The Range of Protons in Human Skullbone

Beams of protons from accelerators in the energy range from 100 to 1000 MIev are being used intracranially for irradiation of neoplastic tissue and for the ablation of specific normal tissues (1). Since protons from such an accelerator have a very well-defined range or depth of penetration in homogeneous material, since they scatter only a few millimeters in penetrating 10 cm of tissue, and since their specific ionization rises sharply toward the end of their range, they may be used to affect small, precisely located volumes in the brain (2-4). An advantage of these techniques is that one can irradiate through the skullbone, so that no open surgery is required. The dosimetry for such irradiations is conveniently carried out with a water phantom. The dose distribution in the patient differs slightly from that in the water phantom because the range-energy relationship in bone and soft tissue differs from that in water. The thickness of tissues to be penetrated can be measured with fair accuracy from diagnostic X-ray films so that approximate corrections can be made if the range-energy relationship is known. Calculations by Tobias et al. (5) yield an estimate of 0.825 for the mass stopping power of bone relative to water. The present experiment was undertaken to check the prediction and to provide enough data for an empirical bone-correction curve.