Photodynamic therapy (PDT) is promising, but there’s still work to do

Photodynamic therapy can be used to treat some forms of cancer (Image © National Cancer Institute)

Just over a week ago, the Sunday Times ran a story about the benefits of a type of cancer therapy called photodynamic therapy, or PDT (link needs subscription).

According to the story, the UK government plans to publish a report in the new year, outlining funding for several new centres to deliver this treatment. This is welcome news, but it’s important not to get too carried away about the potential benefits for cancer patients.

For a start, PDT is only suitable for treating certain types of cancer under certain circumstances. And although there have been promising results from some clinical trials, others haven’t been so positive.  So it’s a bit premature to describe PDT as the “fourth weapon in the battle against cancer alongside … surgery, radiotherapy and chemotherapy.”

Despite these caveats, the technique – which uses laser light to activate cancer drugs within the body – will almost certainly be a useful tool for doctors in the future. Let’s look at PDT in a bit more detail and get the context behind the headlines.

What is PDT?

Photodynamic therapy uses precise beams of light to treat cancer, as part of a two-step process.

First, the patient is given an inert ‘pro-drug’ that builds up in the body, particularly in tumours. The pro-drug is then converted to an active drug when light of a certain wavelength is shone on it. Because the light and the drug only meet each other in the patient’s tumour, the drug is only activated there. This is designed to reduce damage to healthy tissue, keeping side-effects to a minimum.

As we’ve said, PDT is already being used to treat certain types of cancer – such as some skin and oesophageal cancers – and the details of exactly how it works vary depending on the patient’s cancer type. In the case of skin cancer, the pro-drug is applied in a cream. Other times it can be given as an injection.

Similarly, laser light may be shone onto a tumour from a lamp or torch-like device, other times it needs to be inserted inside the body on an endoscope-like tube. There’s detailed information about different types of PDT currently in use on our CancerHelp UK website.

Although being treated with light, rather with surgery or chemotherapy, may instinctively seem ‘better’, it’s worth pointing out that PDT does have significant downsides.

The pro-drug can make patients very sensitive to light so they need to stay in the dark, often for several weeks. This may seem a small price to pay for treating cancer, but sometimes PDT is used to lessen symptoms in people who can’t be cured – in this context, spending their last few weeks in darkness may not be something a patient decides is worthwhile.

Current treatment

As we mentioned above, PDT has been proven effective in treating a few types of cancer at certain stages of their development, and NICE have recommended its use on the NHS to treat people with these forms of cancer if it’s appropriate (see, for example, their guidance on non-melanoma skin cancer).

For example, PDT can be used to treat certain types of non-melanoma skin cancer; various cancers of the airways; and oesophageal cancer. In the latter case, it can be used to attempt to cure very small, early oesophageal cancers – although unfortunately the disease isn’t usually diagnosed until a later stage – or to lessen symptoms in more advanced cancer.

Future research

There have been several recent UK clinical trials of PDT, as a quick browse around our clinical trials database will reveal (search for “photodynamic therapy” and tick the ‘trials with results’ and ‘have finished recruiting’ boxes). Some of these we have helped fund. However, not all of these trials have published their results yet, so it will be interesting to see what new evidence emerges over the next few years.

However, some results have already appeared, and some show that PDT is at least as effective as the treatment with which it was compared. And in most cases it has fewer side effects and is more acceptable to patients. But it’s fair to say that, compared to other treatments overall, the ‘evidence base’ for PDT is relatively patchy. There’s much more work to do, and many questions to answer.

As an example, a recent review of the evidence for using PDT to treat nasopharyngeal cancer concluded that “PDT has the potential to be a very effective local treatment… without the severe side effects seen with radiotherapy”.

But the paper also pointed out that there was still an important problem to be solved: illuminating people’s mouths and throats uniformly was extremely difficult, which could lead to some areas receiving too much treatment, while other patches miss out.

As the authors wrote, “some critical structures are well protected by bone; others, however, like the soft palate and the dorsal oropharyngeal wall would suffer unacceptable damage by this approach”. The scientists responsible are now testing a new device that could solve this problem.

And not all trials have been successful – for example, a recent trial into using PDT to treat bile duct cancer closed early after the PDT group actually did worse than people given standard treatment. The research team, who presented their results at this year’s National Cancer Research Institute conference, and who were funded by Cancer Research UK, are now analysing their results further to try to find out why.

What is Cancer Research UK doing about PDT research?

At Cancer Research UK, we fund research proposals brought to us by scientists (as our colleague Simon previously wrote about). At the moment, we’re not actively funding any ongoing trials into PDT. But if a research proposal was put to our funding committees we would consider it, just as we would any other research proposal.

We do, however, have a strong track record in PDT research. The Paterson lamp, used in PDT for skin cancer, was developed by Cancer Research UK in the 1990s. And in the past we’ve funded the work of UK PDT expert Dr Stephen Pereira, who has carried out clinical trials looking at using PDT to treat bile duct cancer and pancreatic cancer.

Sense of perspective

Like many of the new treatments that are currently being researched, photodynamic therapy has considerable potential. But as the results of recent trials emerge, just as there’s certain to be good news, there will also be more questions raised. So while we’re fully supportive of efforts to improve and research PDT, we think that there’s a need for a sense of perspective about the treatment.

PDT’s not (and probably won’t ever be) a treatment that will work on all cancers – and it’s certainly not a ‘miracle cure’ for the disease. The truth is that the evidence of its effectiveness isn’t yet strong enough to make anything but cautious predictions about its future – whatever you may read in the newspapers.

Henry