No abstract

Purpose Treatment planning system (TPS) dose calculation is sensitive to multileaf collimator (MLC) modeling, especially when treating with intensity‐modulated radiation therapy (IMRT) or VMAT. This study investigates the dosimetric impact of the MLC leaf‐tip model in a commercial TPS (RayStation v.6.1). The detectability of modeling errors was assessed through both measurements with an anthropomorphic head‐and‐neck phantom and patient‐specific IMRT QA using a 3D diode array. Methods and Materials An Agility MLC (Elekta Inc.) was commissioned in RayStation. Nine IMRT and VMAT plans were optimized to treat the head‐and‐neck phantom from the Imaging and Radiation Oncology Core Houston branch (IROC‐H). Dose distributions for each plan were re‐calculated on 27 beam models, varying leaf‐tip width (2.0, 4.5, and 6.5 mm) and leaf‐tip offset (−2.0 to +2.0 mm) values. Doses were compared to phantom TLD measurements. Patient‐specific IMRT QA was performed, and receiver‐operating characteristic (ROC) analysis was performed to determine the detectability of modeling errors. Results Dose calculations were very sensitive to leaf‐tip offset values. Offsets of ±1.0 mm resulted in dose differences up to 10% and 15% in the PTV and spinal cord TLDs respectively. Offsets of ±2.0 mm caused dose deviations up to 50% in the spinal cord TLD. Patient‐specific IMRT QA could not reliably detect these deviations, with an ROC area under the curve (AUC) value of 0.537 for a ±1.0 mm change in leaf‐tip offset, corresponding to >7% dose deviation. Leaf‐tip width had a modest dosimetric impact with <2% and 5.6% differences in the PTV and spinal cord TLDs respectively. Conclusions Small changes in the MLC leaf‐tip offset in this TPS model can cause large changes in the calculated dose for IMRT and VMAT plans that are difficult to identify through either dose curves or standard patient‐specific IMRT QA. These results may, in part, explain the reported high failure rate of IROC‐H phantom tests.

Gold nanoparticles (GNPs) have shown potential in recent years as a means of therapeutic dose enhancement in radiation therapy. However, a major challenge in moving towards clinical implementation is the exact characterisation of the dose enhancement they provide. Monte Carlo studies attempt to explore this property, but they often face computational limitations when examining macroscopic scenarios. In this study, a method of converting dose from macroscopic simulations, where the medium is defined as a mixture containing both gold and tissue components, to a mean dose-to-tissue on a microscopic scale was established. Monte Carlo simulations were run for both explicitly-modeled GNPs in tissue and a homogeneous mixture of tissue and gold. A dose ratio was obtained for the conversion of dose scored in a mixture medium to dose-to-tissue in each case. Dose ratios varied from 0.69 to 1.04 for photon sources and 0.97 to 1.03 for electron sources. The dose ratio is highly dependent on the source energy as well as GNP diameter and concentration, though this effect is less pronounced for electron sources. By appropriately weighting the monoenergetic dose ratios obtained, the dose ratio for any arbitrary spectrum can be determined. This allows complex scenarios to be modeled accurately without explicitly simulating each individual GNP.

The proposed virtual point source model and characterization method can be used to compute absorbed dose in both the homogeneous and heterogeneous block phantoms within of 2%-8% of measured values, depending on the phantom and the beam quality. The authors' results also provide experimental validation for their kV dose computation software, kVDoseCalc.

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.