**3. Biological interactions of US**

**2. Basic physics of waves and ultrasound**

summarized in figure 1 and 2.

2 Updates on Cancer Treatment

**Figure 1.** Physical parameters of sound physics

To understand the basic principle of high intensity focused ultrasound as an ablative treatment modality for prostate cancer, the main properties of sound waves should be known and are

Sound is defined as a mechanical disturbance from a state of equilibrium that propagates through an elastic material medium in the form of vibrating waves. These vibrating waves transport energy from its source to another area as long as a medium is present. The energy transported by sound is known as acoustic energy or sonic energy. Sonic intensity (SI), defined as a time-average rate of sonic energy-flow through a unit area (SI unit: W/cm2), is proportional to sonic pressure square and has a positive correlation with the power and energy of sound. This implies that the amount of energy accumulating at a target area is larger when the sonic pressure or intensity increases. The latter parameter varies with space and time, and it is usually expressed as peak or average intensity, and both quantities can refer to either a spatial or temporal dimension (e.g. ISP = spatial peak intensity, ISATA = spatial average, temporal average intensity). The frequency of a wave is derived from the amount of cycles per unit of time. Ultrasound (US) is a form of sound that has a frequency higher than the frequency detected by human ear (> 20.000 Hz vs. 20-20.000 Hz). HIFU therapy utilizes high intensity US waves that are propagated through human tissues. In contrast to other ablative therapies, HIFU therapy operates without the need of an electrode or antenna to deliver its waves. However, the main challenge of such a technique is focusing energy-accumulation at the target area to

Since the mid 1950, US have been extensively used for diagnostic procedures without any detrimental effect on the human body [21]. However, it must be noted that US waves used for therapeutic purposes could be potentially dangerous. In fact, it carries energy that causes biological reactions in various ways. The most important are the thermal (hyperthermia) and non thermal (acoustic cavitation) effects [22-23]. The thermal effect is tissue heating and protein denaturation can occur at a temperature of 56°C [24-25]. Protein denaturation contributes to a subsequent coagulation necrosis (thermal ablation). The effect is a function of the wave amplitude and pulse frequency. Acoustic cavitation is defined by the creation and develop‐ ment of bubbles or cavities. The violent oscillation and collapse in US fields of these bubbles cause a significant degree of mechanical and thermal effects. Mechanical and cavitation tissue effects are caused by high amplitudes and ultra-short pulse wave, with amplitude reaching 100 Mpa and a pulse of 1-2 ms. Moderate pressure amplitude of 10 Mpa and continuous wave of 1 s are the cause of thermal effects. Short ultrasound pulses and low amplitude up to 1 Mpa causes no significant heating, but cavitation caused by this type of waves are used in some circumstances like enhancing a drug delivery or the transient opening of the blood-brain barrier [22].
