Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

In general, the gonads are very radiosensitive. The testes contain both radiosensitive(e.g., spermatogonia) and radioresistant (e.g., mature spermatozoa) cell populations.The primary effect of radiation on the male reproductive system is temporaryor permanent sterility after acute doses of approximately 2.5 Gy (250 rad) or 5 Gy(500 rad), respectively. Temporary sterility has been reported after doses as low as150 mGy (15 rad). Reduced fertility due to decreased sperm count and motility canalso be seen after chronic exposures of 20 to 50 mGy/wk (2 to 5 rad/wk) when thetotal dose exceeds 2.5 Gy (250 rad). These effects ate not of concern with diagnosticexaminations, because doses exceeding 100 mGy (10 rad) are extremely unlikely.The ova within ovarian follicles (classified according to their size as small, intermediate,or large) are sensitive to radiation. The intermediate follicles are the mostradiosensitive, followed by the large (mature) follicles and the small follicles, which arethe most radioresistant. Therefore, after a radiation dose as low as 1.5 Gy (150 rad), fertilitymay be temporarily preserved owing to the relative radioresistance of the maturefollicles, and this may be followed by a period of reduced fertility. Fertility will recurprovided the exposure was not high enough to destroy the relatively radioresistant smallprimordial follicles. Doses in excess of 6 Gy (600 rad) are typically required to producepermanent sterility; however, sterility has been reported after doses as low as 3.2 Gy(320 rad). In either case it seems that higher doses are required to produce sterility inyounger women. Another major concern is the induction of genetic mutations andtheir effect on future generations. This subject is addressed later in the chapter.The lens of the eye contains a population of radiosensitive cells that can be damagedor destroyed by radiation. Insofar as there is no removal system for these damagedcells, they can accumulate to the point at which they cause cataracts. The magnitudeof the opacity as well as the probability of its occurrence is proportional to the dose.The latent period is inversely related to dose and typically requires at least 1 year afterthe exposure. High-LET radiation is more efficient for cataractogenesis by a factorof 2 or more. Acute doses as low as 2 Gy (200 rad) have been shown to producecataracts in a small percentage of people exposed, whereas doses greater than 7 Gy(700 rad) always produce cataracts. Chronic exposure reduces the efficiency ofcataract formation. For example, the threshold for cataract formation when the exposureis protracted over 2 months is 4 Gy (400 rad), and for 4 months it is 5.5 Gy(550 rad). Cataracts among early radiation workers were common because of theextremely high doses resulting from long and frequent exposures from poorlyshielded x-ray equipment. Today, radiation-induced cataracts are rare and even thehighest exposures to the eyes of radiation workers in a medical setting (typically fromfluoroscopic procedures) do not approach the threshold for effects over an occupationallifetime.A unique aspect of cataract formation is that, unlike senile cataracts,which typically develop in the anterior pole of the lens, radiation-induced cataractsbegin as a small opacity in the posterior pole and migrate anteriorly.As previously discussed, the body consists of cells of differing radiosensitivities anda large radiation dose delivered acutely yields greater cellular damage than the same