Biomedical Engineering Reference
In-Depth Information
and liver cancers and liver metastasis.
216
Thermal ablation is achieved by using
different sources of heating, like laser light, focused ultrasound, microwaves,
radiofrequency field, and magnetic resonance.
3
The most common traditional
method is radiofrequency ablation (RFA) which utilize the natural differences
in properties of the normal and carcinogenic tissues to achieve differential heat
deposition.
3
RFA has low side effects in the treatment of primary and metastatic
liver tumors
3
even though it requires targeting of each individual lesion. In RFA
therapy, complete destruction of the entire tumor with at least a 0.5 cm margin by
heating the tissue between 50-100 °C, forming “coagulation necrosis” is the ulti-
mate goal. Radio waves produce heat through resistive forces by ionic agitation
as they travel from the implanted electrode tip to the ground source placed outside
the body. RF energy has been shown to have low tissue specific absorption rates
(SAR) allowing for excellent whole body tissue penetration with documented
safety in humans exposed to an RF field for 10 min up to several hours. But, the
difference in sensitivity between normal and abnormal tissues is too small, so nor-
mal tissue can be damaged under RF irradiation.
3
Furthermore, the heterogeneity
of electrical conductivity of tissues does not allow selective heating.
To solve these issues, NMs are being studied to increase the contrast between
the two tissues for effective treatment.
3
The use of submicron particles to
enable cellular uptake that cause intracellular hyperthermia has been suggested
to increase the selectivity of the thermal destruction. The use of NMs intro-
duce advantages such as increased sensitivity to radiofrequency energy, lower
required doses and exposure times, selective delivery, and improved homogene-
ity of heat induction.
3
Currently, the NMs being studied for thermal ablation
therapies include superparamagnetic iron oxide nanoparticles (SPIONs), para-
magnetic copper-nickel alloy NPs, magnetite cationic liposomes, carbon par-
ticles (single walled carbon nanotubes and fullerenes), AuNPs and nanoshells.
3
7.5.1 Thermal Ablation Using MNPs
An approach to thermal ablation involves seeding the tumor with MNPs for
selective generation of heat in the tumor.
3
Iron oxide nanoparticles (IONPs)
have very good magnetic properties that allow them to heat up when placed
in an alternating magnetic field by hysteresis loss, induced eddy currents and
Neel relaxation.
217
NMs composed of Fe3O
4
were tested for heat induction in
human breast cancer xenografts in immunodeficient mice by loading the tumors
with 7.7 ± 2.3mg magnetite per 100 mg tissue.
218
When the mice were exposed
to an AC magnetic field for 4 min at an amplitude of 6.5 kA/m and frequency of
400 kHz using a circular coil applicator,
3
the results showed elevated tempera-
tures between 18 and 55 °C at sites containing magnetite agglomerates but there
were heterogeneous particle distribution. Furthermore, migration of particles
from the tumor tissue that were similar to those observed in human Phase I stud-
ies for the treatment of prostate cancer were recorded.
219
These situations must
be addressed during further developments for tumor applications.
