Room: Exhibit Hall
Purpose: To evaluate the accuracy in dose delivery of flattening filter free (FFF) photon beams at low monitor unit settings.
Methods: Relative dose (RD) per MU and the linear error (LE) for different MU settings (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 50, 100, 200, 300 and 500) were evaluated with PTW 0.6 cc ionization chamber for three different fields (4cm x 4cm; 10cm x 10cm; 20cm x 20 cm) and two dose rates with 6 FFF and 10 FFF photon beams. Low-MU stability was analyzed with the dosimeter reading obtained with and without interruption of the beam for 10 MU and 20 MU. Flatness and symmetry was evaluated for different MU settings at four gantry angles with 10cm x 10cm field using PTW Octavius detector array.
Results: For the lowest MU setting (1 MU), RD was found to be 5.1Â±0.5% in 400 MU/min, 6.9Â±0.4% in 1400 MU/min for 6 FFF and 6.8Â±0.5% in 400 MU/min, 7.5Â±0.3% in 2400 MU/min for 10 FFF beams with 10cm x 10cm field. With 10cm x 10cm field, LE was observed to be 6.1Â±0.2%, 6.5Â±0.3% for 400 MU/min and 1400 MU/min respectively for 6 FFF and 6.1Â±0.2%, 6.5Â±0.3% for 400 MU/min and 2400 MU/min respectively for 10 FFF at 1 MU. For both energies the variation in low MU Stability was noticed to be <1% for both dose rates. The stability of 6 FFF and 10 FFF photon beams in terms of flatness and symmetry was found to be within 3% above 3 MU.
Conclusion: The discrepancy in dose delivery for MU <3 was found to be significant for both 6 FFF and 10 FFF beams. The dosimetric characteristics of 6 FFF and 10 FFF beams were observed to be similar at low and high dose rates.