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

Abstract

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.

Figure 1

Beam orientations are shown for primary and boost plan of mixed‐energy. Half of the beams which had to penetrate deeper depths were 15 MV photon beams, while the others were 6 MV photon beams.

Figure 2

Dose volume histograms (DVHs) for the primary and boost planning target volumes (PTVs) from sum plans. The solid lines indicate DVHs of intensity‐modulated radiotherapy (IMRT) with 6 MV photon beams. The one which receives higher dose is DVH for the boost PTV and the other is DVH for the primary PTV. Dashed lines and dotted lines indicate DVHs from 15 MV and mixed‐energy plans, respectively. No clear differences are observed among the three types of plans.

Figure 3

Dose volume histograms (DVHs) for organs at risk (OARs) from sum plans: (a) DVH for rectum wall; IMRT with 15 MV spares more rectum wall in high‐dose regions while 6 MV saves more in low‐dose regions; IMRT with mixed‐energy photons delivers lower dose to rectum wall through all regions than the others; (b) DVH for bladder with no clear differences among three types of plans; (c) DVH for femoral heads shows that IMRT with 6 MV delivers higher dose to femoral heads than the others; (d) DVH for body shows that IMRT with 6 MV delivers higher dose in the region from 10 Gy to 40 Gy. The solid lines indicate DVHs of intensity‐modulated radiotherapy (IMRT) with 6 MV photon beams, while dashed lines and dotted lines indicate DVHs of IMRT with 15 MV and mixed‐energy photons, respectively.