COMPARATIVE ANALYSIS OF CALCULATED AND MEASURED ENTRANCE SKIN DOSE OF COMMON RADIOGRAPHIC PROCEDURES IN BENUE STATE TEACHING HOSPITAL MAKURDI BENUE STATE

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CHAPTER ONE

INTRODUCTION

  1. Background of the Study

Life on earth has developed with ever present background radiation. It is not a new thing invented by the wit of man; radiation has always been there (Hall, 2012). Radiation is a fact of life. It is all around us all the time, hence, we live in a naturally radioactive world. Radiation could be defined as the energy that travels through space or matter in form of electromagnetic waves or photons or streams of radioactive particles (IAEA, 2005; Bushberg et al., 2002). Radiation can either be non-ionizing or ionizing, depending on its ability to ionize matter. Non-ionizing radiation does not produce ionization or ions in the medium through which it passes. It has enough energy to move atoms in a molecule around or cause them to vibrate, but not enough to remove them. Examples of non-ionizing radiation include visible ray, infrared rays among others. Ionizing radiation is a kind of radiation that is capable of transferring energy to the atoms of the material which it interacts with, changing their physical state and leaving them electrically charged or ionized. Ionizing radiation is categorized by the nature of the particles or electromagnetic waves creating the ionizing effect. Ionizing radiation has different ionization mechanisms, and may be grouped as directly or indirectly ionizing. Directly ionizing are those that carry a charge and can, therefore, interact directly with atomic electrons through coulomb forces (IAEA, 2005; Bushberg et al., 2002). Examples of directly ionizing particles are alpha particles, beta particles, electrons, protons and heavy ions. Indirectly ionizing are those that are electrically neutral and do not interact with atomic electrons through coulomb fores. Indirectly ionizing radiation (photons or neutrons) deposits energy in the medium through a two-step process:

● First, a charged particle is released in the medium (photons release electrons or positrons, neutrons release protons or heavier ions)

● Second, the released charged particles deposit energy in the medium through direct Coulomb interactions with orbital electrons of the atoms in the medium. Examples of indirectly ionization particles are X- rays and rays and neutrons (WHO, 2014; IAEA, 2005).

There are basically two sources of radiation: Natural and man-made or artificial radiation. Natural radiation is that which is natural and inevitably present in our environment. Humans are continuously irradiated by internal and external sources. Internal sources include the radionuclides that enter the body through food, water and air. External sources include space or cosmic radiation and terrestrial radiation.

Cosmic radiation consists of fast moving particles that exist in space and originate from a variety of sources, including the sun and other cosmic events in the universe. Cosmic rays are mostly protons but can be other particles or wave energy (CNR, 2012).

Terrestrial radiations emanate from naturally occurring radioactive elements present in varying amounts in all types of water, soils, air, rocks, food and in human body itself (Yussuf et al., 2012; Bushberg et al., 2002). Whatever its origin, radiation is ubiquitous in the environment (HPS, 2010).

Exposure to man-made radiation can also be from medical treatments and activities involving radioactive materials. The following are the most common sources of man-made radiation: medical sources, industrial sources, nuclear fuel cycle and industrial sources (CNR, 2012). However, medical radiation exposure such as diagnostic X-rays, nuclear medicine and radiation therapy constitute the largest man-made source of exposure to ionizing radiation to which humans are subjected (Sharifat and Olarinoye, 2009; Ng et al., 1998). The  average  dose  to  the  population  from  medical  exposure  is  estimated  to  be  about  0.2-2  mSv  per  year  in industrialized countries (UNSCEAR, 2000). Diagnostic X-rays used in hospitals for emergency cases and for routine physical examination are good examples of man-made radiation.

Diagnostic radiology is a rapidly developing branch of modern medicine. It has over the past few decades evolved into a highly sophisticated diagnostic tool; improving the imaging of human internal anatomy and detection of lesions which were previously impossible to detect. (Chougule and Hussain, 1993).  Despite its contribution to diagnosis, diagnostic radiology is burdened with the concern of the safety of patients and radiology staff. This is because the procedures of diagnostic radiology utilize X-ray which is ionizing in nature and it transmits a certain amount of risk to the patient and staff despite its usefulness. The harmful biological effects of X-ray have long been established (Lampinnen, 2000). It was estimated that diagnostic radiology and nuclear medicine contributed 96% to the collective effective dose from manmade sources in the U.K (NRPB, 1993). Similar estimate showed that this contribution was 88% in the U.S.A (NCRP, 1987).

The study of biological effects of X-rays on living tissues started soon after its serendipitous discovery by Wilhelm Roentgen in 1895. To reduce the untoward biological effects associated with X-ray and the dose delivered to patients during medical intervention, the dose has to be kept as low as possible while at the same time trying to obtain optimum image for accurate diagnosis (ICRP, 2005).  

In view of the significant benefits from properly conducted medical exposures, the principal concern in radiological protection is therefore, the reduction of examinations that are either unlikely to be helpful to patient or involve high doses in order to meet specified clinical objectives. In order to achieve this, there is a need to optimize X-ray equipment and radiological techniques Patient dose measurement is an integral part of this optimization procedure (NRPB, 1990; Faulkner et al., 1999). Such optimization procedure will reveal X-ray facilities with high doses after which possible dose reduction measures may be specified. Dose measurement is also necessary so as to establish dose constraints, determine risk to patient and to justify the examination (Faulkner et al., 1999).

