Thermal Therapy - RF/Microwave Interaction with Biological Tissues - page 179

the anaerobic condition, the anaerobic glycolysis becomes active, and also pH
becomes acidic, being lower. Accordingly, this means that hyperthermia is
effective when the conditions of hypoxia (low oxygen partial pressure) and
low pH hold. The efficiency of hyperthermia has been estimated in view of
these environmental factors. The effect of irradiation is considered to be small
for the hypoxic cell.

Period and Hyperthermia Sensitivity of Biological Cell Generally, the bio-
logical cell, which constitutes the organism, repeats the cell division with a
fixed period of time called a cell cycle . Cell division does not repeat forever.
This is because there are times called the M phase (mitotic phase) during which
cells divide and the interphase , when there is no cell division. This one-cell
cycle is separated into four phases: pre-DNA synthesis phase (G 1 ), DNA syn-
thesis phase (S), division preparatory phase (G 2 ), and cell-dividing phase (M).
After all this, the division repeats in terms of initial-phase G 1 phase, S phase,
G 2 phase, and M phase, as shown in Figure 4.29. In each phase, it is in the last
half of the S phase that hyperthermia sensitivity appears the highest. The next
closest is the M phase. On the other hand, the sensitivity to irradiation is high
in the M stage, but it is low in the S phase. Therefore, it is possible to promote
a decrease in the survival rate of the cell in the S phase by hyperthermia while
a large effect is not obtained by irradiation exposure. This is why combining
irradiation and hyperthermia is an effective therapy.

Cell division
phase

M phase

Division preparatory
phase

Synthesis preparatory
phase

G 2 phase G 1 phase

S phase

DNA reproduction
phase

FIGURE 4.29 Cell cycle.