Late-onset events in parosteal osteosarcoma: findings from a two-decade retrospective study

BACKGROUND: Although parosteal osteosarcoma (POS) is generally considered indolent with favorable short-term outcomes, its long-term biological behavior remains poorly understood. This study investigated late-onset events and prognostic factors for POS through an extended follow-up. METHODS: We retrospectively analyzed 84 patients with histologically confirmed POS treated between 2000 and 2020 at our institution. All patients were followed up for a minimum of 5 years. A subgroup analysis was performed for patients followed for more than 10 years to assess very late (> 10 years) local recurrence and distant metastases. Clinical, radiological, and pathological data were also analyzed. The secondary outcomes were survival rates and patterns of disease progression. RESULTS: At a mean follow-up of 10.33 years (range, 5-21 years), local recurrence occurred in 22.6% (19/84) of patients, with 47.37% (9/19) of cases occurring after 10 years. Distant metastases developed in 13.1% (11/84) of patients, with 45.45% (5/11) manifesting after 10 years. Dedifferentiation occurred in 7.1% (6/84) of patients at a median of 8.7 years post-treatment. Multivariate analysis revealed that a tumor size > 8 cm (HR: 3.53, p = 0.018), medullary involvement (HR: 2.52, p = 0.049), and hemicortical resection (HR: 2.94, p = 0.041) were independent predictors of late-stage recurrence. The overall survival rates at 5, 10, and 15 years were 95.2%, 91.7%, and 88.1%, respectively. CONCLUSIONS: POS has significant potential for late recurrence and metastasis beyond the conventional follow-up periods. These findings support the need for extended surveillance, particularly in patients with identified risk factors, and suggest that current follow-up protocols may require revision to account for late-onset events.