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Determining Blocked Equivalent Square Field Size Using An MLC Centroid Method

W Taylor*, K Woods , D DiCostanzo , Ohio State University Medical Center, Columbus, OH


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: The determination of equivalent square field size (ES) for phantom scatter (S(P)) estimation for tertiary collimation (e.g. multi-leaf collimators or MLCs) is of concern in radiotherapy second check dose calculation. It is difficult to determine an ES value with complex MLC patterns. This study proposes using an MLC centroid method, which accounts for differences in S(P) by considering MLC blocking.

Methods: This study retrospectively evaluated 12 clinical treatment plans. The spatial location of each MLC leaf pair was determined within the aperture using the Eclipseᵀᴹ Scripting API. An MLC centroid is calculated parallel and perpendicular to the MLCs (X(centroid),Y(centroid)). The X(centroid) is calculated by averaging the MLC delimited aperture within the secondary collimator aperture. The Y(centroid) is summed for each MLC pair and is a ratio of the MLC aperture and the secondary collimator field size times the physical MLC width. Once the X(centroid) and Y(centroid) are calculated, the ES is determined using Sterling’s approximation. This method was compared with ES calculated using the secondary collimators only. A comparison between calculated monitor units (MU) and S(P) was performed.

Results: Overall, the average percent difference in MU calculation improved from -4.1% ± -1.2% to -1.2% ± 1.3% using the MLC centroid method. The largest differences -7.7% compared to -1.1% were noted in plans utilizing reduced fields. This is to be expected because of the additional MLC shielding employed in this planning technique.

Conclusion: Secondary dose calculations using the MLC centroid method exhibit S(P) values which better represent irregular fields utilizing MLCs. When the MLCs are included in the ES calculation the S(P) factor decreased by -1.3% ± 0.7% for the 40 fields analyzed. In all cases, the physics second check calculation using a commercially available software showed better agreement with the primary dose calculation after applying the MLC centroid method.


Monitor Unit Calculations, MLC, Quality Assurance


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