The Effect of Photon Energy on the Intensity-modulated Radiation Therapy Plan for Prostate Cancer: a Planning Study

In this study, the effect of the beam energy on the intensity modulated radiation therapy (IMRT) plan for prostate cancer was studied for competing IMRT plans optimized for delivery with either 6- or 15-MV photons. This retrospective planning study included 10 patients treated for localized prostate cancer at the Seoul National University Bundang Hospital. A dose of 66 Gy was prescribed in 33 daily fractions of 2 Gy. For inverse IMRT treatment planning, we used a 7-coplanar non-opposed beam arrangement at 0, 50, 100, 150, 210, 260, and 310 degree angles. To ensure that differences among the plans were due only to energy selection, the beam arrangement, number of beam, and close constraints were kept constant for all plans. The dose volume histograms (DVHs) for the 6- and 15-MV plans were compared for the planning target volume (PTV) and for organs at risk (OAR), such as the rectum, bladder and both femoral heads. The conformal index was defined as the ratio of the 95% isodose volume divided by the PTV volume enclosed by the 95% isodose line, because we selected the 95% isodose line as our reference. Doses received by the 95% and 5% volume of the PTV were less than or equal to 1% for 6-MV compared to the 15-MV IMRT plan for 10 patients. Percentage of doses received by the 10% volume of the bladder and rectum were less than or equal to 1%. Percentage of doses received by the 30 and 50% volume of bladder and rectum were 1 similar to 2% higher for 6-MV photons. Also, percentage of dose received by the 10% and 50% volume of the right and the left femur heads were 4 similar to 5% higher for 6-MV photons. The mean homogeneity index for the 6-MV and 15-MV photon plans was 1.06. The mean conformity index of 95% was 1.04 +/- 0.01 and 1.12 +/- 0.02 for 6-MV and 15-MV, respectively, but this difference was not statistically significant. The mean monitor unit was 812 +/- 40 and 716 +/- 33 for the 6-MV and the 15-MV photon plans, respectively. The 6-MV photon plan delivers 13.4% more MU than the 15-MV plan. The-15 MV dose distributions and DVHs of the PTV generated by the clinical treatment planning calculations were as good as, or slightly better than, those generated for 6-MV photons. The organs at risk, such as the rectum, bladder, and femoral head were also similar, with the DVH curve for the 6-MV plan being slightly higher near the low-close region, but lower near the high-dose region. Therefore, we recommend the use of 15-MV photons for IMRT of prostate cancer to achieve better in target coverage, in addition, the integrated dose can be reduce by using IMRT with 15 MV photons.