Page 189 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

188 CHAPTER 8 Ultrasound applications in cancer therapy

remains uncertain [14, 16]. However, the risk of harm such as burns appears to be
low when the modality is properly applied. Overall, ultrasound for physical therapy
has therefore provided a modest level of efficacy and patient benefit, but also a low
level of risk.

8.2.1.2 Hyperthermia
A new method by using ultrasonic waves to heat relatively large volumes of tissue for
the purpose of cancer therapy is developed science 1980s and 1990s. This method of
hyperthermia involves uniformly heating a tumor to about 42°C for periods of about
1 h, which appears to be effective in reducing tumor growth [17]. In some cases of
hyperthermia therapy, the cancerous tissue alone or the tissue and its surrounding
is exposed to high temperature, as high as 44°C and the healthy cell temperature of
the body during the hyperthermia treatment should stay less than 41°C. In clinical
trials, hyperthermia was used with or without radiation therapy and modest efficacy
has been reported [18]. Research suggests that hyperthermia may be advantageous
for drug delivery treatment using nanoparticles. However, the HIFU is applied to
hyperthermia cancer treatment instead of the moderate-temperature hyperthermia in
clinical usage.

8.2.1.3 High intensity focused ultrasound
One of the noninvasive methods of using ultrasound for the treatment of cancer that
has been considered in recent years, is HIFU, that uses nonionizing ultrasonic waves
to heat tissue. HIFU can be used to increase the flow of blood or lymph, or to destroy
tissue, such as tumors, through a various mechanisms. By increasing the tempera-
ture in a short time and in focal zone, destruction of cancer cells without damage
to healthy tissues is possible. This method was initially studied clinically for ther-
mal ablation of inoperable brain tissue for Parkinson’s disease [19]. The continuous
lower frequencies are used to achieve the necessary thermal doses. However, pulsed
waves may also be used if mechanical (such as lithotripsy) rather than thermal dam-
age is desired. The necessary intensity at the target tissue without damaging the
surrounding tissue is achieved by using the acoustic lenses. HIFU may be combined
with other imaging techniques such as medical ultrasound or magnetic resonance
imaging (MRI) to enable guidance of the treatment and monitoring.
In expressing the major advantage of using ultrasound waves in treating cancer
compared to other therapies, can mention as follows [20]:
• Noninvasive
• Increase temperature only in the focal area and reduce the chance of damage to
adjacent tissues and skin
• Nonionizing tissue and no limitation in number of radiation
• Can be used in treating most of the cancerous tissues
In a HIFU system, a signal generator is connected to a focusing transducer, which
2
produces very high local intensities of >1 kW/cm of 0.5–7 MHz ultrasound at the
focal spot (see Fig. 8.3). HIFU application in therapy and treatment of disease is one