Determination of accurate dosimetric parameters for beveled intraoperative electron beam applicators

Intraoperative electron beam therapy requires accurate dose maximum/monitor unit (Dmax/MU) values for both flat and beveled ended applicators (cones). Measurement of Dmax/MU values with either solid or nontilting scanning water systems may give rise to inaccuracies due to the difficulty in locating an accurate position for dmax, since the ionization chamber usually cannot be scanned along the central axis of the beveled intraoperative applicator. A linear one-dimensional scanner (which permits ionization measurements to be made as a function of water depth) has been modified to provide scanning along a line up to 30 degrees from the perpendicular to the phantom surface. This modification has proven helpful in improving the accuracy of certain dosimetric parameters (e.g., Dmax/MU) of beveled applicators. For example, we found inaccuracies which arose when we measured Dmax/MU of beveled intraoperative radiation therapy cones in either solid or other scanning water systems were greatly reduced, especially for the lower electron beam energies (e.g., 6 and 9 MeV).