We present a simple graphical framework to illustrate the potential welfare gains from a "top-up" health insurance policy requiring patients to pay the incremental price for more expensive treatment options. We apply this framework to breast cancer treatments, where lumpectomy with radiation therapy is more expensive than mastectomy but generates similar average health benefits. We estimate the relative demand for lumpectomy using variation in distance to the nearest radiation facility, and estimate that the "top-up" policy increases social welfare by $700-2,500 per patient relative to two common alternatives. We briefly discuss additional tradeoffs that arise from an ex-ante perspective.