Effects of Beam Startup Characteristics on Dose Delivery Accuracy at Low Monitor Units in Step-and-Shoot Intensity Modulated Radiation Therapy

Kazunori Fujimoto, Kunihiko Tateoka, Yuji Yaegashi, Katsumi Shima, Junji Suzuki, Yuichi Saito, Akihiro Nakata, Takuya Nakazawa, Tadanori Abe, Masaki Yano, Masanori Someya, Kensei Nakata, Masakazu Hori, Masato Hareyama, Koichi Sakata

Department of Medical Physics, Graduate School of Medicine, Sapporo Medical University, Sapporo, Japan.
Department of Radiology, Sapporo Medical University, Sapporo, Japan.
Radiation Therapy Research Institute, Social Medical Corporation Teishinkai, Sapporo, Japan.

DOI: 10.4236/ijmpcero.2013.21001                

Abstract

Intensity modulated radiation therapy (IMRT) is a highly accurate technique that is usually implemented in either dynamic or step-and-shoot fashion with many segments each having low monitor units (MUs). The present study evaluated the effects of beam startup characteristics on the dose delivery accuracy for each segment at low MUs for step-and-shoot IMRT with an Elekta Precise accelerator at the highest dose rates. We used a two-dimensional semi-conductor detector for the dose measurements. The field size of each segment was assumed to be 20 ×20 cm² and each segment was set to deliver 1 - 10 MUs. Our results show a variation in dose delivery accuracy between segments for the same IMRT beam, which can be attributed to the beam startup characteristics. This variability is attributed to the changes in the transient changes in the temperatures of the electron gun filament and the magnetron. That is, the transient increase in the temperature of the filament leads to increasing doses with time and that of the magnetron leads to decreasing doses with time during the first few MUs.

Keywords

Startup Performance; Step-and-Shoot IMRT; Low Monitor Units

Conflicts of Interest

The authors declare no conflicts of interest.

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