 At the moment, researchers have shifted their interest in protecting both the patient and staff from harmful effect of radiation to improving the technology of the procedures and constant dose monitoring (Muhohora and Nyanda, 2001). Though, it was once thought that use of digital systems in diagnostic radiology would reduce radiation doses, studies so far conducted suggested otherwise. There is a tendency towards increased radiation doses with use of digital systems. The reasons cited include the fact that overexposure can go undetected, unlike with conventional film-screen combination, where the image turns dark indicating overexposure (Ng and Rehani, 2006). A case of overexposure was reported in some centers which used digital systems, there was an average of 68 exposures per examination in upper gastrointestinal fluoroscopic examinations compared with sixteen (16) exposures with conventional systems. (Axelsson et al., 2000). Also, Reiner et al. (2000), reported that in several United States hospitals the number of examinations per inpatient increased by 82% after transition to digital systems and the number of examinations per outpatient visit increased by 21% while the number of examinations per visit nationally decreased by only 19%. The implication of these statistics is increased radiation risk to both the patient and staff.

From the foregoing it is evident that transition to better technology has improved the diagnostic quality of the images and added more radiation risk, albeit minimally. Thus, medical physicists and radiation workers are left with the option of continually monitoring radiation doses delivered to patients with a view to keeping them reasonably low .One of the methods used for monitoring patient dose is the determination of Entrance Skin Dose (ESD) as proposed by the national protocol for patient dose measurement in diagnostics radiology (NRPB, 1992).

Radiation dose to patients represents an estimate of the likelihood of patients to develop stochastic radiation effects. Thus, the greater the dose absorbed, the greater the chances of stochastic effect, and may even reach a level that can elicit some non-stochastic effects. In view of this, the objective of radiation protection is to keep the probability of developing stochastic radiation effects to a minimum. This is achieved by constant dose monitoring to ensure that doses delivered per examination are within safe limits, but dosimetry is rarely routinely carried out in our radiology departments because of lack of equipment and personnel. Patients dose have always been measured using thermo-luminescence dosimeter (TLD) or ionization chamber. These two dosimeters though accurate, are relatively expensive, time consuming, a cumbersome and may intervene with patient exposure (Hanan, 2007).

Thermo-luminescent dosimeters (TLDs) have the advantage of being physically small, enabling them to be stuck directly and obstructively to the patient’s skin with little interference in patient’s mobility or comfort. They fully measure the radiation backscatter from the patient and do not obstruct useful diagnostic information. However TLD technique requires prolonged annealing and reading process. Furthermore, the use of TLD technique requires special equipment and thorough calibration facilities which may not be available in most X-ray departments (NRPB, 1992).

Ionization chambers are bulky and require connecting cables. They are usually difficult to attach in sufficiently close contact to the patient skin to ensure complete measurement of the backscatter radiation, severely restrict patient mobility and cast interfering shadows on radiographs. They are consequently not recommended for direct measurement of entrance skin dose on patients (NRPB, 1992).

A much easier and cost effective method of dose assessment can be done using calculation methods (Hanan, 2007). The method utilizing calculation by different formulae represents a viable option but the accuracy of these formulae has not been verified empirically. An empirical study which is the main scope of this research is therefore needed to establish the most accurate formulae to use in centers that lack equipment and personnel for dosimetry using various dosimeters.

1.1 Objectives of the Study

The objectives of the study are:

  • To determine the entrance skin dose in common radiographic procedures using LiF TLD chips.
  • To calculate the entrance skin dose in common radiographic procedures using different mathematical formulae.
  • To compare the calculated doses derived from these formulae with doses obtained from LiF TLD chip measurements in order to determine an acceptable method or formula of estimating ESD to patients.
  • To compare the dose measured from LiF TLD chips and those obtained from calculation methods with international reference standards.
  • To investigate the relationship between the applied peak kilovolatge (kVp) and the ESDs. 

 1.2 Justification of the Study

The study will enable practitioners to choose the most accurate and suitable formulae for estimating entrance skin dose with a view to monitoring patient doses, equipment commissioning and as a quality assurance parameter for verification after every major repair. The validated formulae will provide an alternative method of dosimetry to radiology departments without resources and personnel to carry out dosimetry using dosimeters. The study will also provide baseline data on patient radiation doses for use as guide in radiology departments and for further studies.            

CHAPTER TWO

REVIEW OF RELATED LITERATURE

2.0 Theoretical Review

2.1 Production of Characteristics X-ray

 The type of interaction that produces characteristics radiation, involves a collision between the high speed electrons and orbital electrons in the atom. Each electron in the target atom has a binding energy that depends on the shell in which it resides. Closest to the nucleus are two electrons in the K shell, which has the highest binding energy. The L shell with eight electrons, has the next highest binding energy, and so forth. When the energy of an electron incident on the target exceeds the binding energy of an electron of a target atom, it is energetically possible for a collisional interaction to eject the electron and ionize the atom. The unfilled shell is energetically unstable, and an outer shell electron with less binding energy will fill the vacancy. As this electron transits to a lower energy state, the excess energy can be released as a characteristic X-ray photon with an energy equal to the difference between the binding energies of the electron shells. Binding energies are unique to a given element, and so are their differences; consequently, the emitted X-rays have discrete energies that are characteristic of that element .

COMPARATIVE ANALYSIS OF CALCULATED AND MEASURED ENTRANCE SKIN DOSE OF COMMON RADIOGRAPHIC PROCEDURES IN BENUE STATE TEACHING HOSPITAL MAKURDI BENUE STATE