Page 191 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

190 CHAPTER 8 Ultrasound applications in cancer therapy

concomitant cavitation in tissue. Detailed safety considerations should accompany
the introduction of HIFU applications into clinical practice in order to assure benefit
while minimizing risk to the patient.

8.2.2 Non-thermal ultrasound therapeutic applications
8.2.2.1 Extracorporeal shock wave lithotripsy
Extracorporeal shockwave lithotripsy (ESWL) introduced in the 1980s and became
the dominant treatment method. This method relies on nonthermal ultrasound ther-
apy mechanisms [27].
Shock wave devices similar to lithotripters are approved for orthopedic indica-
tions. The shockwaves are used for treating other problems, such as gall bladder
stones, but none have achieved widespread usage. Most lithotripters now are of the
electromagnetic design, however very few lithotripters utilize piezoceramic sources.
All produce about the same waveform: a 1-µs shocked spike of about 50 MPa fol-
lowed by a ∼10-MPa, 4-µs negative pressure tail. The center frequency might be
estimated to be about 150 kHz although. For better treatment efficiency in ESWL
treatment, the source is coupled to the patient by a water pillow and transmission
gel, and in the remaining original lithotripters through a water bath [12]. About 3000
shock waves are triggered at about 2 Hz repetition rate to pulverize the stone so that
the pieces (<2 mm) can pass naturally in urine. Cavitation chips away from the out-
side, adding cracks that grow by dynamic fatigue and further grind down the stone
to passable size [28]. Shockwave Lithotripsy (SWL) works better with some stones
than others. Very large stones cannot be treated this way. Stones that are smaller than
2 cm in diameter are the best size for SWL. The treatment might not be effective in
very large ones.
Lithotripsy has several important biological side effects, such as blood vessel
walls break; Inflammation ensues (i.e., lithotripsy nephritis), scar formation and per-
manent loss of functional renal mass. SWL is more appropriate for some people than
others. Because X-rays and shock waves are needed in SWL, pregnant women with
stones are not treated this way. People with bleeding disorders, infections, severe
skeletal abnormalities, or who are morbidly obese also not usually good candidates
for SWL. In addition to and likely a result of this direct injury cascade, lithotripsy can
lead to an accelerated rise in systemic blood pressure, a decrease in renal function,
onset of hypertension, an increase in the rate of stone recurrence, and an exacerba-
tion of stone disease. The risks of these adverse bioeffects in lithotripsy have stimu-
lated investigation into mitigation methods with some success. For example, a slower
repetition rate (1 Hz) is safer and more effective than the common fast rate (2 Hz)
[29], and a pause early in treatment nearly eliminates injury in animals [30]. Overall,
lithotripsy has been a therapeutic ultrasound method with a high level of efficacy
and patient benefits, but also some important risks particularly for patients requiring
repeated treatments. The development of safer treatment protocols for lithotripsy is
a prime example of the potential value of research on risk mitigation for optimizing
the patient risk/benefit profile in therapeutic ultrasound.