SU-E-T-35: Optimal Clinical Megavoltage X-Ray Beam Quality for Contrast Enhanced RT (CERT)

Abstract

Purpose: To determine the optimal beam quality of MV-x-rays for Contrast Enhanced RT (CERT) based on analysis of a broad database of clinical beams. Methods: EGSnrc was used to calculate spectra and doses for various irradiation conditions (6MV EX linac): flattened (STD) vs unflattened (FFF) beams, open/IMRT fields, in-/out-off-field areas as a function of depth in water and field size. Spectral differences were quantified and related to dose enhancement (DE) effects by introducing two metrics: an energy-dependent dose enhancement ratio DER(E) and an effective DER for the entire spectrum. Using these two metrics the dosimetric impact of spectral changes were studied for different materials (gadolinium-oxysulfide (Gd2O2S) and gold). Results: DER proved to be a good predictor for high-DE effects for the whole spectrum of each beam. However, the DER(E) metric revealed energy-specific effects hidden in the overall analysis. Spectral analysis with DER(E) showed that all DE effects are directly related to the spectral region <∼200KeV. The highest DE effects appear for FFF spectra and out-of-field cases. Differences in DER values can reach over one order of magnitude, depending on the case and the heterogeneity of the medium. For shallower depths behavior of FFF/STD beams is practically the same but as depth increases DE effects of FFF beams undergo more changes than STD beams. Conclusions: DER increases with depth, field size, distance from the CAX and OB vs IMRT due to the increased scatter and concomitant softening of the beam. Differences in DER can be more than one order of magnitude, depending on the case and the medium. The greatest dose enhancement is achieved with FFF (lower effective energy) and with other beams of large scatter content. Complex IMRT is better for CERT compared to conformal plans because of much greater photon scatter from various sources increasing the fluence below 200KeV.