Photothermal Therapy
Encyclopedia
Photothermal therapy is an experimental use of electromagnetic radiation
(most often in the form of infrared
) that is proposed to treat various medical conditions, including cancer
. The basic model for its use is derived in part from photodynamic therapy
, in which a photosensitizer is excited with specific band light. This activation brings the sensitizer to an excited state where it then releases vibrational energy (heat
). The heat is the actual method of therapy that kills the targeted cells.
Unlike photodynamic therapy, photothermal therapy does not require oxygen to interact with the target cells or tissues. Current studies also show that photothermal therapy is able to use longer wavelegth light, which is less energetic and therefore less harmful to other cells and tissues.
Some research has indicated problems with aggregation of the photosensitizers, local shock waves, hyperthermic effects, but otherwise little phototoxicity.
Many of the side effects and complications, as well as the potential applications of photothermal therapy, are unknown.
. This was solved with the recent invention by Cathy Murphy of gold nanorods. The peak absorption of gold nanorods may be tuned from 550 nm up to 1 micrometre by altering its aspect ratio. Once tuned, scientists have learned how to remove the toxic byproduct of CTAB with non-cytotoxic polyethylene glycol
(PEG). The PEG not only keeps the nanorods from aggregating in serum once injected, they also lend to long circulation times for the gold nanorods. Research has shown that the longer the circulation time, the better adsorption of the nanorods into the cancer tumor. This is non-directional (enhanced permeability and retention effect
) and has shown better than 7% accumulation in the cancer tumor from an intravenous injection. Current studies have shown half life circulation times of greater than 15 hours. Once the nanorods have cleared the blood stream, the cancer tumor may be illuminated ex vivo with a diode laser. Nanorods located at distances 10 times their size can still absorb 80% of the incident light energy creating massive heat load to the surrounding cancer tumor. Current studies involve mice but are being extended.
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
(most often in the form of infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
) that is proposed to treat various medical conditions, including cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
. The basic model for its use is derived in part from photodynamic therapy
Photodynamic therapy
Photodynamic therapy is used clinically to treat a wide range of medical conditions, including malignant cancers, and is recognised as a treatment strategy which is both minimally invasive and minimally toxic...
, in which a photosensitizer is excited with specific band light. This activation brings the sensitizer to an excited state where it then releases vibrational energy (heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
). The heat is the actual method of therapy that kills the targeted cells.
Unlike photodynamic therapy, photothermal therapy does not require oxygen to interact with the target cells or tissues. Current studies also show that photothermal therapy is able to use longer wavelegth light, which is less energetic and therefore less harmful to other cells and tissues.
Some research has indicated problems with aggregation of the photosensitizers, local shock waves, hyperthermic effects, but otherwise little phototoxicity.
Many of the side effects and complications, as well as the potential applications of photothermal therapy, are unknown.
Recent Study
One of the biggest recent successes in photothermal therapy is the use of gold nanoparticles. Spherical gold nanoparticles absorptions have not been optimal for in-vivo applications. This is because the peak absorptions have been limited to 520 nm for 10 nm diameter silicon nanoparticles to only 580 nm for gold nanoparticles approximately 100 nm in diameter. Skin, tissues, and hemoglobin have a transmission window from 650 nm up to 900 nm with a peak transmission at approximately 800 nm known as the Near-Infrared WindowNear-infrared window in biological tissue
The near-infrared window defines the range of wavelengths where light has its maximum depth of penetration in tissue. Within the NIR window, scattering is the most dominant light-tissue interaction, and therefore the propagating light becomes diffused rapidly...
. This was solved with the recent invention by Cathy Murphy of gold nanorods. The peak absorption of gold nanorods may be tuned from 550 nm up to 1 micrometre by altering its aspect ratio. Once tuned, scientists have learned how to remove the toxic byproduct of CTAB with non-cytotoxic polyethylene glycol
Polyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...
(PEG). The PEG not only keeps the nanorods from aggregating in serum once injected, they also lend to long circulation times for the gold nanorods. Research has shown that the longer the circulation time, the better adsorption of the nanorods into the cancer tumor. This is non-directional (enhanced permeability and retention effect
Enhanced Permeability and Retention effect
The Enhanced Permeability and Retention effect is the property by which certain sizes of molecules tend to accumulate in tumor tissue much more than they do in normal tissues...
) and has shown better than 7% accumulation in the cancer tumor from an intravenous injection. Current studies have shown half life circulation times of greater than 15 hours. Once the nanorods have cleared the blood stream, the cancer tumor may be illuminated ex vivo with a diode laser. Nanorods located at distances 10 times their size can still absorb 80% of the incident light energy creating massive heat load to the surrounding cancer tumor. Current studies involve mice but are being extended.
See also
- PhotomedicinePhotomedicinePhotomedicine is an interdisciplinary branch of medicine that involves the study and application of light with respect to health and disease. Photomedicine may be related to the practice of various fields of medicine including dermatology, surgery, interventional radiology, optical diagnostics,...
- Light TherapyLight therapyLight therapy or phototherapy consists of exposure to daylight or to specific wavelengths of light using lasers, light-emitting diodes, fluorescent lamps, dichroic lamps or very bright, full-spectrum light, usually controlled with various devices...
- Hyperthermia therapyHyperthermia therapyHyperthermia therapy is a type of medical treatment in which body tissue is exposed to slightly higher temperatures to damage and kill cancer cells or to make cancer cells more sensitive to the effects of radiation and certain anti-cancer drugs...
- Experimental cancer treatmentExperimental cancer treatmentExperimental cancer treatments are medical therapies intended or claimed to treat cancer by improving on, supplementing or replacing conventional methods ....
- Unproven cancer therapyUnproven cancer therapyAlternative cancer treatments describes alternative and complementary treatments for cancer that have not been approved by the government agencies responsible for the regulation of therapeutic goods. They include diet and exercise, chemicals, herbs, devices, and manual procedures...