We investigated two types of materials with very low Shore hardness, silicon rubber (Dragon Skin) and urethane liquid rubber (Clear Flex 30), for use in 3D printing patient-specific boluses. Boluses were manufactured with these materials using a mold casting method. NinjaFlex was also used to manufacture the bolus using a direct printing method. These patient-specific boluses were designed for 3D-printed elaborate human phantoms and their biological, physical, and dosimetric properties were comprehensively assessed. The results of cytotoxicity, skin irritation, and skin sensitization tests showed that Dragon Skin was the most biologically stable material. Furthermore, Dragon Skin exhibited excellent physical properties in terms of flexibility (Shore hardness 10A), durability (tensile strength of 475 psi and elongation at break of 1000 (%)), and preparation (5h curing time). Accordingly, Dragon Skin was finally selected for the bio-compatible patient-specific elastic (BPE) bolus. The dosimetric characteristics were thoroughly investigated with depth dose curves and surface dose. Dragon Skin showed the lowest differences between the calculated dose under virtual bolus and the measured dose at the surface of the phantom head and the lowest amount of unwanted air gap between the bolus and phantom. Overall, Dragon Skin is a suitable material for patient specific elastic bolus, and it could be implemented effectively in the clinic.