Tracking protons for better treatment

Just over two years from now the first patient is expected to be treated in a brand new proton therapy centre at the UMCG hospital. University of Groningen scientists have been awarded a Demonstrator Grant by STW Technology Foundation to build a device for rapid treatment-plan verification. The new method they have developed should be at least three times faster than current ones.

Proton therapy is a relatively new, advanced form of radiotherapy in which cancer patients are irradiated with beams of protons instead of the usual high-energy photons. In contrast to conventional radiotherapy, the beams used in proton therapy deposit the highest dose near the end of their trajectory. This difference means that for selected patients, proton therapy can be a better choice.

Proton beams can be directed very precisely, which allows the radiation oncologist and other medical staff to develop a patient-specific treatment plan. The tumour then receives the prescribed dose while the surrounding healthy tissue is spared as much as possible. ‘The therapeutic benefit of proton beams comes at a price, however’, explains Marc-Jan van Goethem, a physicist at the Radiation Oncology Department of the UMCG.

Accuracy is essential to the clinical outcome, so if the treatment plan is off the mark, it can fail to kill part of the tumour and can damage healthy tissue. All new patient treatment plans must therefore be verified before treatment begins.

‘The treatment plans are verified in water, because the human body is two-thirds water, and water is the standard medium used in radiotherapy dosimetry’, says Oksana Kavatsyuk. She shows us a picture of the most advanced verification device so far: a grid panel that is submerged in a tank of water. ‘The grid measures proton radiation at a large number of positions, providing a 2D dose distribution at a certain depth. You have to repeat the measurement at between five and 20 different depths to obtain a 3D dose distribution.’ This is time-consuming, taking about three-quarters of an hour per treatment plan. As at least two new treatment plans have to be verified every day, this is a considerable burden on precious facility time.

Kavatsyuk, Van Goethem and Sytze Brandenburg, their colleague at the KVI Center for Advanced Radiation Technology (KVI-CART), the University of Groningen accelerator facility, have come up with an idea that will speed up this verification. ‘When the protons release their energy in water, this produces weak ultraviolet light’, says Van Goethem. ‘We can detect this with sensitive UV cameras. By imaging the UV light from different angles, we can reconstruct a 3D dose distribution that can be compared to the treatment plan’.

Moreover, the team has found a compound that increases the amount of detected light. The drawback is that the detected light is no longer directly proportional to the dose deposited by the protons, so some extra corrections are needed.

‘But in any case we believe we can reduce the time needed to check the treatment plan by at least a factor of three’, says Kavatsyuk. Van Goethem adds, ‘We estimate that our method will save half an hour per day in the treatment room, freeing up valuable time in such an advanced treatment facility’.

The scientists have submitted a patent application for their new method. ‘This will allow us to develop the technique’, explains van Goethem. The group has been awarded a € 150,000 Demonstrator Grant by STW Technology Foundation to build a prototype device. ‘We have just started working on the project. A new UV camera arrived last week, so we can start testing.’ Once they have validated their method, they will need industrial partners to transform it into a commercial device.

Van Goethem: ‘We have about a year before the first patient arrives. Working at KVI-CART, which has been involved in proton therapy research for years, gives us a big advantage. A lot of proton therapy research is done in clinical facilities, where the patients always take precedence. Having a proton beam accelerator dedicated to research means we can make much faster progress.’