Photodynamic therapy (PDT) is a treatment that uses a drug, called a photosensitizer or photosensitizing agent, and a particular type of light. When photosensitizers are exposed to a specific wavelength of light, it is theorized they produce a form of oxygen that kills nearby cells. 
Each photosensitizer is activated by light of a specific wavelength. This wavelength determines how far the light can travel into the body. Therefore, doctors use specific photosensitizers and wavelengths of light to treat various areas of the body with PDT.
In addition to potentially killing cancer cells, PDT appears to shrink or destroy tumors in two other ways. The photosensitizer may cause damage to the blood vessels in the tumor, thereby preventing cancer from receiving necessary nutrients. PDT may also activate the immune system to attack the tumor cells.
Photodynamic Therapy (PDT) and Sono-Dynamic Therapy (SDT) have been innovatively combined into the Sono-Photo Dynamic Therapy (SPDT) method. The possibility that SPDT may help even the most advanced cancers, SPDT could provide a powerful and non-toxic method to destroy aberrant cancer cells. 
Sono-Photo Dynamic Therapy combines two therapies suggested to destroy cancer cells. SPDT involves getting an agent into the whole body, (originally by injection, now orally), which adheres to cancer cells, so that when light and now, sound, of the correct frequency is applied, the agent “explodes” into free radical oxygen, essentially killing the cancer cells which cannot survive in oxygen. It is suggested to be followed up by cleansing of toxins from the body, especially including all of the just killed cancer cells.
In this treatment, a patient ingests an oral, non-toxic, photosensitive dietary supplement called SP-Activate (SP-A). SP-A preferentially absorbs into cancer cells. At precise sound (sono) and light (photo) frequencies, SP-Activate produces a high-energy molecule that induces free radical oxygen, destroying the cancer cell. The sound and light frequencies are directed locally at a tumor site, or systemically (treating a large area of the body).
It is important to note that the origin of cancer must still be dealt with.
SPDT is recently approved in the U.S., UK, and has been adopted by the Chinese Government. Experimental projects have recently been published in Toronto, Canada, and other countries.
Sono-Photo Dynamic Therapy has been used in combination with other therapies to treat a number of cancers, including but not limited to the following: 
- Bone Cancer
- Brain Cancer
- Breast Cancer
- Cervical Cancer
- Colon Cancer
- Esophageal Cancer
- Head and Neck Cancers
- Liver Cancer
- Lung Cancer
- Ovarian Cancer
- Pancreatic Cancer
- Prostate Cancer
- Rectal Cancer
- Stomach Cancer
- Thyroid Cancer
- Urinary Bladder Cancer
- Uterine Cancer
- Vaginal Cancer
Research continues on ways to increase PDT effectiveness and expand it to other cancers. Clinical trials are underway to evaluate the use of PDT for cancers of the brain, skin, prostate, cervix, and peritoneal cavity. Other studies are focused on the development of more powerful and targeted photosensitizers that are activated by light which can penetrate tissue and treat deep or large tumors. Researchers are also working toward ways to improve equipment and the delivery of the activating light.