Question

4. What factors are required to estimate the genetically significant dose of an ionizing electromagnetic radiation such as X-rays or gamma rays? 5. What is the difference between the direct versus the indirect effects of radiation damage to biological target molecules? What is the most important biologic target molecule? 6. Using the calculation examples given in your chapter reading, estimate thc cntrance skin exposure for a PA chest X-ray image conducted at 120 kVp/2 mAs. 7. What four (4) factors are taken into consideration when a barrier for a radiographic room is designed and how would making a mistake on each of these factors affect radiation exposure to the clinician and/or patient?

Answer 1

4.Doses will change depending on a number of variables including the specific machine and manufacturer, study techniques (the settings of the machine used to produce the radiation in radiology and, in nuclear medicine, the amount of activity administered and the patient’s metabolism), and other issues.

7.Radiation exposures to the human body, whether from external or internal sources, can involve all or a portion of the body. The health effects of one unit of dose to the entire body are more harmful than the same dose to only a portion of the body, e.g., the hand or the foot. To enable radiation protection specialists to express partial‐ body exposures (and the accompanying doses) to portions of the body in terms of an equal dose to the whole body, the concept of effective dose was developed. Effective dose, then, is the dose to the whole body that carries with it the same risk as a higher dose to a portion of the body.

5.DIRECT EFFECTS: The direct effect theory or target theory proposes that ionizing radiation acts by direct hits on target atoms. All atoms or molecules within the cells, such as enzymatic and structural proteins and RNA, are vulnerable to radiation injury. DNA, however, is the principle target, in which ionizing radiation produces single- or double-stranded chromosomal breaks.

INDIRECT EFFECTS: The direct mechanism theory was found to be inadequate in explaining cellular radiation injuries. The indirect theory proposes that ionizing radiation exerts its effect via radiolysis of cellular water, forming free radicals. These free radicals interact with atoms and molecules within the cells, particularly DNA, to produce chemical modifications and consequently harmful effects. When X-rays interact with water, two types of free radicals are formed

H-----H[hydrogen]+OH[hydroxy]

The presence of an excess of oxygen during irradiation of cells allows the formation of additional free radicals

H+O2----HO2 [hydroxy peroxy free radical]

HO2+HO2----H2O2+O2