviations of circular pallets reading from that

of ion chamber are 3.56% at 1.5 cm depth and 4.91% at 5 cm depth For square chips maximum deviation happened to be 4.38% at 1.5 cm depth and 4.23% at 5 cm depth This measurement shows that TLDs are reliable tool for dosimetry regardless of their shape or manufacturer and they can be used as re-validation tool for ion chamber dosimetry

Ó 2017 The Egyptian Society of Radiology and Nuclear Medicine Production and hosting by Elsevier This

is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/

4.0/)

1 Introduction

Inter comparison is an important activity to ensure the

consis-tency of radiation dosimetry [1] The ionization chamber is a

benchmark of dosimeter, and had been known to provide the most

accurate and reliable results [2] One difficulty with ionization

chamber is that the measured dose can be perturbed by volume

averaging over their relatively large active volume[3] Practically

there are several things to consider, such as the large volume of

active and effective point of measurements[4] They can’t be used

in anthropomorphic for in vivo measurements When ionization

chamber measurements are impractical, it can be replaced by

TLD, especially in in vivo dosimetry The small size of TLD allows

it to be inserted into an anthropomorphic or water phantom for

dosimetry TLDs can be used to measure point doses with greater

precision in volume as their active volume can be made very small

as compared to ionization chamber For fractional dose

measure-ment, the corrections for energy and dose response are taken into account [5] To achieve dose measurement precision, TLD imple-mentation program requires a rigorous annealing and response measurement protocol, and routine QA of the TLD reader and annealing oven temperature control[6]

When radiation is incident on a crystal of thermo luminescent material, it excites it This crystal can’t be de-excited itself and energy of radiation remains trapped If this crystal is heated up

to a certain temperature, it is de-excited and releases the trapped energy in the form photon of visible light This light is detected

by PMTs The output of PMTs is directly related to the light output

of TL crystal, which is itself proportional to the dose absorbed in the crystal by incident radiation If output of PMTs is calibrated against the absorbed dose, TLDs can be used to assess the dose absorbed in TL crystal Once TL dosimeter is read, it can be reused after a process called annealing which eliminates any residual imperfection in crystal[7,8] Thermo-luminescence properties of crystal can be explained theoretically by energy band theory of solids[9]

LiF is the most common crystal used for Thermo luminescence dosimetry because of its tissue equivalence (Zeff= 8.2 compared to 7.4 for tissue) and its energy independent response in the range of

100 keV–1.3 MeV [10,11] TLD-100 is LiF crystal doped with

http://dx.doi.org/10.1016/j.ejrnm.2017.01.012

0378-603X/Ó 2017 The Egyptian Society of Radiology and Nuclear Medicine Production and hosting by Elsevier.

This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer review under responsibility of The Egyptian Society of Radiology and Nuclear

Medicine.

⇑ Corresponding author at: Senior Medical Physicist, Nuclear Medicine Oncology

and Radiotherapy Institute Nawabshah (NORIN), Pakistan.

E-mail address: phy_waqar@yahoo.com (M Waqar).

Contents lists available atScienceDirect

The Egyptian Journal of Radiology and Nuclear Medicine

j o u r n a l h o m e p a g e : w w w s c i e n c e d i r e c t c o m / l o c a t e / e j r n m

Magnesium and Titanium and is usually used for dosimetry in

radiotherapy

The light output of PMTs is plotted against temperature or time

that is called glow curve[12] Various curves plotted in glow curve

shows the released electrons of different energy, which were

trapped in the crystal At room temperature TLD-100 has different

peaks corresponding to different electron energy level [13,14]

Electrons trapped at lower energy levels are prone to fading, which

is loss of trapped charges before readout Factors, which affect the

shape of glow curve are annealing, heating rate and its uniformity,

size and history of sample, threading instrument

TLD-100s are heated before irradiation first at 400°C for one

hour followed by 100°C for two hours or at 800 °C for 24 h (called

annealing) This slow heating relieves trapped electron remained in

lower peaks of glow curve by decreasing their trapping efficiency

These electrons can also be released by post irradiation annealing

by heating at 100°C for 10 min The magnitude of these peaks

rapidly decreases with time after irradiation so they can be

released at lower temperature after irradiation After removing

peaks at lower values of temperature, glow curve shows more

pre-dictable behavior[15]

