MO‐G‐213AB‐04: Investigation of Photon MLC to Deliver Energy and Intensity Modulated Electron Beams for Advanced Radiotherapy

Abstract

Purpose: To implement energy and intensity modulated electron radiotherapy (MERT) using an existing photon multileaf collimator (pMLC) in a modern medical accelerator. Methods: Both Siemens Artiste and Varian Trilogy accelerators have been investigated for MERT using the existing pMLC in the service mode. The limitations of SSD for Varian linac were examined. A Monte Carlo (MC)‐based inverse treatment planning code was used for plan optimization and dose calculation. Water scanning data including PDDs and dose profiles at various depths were acquired for various fields shaped by pMLC for 6, 9, 12 and 15MeV electron energies of both Artiste and Varian linacs and compared with MC calculations. Results: Artiste can treat patient with an SSD down to 61 cm to reduce the field penumbra caused by electron in‐air scatters. This is comparable to the Siemens Primus accelerator that has an SSD about 60cm. The electron beam characteristics are acceptable in terms of flatness and penumbra as compared with those collimated by electron applicators. Varian Trilogy has to use a longer SSD (about 85 to 90cm) due to the current restriction of the table height. Comparisons between measured PDDs and dose profiles, and MC calculations for various energies for pMLC shaped fields of 3 × 3 cm2and 10 × 10 cm2 showed agreement to within 2%/2mm. Conclusions: With pMLC shaped fields, MC dose calculations agree with measured PDDs and dose profiles at various depths, which warrant the dose accuracy for MERT delivered by pMLC using MC‐based dose calculation planning systems. With large SSD on Trilogy, dose profiles deteriorate to some extent as compared with those on Artiste with SSD = 61cm in terms of penumbral width. However the impact of this penumbra difference on the ultimate MERT plan using energy and intensity modulated electron beams is yet to be investigated.