(P 63) DOES TUMOR NECROSIS IMPACT PROGNOSIS IN CHONDROBLASTIC AND OSTEOBLASTIC OSTEOSARCOMA? A STUDY FROM A LARGE TERTIARY CARE ACADEMIC CENTER

Objective: The percentage of tumor necrosis is a crucial prognostic factor in osteosarcoma. Many studies have adopted a 90% cutoff based on osteoblastic osteosarcoma, but these findings are generalized to all conventional subtypes, including chondroblastic osteosarcoma. Studies have reported lower tumor necrosis and worse overall survival (OS) in chondroblastic osteosarcoma compared to osteoblastic osteosarcoma, suggesting different histologic-driven behaviors. However, these studies did not control for differences in tumor characteristics between groups, such as tumor size and grade, that may confound results. Our study aimed to evaluate the relationship between tumor necrosis and survival in osteoblastic and chondroblastic osteosarcoma. In a propensity-score matched cohort, we sought to answer the following questions: (1) Is tumor necrosis ≥ 90% associated with better OS and disease-free survival (DFS) in osteoblastic and chondroblastic osteosarcoma? (2) Does the osteosarcoma subtype impact tumor necrosis? (3) Does the osteosarcoma subtype in “good” responders (tumor necrosis ≥90%) affect OS and DFS?

Methods: We conducted a retrospective study of 156 patients with osteoblastic and chondroblastic osteosarcoma treated at our institution. Demographic, clinical, and treatment variables were displayed using descriptive statistics. Bivariate analysis compared baseline characteristics between osteoblastic and chondroblastic osteosarcoma using Mann Whitney U test (non-parametric) for continuous variables and chi-square for categorical ones. Propensity-score matching was performed to adjust for potential confounders and to ensure comparable cohorts. Kaplan-Meier survival analysis and Cox proportional hazards modeling were performed to identify risk factors for poor chemotherapy response. Primary endpoints were five-year and ten-year OS and DFS. OS and DFS were assessed using the date of surgery for primary tumor resection as the start time.

Results: Patients with osteoblastic osteosarcoma and tumor necrosis ≥90% had higher five- and ten-year OS and DFS compared to those with necrosis < 90%. In chondroblastic osteosarcoma, a trend towards higher OS and DFS was seen in patients with tumor necrosis ≥ 90%; this, however, was not significant. Chondroblastic osteosarcoma was not a risk factor for either tumor necrosis < 90% (p=0.89) or tumor necrosis < 70% (p=0.57). Older age (≥35 years) and larger tumor size (≥7 cm) were associated with higher risk of poor chemotherapy response. Additionally, tumor location in the tibia was associated with a lower risk of poor response. Patients with osteoblastic or chondroblastic osteosarcoma that were deemed “good” responders (tumor necrosis ≥90%) had similar OS and DFS at the five- and ten-year marks.

Conclusion: Conventional osteosarcoma subtype was not a risk factor for “poor” response. Survival outcomes (OS and DFS) were similar between osteoblastic and chondroblastic osteosarcomas with good response to chemotherapy. Further studies pooling data from multiple institutions are required to determine whether the 90% cutoff is also associated with improved clinical outcomes in chondroblastic osteosarcoma. We additionally recommend that studies include more variables into the propensity-score matching algorithm to have more homogeneous populations and better individualize the impact of histology on response to chemotherapy.