The aim of present study is to compare the responses of TLD

dosimeters with the other most widely used dosimeters, ion

cham-ber Ion chambers are commonly used for dosimetry in

radiother-apy Present study focuses on a point that response of TLD

dosimeters is in well agreement with ion chambers and can be

used as a re-validation tool for ion chamber dosimetry

2 Materials and Methods

In this study, two different batches of TLDs (LiF-Mg-Ti) having

different shape, dimensions and manufactured by different

well-known companies (HARSHAW, USA & MTS, Poland) were used

First batch comprised of 125 square shape TLD chips (TLD-100)

having dimensions of 3.2
3.2
0.9 mm3, manufactured by M/s

HARSHAW, USA The second batch comprised of 125 circular

pel-lets (MTS-N) having the dimensions of 4.5
0.9 mm3

manufac-tured by M/s MTS, Poland

The ionization chamber (PTW N30013 Serial number 0114,

PTW-FREIBURG Corporation, Germany) with volume 0.6 cm3was

used in this study It is often used for absolute dosimetry

measure-ment in radiotherapy because it has flat response and better signal

to noise ratio For measurement of the charges produced by photon

in water and PMMA solid phantoms, CNMC11 electrometer (model

No 5232) was used with IC The ionization chamber was calibrated

from Secondary Standard Dosimetry Laboratory (SSDL) PINSTECH,

Islamabad

Linear Accelerator with of 6 MV fixed energy (Primus plus)

manufactured by Siemens Medical systems, USA) and

manually-operated TLD Reader system (Thermo-Scientific Model 3500) with

heating rate 10°C/s was utilized to carry out this study

Before use, TLDs were first grouped according to their Element

Correction Factor (ECC) 125 TLDs of each group are randomly

selected and their ECC were manually calculated HARSHAW

3500 single chip TLD reader was used for this purpose Around

140 TLDs with their ECC close or equal to unity were selected for

the measurement Among 140 TLDs, 70 were square chips and 70

were circular pellets

After grouping, TLDs were calibrated After each use, TLDs were

annealed in Thermo Lyne 47900 furnace, first at 400°C for one

hour and then at 100°C for two hours A time gap of almost 48 h

is given between exposure and reading the TLD to minimize the

fading Background or zero readings were rechecked after

anneal-ing, along with test light reading

A set of annealed TLD-100 were irradiated with field size

10
10 cm2, at 100 cm SSD and at 5 cm depth in a solid water phantom to a dose of 200 cGy with a 6 MV photon beam A calibra-tion factor was measured for each TLD All TLDs were irradiated repeatedly with 50 cGy, 100 cGy and 150 cGy to check the linearity and reproducibility of TLD response All the TLDs showed a linear response with this range of doses

The responses of TLD samples and ionization chamber were observed at SSD 100 cm with field size 10
10 cm2in water phan-tom at depths of 1.5 cm (Dmax) and 5 cm by varying the doses from

25 cGy to 500 cGy Experimental arrangements are shown inFig 1 Relative variation of measured TLD readings (DTLD) and Ioniza-tion chamber readings (DIC) are calculated using following formula [16]:

d ð%Þ ¼ DTLD DIC

DIC

3 Results and Discussion

In this section, the response of both batches of TLDs against irradiation was compared with the ion chamber response sepa-rately It was found that TLD response is in a good agreement with ion chamber readings

3.1 At the Depth of Maximum Dose (1.5 cm) Tables 1 and 2were the measured dose values of TLDs (DTLD) and IC (DIC) for X-ray beams with energy 6 MV measured at depth

of 1.5 cm Average relative variation for square TLD chips from that

of IC readings were calculated The maximum and minimum rela-tive variation was found to be 4.41% and 0.29% for the doses of

250 cGy & 500 cGy respectively This data is plotted inFig 2

Fig 1 Experimental arrangements.

