Now consider dose isolines in this plane.  Using pixel data as we have, where would the 0% line be?  Since everything inside this line must receive 0% or greater, it must lie at the outside boundary.  Similarly, the 100% line would lie at the 100% pixel boundaries.  What about the 50% line? Since the pixels have only 2 values, there can be only two lines, 0% and 100%.  The 50% line can't lie at the 0% boundary, since everything inside is not 50% or greater, so it must lie at the 100% boundary.  In the end, EVERY line except the 0% line would lie at the 100% boundary.  Clearly this is not what we expect to see.  We are used to thinking in terms of a dose continuum which is created by interpolation from a set of points, not pixels.

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