This study aimed to determine the efficacy of amyloid clearance of lecanemab in patients with Parkinson's disease (PD) with amyloid co-pathology. Lecanemab, an anti-amyloid monoclonal antibody, was apporoved by the US FDA in July 2023 and in South Korea in May 2024, as a disease-modifying therapy based on its clinical efficacy and reduction of amyloid plaques in patients with early-stage Alzheimer's disease (AD). AD pathology is also common in PD, and approximately 35% of patients with PD dementia have co-existing AD pathology. Currently, no mediations have been developed to slow the progression of PD. Therefore, this study aimed to determine whether reducing the amyloid burden in patients with PD with co-exsistent AD pathology could potentially slow disease progression. To test it, patients with PD with mild cognitive impairment or early dementia, who were confirmed to have amyloid deposition through amyloid imaging, would be enrolled as a treatment arm, and the degree of reduction of amyloid plaque after 18 months of lecanemab administration would be investigated.