Degradation of the Bragg peak due to inhomogeneities

Abstract

The rapid fall-off of dose at the end of range of heavy charged particle beams has the potential in therapeutic applications of sparing critical structures just distal to the target volume. The effects of highly inhomogeneous regions on this desirable depth-dose characteristic are explored. The proton depth-dose distribution behind a lucite-air interface parallel to the beam was bimodal. A second experiment showed little significant effect on the distal depth-dose of protons having passed through a mosaic of teflon and lucite. Anatomic studies demonstrated significant effects of complex fine inhomogeneities on the end of range characteristics. Monoenergetic protons passing through the petrous ridges and mastoid air cells in the base of the skull showed a dramatic degradation of the distal Bragg peak. Heavy ions showed a corresponding degradation in their ends of range. In the worst case in the base of the skull region, a monoenergetic neon beam showed a broadening of the full width at half maximum of the Bragg peak to over 15 mm. A similar effect was found with carbon ions in the abdomen. The authors address the implications of these data for dose computation with heavy charged particles.