Introduction and purpose <p>Late treatment failure following an initially effective response to targeted therapy may occur. Everolimus, a targeted drug that serves as an mTOR (rapamycin) inhibitor, has been extensively studied for its efficacy and safety in treating subependymal giant cell astrocytomas (SEGA) in pediatric patients with tuberous sclerosis complex (TSC). Clinical studies have demonstrated that Everolimus leads to a sustained reduction in SEGA volume. However, TSC patients may need to continue long term or even lifelong treatment to avoid the risk of progression following termination of treatment. The case report provided here demonstrates that late failure can occur even while in treatment.</p> Case report <p>We hereby describe a 16-year-old female diagnosed with TSC and bilateral large intraventricular SEGA, who displayed sustained tumor volume reduction on Everolimus treatment for 10&#xa0;years. Despite proper adjustment of dosage, confirmed by blood levels, the lesions eventually grew. This bilateral tumor growth subsequently led to symptomatic hydrocephalus. Therefore, at this point, the Everolimus treatment was stopped, and the girl was operated upon. Following subtotal surgical resection, a decision was made to simply follow the residual tumor remnant. When, after a few months, the tumor remnant was found to be slowly growing, the patient was re-started on Everolimus, which proved again to be effective in stabilizing the lesion growth.</p> Discussion <p>Everolimus is a well-known treatment for patients affected by TSC-related SEGA. Maintenance therapy can be continued safely for prolonged periods with continuous tumor volume stability. Nonetheless, late failure of treatment may still occur and require surgery. Potential mechanisms of such late reduction in effectiveness are discussed.</p>