Cancer patients often suffer from various distresses, including cognitive impairment.
 Cognitive impairment during and after cancer diagnosis and treatment are collectively called "Cancer-related cognitive impairment (CRCI)". The number of publications about cognitive impairment due to cancer therapy, especially chemotherapy, hormonal therapy, and radiotherapy, has been growing. Patients often worry not only about their disease condition and therapies, but also experience concerns regarding their memory, attention, and ability to concentrate. Even subtle CRCI can have a significant impact on social relationships, the ability to work, undergo treatment, accomplish meaningful goals, and the quality of life. Longitudinal studies of cancer patients indicated that up to 75% experience CRCI during treatment. Furthermore, CRCI may persist for many years following treatment. However, it is not well understood by most physicians and medical staff.
 CRCI can be mediated through increased inflammatory cytokines and hormonal changes. In addition, the biology of the cancer, stress, and attentional fatigue can also contribute to CRCI. Genetic factors and co-occurring symptoms may explain some of the inter-individual variability in CRCI. Researchers and patients are actively trying to identify effective interventional methods and useful coping strategies.
 Many patients are willing to discuss their disease condition and future treatment with medical staff and/or their families. Some patients also hope to discuss their end-of-life care. However, it is difficult to express their will after developing cognitive impairment. Advance care planning (ACP) can help in such situations. This process involves discussion between a patient, their family, and clinicians to clarify and reflect on values, treatment preferences, and goals to develop a shared understanding of how end-of-life care should proceed.
 The number of cancer patients with cognitive impairment has been increasing owing to the super-aging of society. Psychiatrists need to develop appropriate care for them and understand the value of ACP.
 In this review, an outline of CRCI is given, especially related to cancer therapy such as chemotherapy, hormonal therapy, and radiation therapy. In addition, the importance of ACP to facilitate a living will for patients likely to develop impaired cognitive functions in the future is described.
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