To Study the Impact of Energy Variation On Cone Ratio, PDD, TMR and IMRT Doses for Flattening Filter Free (FFF) Beam of TomoTherapy Hi-Art(TM)
R Tolakanahalli1*, L Robeck2, D Tewatia3, (1) Former Employee of UW Madison, Madison,WI (2) Accuray Inc, Madison, WI, (3) University of Wisconsin Madison, Madison, WISU-E-T-370 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
To study the effect of energy variation on standard IMRT plans, MU chamber ratio (MU1/MU2) and cone ratio (CR) for non-flattened beam e.g., Tomotherapy.
In this controlled experiment, we varied the energy (PDD₁₀) on TomoTherapy Hi-ARTTM from -1.6 % to +1.6% in six steps while keeping calibrated output unchanged. For achieving this, injector current (Injl) and pulse forming network (pfn) voltage were adjusted. The effect of this variation in energy(δE) on standard IMRT plans and energy indicators were studied. To quantify sensitivity of standard IMRT plans with δE, cheese phantom was used with multiple calibrated ionization chambers. ΔD was calculated as a percent variation of measured vs. expected dose. Ratio of ΔDon-axis with ΔDoff-axis for each commissioned field width (FW) was tabulated against δE. The effect of this on other energy indicators was also studied to verify the sensitivity of TMR²⁰₁₀ MU chamber ratio, and detector data-based-CR.
Good agreement between PDD₁₀ vs TMR²⁰₁₀ (R²=0.97, slope=1.01) and PDD₁₀ vs MU chamber ratio(R²=0.99, slope=-0.9) was observed. CR is more sensitive to energy change with slope =-2.5 than PDD₁₀. Ratio of ΔDon-axis with ΔDoff-axis for FW 2.5 cm (R²=0.91, slope =0.63) and FW 1 cm (R²=0.94, slope =0.80) of standard IMRT plan also agreed well with PDD₁₀.
CR is more sensitive to energy changes compared to PDD₁₀, thus CR with MU1/MU2 can definitely be used as quality measure on a daily/weekly basis. Though the on-axis vs off-axis variation is not as sensitive, output across the 6 standard IMRT plans can vary up to 2% for a 1% increase in energy. Hence, it is of utmost important to manage the PDD₁₀ tightly around +/- 0.5%. This study can be easily extended to monitor the calibration and beam quality for other emerging technologies based on flattening-filter-free (FFF) delivery mode e.g., TrueBeamTM.
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