Local relapse patterns after preoperative radiotherapy of limb and trunk wall soft tissue sarcomas: Prognostic role of imaging and pathologic response factors.

Purpose: To retrospectively identify clinical, pathologic, or imaging factors predictive of local relapse (LR) after preoperative radiotherapy (RT) for soft tissue sarcomas (STS). Methods and Materials: This is a retrospective multicenter study of patients who underwent preoperative RT and surgery for limb or trunk wall STS between 2007 and 2018 in French Sarcoma Group centers and were enrolled in the "Conticabase". Patterns of LR were investigated taking into account the multimodal response after preoperative RT. Diagnostic and surgical samples were compared after systematic review by expert pathologists and patients were stratified by tumor grade. Log-rank tests and Cox models were used to identify prognostic factors for radiation response and LR. Results: 257 patients were included; 17 % had low-grade (LG), 72.5 % had high-grade (HG) sarcomas. In HG group, tumors were larger, mostly undifferentiated, and displayed more necrosis and perilesional edema after RT. Median follow-up was 32 months. Five-year cumulative incidence of LR was 20.3 % in the HG group versus 9.7 % in the LG group (p = 0.026). In multivariate analysis, trunk wall location (HR 6.79, p = 0.012) and proportion of viable tumor cellularity ≥ 20 % (HR 3.15, p = 0.018) were associated with LR. After adjusting for tumor location, combination of histotype and cellularity rate significantly correlated with LR. We described three prognostic subgroups for HG sarcomas, listed from the highest to lowest risk: undifferentiated sarcoma (US) with cellularity rates ≥ 20 %; non-US (NUS) with cellularity rates ≥ 20 % or US with cellularity rates < 20 %; and NUS with cellularity rates < 20 %, which shared similar prognostic risks with LG sarcomas. Conclusions: HG and LG tumors have different morphological and biological behaviors in response to RT. Combination of cellularity rate with histotype could be a major prognostic for LR. Patients with undifferentiated HG sarcomas with cellularity rates ≥ 20 % after preoperative RT had the highest risk of LR and disease-specific death.