The dosimetric effect of mixed-energy IMRT plans for prostate cancer.

We investigated the effect of mixing high- and low-energy photon beams on the quality of intensity-modulated radiation therapy (IMRT) plans for patients with prostate cancer. Three different plans for each of twenty patients were generated using either 6 MV or 15 MV alone, and both 6 and 15 MV beams. All the planning parameters, goals, and constraints were set to be identical except beam energy. The dose distributions were similar in terms of target coverage, conformity, and homogeneity regardless of beam energy. The V(70Gy) of rectal wall in 6 MV, 15 MV and mixed-energy plans was 16.7%, 17.9%, and 16.3%, respectively, while V(40Gy) was 55.6%, 53.2%, and 50%. The mean dose to femoral heads in 6 MV, 15 MV, and mixed-energy plans were 31.7 Gy, 26.3 Gy, and 26.2 Gy, respectively. The integral dose of 6 MV plans was 7% larger than those of 15 MV or mixed-energy plans. These results indicated that mixed-energy IMRT plans could take advantage of the dosimetric characteristics of low- and high-energy beams. Even though the reduction of dose to the organs at risk may not be clinically relevant, mixing energy in an IMRT plan for deep-seated tumors can improve the overall plan quality.