Radiation affects normal and cancerous tissues by damaging DNA within the cells and preventing cells from dividing. Rapidly dividing cells that lose the ability to reproduce die almost immediately, whereas more slowly dividing cells may live on for days, months or even years. The timing of cell death without replacement in normal tissues determines when side effects occur.
In the treatment of PCa, the most rapidly dividing cells are in the tissues lining the inside of the bladder and rectum. Since these are the first to die during RT, the initial effect is rectal and bladder irritability. This reaction is temporary and disappears as the tissues regrow during and after RT.
The slowest growing cells are the prostate cancer cells and those that make up the local connective tissues (mainly the wall of the bladder and rectum), and blood vessels that supply oxygen and nutrients to the tumour, muscles of the bladder and rectum, erectile nerves and tissue. These cells may take months or years to die after RT, which explains the gradual fall of PSA. Gradual loss of blood supply to the local tissues can cause permanent scarring that leads to long-term rectal and bladder irritation or bleeding and loss of erectile function. A man going into treatment with already impaired bowel, bladder or erectile function is more likely to experience permanent effects.
The simplest way to minimize radiation injury is to give small doses of radiation over a long period of time. However, the best way to kill prostate cancer cells is to use large daily doses of radiation. The only way to do this without increasing the risk of permanent injury is with sophisticated techniques (image-guided intensity modulated RT or stereotactic body RT) that avoid radiation to the sensitive tissues altogether. Clinical studies are underway to see whether we can use these new techniques to safely compress treatment courses into only a few weeks.
