Cover:
Dose distribution from single proton (left) and 6 MV x-ray (right) beams incident from the left of the figures, illustrating the inherent properties of the two modes of cranial radiation. The colored lines represent relative dose levels, including 50% (dark green), 55% (light orange), 60% (light blue), 65% (light purple), 75% (yellow), 90% (red), 95% (light green), 102% (dark purple), 120% (dark orange), and 150% (dark blue). With protons, the preponderance of the dose is deposited within the target (shown with red shading) with sharp fall-off beyond, whereas for x-rays most of the dose is deposited near the surface with exponential fall-off beyond. Regardless of modality, radiotherapy planning always incorporates multiple beams to conform the dose distribution to the target and minimize the dose to normal tissue. From the article by Petit et al., in this issue, pages 393–399.