Friday, February 8, 2013

Photodynamic Therapy

I am currently teaching Medical Physics (PHY 326) at Oakland University, and for our textbook I am using (surprise!) the 4th edition of Intermediate Physics for Medicine and Biology. In class, we recently finished Chapter 14 on Atoms and Light, which “describes some of the biologically important properties of infrared, visible, and ultraviolet light.”

Once a week, class ends with a brief discussion of a recent Point/Counterpoint article from the journal Medical Physics (see here and here for my previous discussion of Point/Counterpoint articles). I find these articles to be useful for introducing students to cutting-edge questions in modern medical physics. The title of each article contains a proposition that two leading medical physicists debate, one for it and one against it. This week, we discussed an article about photodynamic therapy (PDT) by Timothy C. Zhu (University of Pennsylvania, for the proposition) and E. Ishmael Parsai (University of Toledo, against the proposition):
Zhu, T. C., and E. I Parsai (2011) PDT is better than alternative therapies such as brachytherapy, electron beams, or low-energy x rays for the treatment of skin cancers. Med. Phys. 38: 1133-1135.
When reading through the article, I thought I would check how extensively we discuss of PDT in Intermediate Physics for Medicine and Biology. I found that we say nothing about it! A search for the term “photodynamic” or “PDT” comes back empty. So, this week (with an eye toward the 5th edition) I am preparing a very short new section in Chapter 14 about PDT.
14.8 ½ Photodynamic Therapy

Photodynamic therapy (PDT) uses a drug called a photosensitizer that is activated by light [Zhu and Finlay (2008), Wilson and Patterson (2008)]. PDT can treat accessible solid tumors such as basal cell carcinoma, a type of skin cancer [see Sec. 14.9.4]. An example of PDT is the surface application of 5-aminolevulinic acid, which is absorbed by the tumor cells and is transformed metabolically into the photosensitizer protoporphyrin IX. When this molecule interacts with light in the 600-800 nm range (red and near infrared), often delivered with a diode laser, it converts molecular oxygen into a highly reactive singlet state that causes necrosis, apoptosis (programmed cell death), or damage to the vasculature that can make the tumor ischemic. Some internal tumors can be treated using light carried by optical fibers introduced through an endoscope.”
The two citations are to the articles
Wilson, B. C. and M. S. Patterson (2008) The physics, biophysics and technology of photodynamic therapy. Phys. Med. Biol. 53: R61-R109.

Zhu, T. C. and J. C. Finlay (2008) The role of photodynamic therapy (PDT) physics. Med. Phys. 35: 3127-3136.
The first PhD dissertation from the Oakland University Medical Physics graduate program dealt with photodynamic therapy: In Vivo Experimental Investigation on the Interaction Between Photodynamic Therapy and Hyperthermia, by James Mattiello (1987).

You can learn more about photodynamic therapy here and here. Please don’t confuse PDT with the alternative medicine (bogus) treatment “Sono Photo Dynamic Therapy.”

1 comment:

  1. This is a very interesting debate. While I agree with E. Ishmael Parsai's cautionary statements about PDT, I would say the same for the radiation therapies he mentions. While more is known about how to deliver a homogeneous dose to a specific target (size, shape, depth) what remains unclear in many cases is how to distinguish between the normal and cancerous regions such that the cancer cells are eliminated. In defense of PDT, I would want readers to understand the side effects involved in radiation therapies. It is often painful and unsuccessful.