The present invention offers an alternative for cancer treatment where radiation, thermotherapy, or another therapeutic modality must be delivered to an internal cavity of a subject, for example to treat mouth, anal, cervical, and vaginal cancers. The invention is a new approach that builds on recent results in 3D printing and steerable needle motion planning to create customized implants containing customized curvature-constrained internal channels that fit securely, minimize air gaps, and precisely guide treatment sources through internal printed channels to accurately reach tumors and minimize damage to healthy tissue.