Updated from 3:12 p.m. ESTShares of Impac Medical Systems ( IMPC) were among the best-performing health-related stocks Tuesday, rising 19.4% after the company agreed to be acquired by Sweden-based Elekta for about $250 million in cash. Impac shareholders will receive $24 a share in cash for each share they hold, which represents a 22% premium over Friday's closing price of $19.75. The combined businesses will have relationships with more than 3,000 hospitals and cancer centers around the world, including 1,300 oncology centers; more than 1,100 cancer registry operations; and more than 400 pathology labs in North America. Shares of Impac traded up $3.84 to $23.59. CTI Molecular ( CTMI) rose 2.7% after the company's subsidiary, CTI Mirada Solutions, received Food and Drug Administration clearance to market its Scenium software product. Clinicians use the software to assess and quantify dementia and other neurological diseases. Mirada plans to release the product during the spring of 2005. Shares traded up 36 cents to $13.94. Shares of Maxim Pharmaceuticals ( MAXM) fell 6% after the company said that its leukemia drug would have to undergo an additional phase III trial before it could be approved by the FDA. The FDA said that Maxim's Ceplene, an investigational drug used to treat acute myeloid leukemia, will need further studies before Maxim can apply for regulatory approval in the United States. The expensive testing, the company says, will need to be done in collaboration with a corporate partner, which will help defray costs. Maxim is in discussions with European regulators to determine whether an additional phase III trial is necessary for European approval. Shares traded down 16 cents to $2.51. Other health care movers included Aastrom Biosciences ( ASTM), up 65 cents to $3.35; Pfizer ( PFE), up 5 cents to $25.30; StemCells ( STEM), up 75 cents to $5.99; Wyeth ( WYE), up $2.42 to $44.70; Merck ( MRK), up 43 cents to $31.30; Schering-Plough ( SGP), up 3 cents to $20.33; and Amgen ( AMGN), up 35 cents to $64.16.