Urethra-Sparing Stereotactic Body Radiation Therapy for Localized Prostate Cancer: Dosimetric Results From a Prospective Multicenter Randomized Phase 2 Trial

Purpose/Objective(s): We investigated the dosimetric benefits of gated plans with various duty cycles over the non-gated plans for lung SBRT. We also compared the delivery efficiency for those plans delivered on various Linacs. Materials/Methods: VMAT plans were created to deliver 48Gy to the target in 4 fractions. Those plans were categorized into four types. In the first category, plans were generated on the average intensity projection CTs, where -the ITVs were delineated on the maximum intensity projection CTs. Those plans represented the scenario where patients were treated with free-breathing. The other three types of plans were for the gated treatments delivered with duty cycles of 50%, 30% and 10% respectively. The gated plans were generated on the 50% CT. The corresponding ITVs were the combination of the 30% to 70% GTVs, combination of 40% to 60% GTVs, and 50% GTV only. To investigate delivery efficiency, we created three plans in each category using the following Linacs: Linac1 with 1 cm leave at maximal dose rate of 450 MU/min, Linac2 with 0.5 cm leave at maximal dose rate of 620MU/min, and Linac3 with 0.5 cm leave in FFF mode at a maximal dose rate of 1230 MU/min. The three plans were matched in DVHs on ITV, PTV, and organs at risk. At treatment delivery, beam-on times were recorded. The treatment time for each plan was estimated as the beam-on time divided by its duty cycles. Results: Plan quality across the four categories was compared. The averaged Conformity Index of the PTVs for plans with free-breathing and gating with 50%, 30%, 10% duty cycles was 1.000 0.000, 0.999 0.001, 1.000 0.001 and 1.000 0.001 respectively. The corresponding Heterogeneity Index was 1.057 0.033, 1.053 0.019, 1.050 0.020 and 1.057 0.033. The difference in PTV dose coverage is insignificant. The corresponding averaged lung V20 was 8.29 4.20%, 6.35 2.37%, 6.27 2.44% and 6.25 2.33%. Treatment MUs of plans using different machines were also compared. The averaged MU for the three linacs was 2237.8 347.2, 1991.3 176.3 and 2183.3 112.4, respectively. The mean treatment times for free-breathing plans delivered on the three machines were 5.41 0.44, 3.47 0.09 and 1.72 0.12 minutes. The treatment times for gating plans delivered on the three machines were 11.86 2.69, 6.97 0.58, 3.72 0.15 min for 50% duty cycle, 17.41 2.08, 11.43 1.01, 6.13 0.23 min for 30% duty cycle, and 47.00 2.92, 33.72 1.44, 18.50 0.73 min for 10% duty cycle, respectively. Conclusions: Compared with non-gated therapy, gating improved plan quality. The differences in lung dose for gating with various duty cycles were clinically insignificant. FFF mode offers a dose rate much higher than the conventional dose rate. The significant reduction in treatment time in FFF mode makes it feasible to gate the SBRT with a small gating window. Author Disclosure: J. Wu: None. J. Ye: None. F. Chen: None. G. Hill: None. J. Spiegel: None. V. Mehta: None.