A New Electron IMRT Technique for Breast Cancer: Comparison to Photon IMRT and Conventional Irradiation Based on Static and Dynamic Dose Measurements

Background/Purpose: In radiotherapy of breast cancer, heart and lung receive high doses. When intensity-modulated radiotherapy (IMRT) with photons is applied, parts of the breast volume can be missed due to breathing motion or tissue swelling. Additionally, larger volumes of normal tissue are exposed to a low dose which may induce secondary malignancies. This study compares breast irradiation plans of a new electron IMRT technique to plans achieved with photon IMRT and conventional radiotherapy and investigates the impact of breathing motion on dose delivery. Materials/Methods: Treatment planning for a breast phantom was performed using XiO from CMS for step-and-shoot photon IMRT and conventional tangential photon irradiation as well as a self-designed optimization program for the electron IMRT technique. Our program optimizes the orientation, intensity and energy of electron beams shaped with an accelerator-controlled electron multileaf collimator. The resulting plans were compared by means of dose distributions and dose volume histograms as well as dose measurements in the phantom with and without phantom motion during irradiation. Results: Maximum doses in lung and heart were significantly lower with photon IMRT and electron IMRT compared to the conventional technique. Electron irradiation allowed for considerably better sparing of the heart when using photons. Concerning dose coverage of the breast, the three techniques resulted in similar mean doses. Target coverage by the 95%-isodose was best with electron IMRT. Motion-induced under and over dosages in the target were highest for photon IMRT, followed by electron IMRT and conventional irradiation, while considerable over dosage in the heart was detected in dynamic measurements using photon tangents. Conclusions: The new electron IMRT technique is superior to conventional irradiation. It represents a reasonable alternative to photon IMRT, as it allows satisfying target coverage, better sparing of the heart, and results in less motion-induced under and over dosages in the breast.