1. Department of Radiation Oncology Dana Farber/Brigham and Women's Cancer Center Harvard Medical School Boston MA 02115USA
2. Varian Medical Systems Baden CH‐5405Switzerland
3. Varian Medical Systems Palo Alto CA 94304‐1030USA
4. Radiological Sciences Laboratory Stanford University Stanford CA 94305USA
5. Department of Radiation Oncology University of Massachusetts Lowell MA USA