SU-E-T-110: Small Electron Field Surface Dosimetry Using Solid State Detectors

Solid state detectors from two independent systems have shown similar over-responses to small electron fields used for treatment of surface lesions. We propose to evaluate the accuracy of solid state detector systems for highly blocked electron fields as a method for clinical dosimetry and determine the cause for any deviations from expected dose-delivery calculations.

A QED Diode detector system and a Best-Medical MOSFET dosimeter system were used to measure relative output factors (ROF's) for a range of blocked electron fields with 3 different electron energies. Measurements were taken on the surface of a Solid-Water phantom as well as under varying thicknesses of bolus (surface to dmax) to determine the extent of this effect with increasing depth. Measurements were compared with expected results obtained during machine commissioning with an A-14 micro-ionization chamber.

Field sizes larger than 4×4 cm² showed little significant deviation from expected output factor values. However, fields smaller than this threshold showed as much as a 30% over-response of the solid state detectors compared to ion chamber measurements on the surface. This deviation was consistent over multiple measurements using both systems. The degree of over-response is both energy and effective field size dependent. As the effective depth of measurement increases, the relative differences in responses decrease and eventually converge.

Surface dosimetry using solid state detectors is a straightforward method of verifying dosimetric calculations. Care must be taken when using these systems for surface dosimetry with highly blocked fields as their non-tissue equivalence makes these detectors overly sensitive to low-energy scattered electrons and bremsstrahlung photons produced from the block edge. Measurements taken near dmax will not show a significant deviation from the actual dose and do not need to account for this effect.