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In photodynamic therapy (PDT) a laser is used to excite an injected substance in order to damage tumor cells. Though this method has been known for several decades and there is strong evidence for the potential of photodynamic therapy in treating malignant brain tumors, it is still not a standard procedure. [1][2]

Basic principle

For photodynamic therapy a photosensitizer is injected into the body (e.g. talaporfin sodium). Then a clearance phase follows to accumulate the photosensitizer in the tumor cells. This means the body has some time (ca. 48 h) to wash out the substance, while this process is faster in normal tissue than in the tumor. After that, singlet oxygen is generated by exciting the photosensitizer by irradiation with a laser of a certain wavelength. The toxic singlet oxygen directly injures the tumor cells (necrosis and apoptosis), occludes the tumor vessels and has also been shown to enhance the patient’s immunity. [3]

The basic principle is visualized in the figure below.

 figure: Basic principle of photodynamic therapy. [4]

Problems

  • Photosensitizer might also be excited by visible light 
    → patient has to stay in a dark room during the whole treatment (approx. one week)
  • Though PDT is considered to be less invasive than other treatment options, not only is the DNA of the tumor damaged but also DNA of healthy cells (severe side effects) 
    → trying to irradiate mainly the tumor with the laser lowers this effect
  • Difficult to find right photosensitizer for different applications [3]

Bibliography

1) Quirk B. J. (2015) Photodynamic therapy (PDT) for malignant brain tumors--where do we stand?, https://www.ncbi.nlm.nih.gov/pubmed/25960361

2) Muller P. J. (1990) Photodynamic therapy of malignant brain tumours, Laser Med Sci 5: 245. doi:10.1007/BF02031391, https://link.springer.com/article/10.1007/BF02031391

3) Akimoto J. (2016) Photodynamic Therapy for Malignant Brain Tumors, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4831940/

4) Niemz M. H. (2007) Laser-tissue interactions, 3rd ed., Springer: Berlin, Heidelberg


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