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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.

ABSTRACT: 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 (Xtang.p ) was derived from 50% of the central axis dose created by nominal light field using geometry and mathematical methods. Leaf position (Xmlc.p), defined in the treatment planning system for monitor unit calculation, could be derived from Xtang.p. Offset (correction) could be obtained by the position corresponding to 50% of the central axis dose minus the Xmlc.p position. For SSD from 90 cm to 120 cm at 6 MV and 10 MV, the 50% dose position was located outside of Xmlc,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.

SUBMITTER: Wu JM

PROVIDER: S-EPMC5718526 | biostudies-literature | 2012 Nov

REPOSITORIES: biostudies-literature

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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.

Wu Jia-Ming JM Lee Tsair-Fwu TF Kuo Chung-Ming CM Chen Ching-Jiang CJ Yeh Shyh-An SA

Journal of applied clinical medical physics 20121108 6

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 (Xtang.p ) was derived from 50% of the central axis dose created by nominal light field using geometry and mathematical methods. Leaf position (Xmlc.p), defined in the treatment planning system for monitor unit calculation, could be derived fr ...[more]

PMID: 23149786

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Project description:This dosimetric study investigated the impact of multileaf collimators (MLC) leaf width in volumetric-modulated arc therapy (VMAT) for head and neck cancers (HNC), either with a "standard" simultaneously integrated boost technique (S-SIB) or with a "dose painting" SIB technique (DP-SIB). HNC patients were planned either with an S-SIB comprising three dose levels, from 56 to 70 Gy (16 patients), or with a DP-SIB comprising five dose levels, from 56 to 84 Gy (8 patients), in 35 fractions. Two VMAT plans were calculated for each SIB technique using two Elekta MLCs: MLCi2 with 10 mm leaf width and Beam Modulator (BM) with 4 mm leaf width. Dose distributions were evaluated by comparing doses on PTVs, main OARs, and healthy tissue, and by comparing conformation indexes. Treatment efficiencies were evaluated by comparing the number of monitor units and the number of needed arcs. Comparisons of the two MLCs depending on the two SIB techniques showed: i) Regarding PTVs: Dmean and D2% on lower doses PTV decreased respectively by 0.5 Gy (p = 0.01) and 0.9 Gy (p = 0.01) with BM than with MLCi2 for S-SIB; no significant difference was found for DP-SIB;ii) Regarding OARs: for spinal cord and brainstem, D2% decreased respectively by 1.2 Gy (p = 0.03) and 4.2 Gy (p = 0.04) with BM than with MLCi2 for S-SIB; for controlateral parotid, D50% decreased by 1.5 Gy (p = 0.01) with BM than with MLCi2 for S-SIB; iii) Regarding treatment efficiency: the number of monitor units was 44% (p = 0.00) and 51% (p = 0.01) higher with BM for S-SIB and DP-SIB, respectively. Two arcs were more frequently needed with BM to reach an acceptable dose distribution. This study demonstrated that Beam Modulator (4 mm leaf width) and MLCi2 (10 mm leaf width) MLCs from Elekta provided satisfactory dose distributions for treatment delivery with VMAT technique for complex HNC cases with standard and dose painting prescriptions. OAR sparing was better with BM, mainly for brainstem and spinal cord. However, delivery efficiency of VMAT plans was better with MLCi2.

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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.

Publications

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.

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