Nikkei flat in choppy trade as profit-taking offsets U.S. optimism

* Canon soars after Nikkei says profit likely risen 50 pct * Foreigners net sellers last week By Ayai Tomisawa TOKYO, April 17 (Reuters) - Japan's Nikkei share average was flat in choppy trade on Thursday morning as profit-taking from the previous day's big gains offset strength in U.S. shares which rose on confidence that the Federal Reserve will keep the interest rates low. Canon Inc outperformed the market, surging 2 percent and was the sixth most-traded stock by turnover af