SU‐FF‐T‐394: Start‐Up Characteristics of Elekta SLi‐Linacs: Minimum Number of MU Required for IMRT

Abstract

Purpose: On Elekta SLi‐linacs steering of flatness and symmetry starts after a user‐definable delay of 0.5 to 1 s after beam‐on, which equals 5 to 10 MU at 600 MU/min. The impact of start‐up characteristics of Elekta SLi linacs on dose delivery with small segments during IMRT was investigated and the minimum number of monitor units needed per segment was quantified. Method and Materials: Linearity of the ionisation monitor was quantified by taking ionisation chamber readings for 10×10 cm2 6MV and 10 MV photon beams over the range 1–200 MU. By using the Wellhöfer Beam Imaging System (BIS) dosimetric images of 820×820 pixels of the beam were acquired during the first 10 monitor units for 5 gantry angles (−180°, −90°, 0°, 90°, 180°). Every 120ms images were taken of a 40×40 cm2 field. All data are compared to the reference value after delivery of 10 MU, which is regarded to be the steady‐state value. Results: Deviations in ionisation monitor readings were less than 0.5% for segments larger than 2 MU. For segments of 1MU readings were 1.5% less than expected. Values for symmetry and flatness of the beams were very large during the first MU but decreased within 2MU to twice the steady‐state value at 10 MU. The integrated symmetry (defined as the symmetry of the integrated dose distribution) and integrated flatness decreased within 4MU to twice the steady‐state value. Neither gantry angle nor energy had influence on these results. Conclusion: During IMRT segments of 4MU can be safely used for all gantry angles. Errors in dose are smaller than 1% and errors in symmetry and flatness are less than twice that of 10 MU segments.