Dose linearity and uniformity of a linear accelerator designed for implementation of multileaf collimation system‐based intensity modulated radiation therapy

Abstract

The dose linearity and uniformity of a linear accelerator designed for multileaf collimation system-(MLC) based IMRT was studied as a part of commissioning and also in response to recently published data. The linear accelerator is equipped with a PRIMEVIEW, a graphical interface and a SIMTEC IM-MAXX, which is an enhanced autofield sequencer. The SIMTEC IM-MAXX sequencer permits the radiation beam to be " ON" continuously while delivering intensity modulated radiation therapy subfields at a defined gantry angle. The dose delivery is inhibited when the electron beam in the linear accelerator is forced out of phase with the microwave power while the MLC configures the field shape of a subfield. This beam switching mechanism reduces the overhead time and hence shortens the patient treatment time. The dose linearity, reproducibility, and uniformity were assessed for this type of dose delivery mechanism. The subfields with monitor units ranged from 1 MU to 100 MU were delivered using 6 MV and 23 MV photon beams. The doses were computed and converted to dose per monitor unit. The dose linearity was found to vary within 2% for both 6 MV and 23 MV photon beam using high dose rate setting (300 MU/min) except below 2 MU. The dose uniformity was assessed by delivering 4 subfields to a Kodak X-OMAT TL film using identical low monitor units. The optical density was converted to dose and found to show small variation within 3%. Our results indicate that this linear accelerator with SIMTEC IM-MAXX sequencer has better dose linearity, reproducibility, and uniformity than had been reported.

Dose linearity and uniformity of Siemens ONCOR impression plus linear accelerator designed for step-and-shoot intensity-modulated radiation therapy

“…The symmetry value was obtained with ‘point difference quotient’ method. [5] For 6 MV beam [Figure 8], flatness in both planes is within 3%. The crossplane symmetry for 1 MU of 6 MV is 3.41%; and for 2 MU, 3.16%.…”

“…[5] Beam flatness and symmetry for 6 and 15 MV were evaluated with Kodak diagnostic film. For SFD 100 cm with collimator setting of 10 cm × 10 cm, exposures of 10, 5, 3, 2 and 1 MU were given.…”

Linearity of the Dose Monitor System at Low Monitor Units

“…This problem has been noticed and a number of publications are available [1,2,4,5,10,11,13,17,18,20,22] which analyze the linearity of the dose monitor system of medical electron linear accelerators (linac) down to 1 MU which is the lowest MU setting which can be used at most linacs. The available results show drastic variations: whereas Hansen et al [11] report minor changes of the order of 1-2% of the relative dose at low MU, very recently Ravikumar et al [18] have measured relative doses which are increased by about 20-40% at low MU.…”

Planning quality and delivery efficiency of sMLC delivered IMRT treatment of oropharyngeal cancers evaluated by RTOG H‐0022 dosimetric criteria

“…It is obvious that there are a significant number of segments with 2 or 3 monitor units. assuming the monitor unit linearity error is about 5% for 1 monitor unit, 2% for 2 to 4 monitor units, and 1% for 5 and more monitor units, ( 26 ) the largest error is about 1.5% for one of the IMBs and the average error over 9 IMBs is 1.3%. In this work, we attempted to reduce the number of segments to improve delivery efficiency.…”