A light field-based method to adjust rounded leaf end MLC position for split shape dose calculation correction in a radiation therapy treatment planning system

作者：Wu Jia Ming; Lee Tsair Fwu; Kuo Chung Ming^*; Chen Ching Jiang; Hsiao Kuan Yin; Yeh Shyh An

来源：Journal of Applied Clinical Medical Physics, 2012, 13(6): 3-18.

• multi-leaf collimator
• rounded leaf end
• split shape
• dose correction

We present an analytical and experimental study of split shape dose calculation correction by adjusting the position of the round leaf end position in an intensity-modulated radiation therapy treatment planning system. The precise light field edge position (X-tang.p) was derived from 50% of the central axis dose created by nominal light field using geometry and mathematical methods. Leaf position (X-mlc.p), defined in the treatment planning system for monitor unit calculation, could be derived from X-tang.p. Offset (correction) could be obtained by the position corresponding to 50% of the central axis dose minus the X-mlc.p position. For SSD from 90 cm to 120 cm at 6 MV and 10 MV, the 50% dose position was located outside of X-mlc,X-p in the MLC leaf position range of +8 cm to -8 cm, where the offset correction positively increased, whereas the offset correction negatively increased when the MLC leaf position was in the range of -12 cm to -8 cm and +20 cm to +8 cm when the 50% position was located inside Xmlc, p. The monitor unit calculation could provide underdosage or overdosage of 7.5% per mm without offset correction. Calibration could be performed at a certain SSD to fit all SSD offset corrections. With careful measurement and an accurate offset correction, it is possible to achieve the dose calculation with 0.5% error for the adjusted MLC leaf edge location in the treatment planning system.

• 出版日期2012