  Charge accumulation was measured under calibration conditions in the
spread-out Bragg peak (SOBP) using the calibration bias as well as a range of
voltages from 10V to 500V and a Farmer-style ion chamber. Collection efficiency
was determined by extrapolating to infinite voltage. Similar measurements were
taken in an identical dose distribution with a much shorter spot duration. The
impact of each of the three models on calibration was then quantified using the
TRS-398 protocol. The collection efficiency for the standard calibration was
determined to agree well with the prediction of a continuous beam recombination
correction. The standard calibration field was found to persistently agree with
a continuous beam recombination correction for much lower operating biases. The
collection efficiency result for the short spot duration field did not agree
with either the continuous or pulsed-beam correction. Using the incorrect
recombination model under the standard calibration conditions resulted in a
0.5% calibration difference. We have determined that our spot scanning system
would be most appropriately calibrated using a recombination correction with
continuous beam model. Physicists responsible for the calibration of such
systems are advised to take measurements described here to correctly identify
the applicable recombination model for their clinics.
