Electron beams have a finite range, after which the dose falls off rapidly. As such, electrons are suited to the treatment of superficial lesions such as skin tumours. They can also be used for intraoperative radiotherapy, to provide a boost to the tumour bed, for example, after mastectomy or lumpectomy.

But while electron radiotherapy has been available for decades, technology advancements such as the introduction of intensity-modulated radiotherapy (IMRT) have resulted in a far reduced role for electrons. So is there a case for a similar effort to be focused on developing modern electron therapy technologies, or have electrons become obsolete?

Arguing for the motion that electrons are indeed underused, Mikael Karlsson from Umeå University in Sweden was first to present his case to the ESTRO delegates. The benefits of electrons, Karlsson explained, is that they do not deposit dose behind the target, which significantly reduces normal tissue dose. He argued that if you need a high surface dose, you need to use electrons. And if you have critical structures behind the target tumour, electron therapy is ideal. "Do not spray the patient with radiation," he emphasized.

One downside of electrons is the wide penumbra of a pure electron beam, which increases at depth. According to Karlsson, one way to address this is via the use of combined electron/photon treatments. He presented an example study comparing IMRT with hybrid electron/IMRT, using standard hardware. Results showed that a mixed electron/photon planning technique decreased the normal tissue integral dose compared to a photon-only IMRT plan.

Karlsson also described the potential of combining energy modulation of the electron beam with intensity modulation using a multileaf collimator (MLC) to deliver electron IMRT. To examine this approach further, he has performed Monte Carlo optimizations of the parameters required to adapt a standard accelerator for efficient electron/photon delivery.

"I think that we should use a scanning beam, or at least put the scattering foils at the target position to reduce the penumbra," he said. Planning systems must be developed that can optimize electrons and photons together. Other adaptations include the use of as much helium in the treatment head as possible, to reduce air scatter, and a double-focused MLC for both electrons and photons. Finally, such accelerators must allow for fast energy switching within the same sequence.

Karlsson predicts that we will see a mixed-beam radiotherapy system at ESTRO next year, with only software developments needed to achieve this goal. "I think that all the pieces are there, we just need to put them together," he said. "Then we need to get these tools into the clinic to see how they could be used."

The argument against

Rock Mackie, Director of Medical Devices at the University of Wisconsin–Madison's Morgridge Institute for Research, does not agree. "Electron beams are more trouble than they're worth," he told the assembled crowd.

One problem, Mackie explained, is the increased costs associated with multi-modality radiotherapy accelerators, advanced treatment planning systems that require Monte Carlo-based dose calculations and increased commissioning requirements. Tissue heterogeneities also pose a bigger problem for electrons than photons. But crucially, almost all applications for electron therapy can now be adequately performed using IMRT, brachytherapy or orthovoltage X-rays.

For example, head-and-neck treatments that historically used both photon and electron beams can now be easily and effectively treated with IMRT. Other examples include electron boosts to scars during breast treatments and small lesions treated by complex electron fields, both of which can now be treated with photons. "IMRT can do better than electron beam radiotherapy for most of these indications," he said. "There really is no good case for electrons."

As for modulated electron radiation therapy, according to Mackie, electrons pose "all the difficulties of protons", such as the need to modulate the energy as well as the intensity and no exit beam for verification, but without the benefits of proton therapy's Bragg peak and large penetration depth. "The fundamental thing about electrons is that they are very much like protons, except they're too light. This makes the scattering an order of a million times worse."

In direct opposition to Karlsson's viewpoint, Mackie is firmly set against the idea of a combined system for photon and electron therapy, pointing out that some of the worst radiotherapy accidents occurred due to the existence of an electron mode on a photon linac. He stated that extreme capabilities should not be mixed in equipment, noting that a domestic plug should not be expected to run an X-ray generator, so why should there be two orders of magnitude in electron fluence in a linac?

The final vote

Mackie was not entirely negative on the subject of electron beam therapy, admitting that for intra-operative radiotherapy, there is a good case for the use of a specialized electron machine. "I agree it would be nice to exploit electron beams, they have some advantages if you can control the scattering, but I'm not sure its place is on a linac," he said.

"I completely disagree," rebutted Karlsson. "I think that we should use electrons together with photons. We should use them to not deposit more energy in the patient than is actually needed to treat the tumour."

Speaking in the Q&A session rounding off the debate, Mackie argued that a well-designed specialized electron system does make a lot of sense. But with manufacturers looking for large markets, it may call for a small vendor to build a small machine for superficial and intraoperative treatments. He described his vision for such a system. "I'd start with a robot, use 15 MeV energy and create a fan beam with static magnets so that you can treat wide fields," he said. "The MLC would be close to the patient and the robot would scan the area to treat it. That would be a cool machine."

As for the views of the attending delegates, a rough show-of-hands vote prior to the presentations revealed that just over half of the audience thought that electrons are underused in radiotherapy. At the end of the debate, only a handful had changed their minds. Ultimately, all agreed to disagree.