Verifying monitor unit calculations for tangential breast fields

Abstract

An essential component of quality assurance in radiation therapy is verifying the accuracy of monitor unit calculations. Differences between between sophisticated algorithms using 2.5D or 3D calculations and simpler Monitor unit check algorithms assuming a flat water phantom must be expected. For many anatomical sites such differences are small and of little or no consequence in the context of monitor unit verification. However, for tangential breast fields the discrepancies are considerably larger than those that would generally be considered acceptable. A simple model to reconcile the differences between sophisticated and simple algorithms is presented based on replacing the breast contour with a triangular or elliptical contour and using this to estimate an equivalent rectangle providing equivalent scatter to the prescription point. The elliptical approximation reconciles the observed differences in calculated monitor units. The analysis we present can assist the treatment planning physicist in selecting a method and tolerance window for verifying monitor unit calculations for tangential breast fields.

Figure 1

Histogram of the percent disagreement in monitor units calculated by RadCalc™ compared to Pinnacle™ for 86 tangential breast fields showing an average difference of 4% due to differences in the computational algorithm. $\text{MU}=\text{monitor units}$

Figure 2

Histogram of the percent disagreement in monitor units calculated by RadCalc™ compared to Pinnacle™ for 405 fields applied to sites other than the breast. On average, the agreement is better and the distribution is tighter, demonstrating that tangential breast fields pose a special challenge. $\text{MU}=\text{monitor units}$

Figure 3

A typical breast contour (solid) can be approximated as a triangle (long dashes) or as an ellipse (short dashes). A half‐blocked tangential field is indicated incident from the left side. $\text{pp}=\text{prescription point};\text{iso}=\text{isocenter}$

Figure 4

Replacing the triangle or ellipse in Fig. 3 with a rectangle of “equivalent mass.” $\text{pp}=\text{prescription point};\text{iso}=\text{isocenter}$

Figure 5

Histogram of the percent disagreement in monitor units after RadCalc™'s field dimension is altered to account only for excess field flash

Figure 6

Histogram of the percent disagreement in monitor units after RadCalc™'s field dimension is altered to account both for excess field flash and a triangular estimate of the missing tissue

Figure 7

Histogram of the percent disagreement in monitor units after RadCalc™'s field dimension is altered to account both for excess field flash and an elliptical estimate of the missing tissue