Legendary pop star David Bowie has died at the age of 69, following a secret battle with cancer.

According to the official statement, Bowie died after an 18-month struggle with the disease. Although the type of cancer Bowie had was not disclosed, reports suggest that he may have died of liver cancer, which may have originated as lung cancer and spread to his liver. 

Bowie was a longtime smoker and spoke openly about his drug use, which in his later years he said he had come to regret.

Rumors about Bowie's private health crisis have surrounded the performer for years, beginning in 2004, when he suffered a heart attack on stage.  He had appeared rarely in public over the past few years, although last month, he greeted fans in New York. Although he seemed in good health, his rail-thin appearance seemed alarming, according to the Mirror Online.

Whether Bowie had liver cancer or whether it was lung cancer that had spread to his liver, both these malignancies are among the toughest to treat.

Lung cancer is the biggest cancer killer in the U.S.  This year, an estimated 221,200 adults (115,610 men and 105,590 women) in the United States will be diagnosed with lung cancer. It is estimated that 158,040 (86,380 men and 71,660 women) will die from it this year, according to the American Cancer Society.

The major cause of the most common form of lung cancer is cigarette smoking, which accounts for about 87 percent of cases, the ACS says.

The 5-year survival rate for people with lung cancer is only 17 percent, although this number climbs to 54 percent if the disease is caught in its earliest stages. Unfortunately, only 15 percent of cancers are found at this stage. 

One of the most common sites for lung cancer to spread is the liver, and if it does, the chances of survival are further diminished. If lung cancer spreads to a distant organ the 5-year survival rate drops to only 4 percent.

Liver cancer can also occur as a primary tumor.  This year, an estimated 35,660 adults (25,510 men and 10,150 women) in the United States will be diagnosed with primary liver cancer. An estimated 24,550 Americans (17,030 men and 7,520 women) will die from it, the ACS says.

The overall survival rate for liver cancer is 17 percent.  Most liver cancer is found after the disease has spread, but even for those whose tumor is found at an earlier stage, the survival rate is only 30 percent. If the cancer has spread, the survival rate drops to 11 percent, and only an estimated three percent of people survive if it has spread to a distant part of the body.

The cause of liver cancer is unknown but hepatitis can raise risk. One of the major causes of hepatitis is needle sharing during drug use, however Bowie was not known to have the disease.  Other causes are cirrhosis, whether alcohol-related or not, and age.