An oncology therapy method comprises implanting a radioactive seed (10, 20, 30, 40, 50) in an oncology subject (S). In some embodiments the radioactive seed comprises a radioactive material (12, 32, 33, 42) including at least one radioisotope disposed in a biodegradable host (14, 24, 25, 44) configured to biodegrade over a therapy time period when implanted in the oncology subject. In some embodiments the radioactive seed is implanted in soft tissue of an oncology subject (S), and the radioactive seed comprises a radioactive material (12, 32, 33, 42) including at least one radioisotope disposed in a host material (14, 24, 25, 44) having softness comparable with or softer than the soft tissue into which the radioactive seed is implanted.