The impact of leaf width and plan complexity on DMLC tracking of prostate intensity modulated arc therapy

Abstract

Purpose: Intensity modulated arc therapy (IMAT) is commonly used to treat prostate cancer. The purpose of this study was to evaluate the impact of leaf width and plan complexity on dynamic multileaf collimator (DMLC) tracking for prostate motion management during IMAT treatments. Methods: Prostate IMAT plans were delivered with either a high-definition MLC (HDMLC) or a Millennium MLC (M-MLC) (0.25 and 0.50 cm central leaf width, respectively), with and without DMLC tracking, to a dosimetric phantom that reproduced four prostate motion traces. The plan complexity was varied by applying leaf position constraints during plan optimization. A subset of the M-MLC plans was converted for delivery with the HDMLC, isolating the effect of the different leaf widths. The gamma index was used for evaluation. Tracking errors caused by target localization, leaf fitting, and leaf adjustment were analyzed. Results: The gamma pass rate was significantly improved with DMLC tracking compared to no tracking (p < 0.001). With DMLC tracking, the average gamma index pass rate was 98.6% (range 94.8%-100%) with the HDMLC and 98.1% (range 95.4%-99.7%) with the M-MLC, using 3%, 3 mm criteria and the planned dose as reference. The corresponding pass rates without tracking were 87.6% (range 76.2%-94.7%) and 91.1% (range 81.4%-97.6%), respectively. Decreased plan complexity improved the pass rate when static target measurements were used as reference, but not with the planned dose as reference. The main cause of tracking errors was leaf fitting errors, which were decreased by 42% by halving the leaf width. Conclusions: DMLC tracking successfully compensated for the prostate motion. The finer leaf width of the HDMLC improved the tracking accuracy compared to the M-MLC. The tracking improvement with limited plan complexity was small and not discernible when using the planned dose as reference.

Image-guided radiotherapy and motion management in lung cancer

“…MLC tracking for lung cancer is still in development. [41][42][43] In MLC tracking, the MLC leaves shaping the treatment beam are programmed to move in accordance with the target motion during breathing. This leaf motion can be superposed on intensity-modulated radiotherapy (IMRT) or dynamic arc leaf motion.…”

The first patient treatment of electromagnetic-guided real time adaptive radiotherapy using MLC tracking for lung SABR

“…The collimator was angled to align with the major motion axis of the lesion (superiorinferior) and the arcs rotated between 90 and 270 degrees (Varian IEC). Plan complexity was recorded using the modulation complexity score (MCS) which has been shown to correlate with delivery accuracy and can affect tracking performance above a 0.8 threshold [15,16]. Treatment planning was performed for MLC tracking, and for comparison (and back-up in case of MLC tracking failure) a 'conventional' ITV-based plan was also created.…”

Potential improvements of lung and prostate MLC tracking investigated by treatment simulations

“…As summarized in Table and described more detailed below, simulations were performed for five individual MLC tracking system improvements and two combinations hereof, that is, seven improvements in total. While these improvements could all be investigated and compared using the same set of VMAT plans, other strategies for improved tracking that require replanning (e.g., reduced VMAT plan complexity) was not included in this study.…”

“…For example, Fast et al . used the jaws to help tracking the target motion, Pommer et al . and Falk et al .…”

“…created prostate VMAT plans with the collimator dynamically aligned to the dominant prostate motion axis to reduce motion perpendicular to the MLC leaves. Hardware changes could be a faster real‐time localization system or an MLC with thinner or faster leaves. Software changes are often cheaper and easier to implement than hardware changes, and several linac vendors have research versions of their control software that allow experimental software without conflicting with the clinical software.…”