Older cancer patients are vulnerable to chemotherapy-related cognitive impairment. We prospectively evaluated cognitive impairment and its predictive factors during first-line chemotherapy in elderly cancer patients (≥70 years). Cognitive function was evaluated by the Mini-Mental State Examination (MMSE) with adjusted scores for age and sociocultural level. Multidimensional geriatric assessment was performed at baseline and during chemotherapy including the MMSE, Instrumental Activities in Daily Living (IADL), Mini-Nutritional Assessment (MNA), and the Geriatric Depression Scale (GDS15). Quality of life (QoL) was evaluated using the European Organization for Research and Treatment of Cancer (EORTC) QoL Questionnaire (QLQ-C30). Of 364 patients included, 310 had two MMSE evaluations including one at baseline and were assessed. Among these patients, 86 (27.7%) had abnormal MMSE, 195 (62.9%) abnormal MNA, 223 (71.9%) abnormal IADL, and 137 (43.1%) had depressive symptoms at baseline. MMSE impairment during chemotherapy was observed in 58 (18.7%) patients. Abnormal baseline MNA (odds ratio (OR) = 1.87, p = 0.021) and MMSE (OR = 2.58, p = 0.022) were independent predictive factors of MMSE impairment. These results suggest that pre-existing cognitive impairment and malnutrition are predictive factors for cognitive decline during chemotherapy in elderly cancer patients. Detection and management of these risk factors should be systematically considered in this population before starting chemotherapy.< Leer menos