Background: The prognosis of patients (pts) with relapsed/refractory (R/R) High-Grade Osteosarcoma (HOS) remains dismal without an agreement on systemic therapy. The use of 14IFO with an external pump in outpatient setting (1g/sqm/day x 14 days every 21) in R/R HOS pts is limited. This is the first retrospective cohort study focused on 14IFO activity and toxicity in this setting (NCT04651569). Methods: Five Centers of the Italian Association of Pediatric Onco-Hematology and Italian Sarcoma Group participated to the study. Primary aim is to investigate 14IFO activity in pts with R/R HOS younger than 40 years. Secondary aim is to evaluate toxicity, according to CTCAE v.5, and clinical benefit. Progression Free Survival (PFS) and Overall Survival (OS) analysis are performed using the Kaplan Meier method with 95% confidence interval (CI). Results: Between 2012 and 2021, 26 R/R HOS pts were treated with 14IFO (median follow-up: 17,5 months, range: 4,6 – 83 months). Median age is 19 years at the beginning of 14IFO; eleven pts (42%) are ≤ 18 years. Three pts (12%) have a localized HOS, twelve pts (46%) have pulmonary disease only and eleven pts (42%) have a pluri-metastatic HOS. Eleven pts (42%) have a relapsed HOS with a median disease-free interval (DFI) of 13 months before 14IFO, the remaining (58%) have a refractory disease to two or more treatments. Overall, thirteen pts (50%) receive 14IFO as second line therapy and fifteen pts (57%) are pre-treated with Ifosfamide in first line. Disease Control Rate is 57,5% (5 Partial Response + 10 Stable Disease). Seven pts (27%) receive a local treatment after 14IFO (5 surgery, 1 Radiotherapy (Rt), 1 Carbon Ion Rt). The median PFS is 4,1 months (95% CI 2.13, 7.37). 6-month and 1-year PFS are 38% (95% CI 24-63) and 8% (95% CI 2-29), respectively. The median OS is 13.7 months (95% CI 10.6 – 23.7). 1-year and 2-year OS are 51% (95% CI 35-75) and 22% (95% CI 9.5-49), respectively. Relapsed pts have both longer median PFS and OS compared to pts with a refractory disease [PFS: 7.33 months vs 2.13 months (p = 0.02); OS: 19.4 months vs 10.7 months (p = 0.1)]. Sixteen pts (61%) receive at least 4 cycles and an amount of 101 cycles are evaluated for toxicity. Grade 4 hematological toxicity is reported in 15 cycles (14,8%) as follows: i) white-blood cell decrease in 6 cycles (6%); ii) neutropenia in 7 cycles (7%); iii) thrombocytopenia in 2 cycles (2%). One patient has one episode of febrile neutropenia. No grade 3-4 non hematological toxicities are reported. Conclusions: This trial shows a non inferiority activity of 14IFO compared to other treatments in this setting, despite the small number of pts. Therefore, 14IFO should be considered as a treatment option in R/R HOS, especially for its well tolerated toxicity profile and the home-administration that improve patient’s quality of life and it could significantly reduce cancer care cost. Clinical trial information: NCT04651569.