16-MV Photon Beams Do Not Improve Plan Quality Compared to 6-MV Photon Beams in Prostate Cancer IMRT
R Yaparpalvi*, D Mynampati, W Tome, J Shen, L Hong, H Kuo, M Garg, W Bodner, S Kalnicki, Montefiore Medical Center, Bronx, NYSU-E-T-604 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
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 data-base, 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.