Table 1 Responses of TLD (square chips) and Ion chamber dosimeter at the depth of 1.5 cm Dose (cGy) Square chips 1.5 cm

D TLD (cGy) D IC (cGy) Relative variation (%)

150 146.50 147.08 0.40

200 190.15 196.11 3.04

250 234.44 245.25 4.41

300 281.43 294.33 4.38

350 336.91 343.36 1.88

400 376.94 392.33 3.92

450 433.16 441.55 1.90

500 489.37 490.77 0.29

For circular pallets, the maximum and minimum relative

varia-tion was found to be 3.56% and 0.25% at dose values of 500 cGy &

150 cGy respectively as tabulated inTable 2and plotted inFig 3

For 1.5 cm depth, relative variation for square chips and circular

pallets with respect to ionization chamber was observed to be

within ±4.5% and ±4% respectively

3.2 At Dose Depth (5 cm) Measurements were also performed at dose depth of 5 cm Dose values of TLD’s (DTLD) and IC (DIC) for X-ray beams with energy

6 MV measured at depths of 5 cm for the square and circular chips with ionization chamber are shown in Tables 3 and 4 Average

Table 2

Responses of TLD (circular pellets) and Ion chamber dosimeter at the depth of 1.5 cm.

Dose (cGy) Circular Pellets 1.5 cm

D TLD (cGy) D IC (cGy) Relative variation (%)

150 147.45 147.08 0.25

200 197.18 196.11 0.55

250 253.54 245.25 3.38

300 300.03 294.33 1.94

350 346.06 343.36 0.79

400 397.92 392.33 1.42

450 427.95 441.50 3.07

500 473.30 490.77 3.56

Fig 2 Comparison of TLD (square chips) and ion chamber response at the depth of 1.5 cm.

Fig 3 Comparison of TLD (circular chips) and ion chamber response at the depth of 1.5 cm.

Table 3 Responses of TLD (square chips) and Ion chamber dosimeter at the depth of 5 cm Dose (cGy) Square chips 5 cm

D TLD (cGy) D IC (cGy) Relative variation (%)

150 127.40 127.79 0.30

200 164.71 170.36 3.31

250 204.40 212.94 4.01

300 245.33 255.73 4.07

350 285.73 298.36 4.23

400 340.05 341.05 0.29

450 377.92 383.68 1.50

500 417.08 426.31 2.16

relative variation for square TLD chips from that of IC readings

were calculated The maximum and minimum relative variation

was found to be 4.23% and 0.29% for the doses of 350 cGy &

400 cGy respectively The data is given inTable 3and plotted in

Fig 4 For 5 cm dose depth, the relative variations within ±4%

and ±5% for square chips and circular pellets respectively as shown

inFig 5 While comparing IC response with TLD’s response in water phantom at different depths, it was found that TLD is a suitable detector for quality assurance or re-validation purpose, especially for the energies of therapeutic range, provided that they have been properly calibrated The dose re-validation is very important because of the direct involvement of human beings Very limited and insufficient literature was available to support the procedure adopted in this section Banjade et al.[17]proposed that accuracy

of better than 5% of measured dose can be achieved He cited the results of Marshall et al for the measurement of depth dose with accuracy of 5.3% According to ICRU recommendation the variation

of 3–5% between the delivered and prescribed dose is allowed in radiotherapy [18] At 10 cm depth and energies of 6 MV and

18 MV, variation is shown to about 0.1–1.3%[16] At low energy

in mammography the variation of results between ion chamber and TLS is 4–8%[19] Present measurements have shown relative variation within ±4% and ±5% of ion chamber measurement at 1.5 cm & 5 cm respectively It is also suggested that more studies must be conducted, for comparison of these two dosimeters

Table 4

Responses of TLD (circular pellets) and Ion chamber dosimeter at the depth of 5 cm.

Dose (cGy) Circular Pellets 5 cm

D TLD (cGy) D IC (cGy) Relative variation (%)

150 122.59 127.79 4.07

200 163.63 170.36 3.95

250 219.28 212.94 2.98

300 259.92 255.73 1.64

350 298.46 298.36 0.03

400 352.40 341.05 3.33

450 395.27 384.67 2.76

500 438.37 426.31 2.83

Fig 4 Comparison of TLD (square chips) and Ion chamber response at the depth of 5 cm.

