Study of Radiation Protection for Elekta Medical Linear Accelerator

The incidence of cancer throughout the world is increasing with the prolonged life expectancy that has resulted from improvements in standards of living. About half of all cancer patients receive radiotherapy, either as part of their primary treatment or in connection with recurrences or palliation. The International Atomic Energy Agency (IAEA) has estimated that approximately 2500 teletherapy machines were in use in 1998 in developing countries and that 10000 such machines may be needed by 2015. The preparation of this Safety paper was initiated as a result of an expected increase in the construction of radiotherapy facilities, and in response to Member States that have requested practical guidance regarding the design and shielding of such facilities. In this work, The Harshaw Model 4500 Manual TLD Reader was used for Thermo Luminescence Dosimetry (TLD) by using a set of 20 chips Dosimeter TLD-100. A comprehensive set of measurements were performed for five different sites inside and outside the accelerator. The obtained results showed agreement with published data in some location while the results of the working staff site were within the limits of the recommended permissible dose where it was about 0.5 m Sievert per year.


INTRODUCTION
Radiation exposure limits or standards were introduced as early as the start of this century when the potential hazards of radiation were realized.One of the first standard setting bodies was the International Commission on Radiological Protection (ICRP), which continues its function through its series of publications.
These reports form the basis for many national protection guidelines.In the United States, the National Council on Radiation Protection and Measurements (NCRP) has functioned as a primary standard-setting body through its separate publications.
One of the agencies with regulatory powers in this country is the Nuclear Regulatory Bull High Inst Public Health Vol.38 No. 2 [2008]   Commission (NRC), which has control over the use of all reactor-produced materials (e.g., 60 CO and 192 Ir).The naturally occurring radioactive materials (e.g., radium and radon) and x-ray machines are regulated by individual states. 2

1-Dose equivalent
Because the biological effects of radiation depend not only on dose but also on the type of radiation, the dosimetric quantity relevant to radiation protection is the dose equivalent (H).It is defined as:

H=D.Q
Where D is the absorbed dose and Q is the quality factor for the radiation.
The SI unit for both dose and dose equivalent is joules per kilogram, but the special name for the SI unit of dose equivalent is sievert (Sv).
If dose is expressed in units of rad, the special unit for dose equivalent is called the rem.

H (rem) = D (rad).Q
Because Q is a factor and has no units, The use of quality factor in radiation protection is analogous to the use of relative biological effectiveness (RBE) in radiobiology.

2-Low-level radiation effects
A vast literature exists on the biological effects of radiation.predicting low dose effects.][8] 3 Effective dose equivalent limits

B Read Dosimeters
The subsets of dosimeters were read out.After the reading process complete, the RCF are Generated and by accepting the results the RCF is applied to the database.
After Generated ECC and RCF are applied, the entire set of dosimeters now called Field Dosimeters.Thus, these chips are ready to use for radiation measurements

Protection measurement
The gamma and neutron dose measurements were determined by using the TLD ships For this purpose five locations were chosen to estimate the radiation doses using TLD detector as shown in Fig. (5).The first one was placed on the patient bed, the second was placed on the wall facing the accelerator head in the accelerator room at 2 m height and 2 m away from the isocenter, the third was

NCRP 4
recommendations on exposure limits of radiation workers are based on the following criteria: (a) at low radiation levels, the nonstochastic effects are essentially avoided; (b) the predicted risk for stochastic effects should not be greater than the 'average risk of accidental death among workers in "safe" industries; and (c) ALARA principle should be followed, for which the risks are kept as low as reasonably achievable, taking into account social and economic factors.

3. 1 Fig. ( 1 )
Fig. (1) with 4,6 and 15 MV photons and from 4 till 18 MeV electrons.It provides variable field sizes from 4x4 to 40x40 at 100 cm SSD.Variable X-ray dose rate can be changed manually or via the digital interface protocol.Multileaf collimators (MLC) and asymmetric jaws provide manual and automatic beam shaping for conformal therapy.There is availability of arc therapy.Virtualwedge is also present.

Fig. ( 4 ) 1 . 2 . 3 . 1
Fig. (4) Handling and preparation of Chips Dosimeter TLD-100 upper and lower limits of ECC values (around the mean 1.0) the more deviated chips are rejected from the set and the accepted chips are called Calibration Dosimeters.chips randomly selected from the entire set of dosimeters (from the Calibration Dosimeters) and exposed to a well known dose (100 rev.= 4.35 mGy ) by Bicron Model 2210 TLD Irradiator.A Store Dosimeters The Dosimeters Stored for the time established (fade time = 24 hours) in a subdued UV environment at a temperature no higher than 30 o C.