doubling time
Meaning of doubling time: Not how long one cell takes to divide, but how long a whole population takes to double, which is usually far longer.
Definition of doubling time
Doubling time is the interval a growing population takes to double, whether that population is bacteria in a flask, cells in a culture, or the volume of a tumor. It is worked out from two measurements taken at different times, assuming growth continues at the same proportional rate in between. The same calculation describes how fast a substance measured in blood is rising.
Why is it longer than the time a cell takes to divide?
Because a population is not a single cell. In a tumor only some cells are actively cycling; others sit dormant, and a substantial share of the new cells produced die before they can divide again. Both effects drain away growth that the cell cycle is generating. A cell may complete a full division in about a day while the tumor around it takes months to double in bulk, and the gap between those two figures is the arithmetic of dormancy and cell death. Confusing the two is the commonest error made with the term, and it makes tumors sound as though they grow far faster than they do.
What does a doubling look like on a scan?
Less than expected. Volume rises as the cube of the diameter, so doubling the volume of a round lesion increases its width by only about a quarter. A nodule that has genuinely doubled can therefore look almost unchanged to the eye on two scans side by side, which is why comparison is done by measured volume rather than by impression. The rate matters as much as the size: lesions that double in days are usually inflammation or infection, ones that barely change over years are usually harmless, and the middle range is where cancer concentrates.
Where is the measure used?
| Measure | What is being timed | Where it is used |
|---|---|---|
| Cell cycle time | One cell completing a division | Laboratory culture |
| Volume doubling time | A mass doubling in bulk | Comparing scans of a lung nodule |
| Marker doubling time | A blood level doubling | Following a rising tumor marker |
| Generation time | A bacterial population doubling | Microbiology |
All of these assume steady exponential growth, and real tumors depart from it. Growth slows as a mass enlarges and outstrips its blood supply, so a doubling time measured now cannot be projected far forward, nor run backward to date when the tumor began. Treatment changes it again, which is one reason a lengthening interval is watched alongside time to progression rather than in place of it.
Also known as
Used in a sentence
The tumor volume doubling time was estimated from the two scans.
Good to know
Doubling time describes a population rather than a single cell and runs longer than one cell division, because many cells are dormant and many newly made cells die; growth also slows as a mass enlarges, so a rate measured now cannot be projected far in either direction.