PURPOSE: Treatment of sacral tumors requires either total or partial sacrectomy. Partial sacrectomy creates heterogeneous defects that are potentially amenable to a greater variety of reconstructive techniques. It has not been determined which factors best guide decision-making for managing these defects. We hypothesized that partial sacrectomy defect volume is the major factor determining selection of reconstructive technique to optimize outcomes. METHODS: We conducted a 15-year retrospective review of all partial sacrectomy reconstructions at a major U.S. cancer center. We evaluated the relationship of patient, tumor, and treatment factors and defect volume to flap choice and surgical outcome. Defect volume was categorized into three groups: small (< 400 cm3), moderate (400 – 2000 cm3), and large (> 2000 cm3). Reconstructions were classified into five groups: gluteus-based, gluteal thigh, vertical rectus abdominis musculocutaneous (VRAM), paraspinous muscle, and other flaps. Univariate and multivariate regression analysis was used to evaluate predictive factors for surgical outcomes. RESULTS: Fifty patients underwent partial sacrectomy reconstruction: 25 (50%) gluteus-based, 13 (26%) VRAM, 4 (8%) gluteal thigh, 4 (8%) paraspinous, and 4 (8%) other flaps. Average follow up was 26.2 months. For flap selection, surgeons employed a reconstructive strategy in which greater magnitude and complexity reconstructions were used for larger volume defects. The distribution of cases with small, medium, and large defect volumes was 15 (30.0%), 25 (50.0%), and 10 (20.0%), respectively. Resection volume as a continuous variable (p=0.023) and by category (i.e., small, medium, and large; p=0.0163) was a significant predictor for flap chosen. Sacrectomy resection level was not significantly associated with flap choice. Overall, the patient complication rate was high (44%), but no factors, including flap choice, predicted complications. Outcomes specifically related to function, such as ambulatory or continence status, length of hospitalization, tumor recurrence, and mortality, increased in proportion to increasing defect volume. However, complications related to the surgical site, such as abscess, dehiscence, or delayed wound healing, were similar irrespective of defect volume. CONCLUSION: Resection volume was the major factor determining flap selection in this series. Despite the worsened functional morbidity and oncologic prognosis associated with increased resection volumes, wound-related complications were similar among defect volume groups. Thus, using defect volume to select the partial sacrectomy reconstruction technique will enable surgeons to optimize patient outcomes.