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.PACS number: 87.53.Tf, 87.55.x, 87.55.D, 87.55.dk
Highlights
The projection of the nominal light field at an SSD of 100 cm was adopted for dose profile measurements, but the dose profile from the nominal light field edge could not quantitatively determine the geometry of the tangential edge (Xtang,p) for the derivation of Xmlc,p; the precise light field edge (Xtang.p) was derived from 50% of the central axis dose by geometry and mathematical methods (Eq [1])
The results indicate that a longer SSD and higher photon energy increases the distance of the 50% dose position from the crosshair isocenter
It is critical for high-quality radiation therapy that planned and delivered dose measurement should be at an appropriate level
Summary
The projection of the nominal light field at an SSD of 100 cm was adopted for dose profile measurements, but the dose profile from the nominal light field edge could not quantitatively determine the geometry of the tangential edge (Xtang,p) for the derivation of Xmlc,p (planning leaf position); the precise light field edge (Xtang.p) was derived from 50% of the central axis dose by geometry and mathematical methods (Eq [1]). This point could be derived from Xtang.p. Once the Xmlc.p was decided,
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