Room: Track 5
Purpose: Wet-type (neovascular) AMD is the leading cause of blindness in the USA. The mainstay treatment requires monthly intravitreal injection of anti-VEGF drugs, associated with high costs and the risk of surgical injury and infection. Anti-VEGF drugs inhibit the formation of neovasculature, but do not attack it. Radiotherapy can destroy neovasculature and also inhibit wet-AMD associated inflammation and fibrosis not addressed by VEGF inhibitors. However, the current collimation-based radiotherapy device uses fixed 4mm beams, prone to overtreat or undertreat the choroidal neovascularization (CNV) lesions because of their various sizes and shapes. This study develops and evaluates personalized conformal treatment with focused kV radiation using cutting-edge polycapillary x-ray lens.
Methods: A phase-space file modeling the focused photons was generated via MC-based ray tracing and verified by phantom measurements. The ultrasmall ~0.2 mm beam focal spot perpendicular to the beam direction enables spatially fractionated grid therapy, which has been shown to preferentially damage abnormal neovascular blood vessels vs normal ones. Geant4-based MC simulations of multi-arc scanning were performed to conformally treat three clinical cases of large, medium, and small CNV lesions with regular and grid deliveries. Dose delivery uncertainties due to positioning errors were analyzed.
Results: The phantom measurements using 45 kVp x-rays are within 10% of the simulated focal spot sizes and depth doses. The CNV simulations show highly-conformal delivery of dose to the lesion plus margin with sharp dose fall-offs and controllable spatial modulation patterns. The 90%-10% isodose penumbra is ~0.5-0.7 mm. With a 16 Gy prescription dose to the lesions, the critical structure doses are well below the tolerance. The average dose varies within 4% due to 0.75mm linear displacements of the eyeball.
Conclusion: Focused kV technique allows personalized treatment of CNV lesions and reduces unwanted radiation to healthy tissues. The simulated dose distribution is superior to existing techniques.