s. Overall expression of the tight junction proteins, occludin and claudin-5, which was significantly reduced after CONV irradiation, remained unchanged in the FLASH-irradiated brains at one and four weeks postirradiation. Our data further confirm that, compared to isodoses of CONV irradiation known to elicit detrimental effects, FLASH does not damage the normal vasculature. These data now provide the first evidence that FLASH preserves microvasculature integrity in the brain, which may prove beneficial to cognition while allowing for better tumor control in the clinic.The combination of radiotherapy and immunotherapy may generate synergistic anti-tumor host immune responses and promote abscopal effects. Spatial fractionation of a radiation dose has been found to promote unique physiological responses of tumors, which might promote synergy with immunotherapy. To determine whether spatial fractionation may augment immune activity, whole-tumor or spatial fractionation grid radiation treatment (GRID) alone or in combination with antibodies against immune checkpoints PD1 and CTLA-4 were tested in an immunocompetent mouse model using a triple negative breast tumor (4T1). Tumor growth delay, immunohistochemistry and flow cytometry were used to characterize the effects of each treatment type. Whole-beam radiation with immune checkpoint inhibition significantly restrained tumor growth in the irradiated tumor, but not abscopal tumors, compared to either of these treatments alone. In mice that received spatially fractionated irradiation, evidence of abscopal immune responses were observed in contralateral tumors with markedly enhanced infiltration of both antigen-presenting cells and activated T cells, which were preceded by increased systemic IFNγ production and led to eventual tumor growth delay. These studies suggest that systemic immune activation may be triggered by employing GRID to a primary tumor lesion, promoting anti-tumor immune responses outside the treatment field. Interestingly, PD-L1 was found to be upregulated in abscopal tumors from GRID-treated mice. Combined radio-immunotherapy therapy is becoming a validated and novel approach in the treatment of cancer. With the potential increased benefit of GRID to augment both local and metastatic disease responses, further exploration of GRID treatment as a part of current standards of care is warranted.In the novel and promising radiotherapy technique known as FLASH, ultra-high dose-rate electron beams are used. As a step towards clinical trials, dosimetric advances will be required for accurate dose delivery of FLASH. The purpose of this study was to determine whether a built-in transmission chamber of a clinical linear accelerator can be used as a real-time dosimeter to monitor the delivery of ultra-high-dose-rate electron beams. This was done by modeling the drop-in ion-collection efficiency of the chamber with increasing dose-per-pulse values, so that the ion recombination effect could be considered. The raw transmission chamber signal was extracted from the linear accelerator and its response was measured using radiochromic film at different dose rates/dose-per-pulse values, at a source-to-surface distance of 100 cm. An increase of the polarizing voltage, applied over the transmission chamber, by a factor of 2 and 3, improved the ion-collection efficiency, with corresponding increased efficiency at the highest dose-per-pulse values by a factor 1.4 and 2.2, respectively. The drop-in ion-collection efficiency with increasing dose-per-pulse was accurately modeled using a logistic function fitted to the transmission chamber data. The performance of the model was compared to that of the general theoretical Boag models of ion recombination in ionization chambers. The logistic model was subsequently used to correct for ion recombination at dose rates ranging from conventional to ultra-high, making the transmission chamber useful as a real-time monitor for the dose delivery of FLASH electron beams in a clinical setup.FLASH radiation therapy (FLASH-RT) reference dosimetry to obtain traceability, repeatability and stability of irradiations cannot be performed with conventional dosimetric methods, such as monitor chambers or ionization chambers. https://www.selleckchem.com/ Until now, only passive dosimeters have provided the necessary dosimetric data. Alanine dosimetry is accurate; however, to be used for FLASH-RT in biological experiments and for clinical transfer to humans, the reading time needs to be reduced, while preserving a maximum deviation to the reference of ±2%. Optimization of alanine dosimetry was based on the acquisition of electron paramagnetic resonance (EPR) spectra with a Bruker spectrometer. Reading parameters such as the conversion time, the number of scans, the time constant, the microwave power and the modulation amplitude of the magnetic field were optimized as a trade-off between the signal-to-noise ratio (SNR) and the reading time of one measurement using the reference 10.1 Gy alanine pellet. After optimizing the parameters, wGy and 100 Gy, the optimized acquisition parameters made it possible to keep the average differences between the reference and the measured doses below ±2%, for a reading time of 7.8 min. This enabled an accurate and fast dose determination for biological preparations as part of FLASH-beam irradiations.Radiation chemists have been routinely using high-dose microsecond-pulsed irradiation for almost 60 years, involving many thousands of studies, in the technique of "pulse radiolysis". This involves dose rates broadly similar to the FLASH regimen now attracting interest in radiotherapy and radiobiology. Using the experience gained from radiation chemistry, two scenarios are examined here that may provide a mechanistic basis for any differential response in normal tissues versus tumors in FLASH radiotherapy. These are 1. possible depletion of a chemical critical to the response to radiation, and 2. radical-radical reactions as a possible cause of effects occurring mainly with high-intensity pulsed radiation. The evidence for changes in relative levels of so-called "reactive oxygen species" produced after irradiation using FLASH versus conventional irradiation modalities is also examined.