A tumor which is 100 hypoxic is irradiated with a single Xr

A tumor which is 100 % hypoxic is irradiated with a single X-ray dose of 18 Gy. The surviving fraction from this dose is measured to be 0.05. When a similar tumor in an air-breathing animal is given the same dose, the surviving fraction is found to be 0.01 in the second tumor. Calculate the fraction of hypoxic ceils the second tumor.

Solution

When tumor cells are deprived of oxygen they are referred as hypoxic tumor cells. These hypoxic tumor cells are resistant to radiation therapy and many chemotherapeutic drugs. In response to radiation, surviving fraction of the hypoxic tumor cell is high compared to air-breathing tumor cells. Anoxic cells are 3 times less resistant to radiation than a hypoxic tumor cell, this is because oxygen is a good chemical radiosensitizer which promotes DNA damage after absorbing energy from radiation.

In the above case 1: 100% Hypoxic cell

Surviving fraction= 0.05

Case 2: Anoxic cells

Surviving fraction= 0.01

Fraction of hypoxic cell in case 2 tumor= (Surviving fraction in case:2 Anoxic cell)/(Surviving fraction in case:1 100% hypoxic cell)

=0.01\\0.05

= 0.2