Radiation Damage to Tissues

Some forensic evidence is easy to detect. Gunshot and knife wounds and the burns inflicted by chemicals or fire are obvious examples. However, other causes of injury or death are not as easily detected, at least in their early stages. An example of the latter is exposure to radiation. While exposure to a high level of radiation can cause rapid death and massive burning of the skin, the exposure to less immediately harmful levels of radiation cause subtle internal changes in the body. Knowledge of these changes can be useful to a forensic investigator.

Certain types of radiation exposure may cause mutations (DNA damage and genetic alterations) or accelerate the types of mutations that occur spontaneously at a very low rate. Ionizing radiation was the first mutagen that efficiently and reproducibly induced mutations in a multicellular organism. Direct damage to the cell nucleus is believed to be responsible for both mutations and other radiation-mediated genotoxic effects like chromosomal aberrations and lethality. Free radicals generated by irradiation of the cytoplasm are also believed to induce gene mutations even in the non-irradiated nucleus.

There are many kinds of radiations that can increase mutations. Radiation is classified as ionizing or non-ionizing depending on whether ions are emitted in the penetrated tissues or not. X rays, gamma rays, beta particle radiation, and alpha particle radiation (also known as alpha rays) are ionizing forms of radiation. An example of non-ionizing radiation is sunlight, more specifically the ultraviolet component of the visible light spectrum of wavelengths.

Critical lesions leading to mutations or killing of a cell include breaks in the DNA strands, damaged bases (the building blocks of DNA: adenosine, thymine, cytosine, guanine) and sites where a base is deleted. Large chromosomes can also be deleted when cells damaged by radiation are replicating. Except for large deletions, most of these lesions can be repaired to a certain extent, and the lethal and mutagenic effect of radiation is assumed to result principally from incompletely or incorrectly repaired DNA. This view is supported by experimental studies, which showed that mice given a single radiation dose, called an acute dose, develop significantly higher levels of mutations than mice given the same dose of radiation spread over a period of weeks or months, allowing time for DNA repair.

Biologically, the different effects produced by the different types of radiation involve the way energy is distributed in irradiated cell populations and tissues. For example, alpha radiation ionizations occur every 0.2–0.5 nanometers (nm), which leads to an intense localized deposition of energy. Accordingly, alpha radiation particles will travel only about 50 nm before expending of their energy. Primary ionization in x rays or gamma radiation occurs at intervals of 100 nm or more and traverses centimeters into tissues. This penetration leads to a more even distribution of energy as opposed to the more concentrated or localized alpha rays.

Thus, in a forensic examination, the pattern of radiation damage can be a clue to the type of radiation that was involved.

see also Chromosome; DNA; Dosimetry; Radiological threat analysis.