Basic data on tissue-equivalent compensators for 4-MV x-rays

The compensator technique to correct for irregular and sloping body surfaces was examined for 4 MV x-rays. Thickness ratios for tissue-equivalent compensators were determined from central axis depth dose measurements and ratios were obtained for various compensator-to-surface distances. Use of a single thickness ratio to compensate over a range of field sizes, at different depths and for various deficit thickness was examined; agreement within +-4% was achieved between central axis depth doses of the compensated and normally incident beams. Similar agreement down to a depth of 20 cm was obtained between isodose distributions of a normally incident beam and an obliquely incident beam that was compensated for a non-uniform thickness of missing material. Errors in depth doses caused by a failure to correct for the reduced compensator thickness were also assessed