Fig 5 Comparison of TLD (circular chips) and ion chamber response at the depth of 5 cm.

responses at specified depths (1.5 cm & 5 cm), to have a better

understanding

4 Conclusion

Ion chambers are primary radiation detectors for radiation

dosimetry in radiotherapy Their relatively large active volume

and their vulnerability to changes in atmospheric conditions,

sometimes, make their results unreliable In present study, TLDs

showed the results of within 5% deviation from those of ion

cham-ber which is in a good agreement The responses of both types of

TLD lie within acceptable limits regardless of shape and

manufac-turer It can be concluded that TLDs can be used as a re-validation

tool for ion chamber dosimetry

Conflict of interest

Authors have nothing to disclose

Acknowledgement

The authors are grateful to Mr Muhammad Shahban, NORIN

cancer hospital Nawabshah, Mr Shamim Haider and Tauseef

Reh-man, KIRAN cancer hospital Karachi for their assistance and help

References

[1] Zˇivanovic´ MZ, Lazarevic´ ÐR, Ciraj-Bjelac OF, Stankovic´ SJ, C´eklic´ SM, Karadzˇic´

KS Intercomparisons as an important element of quality assurance in

metrology of ionising radiation Nucl Technol Radiat Prot 2015;30:225–31

[2] Nalbant N Pre-treatment dose verification of IMRT using Gafchromic EBT3

film and 2D array J Nucl Med Radiat Ther 2014;2014

[3] Fitriandini A, Wibowo W, Pawiro S Comparison of dosimeter response:

ionization chamber, TLD, and Gafchromic EBT2 film in 3D-CRT, IMRT, and SBRT

techniques for lung cancer J Phys: Conf Ser 2016;012006.

[4] McEwen M, Kawrakow I, Ross C The effective point of measurement of ionization chambers and the build-up anomaly in MV X-ray beams Med Phys 2008;35:950–8

[5] Olko P Advantages and disadvantages of luminescence dosimetry Radiat Meas 2010;45:506–11

[6] Low DA, Moran JM, Dempsey JF, Dong L, Oldham M Dosimetry tools and techniques for IMRT Med Phys 2011;38:1313–38

[7] Pradhan A, Bakshi A Calibration of TLD badges for photons of energy above

6 MeV and dosimetric intricacies in high energy gamma ray fields encountered

in nuclear power plants Radiat Prot Dosim 2002;98:283–90 [8] Rosenwald J-C, Nahum A, Chavaudra J, Carlsson GA, Dance D, Bielajew A, et al Handbook of radiotherapy physics theory and practice; 2008.

[9] Siddique R, Uddin Z, Hussain M Dosimetric evaluation and verification of external beam 3-D treatment plans in humanoid phantom using thermoluminescent dosimeters (TLDs) J Basic Appl Sci 2012;8:690–5 [10] Cherry P, Duxbury A Practical radiotherapy: physics and equipment John Wiley & Sons; 2009

[11] Nette H, Onori S, Fattibene P, Regulla D, Wieser A Coordinated research efforts for establishing an in international radiotherapy dose intercomparison service based on the alanine/ESR system Appl Radiat Isot 1993;44:IN1–IN11 [12] Taylor G, Lilley E The analysis of thermoluminescent glow peaks in LiF (TLD-100) J Phys D: Appl Phys 1978;11:567

[13] Anacak Y, Arican Z, Bar-Deroma R, Tamir A, Kuten A Total skin electron irradiation: evaluation of dose uniformity throughout the skin surface Med Dosim 2003;28:31–4

[14] Delgado A, Ros JG Evolution of TLD-100 glow peaks IV and V at elevated ambient temperatures J Phys D: Appl Phys 1990;23:571

[15] Derreumaux S, Chavaudra J, Bridier A, Rossetti V, Dutreix A A European quality assurance network for radiotherapy: dose measurement procedure Phys Med Biol 1995;40:1191

[16] Swinnen A Quality assurance in radiotherapy: development and validation of

a mailed dosimetry procedure for external audits using a multipurpose phantom and in vivo dosimetry Katholieke Universiteit Leuven; 2005 [17] Banjade D, Raj TA, Ng B, Xavier S, Tajuddin A, Shukri A Entrance dose measurement: a simple and reliable technique Med Dosim 2003;28:73–8 [18] Andreo P, Burns DT, Hohlfeld K, Huq MS, Kanai T, Laitano F, et al Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water IAEA TRS 2000;398

[19] Stanton L, Day J, Brattelli S, Lightfoot D, Vince M, Stanton R Comparison of ion chamber and TLD dosimetry in mammography Med Phys 1981;8:792–8