Radiation shielding and safety implications following linac conversion to an electron FLASH‐RT unit

Abstract

Purpose Due to their finite range, electrons are typically ignored when calculating shielding requirements in megavoltage energy linear accelerator vaults. However, the assumption that 16 MeV electrons need not be considered does not hold when operated at FLASH‐RT dose rates (~200× clinical dose rate), where dose rate from bremsstrahlung photons is an order of magnitude higher than that from an 18 MV beam for which shielding was designed. We investigate the shielding and radiation protection impact of converting a Varian 21EX linac to FLASH‐RT dose rates. Methods We performed a radiation survey in all occupied areas using a Fluke Biomedical Inovision 451P survey meter and a Wide Energy Neutron Detection Instrument (Wendi)‐2 FHT 762 neutron detector. The dose rate from activated linac components following a 1.8‐min FLASH‐RT delivery was also measured. Results When operated at a gantry angle of 180° such as during biology experiments, the 16 MeV FLASH‐RT electrons deliver ~10 µSv/h in the controlled areas and 780 µSv/h in the uncontrolled areas, which is above the 20 µSv in any 1‐h USNRC limit. However, to exceed 20 µSv, the unit must be operated continuously for 92 s, which corresponds in this bunker and FLASH‐RT beam to a 3180 Gy workload at isocenter, which would be unfeasible to deliver within that timeframe due to experimental logistics. While beam steering and dosimetry activities can require workloads of that magnitude, during these activities, the gantry is positioned at 0° and the dose rate in the uncontrolled area becomes undetectable. Likewise, neutron activation of linac components can reach 25 µSv/h near the isocenter following FLASH‐RT delivery, but dissipates within minutes, and total doses within an hour are below 20 µSv. Conclusion Bremsstrahlung photons created by a 16 MeV FLASH‐RT electron beam resulted in consequential dose rates in controlled and uncontrolled areas, and from activated linac components in the vault. While our linac vault shielding proved sufficient, other investigators would be prudent to confirm the adequacy of their radiation safety program, particularly if operating in vaults designed for 6 MV.

Technical note: Characterization and practical applications of a novel plastic scintillator for online dosimetry for an ultrahigh dose rate (FLASH)

“…Although FLASH‐RT systems offer promising results, their UHPDR deliveries introduce a number of dosimetric challenges. Not least of which is the recombination and polarity correction factors in ionization chambers that can no longer be estimated using the traditional linear estimations from the AAPM TG‐51 formalism when dose per pulse exceeds ∼1 mGy/pulse 15,16,26 . Although the impact of the recombination effects depends on the precise time‐pulse structure and instantaneous dose rate, and can be estimated, 24 this is difficult to do in practice.…”

“…The process in which we converted a Varian 21EX Cseries linear accelerator to deliver 16-MeV electrons at UHPDR was previously described. 16 Briefly, maximizing UHPDR was achieved by tuning the electron beam current and the radio-frequency driver. An external circuit counts the pulses created by the pulse-forming network and asserts an external (EXT) interlock when the desired number of pulses is delivered.…”

“…In this "physics" geometry (see Figure 1) at isocenter, the dose per pulse becomes ∼0.8 cGy/pulse, where recombination effects are on the order of just a few percent. 16 The ionization chamber is therefore used as a relative counter to assess linearity and reproducibility of the plastic scintillator response. The plastic scintillator and the EBT-XD film are placed at a water-equivalent depth of 3 cm (nominal depth of maximum dose for 16 MeV) comprising 2 cm of Plastic Water and 1.14 cm of a ClearSight Bolus (Clearsight RT, Durham, NC).…”

“…Electrometers are saturated, and ion chambers suffer from high ion recombination beyond that for which the ion recombination factor P ion can be estimated using equation 12 from TG-51. [15][16][17] Although some investigators propose methodologies for which to estimate the ion chamber recombination factor at UHPDR, these methodologies largely rely on using other trusted dosimeters such as radiochromic film 18 or calorimeters. 19 Most investigators tend to use a Gafchromic (radiochromic) film, [20][21][22][23] which is insensitive to a dose rate below ∼10 6 -10 9 Gy/s.…”

Three discipline collaborative radiation therapy (3DCRT) special debate: FLASH radiotherapy needs ongoing basic and animal research before implementing it to a large clinical scale

“…Third, while we agree with our opponents on the need for FLASH‐specific measurement and quality assurance technology, and clinical practice guidelines, there is a rapidly growing body of work in this space. Independent groups have published on FLASH dosimetry (definition and measurement), 21 , 46 radiation safety, 47 beam commissioning, 48 and treatment planning. 20 More recently, the American Association of Physicists in Medicine (AAPM) has instituted Task Group 359 for FLASH radiation dosimetry.…”