FLASH and minibeams in radiation therapy: the effect of microstructures on time and space and their potential application to protontherapy.

After years of lethargy, studies on two non-conventional microstructures in time and space of the beams used in radiation therapy are enjoying a huge revival. The first effect called "FLASH" is based on very high dose-rate irradiation (pulse amplitude ≥106 Gy/s), short beam-on times (≤100 ms) and large single doses (≥10 Gy) as experimental parameters established so far to give biological and potential clinical effects. The second effect relies on the use of arrays of minibeams (e.g., 0.5-1 mm, spaced 1-3.5 mm). Both approaches have been shown to protect healthy tissues as an endpoint that must be clearly specified and could be combined with each other (e.g., minibeams under FLASH conditions). FLASH depends on the presence of oxygen and could proceed from the chemistry of peroxyradicals and a reduced incidence on DNA and membrane damage. Minibeams action could be based on abscopal effects, cell signalling and/or migration of cells between "valleys and hills" present in the non-uniform irradiation field as well as faster repair of vascular damage. Both effects are expected to maintain intact the tumour control probability and might even preserve antitumoural immunological reactions. FLASH in vivo experiments involving Zebrafish, mice, pig and cats have been done with electron beams, while minibeams are an intermediate approach between X-GRID and synchrotron X-ray microbeams radiation. Both have an excellent rationale to converge and be applied with proton beams, combining focusing properties and high dose rates in the beam path of pencil beams, and the inherent advantage of a controlled limited range. A first treatment with electron FLASH (cutaneous lymphoma) has recently been achieved, but clinical trials have neither been presented for FLASH with protons, nor under the minibeam conditions. Better understanding of physical, chemical and biological mechanisms of both effects is essential to optimize the technical developments and devise clinical trials.

Biological and Mechanical Synergies to Deal With Proton Therapy Pitfalls: Minibeams, FLASH, Arcs, and Gantryless Rooms.

Proton therapy has advantages and pitfalls comparing with photon therapy in radiation therapy. Among the limitations of protons in clinical practice we can selectively mention: uncertainties in range, lateral penumbra, deposition of higher LET outside the target, entrance dose, dose in the beam path, dose constraints in critical organs close to the target volume, organ movements and cost. In this review, we combine proposals under study to mitigate those pitfalls by using individually or in combination: (a) biological approaches of beam management in time (very high dose rate "FLASH" irradiations in the order of 100 Gy/s) and (b) modulation in space (a combination of mini-beams of millimetric extent), together with mechanical approaches such as (c) rotational techniques (optimized in partial arcs) and, in an effort to reduce cost, (d) gantry-less delivery systems. In some cases, these proposals are synergic (e.g., FLASH and minibeams), in others they are hardly compatible (mini-beam and rotation). Fixed lines have been used in pioneer centers, or for specific indications (ophthalmic, radiosurgery,…), they logically evolved to isocentric gantries. The present proposals to produce fixed lines are somewhat controversial. Rotational techniques, minibeams and FLASH in proton therapy are making their way, with an increasing degree of complexity in these three approaches, but with a high interest in the basic science and clinical communities. All of them must be proven in clinical applications.

Treatment with 5-fluorouracil enhances radiosensitivity of the human pancreatic cancer cell line MiaPaCa-2.

BACKGROUND AND PURPOSE: Several clinical studies have suggested that the combination of radiation therapy and 5-fluorouracil (5-FU) may improve outcome of patients with pancreatic cancer. However, there are few experimental studies supporting this treatment. AIM OF THE STUDY: To examine the radiosensitivity of human pancreatic cancer cells and its modulation by 5-FU. MATERIAL AND METHODS: MiaPaCa-2, PANC-1 and NP-18 cells growing as monolayer culture were treated with radiation and 5-FU. In addition, 5-FU was studied administered either pre- or postradiation, both as pulse or continuous exposure. Cell survival was determined by the in vitro clonogenic assay. RESULTS: In MiaPaCa-2 cell line, both radiation and 5-FU alone reduced cell survival. The addition of 5-FU to radiation caused a significant net decrease of cell survival. Pulse exposure of 5-FU decreased survival after 2 Gy and mean inactivation dose by 1.64; continuous exposure decreased survival after 2 Gy and mean inactivation dose by about 2.4. Timing of 5-FU exposure did not modify survival. However, when adjusting for 5-FU killing effect and cell multiplicity, only continuous exposure significantly enhanced radiation cell killing. CONCLUSION: Both pulse and continuous exposure increase radiation cell killing, but only continuous exposure may radiosensitize MiaPaCa-2 cells.