Localized osteosarcoma analysis of very poor responders subgroup (Huvos I)

e22010 Background: Osteosarcoma is a malignant primitive bone tumor whose prognosis is not changed since 4 decades, after the introduction of neoadjuvant chemotherapy with Methotrexate, Cisplatin, Doxorubicine and Ifosfamide. Histologic response to preoperative chemotherapy is a significant prognostic factor. Huvos I (necrosis ≤ 50%) has worst prognosis . Previous studies reported a 3 years EFS of this Huvos I patients around 25% (Tsuda Y,2020). In order to evaluate if survival has changed in recent years in this unfavourable prognostic group we evaluated the outcome of osteosarcoma patients with Huvos I. Methods: from our Pathology archieves we retrieved all cases of localized osteosarcoma treated at Rizzoli with neoadjuvant chemotherapy who reported an histologic necrosis below or equal to 50% (Huvos I grade) after preoperative chemotherapy MAP (Ethical C. Approval 917/2020/Oss/IOR). Results: from 2003 to 2019 we had 70 cases of localized osteosarcoma with Huvos I necrosis after neoadjuvant chemotherapy ( MAP in 66 and MAPI in 4) evaluable. Median age 21,5 (3-70); M:F = 44:26. 10/70 had axial localization vs extremity(60), subhistotype distribution:46 osteoblastic,11 chondroblastic, 7 fibroblastic, 5 teleangectatic, one not classified. In 24 cases PgP was available(14 PgP positive). With a median follow up of 86.7 ms (IQR 41-136) 43/70 had already relapsed. The median EFS was 25 ms (95% CI 15-42) and the 3 yrs EFS was 40.6% (95% CI 29-52). The 3 yrs overall survival was 80% (95%CI 68-88) and median OS was not reached. Axial tumor site was associated with significant inferior EFS (P = .004). Conclusions: these data confirm the poor prognosis of patients with necrosis ≤50% and the need of new drugs to improve their survival in this sub-group.