Volume versus Threshold Curves. A typical volume versus threshold curve (dots) from the data described in . The tumor volume is defined to be those voxels with activity above 40% of the maximum activity. The slope of the line (-0.37 cc/%) of best fit between 40% and 80% was then used as a measure of intra-tumoral heterogeneity. This is the slope which we now argue does not predict disease outcome as was claimed in . For reference, the best-fit exponential curve is also shown (dashed).