Page 203 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

202 CHAPTER 8 Ultrasound applications in cancer therapy

area. Microspheres have four different types: Bioadhesive microspheres that exhibit
mucoadhesive property and permits the drug-coated on the surface of the polymer to
stick to the targeted organ, resulting in prolonged delivery of the therapeutic agents
to the diseased site. Magnetic microspheres which can be used for both diagnos-
tic purposes and drug delivery. The drugs within these particles can be targeted to
the diseased area using an external magnetic source [68]. Also, they are commonly
utilized for magnetic hyperthermia in tumor tissues. Floating microspheres that are
meant to release the drugs loaded in them in gastric content. Radioactive particles
which are used for the therapeutic purpose. They directly inject in the veins and link
to the targeted organ or tissue.

8.4.2 Sonodynamic therapy (SDT)
Sonodynamic therapy (SDT) is an emerging approach that involves a combination
of low-intensity ultrasound and specialized chemical agents known as sonosensitiz-
ers. This method has been developed as a novel promising noninvasive approach
derived from photodynamic therapy (PDT), that is mediated via ultrasound-induced
cavitation and sonosensitizers to produce free radicals to kill dividing cancer cells.
The SDT method is used to treat deeply located tumors, however, PDT utilizes vis-
ible light, which has limited penetration, and can only be employed superficially
or intraoperatively. Jin et al. found that SDT inhibited tumor growth by 77%, com-
pared with 27% for PDT in a subcutaneously located murine squamous cell carci-
noma [69]. To improve the efficacy of treating solid tumors, it is important that the
sonosensitizer is injected intravenously prior to insonation, rather than directly into
the tumor, so that it is more fully and evenly distributed throughout the neoplasm
[70].In SDT, the sonication parameters (usually 1.0–2.0 MHz at an intensity of
0.5–3.0 W cm) have been selected to produce inertial cavitation in a cell culture or
tumor, where a bubble in a liquid rapidly collapses, producing a shock wave which
produces free radicals and a cascade of molecular events that activate the sonosen-
sitizer and in turn damage the cancer cells [71]. Gao et al. reported that SDT also
had an antivascular effect and inhibited tumor neovascularization [72]. Combining
PDT with SDT had a synergistic effect in solid tumors with additional posttherapy
tumor necrosis, inhibition of tumor growth and increased survival times [71]. The
combining of the sonosensitizer with a microbubble contrast agent provides new
developments in SDT [73].
The sonication parameters in SDT (usually 1.0–2.0 MHz at an intensity of
2
0.5–3.0 W/cm ) have been selected to produce cavitation in a cell culture or tumor.
Cavitation process involves the nucleation, growth, and implosive collapse of gas-
filled bubbles under the appropriate ultrasound conditions. It may be essentially
classified into stable and inertial cavitation 14. Bubbles of stable cavitation oscil-
late, creating a streaming of the surrounding liquid which results in a mixture of
the surrounding media while the gas bubbles in inertial cavitation process grow to a
near resonance size and expand to a maximum before collapse violently 18. Further,
insonation of the microbubble may lead to an additional significant local thermal