One form of cancer treatment has left patients with an unusual side effect: the ability to see in the dark. Scientists have been researching to learn why this cancer treatment night vision occurs. Now, they have an answer. (1)

Photodynamic Therapy

Photodynamic therapy uses a drug called a photosensitizer to kill cancer cells. When the photosensitizer is exposed to specific wavelengths of light, it generates a type of oxygen that kills cells. The light used for photodynamic therapy most often comes from a laser. For the treatment, doctors inject a photosensitizing agent into a patient’s bloodstream. Cells throughout the patient’s body absorb the photosensitizer. Since the photosensitizer remains in cancerous cells for longer than normal cells, doctors wait before activating the photosensitizer. Between 24 and 72 hours after the injection, the photosensitizer has left healthy cells but remains in cancerous ones. Doctors then expose the tumor to light. When the photosensitizer in the tumor reacts to the light, it creates an active oxygen form that kills the cancer cells. (2)

Besides killing off cancerous cells, photodynamic therapy destroys tumors in two other ways. First, the photosensitizer can damage blood vessels in the tumor, cutting off the nutrients it needs to continue growing. Additionally, it may trigger the immune system to attack cancerous cells. (2)

Photodynamic therapy is used to treat esophageal cancer and non-small cell lung cancer. It’s an outpatient procedure that can be repeated. Patients can receive photodynamic therapy in conjunction with chemotherapy, radiation therapy, and surgery. (2)

Cancer Treatment Night Vision

One odd side effect of photodynamic therapy is improved night vision. Although this might initially seem like a benefit, the side effect was upsetting to some patients. They began to see silhouettes and outlines in the dark. (1)

Curious as to why this unexpected night vision was happening, scientists began researching. Using a computer, they ran molecular simulations that imitate biochemical reactions. They learned that retinal, an active compound in the retina, is modified during photodynamic therapy. It begins to act the same way in the dark as it does in the light. (1)

“Thanks to experiments carried out by biologists in recent years, we now know that under infrared light the chemical structure of retinal is modified after the injection of chlorine (isomerization) in the same way as when it receives visible light. This explains the increase in night-time visual acuity,” scientist Antonio Monari explained to CNRS News. “However, we did not know precisely how rhodopsin [a photosensitive protein] and its active retinal group interacted with chlorine. It is this mechanism that we have now succeeded in elucidating via molecular simulation.” (1)

Future Possibilities

Now that scientists understand why this cancer treatment night vision occurs, there are many future possibilities for this knowledge. First, now that they understand the source of the night vision, they can work to reverse it in cancer patients. Additionally, they can use this information to develop treatments for ocular impairments, such as blindness and hypersensitivity to light. (1)

“One could thus imagine reproducing this mechanism and improving it to allow people to see at night, but intentionally this time,” Monari said. (1)