FLASH: Revolution in RadiationTherapy

FLASH. Have you already heard of it? This treatment method for tumor diseases is a new, promising approach in radiationtherapy. In FLASH therapy an ultra-high dose rate is applied in less than one second. FLASH radiations place special demands on dosimetric measurement technology. 3 questions and 3 answers to Dr. Rafael Kranzer, R&D Project Manager, and Dr. Frank Schwamm, Product Manager, who develop FLASH dosimetry solutions at PTW.

In 2019, the EU launched a project with the aim of developing a method for measuring dose at the ultra-high pulse dose rates involved in FLASH radiotherapy. What was this project about, and what role did PTW play in this project?

Dr. Rafael Kranzer:
FLASH is a new promising treatment technique in radiation therapy. Before this treatment method can be implemented in clinical practice, there must be a procedure that makes it possible to precisely measure radiation dose at ultra-high pulse dose rates. The European Union launched a research project called UHDpulse to pave the way for it.

Among other things, the objectives of the project were: …

… to develop a metrological framework for dose measurements at ultra-high pulse dose rates.

… to examine existing, but also newly developed, detector systems for their suitability for dosimetry of beams with ultra-high dose rate or ultra-short pulse duration.

… to develop reference standards for dose measurements at ultra-high pulse dose rates and suitable measurement methods.

PTW was a partner of this project and involved in various work packages. One of these packages was the development of suitable detectors for FLASH dosimetry.

In this context, PTW pioneered the development of a new detector. What distinguishes this detector and why can’t existing detectors be used for dose measurement in FLASH therapy?

Dr. Frank Schwamm:
The dose measurement of FLASH radiation beams places special demands on detectors. PTW recognized this early on and was looking for suitable solutions for active real-time dosimetry. New challenges arise for both reference and relative dosimetry.

In reference dosimetry on conventional linear accelerators, the absolute dose is typically measured with ionization chambers. Since FLASH radiation uses ultra-high dose rates, recombination effects occur in the ionization chambers resulting in erroneous measurement values. PTW thoroughly investigated this problem, developed suitable detectors for dose measurement of FLASH radiation and has already tested them for this particular application.

Initial solutions for FLASH dosimetry: Dose measurements with flashDiamond at University Hospital Heidelberg, photo: courtesy of University Hospital Heidelberg

However, suitable measurement technology is also required for relative dosimetry of FLASH radiotherapy. Relative dose distributions are usually measured in a 3D water phantom and require detectors with a very high spatial resolution. Conventional solid-state detectors, which are mostly based on silicon diodes, are unsuited because of insufficient dose stability and linearity.

In collaboration with the University of Rome Tor Vergata, PTW developed a new diamond detector, the flashDiamond. It is characterized by high dose stability and also measures very precisely at the ultra-high dose rates delivered during FLASH radiotherapy. This also applies to very small field sizes.

The clinical introduction of FLASH radiotherapy places high demands on dosimetry. The AAPM (American Association of Medical Physics) has established a new working group with the aim of developing dosimetry standards for FLASH applications. What role does PTW play in this?

Dr. Rafael Kranzer:
The AAPM Task Group 359 has been formed to develop guidelines for dosimetry in FLASH radiotherapy. In this context, radiation measuring devices are evaluated for their suitability for FLASH radiation and practical recommendations for calibration, dosimetry and documentation are developed. PTW is also represented on this expert committee. We are excited to be able to work on the dosimetry standards for FLASH radiotherapy and contribute our know-how in dosimetry.