Abstract It is difficult to treat patients with an inoperable sarcoma adjacent to the gastrointestinal (GI) tract using carbon ion radiotherapy (C-ion RT), owing to the possible development of serious GI toxicities. In such cases, spacer placement may be useful in physically separating the tumor and the GI tract. We aimed to evaluate the usefulness of spacer placement by conducting a simulation study of dosimetric comparison in a patient with sacral chordoma adjacent to the rectum treated with C-ion RT. The sacral chordoma was located in the third to fourth sacral spinal segments, in extensive contact with and compressing the rectum. Conventional C-ion RT was not indicated because the rectal dose would exceed the tolerance dose. Because we chose spacer placement surgery to physically separate the tumor and the rectum before C-ion RT, bioabsorbable spacer sheets were inserted by open surgery. After spacer placement, 67.2 Gy [relative biological effectiveness (RBE)] of C-ion RT was administered. The thickness of the spacer was stable at 13–14 mm during C-ion RT. Comparing the dose–volume histogram (DVH) parameters, Dmax for the rectum was reduced from 67 Gy (RBE) in the no spacer plan (simulation plan) to 45 Gy (RBE) in the spacer placement plan (actual plan) when a prescribed dose was administered to the tumor. Spacer placement was advantageous for irradiating the tumor and the rectum, demonstrated using the DVH parameter analysis.