London researchers shine light on skin cancer treatment volumes

London researchers are using a novel technique to more precisely define the extent of skin cancer. In the new Lawson Health Research Institute (Lawson) study, scientists found that the standard radiation margins used for treating non-melanotic skin cancer may not be large enough for all patients.

Squamous cell carcinoma and basal cell carcinoma, also known as non-melanotic skin cancers, are the most commonly diagnosed cancers worldwide. While radiation is a common and effective therapy, it can be hard to determine the precise area that requires treatment. This is because the borders around some tumours are not visible to the naked eye. These are called poorly demarcated tumours.

In an effort to improve patient outcomes while sparing patients from unnecessary radiation, a standard margin of 10 mm is extended beyond the visible tumour. This marks the area that will be treated with radiation in the hopes of treating the entire disease.

“Unfortunately, the cancer returns in 10 to 20% of these patients, suggesting the standard margin may not be adequate,” said Dr. Stephanie Casey, Clinical Fellow in Radiation Oncology at the London Regional Cancer Program (LRCP), London Health Sciences Centre (LHSC). “My colleagues and I wanted to more precisely determine the reach of these tumours to see if we could improve radiation treatment and patient outcomes.”

In this study, Dr. Casey and her team used a technique called protoporphyrin fluorescence at the LRCP Photodynamic Therapy clinic. Photodynamic therapy (PDT) is a treatment that uses photosensitizers like amino-levulinic acid (ALA) to treat some skins cancers. “Photosensitizers are drugs that react to light. When photosensitizers are exposed to the proper colour of light, they produce a form of oxygen that kills nearby cells, including cancerous ones,” said Dr. Kevin Jordan, Scientist at Lawson and Clinical Physicist at LRCP.

Photosensitizers can also act in a second way. Blue light can be absorbed by the photosensitizer and then released as red fluorescent light. Since ALA is more strongly absorbed by cancerous cells, they shine brighter than healthy cells and this method can be used to visualize the tumour’s borders and any smaller sites of disease.

The team of Lawson scientists studied protoporphyrin fluorescence with a number of non-melanotic skin cancer patients, including those who had tumours with distinctly visible borders. The photosensitizer was mixed with cream and applied to the skin cancer lesion and the surrounding area. After two hours, the excess cream was removed and the patient’s tumour was examined under blue light. The full area affected by disease was marked and results were photographed and examined, both in normal light and under blue light. This full process is called photo-delineation.

In total, 62% of patients with poorly demarcated tumours had disease that extended beyond the standard 10 mm margin. Results from the study suggest that a standard margin of 15 mm would be more effective for patients with poorly demarcated tumours. The extent of disease was not consistent around the visible disease site. This means that radiation treatment does not need to be given in a uniform shape around the visible disease. Rather, with the use of protoporphyrin fluorescence, some areas can be spared from radiation.

In contrast, patients who had tumours with distinctly visible borders did not have disease that extended beyond the standard 10 mm margin.

Patients’ radiation treatments were customized based on results. The study followed patients for an average of 22 months following treatment. These patients had a local control rate of 93.9%, meaning they did not show signs of their cancer recurring.

“We believe we are the first cancer clinic to use protoporphyrin fluorescence in this way. The results are promising and illustrate the usefulness of protoporphyrin fluorescence in determining appropriate radiation treatment volumes,” said Dr. Edward Yu, Scientist at Lawson, Radiation Oncologist at LRCP, and Principle Investigator of this study.

The team at LRCP will continue using protoporphyrin fluorescence for patients with poorly demarcated tumours. It is their hope that other cancer clinics in Canada will follow suit.

The paper, “Use of Protoporphyrin Fluorescence to Determine Clinical Target Volume for Non-melanotic Skin Cancers Treated with Primary Radiotherapy”, is published online on Cureus. The study received partial funding from donations to the LRCP Small Grants Fund.