Linearity of the dose monitor system at low monitor units

Background and Purpose: The accuracy of the dose monitor system of a medical electron accelerator is of crucial importance for the precise delivery of the prescribed dose. Special attention is given to the case of low monitor units (MU). The long-term stability of the dose monitor system is analyzed. Material and Methods: Photon and electron fields with monitor units between 1 and 500 MU were measured with field sizes of 10 x 10 cm2 and 20 x 20 cm2 at source-skin distances (SSDs) of 100 cm and 95 cm, respectively. The relative dose per MU r D(x MU) / D(200 MU) x 200 / x was determined for an Elekta linear accelerator. Results: The relative dose per MU was measured to be constant within about 1% down to fields with only 2 MU. The deviation remained below 2% down to 1 MU which is the minimum setting of the accelerator. The long-term stability is better than 0.2% per month. Conclusion: No therapeutically relevant deviation of the prescribed dose could be detected for the Elekta linear accelerator. This is a prerequisite for the implementation of intensity-modulated radiotherapy (IMRT) techniques