  Purpose: In this study, procedures were developed to achieve efficient
reversible conversion of a clinical linear accelerator (LINAC) and deliver
electron FLASH (eFLASH) or conventional beams to the treatment room isocenter.
Material & Methods: The LINAC was converted to deliver eFLASH beam within 20
minutes by retracting the x-ray target from the beam's path, positioning the
carousel on an empty port, and selecting 10 MV photon beam energy in the
treatment console. Dose per pulse and average dose rate were measured in a
solid water phantom at different depths with Gafchromic film and OSLD. A pulse
controller counted the pulses via scattered radiation signal and gated the
delivery for preset pulse count. A fast photomultiplier tube-based Cherenkov
detector measured per pulse beam output at 2 ns sampling rate. After conversion
back to clinical mode, conventional beam output, flatness, symmetry, field size
and energy were measured for all clinically commissioned energies. Results:
Dose per pulse of 0.86 +/- 0.01 Gy (310 +/- 7 Gy/s average dose rate) were
achieved at isocenter. The dose from simultaneous irradiation of film and OSLD
were within 1%. The PMT showed the LINAC required about 5 pulses before the
output stabilized and its long-term stability was within 3% for measurements
performed at 3 minutes intervals. The dose, flatness, symmetry, and photon
energy were unchanged from baseline and within tolerance (1%, 3%, 2%, and 0.1%
respectively) after reverting to conventional beams. Conclusion: 10 MeV FLASH
beams were achieved at the isocenter of the treatment room. The beam output was
reproducible but requires further investigation of the ramp up time in the
first 5 pulses, equivalent to <100 cGy. The eFLASH beam can irradiate both
small and large subjects in minimally modified clinical settings and dose rates
can be further increased by reducing the source to surface distance.
