AbstractID: 7990 Title: Mono-isocentric technique for photon and electron fields in treatment of head and neck cancer The conventional radiation treatment for head and neck tumors often require the use of lateral fields with anterior photons and posterior electrons. The posterior electron fields are often set up at extended source skin distance (SSD). The anterior neck treated with photons can be set either as an SSD or an isocentric set up. Regardless whether or not the technique could provide homogeneity in the junctional region, the patient must be repositioned when switching between the photon and electron treatments, and this introduces a position error. This work describes a mono-isocentric technique for both photon and electron fields, thus simplifying patient setup and minimizing position error. This technique uses cerrobend cutouts mounted on the photon block tray instead of using electron cones. The methods presented in this work modify photon blocks to provide an extended photon dose falling off slope to match that of “no-cone” electrons. Stacked MLC fields mimicking the modified blocks also provide similar results. A 10-degree up gantry rotation of the electron beam is used for positioning the isodose lines of its Dmax region in the same direction as that of photons. With the presented technique, dose uniformity is achieved through the extended depth at the joint region of abutting photon and electron fields. This method is applicable to the typical energy combinations used for head and neck patients. Not only does this mono-isocentric technique itself minimize position error, it also decreases the position error at the abutting region because of the nonsharp dose falling off.