Photothermal Therapy
Photothermal therapy (PTT) 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.
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. 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.
See also
- Photomedicine
- Light Therapy
- Hyperthermia therapy
- Experimental cancer treatment
- Unproven cancer therapy
