Measurement of the dosimetric parameters for low monitor units in step-and-shoot IMRT delivered by Siemens Artiste linear accelerators

Absorbed dose linearity and beam stability, both for low monitor units, are important factors for ensuring planned dose delivery in step-and-shoot IMRT. For Siemens Artiste linear accelerators, under IMRT stable irradiation conditions and for a single segment of 20 cm x 20 cm field size, the linearity of the absorbed dose with the monitor units, field flatness and symmetry have been measured for the range between 1 and 10 monitor units. We have found that absorbed dose linearity with monitor units is within 2% down to 2 monitor units and it is about 9% for 1 monitor unit. Flatness and symmetry values show variations within 1% down to 2 monitor units and increase by 9% for lower values. Using our monitor unit distribution per segment in IMRT we estimate that the uncertainty in absorbed dose for a whole treatment due to these factors is less than 1% (k= 3). (Author) 13 refs.