Ultrasounds are a non-ionizing mechanical wave with frequencies higher than 15kHz, beyond the audible range of the human ear. When an ultrasound is given, the ultrasound machine conducts an electrical signal through crystals found in the head of the ultrasound probe. The crystals vibrate and create mechanical waves which transfer energy to the surface of the human body and can target soft tissue deep to the epidermis. Medical ultrasounds fall into two categories: diagnostic and therapeutic. Diagnostic ultrasound uses sound waves to create images of the body for diagnostic purposes. In ultrasound therapy, the sound waves interact with tissues in the body so that they are either modified or destroyed. Modifications include moving or pushing tissue, heating tissue, dissolving blood clots, or delivering drugs to specific areas of the body.
Ultrasound is best known for its use in imaging, but it was first used therapeutically when medical uses for ultrasound began to be explored in the 1930’s. It was recognised in 1927 that ultrasound could produce lasting changes in biological systems, and this was the beginning of research into its safety and therapeutic uses. Low-frequency ultrasounds have been used since the 1950s in physiotherapy to treat tendinitis or bursitis, and scientific advances allowed a more effective treatment of Meniere’s disease by destroying the vestibular nerve, and of Parkinson’s disease using focused ultrasound for localized tissue destruction in the brain. In the 1980s they were used to treat kidney stones, and research continues on more difficult applications in neurosurgery and cancer treatment.
Types of ultrasound therapy:
- High power: High intensity focused ultrasound. Megahertz ultrasound in tissue allows a high powered device to be brought to a tight focus at a distance from its source. This generates high temperatures within the focal region, with the surrounding areas left unheated. This therapy leads to instantaneous cell death.
- Low power: Low intensity pulsed ultrasound is a form of ultrasound that delivers waves at a much lower intensity than traditional ultrasound energy. Includes physiotherapy, fracture repair, fracture repair, sonophoresis, sonoporation, and gene therapy. Its potential for cancer treatment is currently being explored.
- Thermal: The ultrasound wand heats the skin and muscles through vibration. This therapy is used to treat soft tissue pain, skin problems, and other musculoskeletal issues, as well as uterine fibroids and prostate cancer.
- Mechanical: Also known as cavitation ultrasound therapy, it involves the ultrasound waves creating pressure differences in tissue fluids which forms bubbles. As the bubbles interact with solid objects, they burst and create shockwaves. This method can be used to treat everything from kidney stones to cancer.
Ultrasound therapy for cancer
Multiple types of ultrasound therapy are being studied for their effects on cancer. Ultrasound treatments of tumors rely on the activation of multiple pathways to cause cell apoptosis and necrosis. For instance, high‐intensity focused ultrasound (HIFU), high‐intensity pulsed ultrasound, and low‐intensity pulsed ultrasound have been used to ablate tumors. Moreover, ultrasound has been used in combination with other treatments like hyperthermia, chemotherapy, and sonodynamic therapy to enhance the efficacy of tumor treatment.Ultrasound waves can disrupt the structure of cancer cells, (microtubules) while leaving normal cells alone. Ultrasound treatment alone disrupted the microtubule network (eightfold reduction in the number of intact microtubules per 10 μm in comparison to the non‐treated cells (~40 vs. 5). However, normal cells like epithelial cells and fibroblasts showed minimal microtubule disruption upon ultrasound treatment, indicating that they were largely unaffected by the ultrasound forces.
High intensity focused ultrasound (HIFU) is a cancer treatment that aims to kill cancer cells with the high frequency sound waves. Researchers are currently looking at HIFU as a treatment for different cancer types. Most of the research so far has been for prostate cancer, but it is being studied for its effects on:
- rectal cancer
- cervical cancer
- vaginal cancer
- womb cancer
- soft tissue sarcoma
- pancreatic cancer
- breast cancer
Researchers around the world have also looked at HIFU as a treatment for other cancers including kidney cancer and liver cancer. HIFU doesn’t pass through solid bone or air, so it’s not effective for every cancer.
Histotripsy
Histotripsy is a new technology, recently approved by the FDA for the treatment of liver tumors, that uses focused ultrasound energy to destroy tumors. It works through a process called cavitation, or the creation of air pockets in a substance. The focused energy created by the ultrasound machine generates enough force to destroy the liver tumor. It can treat any type of liver tumor, whether a primary tumor (a tumor that begins in the liver) or another type of cancer that has spread, or metastasized to the liver. It is currently being tested for tumors in other organs, but is not currently approved for the treatment of tumors outside the liver.
Oncotripsy
Oncotripsy is a type of treatment that has the ability to selectively target cancer cells through the ultrasound harmonic excitation at their specific resonance frequency. Recent studies suggest that low intensity ultrasound waves at specific resonance frequencies help to disintegrate the cancerous cells without affecting non-cancerous cells. This effect occurs through the ultrasound causing resonant oscillations in cellular membranes that lead to membrane rupture.
The resonance frequencies are a function of cellular mechanical properties and thus have different values in cancerous and non-cancerous cells. This predisposes cancer cells to be vulnerable to certain frequencies of ultrasound that have no effect on non-cancerous cells. This type of therapy is promising as it can target the tumor without harming the rest of the body.
When studying the effects of low intensity pulsed ultrasound, researchers found that ultrasound applied at a frequency of 0.5–0.67 MHz and a pulse duration of >20 ms caused selective disruption of a panel of breast, colon, and leukemia cancer cell models in suspension without significantly damaging healthy immune or red blood cells. The formation of acoustic standing waves and the emergence of cell-seeded cavitation lead to cytoskeletal disruption, expression of apoptotic markers, and cell death in cancer cells.
Some Takeaways…
- Ultrasound therapy has a wide variety of promising uses and is something to keep an eye on.
- Therapies that specifically target exclusion zone water show a lot of promise when it comes to cancer, an unstructured state.
- Many clinics and hospitals are starting to offer ultrasound therapy as a non-invasive alternative to traditional treatments.
- Many are experimenting with ultrasound therapy for the treatment of other conditions with great success.
- Home devices are becoming more accessible and affordable.