VARIATION OF ELECTRON-BEAM UNIFORMITY WITH BEAM ANGULATION AND SCATTERER POSITION FOR TOTAL SKIN IRRADIATION WITH THE STANFORD TECHNIQUE

Purpose: The influence of different scatterer-degraders and beam angul ations on beam uniformity for total skin electron irradiation using th e six dual beam Stanford technique is investigated. Methods and Materi als: The 6 MeV high dose rate total skin electron irradiation mode on a linear accelerator was used. Beam profiles and percentage depth dose s in the patient plane for single, dual, and six dual beams were measu red for different dual beam angulations and acrylic scatterer-degrader s of different thicknesses mounted on the treatment head or in front o f the patient in the treatment plane. Results: It is demonstrated that , with the same electron nominal energy, total skin irradiation techni ques with different beam penetrations can be obtained by inserting var ious beam scatterer-degraders into the beam, either mounted on the acc elerator head or close to the patient. For our patient treatment, a be am penetration was selected so that the 80% dose lay at 8-9 mm and the 50% dose at 15-16 mm depth. This was achieved by mounting a 0.32-cm t hick acrylic beam scatterer-degrader on the accelerator head. A unifor m vertical profile was obtained for gantry angulations of +/-21 degree s. Conclusions: To implement a total skin electron irradiation techniq ue using the Stanford method, the required depth of penetration needs to be selected. Based on this, the appropriate combination of scattere r-degraders and dual beam angulations to produce a uniform beam in the treatment plane needs to be determined. Different techniques with dif ferent beam penetrations can be developed using the same high dose rat e mode on the linear accelerator by a proper choice of scatterer-degra ders and beam angulations.