Thermal Therapy - RF/Microwave Interaction with Biological Tissues - page 177

of uterine cervix in 1898 and the effectiveness was confirmed [34]. In the early
twentieth century, applied research was carried out together with basic
research. However, since the heating method and temperature-measuring
technology, for example, did not develop sufficiently at that time, the positive
clinical application of hyperthermia treatment was not carried out. Therefore,
surgery, radiotherapy, chemotherapy, and so on, were dominant as therapy of
tumors. Under such situations, a selective antitumor effect for a tumor treated
at a temperature of 40-45°C was reported by Westermark. Later, hyperther-
mia attracted attention and its effect was supported by Westra, Overgaard, and
others [35, 36]. Especially after the report of an enhanced effect when hyper-
thermia and radiotherapy are combined by Crile [37] and Ben-Hur [38], the
interest in hyperthermia heightened, and research also rapidly developed. In
the 1970s, hyperthermia research activity started at a hyperthermia institute.
A symposium organized by the cancer center in the United Sates was held in
Washington in 1975 and the activity has been pursued. In 1977, the applica-
tion to cancer treatment opened a session of the International Microwave
Symposium of the Microwave Theory and Techniques (MTT) Society of the
IEEE, and a special issue of the MTT Transactions was published on the
subject the next year [39]. In the United Sates, a hyperthermia group was
formed in 1981 while the European Hyperthermia Institute was formed in
Europe in 1983. In Japan, hyperthermia research started in 1978 and The
Japanese Society of Hyperthermia Oncology was established in 1984. Research
on multidisciplinary therapy in which hyperthermia is used jointly with radio-
therapy, chemotherapy, surgical treatments, immunotherapy, and so on, has
become popular recently.

4.3.1 Biological Background of Hyperthermia

This section gives some reasons for the effectiveness of hyperthermia in treat-
ing tumors based on biological results on culture cells and tumors of labora-
tory animals.

Survival Rate and Hyperthermia Sensitivity of Cell Generally, living cells are
submitted to an effect of temperature rise: When the temperature increases,
the survival rate of the cell becomes lower. The survival rate of the cell is
defined as the ratio of the cell population submitted to heating to the cell pop-
ulation before heating. The result of measuring this survival rate as a function
of time is called the survival rate curve [40].
Figure 4.27 is a survival rate curve for the Chinese hamster ovarian cell. It
is observed that the survival rate rapidly decreases above 42.5°C and decreases
when heating time increases. In short, the cell population begins to decrease
depending on these factors. The effect due to heating, that is, this hyperther-
mia sensitivity, depends on various kinds of factors. It has been said that there
is no difference in hyperthermia sensity between a normal cell and a cancer
cell in every phase of cell growth. However, one feature of the normal cell is