Impact of Irradiation on Post-Surgical Residuals of WHO Grade I Meningioma

<b>Background:</b> Radiotherapy is widely used to control postoperative residuals of WHO grade I meningiomas, with favorable outcomes reported from relatively short follow-ups. The aim of this study is to evaluate the effect of radiation on extended long-term outcome of benign meningiomas, comparing radiated to non-radiated post-surgical residuals. <b>Methods:</b> A retrospective observational record analysis of 2499 consecutive meningiomas treated from 1990 through 2023 identified 436 WHO grade I meningiomas with post-surgical residuals after subtotal resection (STR); of these, 176 received radiotherapy. Progression-free survival, cause-specific overall survival, and mortality were analyzed. Clinical control was defined as the absence of post-irradiation intervention. Malignant transformation was confirmed histologically. <b>Results:</b> At a median and mean follow-up of 103.5 and 127.28 months, the 3-, 5-, 10-, and 15-year progression-free survival were 91%, 85%, 77%, and 70% following STR alone, and 59%, 43%, 23%, and 16% after STR plus radiotherapy. The cause-specific overall survival at 5, 10, 15, and 20 years was 97.6%, 97.6%, 97.6%, and 96% for STR and 97%, 93%, 85%, and 76% for STR with irradiation, respectively. Mortality was 26% in the irradiated group, compared to 4%. Clinical control was achieved in 87% and 37% in the surgery and irradiation groups, respectively. Malignant transformation occurred in 28% of the irradiated group and 1% after surgery alone. <b>Conclusions:</b> This study revealed that with a follow-up beyond 10 years, irradiation of residual WHO I meningiomas was associated with increased recurrence, worse survival, less clinical control, and increased malignant progression.