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 angulations on beam uniformity for total skin electron irradiation using the six dual beam Stanford technique is investigated. METHODS AND MATERIALS: The 6 MeV high dose rate total skin electron irradiation mode on a linear accelerator was used. Beam profiles and percentage depth doses in the patient plane for single, dual, and six dual beams were measured for different dual beam angulations and acrylic scatterer-degraders of different thicknesses mounted on the treatment head or in front of the patient in the treatment plane.
RESULTS: It is demonstrated that, with the same electron nominal energy, total skin irradiation techniques with different beam penetrations can be obtained by inserting various beam scatterer-degraders into the beam, either mounted on the accelerator head or close to the patient. For our patient treatment, a beam 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 thick acrylic beam scatterer-degrader on the accelerator head. A uniform vertical profile was obtained for gantry angulations of +/- 21 degrees.
CONCLUSIONS: To implement a total skin electron irradiation technique using the Stanford method, the required depth of penetration needs to be selected. Based on this, the appropriate combination of scatterer-degraders and dual beam angulations to produce a uniform beam in the treatment plane needs to be determined. Different techniques with different beam penetrations can be developed using the same high dose rate mode on the linear accelerator by a proper choice of scatterer-degraders and beam angulations.