A new radiation phantom with humanoid shape and adjustable thickness (RPHAT) has been developed. Unlike the RANDO phantom which is a fixed thickness, this newly designed phantom has adjustable thickness to address the range of thicknesses of real-world patients. RPHAT allows adjustment of the body thickness by being sliced in the coronal (instead of axial) direction. Centre slices are designed so that more sections can be added or removed while maintaining the anthropomorphic shape. A prototype of the new phantom has been successfully used in a study investigating peripheral dose delivery, where the amount of scatter within the patient, and therefore the patient thickness, plays a critical role in dose deposition. This newly designed phantom is an important tool to improve the quality of radiation therapy.