There are a number of descriptors of tumour growth. This section is based on Radiobiology for the Radiologist[1]

## Cell Cycle Time (T_{c})

T_{c} describes the average length of time for tumour cells to progress through the cell cycle. A shorter T_{c} leads to more rapid tumour growth. Cell cycle time is typically measured by using flow cytometry to estimate the number of cells in S-phase, and extrapolating from there the length of the cell cycle. S phase is typically 12 hours but may vary widely in 25% of tumours.

## Growth Fraction (GF)

GF describes the number of cells that are actively participating in the cell cycle. Cells in G_{0} or which are terminally differentiated do not contribute to the growth fraction. The percentage of cells in the cell cycle can be measured using Ki-67 immunohistochemistry.

## Potential Volume Doubling Time (T_{pot})

T_{pot} is the time required for a tumour to double in size, taking into account the number of cells in the cell cycle and the speed of progression through the cell cycle.

T_{pot} is found using the growth fraction and the cell cycle time:

T_{pot} varies from 4 to 34 days for most tumours.

## Volume Doubling Time (VDT)

This is the amount of time the tumour takes to double in size. It is dependent on the number of cells in the cell cycle, the speed of progression through the cell cycle, and the number of cells that are lost from the tumour mass.

VDT is usually constant for an individual tumour, but often falls with larger tumours as the number of cells lost to hypoxia and necrosis increases.

VDT is usually much less than the T_{pot} in larger tumours, due to the large number of cells that are lost.

## Cell Loss Factor (CLF)

The cell loss factor describes the percentage of cells within the tumour that die and reduce the tumour size. It corrects for the discrepancy between T_{pot} and the VDT.

Cell loss may be due to migration of cells away from the cell mass (metastasis) or death of cells (apoptosis or necrosis).