Total skin electron therapy at two energies on a linear accelerator.

We have implemented the AAPM Task Group 30 recommendations with respect to Total Skin Electron Therapy (TSET) for the treatment of Mycosis Fungoides. Extensive measurements have been performed on the Varian 2100c linear accelerator with a water equivalent phantom, cylindrical phantoms and an anthropomorphic phantom to validate the 12 beam technique and to measure calibration factors. The technique was developed at 2.5 MeV and 3.5 MeV (mean energy at the surface) with the energy degrading screen at 30 cm from the patient. Field flatness over the treatment area was within +/- 3% with no significant regional variation of energy. The bremstrahlung contamination was .06-1.2%. The absolute calibration of absorbed dose to the patient required the measurement of the ratio "skin dose to calibration point dose", this was confirmed by measurements with a parallel plate ionization chamber and thermoluminescence dosimetry. The results indicate the care that must be taken with ionization chamber measurements under TSET condition to avoid erroneous readings due to the "polarity effect". A protocol is suggested for the practical implementation and quality assurance of the technique. The results of two successful treatments are presented and compared with the predicted results from phantoms.