Sen. Edward M. Kennedy was diagnosed with a cancerous brain tumor Tuesday in what could be the grim final chapter in a life marked by exhilarating triumph and shattering tragedy. Some experts gave the liberal lion less than a year to live. Doctors discovered the tumor after the 76-year-old senator, the sole surviving son of America's most storied political family, suffered a seizure over the weekend. Kennedy's doctors at Massachusetts General Hospital said he had a malignant glioma in the left parietal lobe, a region of the brain that helps govern sensation, movement and language. Malignant gliomas are diagnosed in about 9,000 Americans a year. In general, half of all patients die within a year.