Exhibit Hall | Forum 4
Purpose: Combining methods of conformal dose delivery with ultra-high dose rate (UHDR) beams can achieve the benefits of the FLASH effect in clinical treatments. We investigate the deployment of intensity modulation (IM) in passive electron FLASH radiotherapy through treatment plan optimization.
Methods: The .decimal ElectronRT treatment planning system (TPS) was validated with film measured lateral and percent-depth-dose (PDD) profiles for an electron FLASH irradiator beam model. Plans were developed comparing collimated open fields and IM electron beams achieved with metal compensators that vary in spacing and size. Homogeneity and conformity were quantified for plans developed in a water phantom and anonymized patient cases considering constraints in prescribed dose to the target and minimizing dose to organs-at-risk (OAR).
Results: The measured profiles and TPS beam model agreed on average to within 1% and 2% for lateral and PDD profiles, respectively. For a large 15x15cm2 field in a water phantom, IM improved the beam flatness (30% to 6%) and penumbra (35mm to 15mm) at 3 cm depth while retaining UHDR in the treatment field with a 38% reduction in central axis output (~180Gy/sec) and <2% change to the PDD. In the rib metastasis plan, the modulated plans treated the tumor volume with a homogeneity index (HI) improvement of 0.2. In the facial orbital plan, the conformity index improved by 8%, HI improved by 0.05 and achieved comparable dose to OARs.
Conclusion: IM improved homogeneity of prescribed dose with proper treatment constraints while reducing hot spots for clinical plans. Depending on the treatment case IM can also generate superior dose distributions while retaining the UHDR conditions to best exploit the FLASH effect. Future study will validate the treatments plans utilizing IM apertures and will model dosimetric benefits by tumor control and normal tissue complication probability models in a larger number of relevant cases.