Evaluation of intensity‐modulated electron FLASH radiotherapy in a clinical setting using veterinary cases

Abstract

AbstractPurposeThe increased normal tissue tolerance for FLASH radiotherapy (FLASH‐RT), as compared to conventional radiotherapy, was first observed in ultra‐high dose rate electron beams. Initial clinical trials in companion animals have revealed a high risk of developing osteoradionecrosis following high‐dose single‐fraction electron FLASH‐RT, which may be related to inhomogeneities in the dose distribution. In the current study, we aim to evaluate the possibilities of intensity‐modulated electron FLASH‐RT in a clinical setting to ensure a homogeneous dose distribution in future veterinary and human clinical trials.MethodsOur beam model in the treatment planning system electronRT (.decimal, LLC, Sanford, FL, USA) was based on a 10‐MeV electron beam from a clinical linear accelerator used to treat veterinary patients with FLASH‐RT in a clinical setting. In electronRT, the beam can be intensity‐modulated using tungsten island blocks in the electron block cutout, and range‐modulated using a customized bolus with variable thickness. Modulations were first validated in a heterogeneous phantom by comparing measured and calculated dose distributions. To evaluate the impact of intensity modulation in superficial single‐fraction FLASH‐RT, a treatment planning study was conducted, including eight canine cancer patient cases with simulated tumors in the head‐and‐neck region. For each case, treatment plans with and without intensity modulation were created for a uniform bolus and a range‐modulating bolus. Treatment plans were evaluated using a target dose homogeneity index (HI), a conformity index (CI), the near‐maximum dose outside the target (), and the near‐minimum dose to the target ().ResultsBy adding intensity modulation to plans with a uniform bolus, the HI could be improved (p = 0.017). The combination of a range‐modulating bolus and intensity modulation provided a further significant improvement of the HI as compared to using intensity modulation in combination with a uniform bolus (p = 0.036). The range‐modulating bolus also improved the CI compared to using a uniform bolus, both with an open beam (p = 0.046) and with intensity modulation (p = 0.018), as well as increased the (p = 0.036 with open beam and p = 0.05 with intensity modulation) and reduced the median (not significant).ConclusionsBy using intensity‐modulated electron FLASH‐RT in combination with range‐modulating bolus, the target dose homogeneity and conformity in canine patients with simulated tumors in complex areas in the head‐and‐neck region could be improved. By utilizing this technique, we hope to decrease the dose outside the target volume and avoid hot spots in future clinical electron FLASH‐RT studies, thereby reducing the risk of radiation‐induced toxicity.