## 1 Introduction  
In orthopaedic oncology, limb salvage procedures for the pelvis requires tumor resection by an appropriate excision followed by careful corresponding reconstruction of the affected bone and soft tissues [1]. Pelvic resections and reconstructions are classified by tumor extension and the section of bone to be resected, i.e., iliac or periacetabular or public location [2]. In particular, according to the [PERSON] classification [2], type I involves the iliac region, type II the periacetabular region, type III the pupils or ischiuum, and type IV the lateral part of the sacrum. Most of these reconstructions involve the acetabulum, which implies the replacement of the hip joint [3,4]. For all pelvic resection and reconstruction procedures, the primary goals are the restoration of the physiologic joint motion and the maintenance of good quality of life [4]. To date, surgical reconstruction after tumor resection in the pelvis remains a challenge, because of the critical and complex anatomical structures involved. A major critical aspect is the bone cut, which must be performed to achieve an adequate margin around the tumor, but also to preserve as much skeletal structure and joint function as possible, including adequate bone stock and soft tissues such as ligaments and tendons [1, 5, 6, 7]. Important advancements in pelvic reconstruction using biological reconstruction such as structural pelvic allografts or autografts, arthrodeis and endoprostheses have been shown [8], though a high rate of complications, including infection, dislocation and mechanical failure, have been reported [5; 9; 10; 11; 12]. More recently, there has been a strong interest in custom-made prostheses, particularly after the great developments of additive manufacturing, also known as 3D-printing. A custom-made prosthesis is a fully personalized implant, which is aimed at achieving a more anatomical reconstruction, i.e., more respectful of the original anatomy, and a better match with patient's residual bone, and thus a smaller risk of loosening, infection, fractures, and any possible mechanical failure. It also allows a precise pre-operative planning of the surgical procedure, this including the positioning of the prosthesis, and the setting of corresponding bone cuts. The potential better long-term clinical and functional results are supported by the good short-term results [3; 5; 7; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. 3D printed implants have attracted much attention nowadays also because of faster production and lower costs, together with the accurate optimization and control of the overall geometry, both in terms of the external roughness and internal topology [25]. Surgical reconstructions at the pelvis using custom-made implants are showing encouraging results, both at the acetabulum, ileum, and sacrum sections, with low complication rates, although wound healing problems have been reported [17; 26]. On the other hand, any custom-made implant requires careful medical imaging and time-consuming modelling and design [6]. Nevertheless, this digital process allows a precise identification of the tumor and, thus, a careful computer-based pre-surgical planning of the bone resection [5; 24; 27], though this virtual surgery depends on the quality of medical imaging and the software tools utilized and the experience and ability of the surgical team. As mentioned, computer-based design and 3D printing of these implants have the potential to result in advanced porous metal implants, with different geometries of the internal and external structures, and with no limits to their complexity. It is also possible to use a number of different materials, with mechanical properties similar to those of the natural anatomical structures [25; 28; 29]. This further technical advancement has been assessed already also in custom-made implants for complex pelvic reconstructions [13; 17; 30; 31].