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The risk associated with diagnostic x-ray examination is determined by the amount of radiation dose (effective dose) received by patient during the procedure.

Risk is the probability of an undesired consequence of an event, such as radiation

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exposure, within a specified period of time. Risk is defined as a quantitative function of exposure and biological effect (Rutherford, 2002). Some of the risks associated with radiation exposure are radiation sickness, cell damage, vascular damage, deaths and cancer induction.

Current radiation protection standards and practices (that is, the philosophy of radiation protection) are based on the premise that any radiation dose, no matter how small, can result in detrimental health effects, such as cancer and hereditary damage (Kenneth et. al, 1996). Although in radio-diagnosis, an optimally restricted field sizes and limited beam penetration are expected for a given examination, yet exposure of surrounding organs, which have relatively high degree for cancer inductions cannot be avoided in some instances. In such a situation, great care is taken to minimize the radiation dose to such critical organs.

2.4.1 RADIATION RISK MODELING

Several researches, from where risk models are formulated, had been conducted to improve the scientific basis of radiation risk assessment. The findings from these researches are used to develop highly effective management strategies for solving radiation related problems (NAP, 2006). One of the recent developments in radiation protection researches, which has implication for risk assessments is the use of mathematical models for cancer induction/formation. These models are based on a multistage mechanism. In addition to meeting the requirements for being the primary purpose of representing radio-epidemiologic data, these models also conform to the established radio-biologic principles.

However, in the current cancer risk model, the risk estimates obtained are in apparent conflict with radio-biologic principles in two main aspects namely the shape of the dose-response curve (linearity Vs curvilinearity) and dependence of Radio Biological Effectiveness (RBE) on doses. These conflicts are expressed to verify whether the occurrence of excess cancer rates is threshold dependence or linear no-threshold dependence.

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2.4.2 LINEARITY VS CURVI-LINEARITY (NON LINEARITY) OF DOSE- EFFECTS RELATIONSHIP

One of the major disagreements between radio-biologic observations and estimates of the current risk model is the shape of the dose-response curve for gamma radiation. In the majority of radio-biologic observations of specimen after photon irradiation, the dependence of cancer induction on dose is found to be curvilinear which is often described as linear-quadratic dependence. However, between low to moderate doses (<< 2 Gy) of radiation, there is no apparent deviation from linearity in the excess rates of solid cancer among the atomic-bomb survivors, which were the primary source of risk estimates. As a result of this finding, in current models, dose proportionality is used instead of linear-quadratic dose dependence. However, the International Commission on Radiological Protection (ICRP, 1991) resolved the argument by declaring that linear dependence of cancer induction/development on radiation dose is inconsistent with radio-biologic findings. That means, the dependence of cancer induction on radiation doses is linear-quadratic in shape.

2.4.3 LINEAR NON THRESHOLD DOSE MODEL (LNT)

The linear non threshold model (LNT) is a model used to express the detriment caused by ionizing radiation. The model proposed that the response of a biological medium to radiation exposure is linear (i.e. directly proportional) at all levels of dose, no matter how small the exposure is. Thus LNT asserts that there is no threshold of exposure below which the response ceases to be linear. The LNT Model stands in contrast to theories, which states that below a certain radiation level, radiation is harmless or that there is a threshold dose for radiation damage. One of such theories is radiation hormesis, which states that radiation is beneficial at very low doses but harmful at large doses.

For the purpose of radiological protection, the assumption of LNT model is that the underlying dose-response relationship is linear-quadratic with no threshold dose (NCRP, 2002). From the epidemiological point of view, the dose-response

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relationship for leukaemia is linear-quadratic while for solid tumours, the response is consistent with linearity down to about 100 mSv.

LNT model was initially rejected by the United States Department of Energy (USDOE) because they felt it was “inconvenient” (Gofman, 1994). In order to put the record straight, the report published by the National Academy of Sciences (National Academies Press, 2006) declared that there is no safe level of radiation. Earlier reports had presented equivocal and inconclusive discussions on the issue (National Research Council, 2005).

2.4.4 CONTROVERSY ABOUT LNT MODEL

In recent time, the accuracy of the LNT model at low dose of radiation has been questioned. Many believed that when radiation is distributed uniformly within a biological medium such that its intensity is comparable to natural/background level, then it would have no harmful health effects on the medium.

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) in its most recent report (UNSCEAR 2000) stated that “Until the uncertainties on low-dose response are resolved, an increase in the risk of cancer induction which is proportionate to radiation dose is reasonable with developing knowledge and is the most scientifically acceptable approximation of low-dose response. However, a strictly linear dose response should not be expected at all circumstances”.

The risk coefficient published by the ICRP (ICRP, 1990), being widely accepted at the time of this study, was used to estimate the risk associated with diagnostic x-ray examinations at four different diagnostic centres in Nigeria.

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Some of the measurable quantities that could be used to estimate radiation risk in diagnostic x-ray examinations and especially in film radiography are itemized below:

i. Incident Air kerma/Exposure ii. Dose-area product (DAP) iii. Entrance Skin Dose (ESD) iv. Energy imparted

v. Organs‟ absorbed dose vi. Effective dose

Of all these itemized quantities, the effective dose is the preferred quantity for estimating radiation risk in most medical x-ray examination studies. This is because effective dose accounts for distribution of dose among the radiosensitive organs of the exposed person by summing up their individual doses, having weighted each organ according to its relative sensitivity to radiation effect (Wall, 1997; Broerse and Geleijns, 1998). In addition to this property, effective dose allows the complex dose pattern within the body to be expressed in terms of a simple quantity that is roughly proportional to the probability (risk) of radiation harm.