Tycoon Warren Buffett announced Tuesday that he has early-stage prostate cancer, and indicated he had no intention of stepping down as head of his investment firm Berkshire Hathaway. Buffett, in a letter to Berkshire shareholders, said he had been diagnosed with Stage I prostate cancer last Wednesday and would undergo radiation treatment beginning in mid-July. Imaging tests revealed no cancer elsewhere in his body, he said. \"The good news is that I\'ve been told by my doctors that my condition is not remotely life-threatening or even debilitating in any meaningful way,\" the 81-year-old investment guru said. The letter was released after the stock markets closed. Buffett said he and his doctors had decided on a two-month treatment of daily radiation, which would restrict his travel \"but will not otherwise change my daily routine.\" \"I feel great -- as if I were in my normal excellent health -- and my energy level is 100 percent,\" said Buffett, dubbed the Oracle of Omaha for his shrewd investments that have raked in billions. The cancer was discovered during a routine medical check-up that showed an unusually large jump in his blood level of PSA -- a protein that is a prostate-specific antigen produced by the cells of the prostate gland. \"A biopsy seemed warranted,\" Buffett said. The PSA test is recommended by many doctors to help determine prostate cancer in men age 50 and older. \"I will let shareholders know immediately should my health situation change. Eventually, of course, it will; but I believe that day is a long way off,\" he said. Berkshire shares rose 1.4 percent Tuesday but were down 1.8 percent in post-market trading.