Radiologic Physics Lecture 1

Study Notes

Deoxyribonucleic acid (DNA) encodes genetic instructions essential for all living organisms and many viruses.
DNA consists of two strands known as polynucleotides, made up of nucleotides.
Each nucleotide comprises a nitrogenous base (guanine, adenine, thymine, cytosine), a deoxyribose sugar, and a phosphate group.
Nucleotides are linked by covalent bonds forming a sugar-phosphate backbone.
Base pairing dictates that adenine pairs with thymine, and cytosine pairs with guanine, stabilizing the double-helix structure.
In cells, DNA is organized into chromosomes, which are duplicated during cell division through DNA replication.

Single Strand Break: Often results in point mutations, primarily from low-LET (Linear Energy Transfer) radiation. Cells can often repair this damage.
Double-Strand Break: More frequent with high-LET radiation, this type of break poses a greater risk to cell survival.
Double-Strand Break in Same Rung: Leads to cleaved chromosomes, potentially causing cell death or impaired function.

Cells are divided into two main parts: the nucleus, which manages cellular activity, and the cytoplasm where metabolism occurs.
Cytoplasm includes organelles: cell membrane, endoplasmic reticulum, ribosomes, mitochondria, lysosomes, Golgi complex.
Nuclear structures consist of the nuclear envelope, chromosomes, and DNA.

Mitosis: Somatic cell division resulting in two identical daughter cells, encompassing four phases: prophase, metaphase, anaphase, telophase.
Meiosis: Specialized division reducing chromosome number from diploid (2n) to haploid (n), producing four gametes from an original germ cell.

Radiosensitivity refers to the vulnerability of cells to radiation damage; it's directly proportional to the rate of cell division and inversely proportional to cell differentiation.
Actively dividing and immature cells are most radiosensitive.
High division rates, high metabolic activity, non-specialized nature, and good nourishment enhance radiosensitivity.

Fully differentiated, non-dividing cells (nerve/muscle cells) are radioresistant.
Partially differentiated cells (liver/glandular cells) show moderate sensitivity.
Intermediate cells support other tissue cells and have intermediate radiosensitivity.
Dividing cells in the early stages of differentiation (myelocytes, spermatocytes) are fairly radiosensitive.
Continuously dividing stem cells (e.g., bone marrow, intestinal, skin, testes) are very radiosensitive but can rapidly aid recovery if not killed.