Anthropomorphic radiotherapy phantoms require tissue-equivalent materials to achieve Hounsfield units (HU) that are comparable to those of human tissue. Traditional manufacturing methods are limited by their high-cost and incompatibility with patient-specific customization. Additive manufacture (AM) provides a significant opportunity to enable manufacture of patient-specific geometries at relatively low cost. However, AM technologies are currently limited in terms of available material types, and consequently enable very little variation in achievable HU when standard manufacturing parameters are used. This work demonstrates a novel method whereby the partial volume effect (PVE) is utilized to control the HU of an AM material, in particular, enabling low HU in the range typical of lung tissue. The method enables repeatable design of lung HU and is compatible with commercial machines using standard print parameters. A custom algorithm demonstrates the clinical application of the method, whereby patient-specific computed tomography (CT) data are algorithmically calibrated according to AM print parameters and confirmed to be robust as a custom anthropomorphic radiotherapy phantoms.