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FLASH – One Session For One Treatment

Recent in-vivo studies demonstrated that electrons delivered at ultra-high dose rates within 0.5 s (FLASH) to cancerous tissues inhibit tumour growth as effectively as in conventional therapy, but with significantly more sparing of surrounding healthy tissues (Favaudon, Caplier, et al. 2014). Healthy tissue sparing with FLASH enables a dose increase and extreme reduction of therapy time without additional complications.

FLASH can be delivered with x-rays, electrons, protons, and light ions, but FLASH with protons is expected to offer additional healthy tissue sparing from beam stopping in the tumour.

Feasibility of FLASH in current proton therapy systems

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Legacy cyclotrons are currently used for proton FLASH investigations. The current proton FLASH pre-clinical (mice) studies are conducted at the cyclotron extraction (maximum) energy corresponding to a depth of about 32 cm in a patient. FLASH delivered in this way is only clinically effective for deep-seated tumours. Approximately 25 % of tumours are designated as deep seated.

This is a significant therapeutic limitation because cyclotron dose rates decrease rapidly at lower energies. In addition, the tumours with the greatest potential for therapeutic improvement are located at mid, not deep depths. Consequently, cyclotron generated FLASH radiotherapy fails to meet the clinical demand of both the patient population and the radiation oncologist specialty. The translation from pre-clinical studies to human trials is imminent. Anticipated human FLASH targets with the potential for significantly improved patient outcomes are expected to be lung, head and neck, and brain tumours. These are mid-depth (10-20 cm), not deep tumours. Consequently, the legacy cyclotron technology is not expected to be able to provide FLASH treatments for these clinical targets.

LIGHT is perfectly suited for FLASH

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The Advanced Oncotherapy LIGHT system is fundamentally different in this regard and is designed to provide the needed clinical FLASH tool. Critically, LIGHT maintains full output at any energy (see figure below cf. cyclotrons).

For comparison to legacy cyclotrons, we select a 2x2x2 cm3 volume with energy layer scanning. Our modelling represents a full volumetric proton scanning delivery, including the target depth. The comparison with other proton therapy systems is below. The cyclotron and synchrocyclotron values are as publicly reported by the provider. Comparatively, for the expected clinical application of proton FLASH, LIGHT is designed to provide superior performance in comparison to all other systems.