Dosimetric Comparison of 6 MV and 15 MV Single Arc Rapidarc to Helical TomoTherapy for the Treatment of Pancreatic Cancer

We conducted a planning study to compare Varian's RapidArc (RA) and helical TomoTherapy (HT) for the treatment of pancreatic cancer. Three intensity-modulated radiotherapy (IMRT) plans were generated for 8 patients with pancreatic cancer: one using HT with 6-MV beam (Plan_HT6), one using single-arc RA with 6-MV beam (Plan_RA6), and one using single-arc RA with 15-MV beam (Plan_RA15). Dosimetric indices including high/low conformality index (CI100%/CI50%), heterogeneity index (HI), monitor units (MUs), and doses to organs at risk (OARs) were compared. The mean CI100% was statistically equivalent with respect to the 2 treatment techniques, as well as beam energy (0.99, 1.01, and 1.02 for Plan_HT6, Plan_RA6, and Plan_RA156, respectively). The CI50% and HI were improved in both RA plans over the HT plan. The RA plans significantly reduced MU (MURA6 = 697, MURA15 = 548) compared with HT (MUHT6 = 6177, p = 0.008 in both cases). The mean maximum cord dose was decreased from 29.6 Gy in Plan_HT6 to 21.6 Gy (p = 0.05) in Plan_RA6 and 21.7 Gy (p = 0.04) in Plan_RA15. The mean bowel dose decreased from 17.2 Gy in Plan_HT6 to 15.2 Gy (p = 0.03) in Plan_RA6 and 15.0 Gy (p = 0.03) Plan_RA15. The mean liver dose decreased from 8.4 Gy in Plan_HT6 to 6.3 Gy (p = 0.04) in Plan_RA6 and 6.2 Gy in Plan_RA15. Variations of the mean dose to the duodenum, kidneys, and stomach were statistically insignificant. RA and HT can both deliver conformal dose distributions to target volumes while limiting the dose to surrounding OARs in the treatment of pancreatic cancer. Dosimetric advantages might be gained by using RA over HT by reducing the dose to OARs and total MUs used for treatment.