Application of the spirometer in respiratory gated radiotherapy

~Received 23 May 2003; revised 9 September 2003; accepted for publication 23 September 2003;published 20 November 2003!The signal from a spirometer is directly correlated with respiratory motion and is ideal for targetrespiratory motion tracking. However, its susceptibility to signal drift deters its application inradiotherapy. In this work, a few approaches are investigated to control spirometer signal drift fora Bernoulli-type spirometer. A method is presented for rapid daily calibration of the spirometer toobtain a flow sensitivity function. Daily calibration assures accurate airflow measurement and alsoreduces signal drift. Dynamic baseline adjustment further controls the signal drift. The accuracy ofthese techniques was studied and it was found that the spirometer is able to provide a long-termdrift-free breathing signal. The tracking error is comprised of two components: calibration error andstochastic signal baseline variation error. The calibration error is very small ~1% of 3 l! andtherefore negligible. The stochastic baseline variation error can be as large as 20% of the normalbreathing amplitude. In view of these uncertainties, the applications of spirometers in treatmenttechniques that rely on breathing monitoring are discussed. Spirometer-based monitoring is notedmost suitable for deep inspiration breath-hold but less important for free breathing gatingtechniques. © 2003 American Association of Physicists in Medicine. @DOI: 10.1118/1.1625439#I. INTRODUCTIONVarious respiratory motion control treatment techniques in-cluding gating,