Thermal Therapy - RF/Microwave Interaction with Biological Tissues - page 178

that it reaches the fixed size and there is a period of time when cell breeding
stops. During this period, hyperthermia sensitivity becomes low, compared to
a period of normal cell growth. On the other hand, cancer cell growth does
not stop, and it tends to keep increasing indefinitely. Accordingly, the hyper-
thermia treatment is considered to affect more the cancer cell.

Oxygen Partial Pressure, pH, and Hyperthermia Sensitivity of Cell The
hyperthermia sensitivity of the cell is influenced by oxygen partial pressure
and pH, which are environmental factors of the cell. Generally, in the tumor,
it is considered that the blood vessel growth cannot follow the tumor growth,
and the tumor becomes a no-development condition. Therefore, it is in the
anaerobic condition where the oxygen from blood flow is not sufficiently sup-
plied. In this condition, hyperthermia sensitivity is high, as shown in Figure
4.28. It is also observed in the same figure that the sensitivity becomes lower
when pH is higher. Generally, when we consider the gross tumor, it tends to

1.0

Oxic
pH 7.2

0.1

Hypoxic

pH 7.2

0.01

Hypoxic

pH 6.4

Oxic
pH 6.4

0.001

0 30 60 90 120

Heating Time (at 42.5)

FIGURE 4.28 Survival rate depending on cell environmental factor (oxygen partial
pressure and pH).