Radiotherapy uses high-energy radiation to eradicate cancer cells and requires high precision and accuracy. However, the negative psychological and physiological conditions of cancer patients could introduce instability in the treatment. In the study, we explore the connection between mental stress and the severity of the instability. A smartphone is utilized to perform simultaneous measurements and a specific experimental setting and data analysis method are designed. A potential advantage of the assessment is that if a cancer patient cannot maintain good stability in the test, it is also not safe for him/her to receive the radiotherapy due to the higher risk of missing the cancerous site and damaging the normal tissue.