Dynamic optical modulation of the electron beam for the high performance intensity modulated radiation therapy

The radiation therapy of cancer is developing to non-uniform irradiation as intensity modulated radiation therapy (IMRT), reducing the dose to normal tissue and concentrating the dose to cancer tissue. A photocathode RF gun is able to generate a low emittance electron beam pulse using laser light. We thought that a photocathode RF gun can generate an intensity-modulated electron beam by optical modulation at the incident optics dynamically. Because of a low emittance, the modulated electron beam pulse was able to accelerate and keep its shape. The human body is always moving, for example during breathing, thus the cancer is also always moving. For radiation therapy, the electron beam must be synchronized to the breathing. Toward high performance (high speed modulation and high spatial resolution, etc.) IMRT, dynamic optical modulation of the electron beam pulse were studied using a photocathode RF gun. Images on photo masks were transported to a photocathode surface by optical relay imaging. Dynamic optical control of the electron beam was carried out by a remote mirror. The modulated electron beam had fine spatial resolution, good enough to use in radiation therapy. Spatial separation spot-to-spot is about 0.3 mm. The moving electron beam images like an electron beam movie were measured, something that was impossible by former methods. Modulated and moving electron beams are reported here.