noun
BAD : Her husband died 10 years ago of a lung cancer.
GOOD : Her husband died 10 years ago of lung cancer.
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cancer (uncountable) = a type of serious disease: 'It is generally believed that diet plays an important role in the prevention of cancer.' 'Skin cancer is related to prolonged exposure to sunlight.'
a cancer (countable) = an abnormal growth in someone's body which is caused by this disease; tumour: 'Some small cancers may be destroyed by the body's defence mechanisms.'