Beam spoilers versus bolus for 6 MV photon treatment of head and neck cancers.

When parallel opposed 6-MV x-ray beams are used for treatment of head and neck tumors, superficial tissues and lymphatics at shallow depths of < or =4 to 6 mm may be at cancer risk but receive less than full radiation dose. In these cases, the use of either a beam spoiler or bolus material can increase dose to superficial tissues. The potential benefit of a beam spoiler relative to bolus is preservation of skin-sparing characteristics for cases in which the skin surface does not require full dose. In this study, we evaluate the application of a beam spoiler and compare it to bolus for standard treatments of head and neck tumors. Measurements of both depth dose in-water and in-air profiles were made with a beam spoiler for a 6-MV photon beam. The measurements were combined with Monte Carlo calculations to obtain the energy spectrum of the spoiler-generated electrons. An in-house pencil beam treatment-planning algorithm was used to calculate the dose distribution with spoiler. The dose distribution in the buildup region was then studied with and without the spoiler for a typical head and neck treatment with parallel-opposed beams. Dose distributions and partial-volume dose histograms (PVDH) demonstrate the benefits provided by spoilers for the head and neck treatments and the limitations of their use. The beam spoiler is useful in treating the superficial lymphatics in the buildup region near head and neck tumors. Guidelines for use of beam spoiler versus bolus are discussed.