Management of the interplay effect when using dynamic MLC sequences to treat moving targets

Abstract

Interplay between organ motion and leaf motion has been shown to generally have a small dosimetric impact for most clinical intensity-modulated radiation therapy treatments. However, it has also been shown that for some MLC sequences there can be large daily variations in the delivered dose, depending on details of patient motion or the number of fractions. This study investigates guidelines for dynamic MLC sequences that will keep daily dose variations due to the interplay between organ motion and leaf motion within 10%. Dose distributions for a range of MLC separations (0.2-5.0 cm) and displacements between adjacent MLCs (0-1.5 cm) were exported from ECLIPSE to purpose-written software, which simulated the dose distribution delivered to a moving target. Target motion parallel and perpendicular to the MLC motion was investigated for a range of amplitudes (0.5-4.0 cm), periods (1.5-10 s), and MLC speeds (0.1-3.0 cm/s) with target motions modeled as sin. Results were confirmed experimentally by measuring the dose delivered to an ion chamber array in a moving phantom for different MLC sequences. The simulation results were used to identify MLC sequences that kept dose variations within 10% compared to the dose delivered with no motion. The maximum allowable MLC speed, when target motion is parallel to the MLC motion, was found to be a simple function of target period and MLC separation. When the target motion is perpendicular to MLC motion, the maximum allowable MLC speed can be described as a function of MLC separation and the displacement of adjacent MLCs. These guidelines were successfully applied to two-dimensional motion, and a simple program was written to import MLC sequence files and evaluate whether the maximum daily dose discrepancy caused by the interplay effect will be larger than 10%. This software was experimentally evaluated, and found to conservatively predict whether a given MLC sequence could give large daily dose discrepancies.

Use of reduced dose rate when treating moving tumors using dynamic IMRT

“…${\text{Sin}}^6$ was selected because previous work showed that the interplay effect is worse for ${\text{sin}}^6$ than ${\text{sin}}^2$ and ${\text{sin}}^4$ motion. ( 1 , 10 ) The majority of motion in the lung is in the superior‐inferior direction. ( 16 , 18 ) Real patient motion is, however, not as regular as this.…”

“…( 2 , 10 ) These effects are smallest for large MLC separation, small displacement between adjacent MLCs, slow MLC speeds, short target periods and small target amplitudes. A number of groups have provided specific recommendations on the MLC parameters that will minimize the interplay effect.…”

“…Yu et al ( 2 ) recommends keeping the speed of collimator (or MLC) motion below $0.5\hspace{0.17em}\text{cm}\hspace{0.17em}{\mathrm{normals}}^{-1}$ for apertures larger than 1 cm. Court et al ( 10 ) gives recommendations for the maximum collimator speed as a function of various parameters such as field aperture and target period. For apertures larger than 1 cm the results of Court et al are similar to those of Yu et al For smaller apertures ($<1\hspace{0.17em}\text{cm}$), they showed that the collimator speed should be kept to $0.1-0.2\hspace{0.17em}\text{cm}\hspace{0.17em}{\mathrm{normals}}^{-1}$.…”

Minimizing dose variation from the interplay effect in stereotactic radiation therapy using volumetric modulated arc therapy for lung cancer

“…The results of this study are consistent with their findings. Previous papers have reported several solutions to reduce the dosimetric impact of the interplay effect: adjusting the maximum MLC speed14 or dose rate 12. These solutions require modifications to the plans; consequently, it might restrict planning flexibility.…”

“…Some studies14, 18, 19 investigated whether plan complexity caused the dosimetric impact of the respiratory motion in the VMAT, and found that the interplay effect was greater for more modulated plan. Ehrbar et al 20.…”