Prompt gamma rays emitted from nuclear excitations at the beam line exit window were measured using a gamma detector comprised of a NaI(TI) crystal coupled to a photomultiplier tube biased at 900 V. Irradiated film was analyzed using ImageJ and Matlab software. Beam width, flatness, and symmetry were evaluated at the target position using EBT3 film scanned in an Epson 1000XL flatbed scanner. (E) Representative dose rate time structure delivered throughout each pulse for standard (18 beam pulses, upper panel) and FLASH (20 beam pulses, lower panel) dose rates.ĭepth dose profiles were measured with the Markus chamber at multiple depths (FLASH dose rate) and with a multilayer ionization chamber, Zebra (standard dose rate). (C) Schematics of proton flux modulation by fixed versus variable pules sent to the beam current regulation unit of the IBA system and (D) linearity of dose versus pulse width or monitor unit count for these designs. (B) Comparison of proton depth dose distribution using FLASH versus standard dose rate. Custom brass collimators of proton stopping thickness can be inserted into the acrylic column for variation of beam size. Downstream from the secondary IC is the secondary scatterer consisting of a 5 mm diameter lead shot embedded in a plastic torus which itself is inserted into an acrylic column providing adjustment of distance between scattering centers for optimal beam profile parameters. Adjacent to the primary IC exit window, a NaI-gamma detector was introduced for time structure measurements of proton flux. After the primary IC is a 2 mm lead first scatterer and secondary IC providing online dose verification. Beam line vacuum ends at the primary ionization chamber (IC) with dual channel readout electrometers interlocked to the primary dose counter and the cyclotron rf chain for a positive failsafe shutoff. Schematic diagram of FLASH proton radiation therapy set up (A). Methods and Materials Proton delivery and beam modulationįig. 1 Proton radiation therapy set up. Using this system, we provide the first evidence that compared with standard PRT, FLASH PRT elicits reduced levels of both acute and chronic gastrointestinal damage without radioprotection of pancreatic cancer flank tumors in the same mouse model. We use the single pencil beam in our research room to create a double scatter system so that we can provide a relatively uniform and expanded field size without adding another variable with spot scanning speed. For this reason, we have designed and constructed a novel RT apparatus that can deliver either FLASH (60-100 Gy/s) or standard (0.5-1 Gy/s) dose rates using double scattered protons in a computed tomography (CT)–defined geometry. Although we are capable of performing pencil beam scanning in our clinical treatment rooms, it is important to understand the basic dosimetry and biological effects of dose rates in experimental settings. The main goal of this research is to understand the biological effects of proton beams. Standard PRT pencil beam scanning dose rates are higher than conventional electron or photon RT dose rates, with localized areas achieving dose rates of 0.5 to 1 Gy/s. These results suggest the exciting hypothesis that the unique temporal effects of the FLASH-PRT dose rate could augment PRT-intrinsic spatial advantages.
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