Exploratory study of proton radiation therapy using large field techniques and fractionated dose schedules

Three patients have been treated with 160-MeV protons combined with high-energy photons to examine the advantages and difficulties associated with the clinical implementation of a program of large-field, fractionated-dose, proton-radiation therapy. We have found it necessary to: 1) obtain an accurate three-dimensional determination of the treatment volume including the density of all tissues in the beam path; 2) construct an adequate bolus to compensate for tissue heterogeneities; 3) use much more precise and accurate immobilization and patient positioning devices than used in photon irradiation; 4) treat with both protons and photons so as to keep the skin dose within an acceptable level. In tissues without significant inhomogeneities due to bone and air spaces we have delivered a well-defined dose to involved tissues while sparing distal sensitive structures. However, in those regions where there is much “fine structure” of tissue density, it has been difficult to compensate satisfactorily for the inhomogeneities.