SU‐E‐T‐604: 16‐MV Photon Beams Do Not Improve Plan Quality Compared to 6‐MV Photon Beams in Prostate Cancer IMRT

Purpose: Photon energies 10 MV or higher are generally considered optimal for treatment of deep‐seated pelvic targets. We performed dosimetric quality assessment of Prostate IMRT plans in patients treated with 6‐MV and 16‐MV Photons, to discern if 16‐MV plan quality was superior to 6‐MV treatment plans. Methods: From our institutional database, treatment plans of 84 patients previously treated for early stage prostate cancers were included in this retrospective study. Forty‐two patients were treated with 6‐MV and forty‐two with 16‐MV. Beam energy choice was based on linac capability, physician preference and not on patient separation. All patients were planned with a coplanar 7‐F IMRT technique. The prescription dose (75.6‐Gy), optimization technique and planning objectives were similar in all patients. Dose distributions were evaluated using various indices‐Conformity‐Index (CI), Healthy‐Tissue Conformity Index (HTCI), Homogeneity‐Index (HI), Gradient‐Index (GI), Conformity‐Number (CN), Normal‐Tissue Integral Dose (NTID), Body‐mass‐index (BMI) and quality of coverage (QC). Rectal and Bladder dose‐volume indices were evaluated per RTOG guidelines. Non‐parametric Mann‐Whitney test was applied in the statistical analysis and for a p‐value <0.05, the null hypothesis is rejected. Results: Mean PTV was 197.9cc (±13.1) for the 6‐MV group and 191.8cc (±10.3) for the 16‐MV group. MUs per fraction were 905 (±32) for 6‐MV and 862 (±41) for 16‐MV plans. The CI, HTCI and GI were statistically similar between 6‐MV and 16‐MV plans (p=0.22). Indices HI, QC and CN all showed statistically significant improvement for 6‐MV plans compared to 16‐MV plans (p<0.03). NTID was slightly lower for 16‐MV plans, but not statistically significant, compared to 6‐MV plans. NTID correlated with BMI for 16‐MV group (r=0.70) and weakly for 6‐MV group (r=0.28). Rectal V65, V40 and Bladder V65 were similar between 6‐MV and 16‐MV plans. Conclusion: We conclude that 16‐MV photon beams do not provide additional dosimetric advantage compared to 6‐MV photon beams in Prostate IMRT.