The desirability of treatment with multiple fields in charged heavy particle therapy

Many parallels exist in treatment planning for charged heavy particles, X-rays and neutrons. The charged particles, however, offer improved dose distributions because of their finite range. Parallel opposed, as compared to single port, irradiation with pions, helium ions and heavier ions is advantageous because of improved tumour-to-entrance-region dose ratios and more uniform distributions of the high LET radiation events throughout the tumour volumes. The clinical significance of the variations of RBE and OER with pion star fraction (or density of high LET events with the other charged heavy particles) remains to be elucidated for fractionation schemes which might be used for patient treatment. This will also be important for the evaluation of mixed heavy ion and X-ray irradiation if it is necessitated by the high RBE in the entrance region with neon and other heavy ions, or is shown to be an effective way to use heavy ions. The advantages of irradiation with the cylindrical geometry of the Stanford Medical Pion Generator (SMPG) compared to single or opposed pion beam irradiation are not only higher pion star fractions and tumour-to-entrance-region dose ratios for comparable tumour dimensions, but simpler patient immobilisation and tumour localisation. Similar benefits are expected with multiport irradiation with other charged heavy particles.