FLASH Radiotherapy:
Dosimetric Challenges and Solutions

Sparing of healthy tissue while maintaining or improving tumor control is the primary goal of radiotherapy. In recent decades, this goal has been advanced primarily through technological innovations. Since the rediscovery of the so-called FLASH effect in 2014, many preclinical studies have shown that ultra-high dose rate (UHDR) irradiation (mean dose rate > 40 Gy/s) results in significant healthy tissue sparing. Accurate dosimetry of these ultra-high dose rates is a major challenge and crucial for the safe transition of FLASH radiotherapy (FLASH-RT) into clinical practice. To address this need, PTW had joined the European UHDpulse project to provide a framework for dose measurements at UHDR¹, which was successfully completed in February 2023.

Ionization chambers need to be adapted to operate under high dose rate conditions

Vented ionization chambers (ICs) are considered as the gold standard for reference dosimetry and are most commonly used as secondary standards. Unfortunately, established ICs show significant saturation effects due to ion recombination at UHDR in pulsed beams (Figure 1).

In addition, other conventional active real-time detectors, such as solid-state detectors based on semiconductors, start to fail at UHDR. For this reason, passive dosimeters, such as alanine, radiochromic films, or luminescence dosimeters (TLDs and OSLDs), are mainly used under these conditions. However, the use of passive dosimeters is complex, very time-consuming, and usually lacks the traceability to primary standards. There is therefore a need for active real-time dosimeters and traceability. It is also desirable to use ionization chambers as a secondary standard under UHDR conditions.

To find a solution to this challenge, the role of the distance between the electrodes of vented ICs and its impact on ion recombination has been analyzed experimentally and by numerical simulation². The results show that this parameter is the most relevant to achieve negligible recombination losses. Using a very small electrode distance of 0.25 mm in vented ICs (called ultra-thin ICs (UTIC)) leads to a charge collection efficiency higher than 99% for a 2.5 µs pulse of 5 Gy (Figure 1). Ionization chambers are therefore excellent candidates as secondary standard for reference dosimetry in UHDR beams because they are also waterproof, easy to handle and can be used according to the existing methodology in current codes of practice.

flashDiamond – the first  detector for FLASH dosimetry

However, ICs are limited in spatial resolution due to the diameter of the sensitive volume, especially when measuring lateral dose distributions and in small fields.

A detector that is particularly well suited for these applications, which require high spatial resolution, is the well-established microDiamond detector by PTW (type T60019).It can also be used for electron radiation, including high dose per pulse (DPP) applications such as used in IOeRT^3,4.  Due to the high water equivalence of its sensitive volume in terms of effective atomic number, no conversion from ion dose to absorbed dose to water is necessary when determining depth dose curves for electron beams. This characteristic, along with its good stability of response as a function of the accumulated dose, made the microDiamond detector a promising detector for use in UHDR¹ environments. However, the detector has shown to exhibit saturation behavior at UHDR pulsed electron beams⁵. A thorough investigation under ultra-high pulse dose rate (UHPDR) conditions using different samples and modified designs was performed by Marinelli et al.⁶ and Kranzer et al.⁷. These studies confirmed the observed saturation behavior (Figure 2) and investigated the influencing parameters.