  Technical Note describing studies, in 2011, of time resolved diode dosimetry
in an anthropomorphic phantom at the Francis H. Burr Proton Therapy Center,
Massachusetts General Hospital, Boston, MA. This technique measures the water
equivalent path length (WEPL) to a diode dosimeter with sub-millimeter
precision, using very little dose. It may allow one to use the proton stopping
point (otherwise too uncertain) to cut between tissue to be treated and tissue
to be spared, in cases (such as the prostate) where a diode can be placed in a
cavity (the rectum) distal to the target.
  A potential problem is range mixing, where (because of scattering) protons
may arrive at a diode with different energy-loss histories. The WEPL reported
by such a diode is meaningless and must be ignored. We therefore use an array
of diodes, of which some are likely to be at more favorable locations. In
addition to the overall analysis of time resolved data, this Note describes in
detail how range mixed diodes can be identified by looking at the skewness and
kurtosis of the time dependent signal.
