AbstractID: 9262 Title: A Smart Interpolation Formalism to Determine the Gy/MU-Factor for
Open and Blocked Radiation Fields from Full-Scatter Output Measurements
Purpose: We wish to determine the Gy/MU-Factor (GMF) from a limited number of measured data. Method and Results: The GMF at the
central axis of a square field depends, for each beam-energy, on the source-skin distance (SSD), the fieldsize at the isocentre, and the depth.
For monitor-units calculations the GMF at every point in the (mathematical) three-dimensional SSD-Fieldsize-Depth-space must be known.
We show that  for each fieldsize  the measurement of the GMF at two different SSDs, and one measurement of the Percentage-DepthDose are sufficient. The correction for blocking is a function of the blocked fraction, SSD, Fieldsize and Depth, and requires 12 additional
GMF-measurements. To fully characterize a beam for 15 fieldsizes, one needs 42 GMF-measurements and 15 PDD-measurements. Using
these data, a sophisticated interpolation in the three-dimensional SSD-Fieldsize-Depth-space gives the GMF-value for all situations. The
formalism is easily incorporated in a spreadsheet, whose results are virtually identical to the standard way of calculation. If needed, values
for the phantom-scatter factor (Sp ) and collimator-scatter factor (Sc) can be derived from the measurements by a stepwise procedure.
Conclusion: Essential in our interpolation approach is: “What You Measure Is What You Need”. The method requires a minimum of
measurements, which are all performed under full-scatter conditions and are therefore directly relevant to the clinical situation. The method
is conceptually straightforward and no additional concepts (Sp , Sc ) are needed. Because no measurements with a mini-phantom are needed,
the formalism is suitable for the small fieldsizes (<4 cm) used in modern IMRT-treatments.