Radar Online reports the 49-year-old R&B star Janet Jackson recently postponed her Unbreakable World Tour, originally scheduled to start on Jan. 9. Sources indicate Janet Jackson has a tumor that could be cancerous, her condition requires immediate medical attention. "Doctors found a growth on Janet's vocal cords that could be serious," a source added. "It's a medical condition that has to be taken care of immediately."According to Dr. Stuart Fischer, if Jackson is found to be suffering from cancer of the larynx, it could spell the end of her long career. "That would abruptly end her ability to ever perform again," he said. Cancer of the larynx typically requires immediate surgery, and a patient often needs an electrolarynx, or artificial voice box, just to be able to speak, noted Fischer."Singing would be impossible," the doctor added. The singer herself recently took to social media to alert fans of her situation, and to ask for prayers for her and her family. Experts noted treatment for any cancerous growth would almost certainly be extremely aggressive and require intense aftercare. "Surgery would be very complex, and followed by months of radiation treatment," Fischer said. The delay marks at least the second time performances for the Unbreakable Tour have been pushed back. In October, shows were canceled so Jackson could give her voice a rest. "Janet said she knows that with prayers and her will, she'll be back out on the road in the spring or early summer," a source added.