Delivery of modulated electron beams with conventional photon multi-leaf collimators

Electron beam radiotherapy is an accepted method to treat shallow tumors. However, modulation of electrons to customize dose distributions has not readily been achieved. Studies of bolus and tertiary collimation systems have been met with limitations. We pursue the use of photon multi-leaf collimators (MLC) for modulated electron radiotherapy (MERT) to achieve customized distributions for potential clinical use. As commercial planning systems do not support the use of MLC with electrons, planning was conducted using Monte Carlo calculations. Segmented and dynamic modulated delivery of multiple electron segments was configured, calculated and delivered for validation. Delivery of electrons with segmented or dynamic leaf motion was conducted. A phantom possessing an idealized stepped target was planned and optimized with subsequent validation by measurements. Finally, clinical treatment plans were conducted for post-mastectomy and cutaneous lymphoma of the scalp using forward optimization techniques. Comparison of calculations and measurements was successful with agreement of ±2%/2 mm for the energies, segment sizes, depths tested for delivered segments for the dynamic and segmented delivery. Clinical treatment plans performed provided optimal dose coverage of the target while sparing distal organs at risk. Execution of plans using an anthropomorphic phantom to ensure safe and efficient delivery was conducted. Our study validates that MERT is not only possible using the photon MLC, but the efficient and safe delivery inherent with the dynamic delivery provides an ideal technique for shallow tumor treatment.