The results of a TLD Therapy Dosimetry Quality Assurance Program for Dong Nai General Hospital, Vietnam (Period 2013-2017)

The objective of this study is to perform a thermo luminescent dosimetry (TLD) postal dose audit provided the International Atomic Energy Agency (IAEA) for a linear accelerator in Dong Nai General Hospital. The TLD were irradiated with an absorbed dose to water of 2 Gy for 6 MV and 15 MV photon beams generated by a linear accelerator (LINAC) (Siemens Medical Solutions, Concord, CA). The percentage deviation relative to IAEA measured dose was 0.4% and 3.4% for 6 MV and 16 MV, respectively. Agreement within ± 5% between the users stated dose and the IAEA measured dose is considered satisfactory. The results showed that all photon beams are within the IAEA’s acceptance limit of ± 5%. As a conclusion, the beam output of Siemens Primus is accuracy in delivery of absorbed dose within ± 5%.


Introduction
Radiation therapy is one of most commonly used for treatment cancer to deliver as high doses to the tumor while sparing the surrounding healthy tissue and organs.Therefore, it is important to have high accuracy in the dose delivered of LINAC.Dong Nai General Hospital was equipped with a LINAC for this purpose since 2009.Quality assurance (QA) of the beam outputs is recommended for safe use of linear accelerators for radiotherapy.The International Atomic Energy Agency (IAEA) and the World Health Organization (WHO) operate the IAEA/WHO TLD postal dose audit program [1].The purpose of the program is to verify the beam calibration in radiotherapy centers and to improve the accuracy and consistency of clinical dosimetry in radiotherapy hospitals worldwide.The TLD audit is cost free to participants.It can be checked the calibration of clinical external beams therapy such as Co-60 and megavoltage beams from accelerators.It is not designed for small beams used in radiosurgery, electron beams, brachytherapy or orthovoltage X-ray beams, or stereotactic radiosurgery equipment, a Gamma-knife, X-ray knife, Cyber-knife, or similar.The advantages of TLD are small, reasonably tissue equivalent and not attached to measuring equipment with cables or wires.In Vietnam, this program in collaboration with the Institute of Atomic Energy Vietnam has been conducted in most of the radiation facility in the country.Recently, Dong Nai General Hospital have participated this program since 2013.The irradiated TLDs should arrive at the IAEA laboratory not later than 6 weeks after irradiation; otherwise the hospitals will have to wait for their results due to queues for the TLD reader arising from irregular return of TLDs.This work show technical note and result of a TLD Therapy Dosimetry Quality Assurance Program for Dong Nai General Hospital, Vietnam (period 2013-2017).The TLD results are sent to the Dong nai Hospital within 8 weeks of receiving the irradiated TLDs at the IAEA, depending on the queue for the TLD reader.Our hospital receives individual result certificates for each beam checked with TLDs.If the results within a 5% limit are considered acceptable, the next TLD audit will be recommended after two years later.Otherwise, if the results outside the 5% acceptance limit are provided with a second, the repeat irradiation TLD is immediately required.If the second TLD result is still not acceptable, an expert will visit to resolve the problem, and the next TLD audit is recommended for one year later.After getting the bad result, a hospitals participating in the IAEA/WHO TLD postal dose audits should have to revised their dosimetry procedures and upgrade their instrumentation to prevent radiological accidents.

Treatment machines and equipments
Measurements have been performed on the Siemens Primus LINAC (Siemens Medical Solutions, Concord, CA).The beam qualities of photons, D20/D10 are 0.57 and 0.642 for 6 MV and 15 MV respectively.Dose 1 dosimeter, Ionization chamber FC 65-P, Blue phantom (IBA Dosimetry, Germany) and plastic phantom.
TLD samples have provides by IAEA.The TL material currently used is LiF: Mg, Ti, type TLD-700.The TL powder is annealed before it is used for dose measurements in order to optimize the LiF characteristics, and to achieve better stability of powder sensitivity and lower fading.
Setting up TLD capsules [2] • Assemble the TLD holder • Place the holder in a water tank on the treatment table

Nuclear Medicine & Radiation Therapy
• Filling the water tank exactly to the level of the top of the holder (Figure 1) Figure 1: TLD holder and Siemens Primus LINAC.

Calculation of time/monitor setting
For example of 15 MV photons, we have used the blue water phantom (IBA).We record the value of absorbed dose in water at depth=10 cm, SSD=100 cm, field size 10 cm × 10 cm with LINAC delivered of 200 MU.The result was 1.573 Gy.To receive 2 Gy at this depth, we must set the Monitor Unit as 254.5 MU [3].

Irradiation of TLD capsules
The TLD capsules were irradiated at 10 cm depth in water using a field 10 × 10 cm 2 at a distance 100 cm with SSD fixed source skin distance (Figure 2).A relative deviation with positive sign indicates that the user estimates higher dose than what is measured; a patient would therefore receive higher dose than calculation.Results for the TLD audit are shown in Table 1.

Discussion
A main aspirational goal of this work was to report the result of A TLD audit for Dong Nai General Hospital and to share a technical note with other center oncology which would implement TLD audit in the future because TLD audit for dosimetry in radiotherapy has proved to be effective in assuring the quality of dose determination in radiotherapy.
Several authors [1,[5][6][7][8] have reported a perspective of dosimetry practices at hospitals in developing countries and results.Their study aimed checks the accuracy output factor of radiotherapy beams with a TLD in reference conditions given by IAEA.Some participants did not report regarding the dosimetry protocol used and their dose calculation procedures and did not include step by step procedure.There are several protocols as the German DIN-6800, the IAEA TRS-398, and the AAPM TG51.Our study used the IAEA TRS-398.The purpose of the current study was to perform the step by step of measurement TLD in water phantom by using the IAEA TRS-398 protocol.
It is interesting to note that the agreement between the Dong Nai hospital dose and the IAEA measured for 6 MV and 15 MV photon beams are within ± 5%, this result is considered satisfactory.The percentage deviation relative for 15 MV was higher than 6 MV.The IAEA/WHO TLD postal program for radiation therapy beams at Dong Nai hospital has been strengthened with new procedures to improve the overall efficiency of the treatment.During recent years, we attended and got the good results.

Figure 3 :
Figure 3: Inserting and removing the TLD capsule.
• Set a 10 cm × 10 cm field size • Align the holder tube with the central axis of the beam J Nucl Med Radiat Ther, an open access journal ISSN:2155-9619 Volume 8 • Issue 2 • 1000330

Table 1 :
Results for the TLD audit for 6 MV and 15 MV.