RBE or effective dose to evaluate the effect of different beam energies for intensity modulated radiation therapy

The low dose limit relative biological effectiveness (RBE_m) and effective dose (previously reported by these authors) were used to compare 6, 15, and 18 MV conventional and IMRT radiotherapy for prostate cancer. In a comparison of the three beam energies used for the IMRT treatments, 6 MV resulted in the lowest effective dose, while 18 MV resulted in the highest effective dose. Comparing IMRT and conventional treatments of the same beam energy, IMRT resulted in increased equivalent dose to peripheral organs but lower effective doses. Dose to organs in close proximity to the treatment volume were orders of magnitude greater than dose to peripheral organs, thereby dominating the effective dose values. When RBE_m is used to weight the doses to organs close to the treatment volume, IMRT resulted in lower Gy-equivalent doses compared to the conventional treatments, and thus lower deterministic effects for the colon, gonads, and bladder. The RBE_m, does not consider the effects to peripheral organs, painting an incomplete picture for inter-comparison of different energies and different treatment techniques. Although the neutron radiation weighting factors are not fully applicable at some of the high organ doses, effective dose (and associated organ equivalent doses) still represents a useful measure for comparing radiation delivery methods. Perhaps it is reasonable (with full understanding of the limitations of each) to report both effective dose and RBE_m weighted doses when comparing different radiation treatment techniques.