Why is Chemotherapy Given in Cycles?
A friend recently asked why chemotherapy infusions are spaced out. Is it to allow the body a chance to recover between treatments, or is chemo actually more effective when given in cycles? How does it work?
The answer is a bit of both. Traditional chemotherapy involves drugs that are toxic for rapidly dividing cells
. The theory is that because cancer cells are dividing very quickly, they will succumb to the toxic chemotherapy quicker than normal cells would. The fact that chemo affects rapidly dividing cells also explains their typical side effects
; hair loss because hair follicle cells are constantly dividing, nausea and diarrhoea because the cells lining the digestive system are constantly dividing and so on. So if you take too much chemo too soon, then normal cells would be affected even more, which in turn could lead to even more nasty side effects.
But it’s also important to remember that the cancer cells in a tumour are at various stages of the cell cycle
. The cell cycle is a programme that every cell goes through during growth and division - cells first need to double their DNA and then divide into two.
The way chemo works is by causing DNA damage
, so that cancer cells kill themselves. This happens through a process known as ‘apoptosis’
. So a chemotherapy infusion kills all the cells that are in the DNA synthesis phase of the cell cycle, but doesn’t affect cancer cells that are in the other stages. By spacing it out and giving an infusion a few weeks later, the doctors are able to target the cells they couldn’t get the first time around
. Normal cells usually repair the damage from chemotherapy more effectively than cancer cells, so damage to cancer cells should progressively build up without causing permanent damage to normal cells
When I explained this to my friend, she mentioned how "it’s a lot like waiting for dandelions to get big enough so they can be ripped out effectively. It’s much harder when they are tiny. But you want to actually get to them before the flowers set seed"
. It’s a brilliant analogy, and explains really well why chemotherapy is given in cycles.
Image credit: https://www.flickr.com/photos/swallowtailgardenseeds/15644450971/
H/T to +Mary Anne Mohanraj
for the wonderful analogy! #ScienceEveryday