Dosimetric characteristics of linear accelerator photon beams with small monitor unit settings

Abstract

Several studies on the effect of tumor cell killing by dose rate variation have implied that the use of a shorter treatment time is more favorable for intensity modulated radiation therapy (IMRT). Aiming at step-and-shoot IMRT with higher dose rates, the stabilities of beam output and profiles with small monitor unit (MU) settings were investigated for various dose rates. With the use of a Varian 21EX (Varian Medical Systems Inc., Palo Alto, CA), static and step-and-shoot IMRT beam output along with profiles were measured by use of an ion chamber and a two-dimensional diode array detector as a function of monitor units and dose rates. For a static case, as the MU approached 1, the beam output increased up to 2% for 300 MU/min and 4.5% for 600 MU/min, showing a larger overdose as the dose rate increased. Deterioration of the beam symmetry and flatness were also observed as the MU decreased to 1 monitor unit. For the step-and-shoot IMRT case, a large dosimetric error of more than 10% was also detected with the use of a small MU segment. However, no definite correlation with the dose rate was observed due to the combined beam start-up effects by the grid pulse and finite communication time between the machine console and multileaf collimator (MLC) controller. For step-and-shoot IMRT with higher dose rates, beam output and beam profile stability with small MU needs to be checked, and adequate MU limitation where segments are not allowed need to be reflected in the step-and-shoot IMRT planning.

Characteristics of flattening filter free beams at low monitor unit settings

“…The small variations in flatness of the FFF beams also indicate the stability of the profiles. Kang et al 21 reported that the symmetry and flatness did not show any correlations with energy and dose rate. In our results, the characteristics of the FFF beams at 2000 MU/min dose rate were similar to those at 500 MU/min.…”

Evaluation of delivered monitor unit accuracy of gated step‐and‐shoot IMRT using a two‐dimensional detector array

“…A total of 68 segments were used, and the minimum segment MU was set to 3 MU in order to prevent the problems reported by several authors that arise from a small MU setting. 8,9 To deliver gated beams, we used respiration signals acquired from the real-time position management ͑RPM͒ ͑Varian Medical Systems, Palo Alto, CA͒ system and a homemade one-dimensional moving phantom. Figure 1 shows the moving phantom and the experiment setup.…”

Improvements in dose accuracy delivered with static‐MLC IMRT on an integrated linear accelerator control system

“…For SMLC-IMRT plans involving a large number of low MU segments and high dose rates this can lead to unacceptable dosimetric errors. [1][2][3][4][5] Especially noticeable is that the first and last segments tend to have higher and lower dosages, respectively, than the other intermediate segments. This issue is commonly referred to as the "overshoot phenomena," where an "undershoot" of dose in the final segment occurs to compensate for the overshoot of dose in the initial segment.…”