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Optimal Beam Selection for Intensity Modulated Radiation Therapy: High Intensity Flattening-Filter Free Versus Conventional Flattened Beams

J Duan

J Duan*, S Shen, Y Yuan, X Wu, R Cardan, R Popple, I Brezovich, Univ Alabama Birmingham, Birmingham, AL

SU-E-T-601 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To investigate the effect of target size and magnitude of intensity modulation on the required number of monitor units (MU) in IMRT delivered by high intensity flattening filter free (HIFFF) beam. To provide guidelines for the selection of beams that optimizes delivery time.

Methods: A 29cmx1cmx5cm target in the center of a 20cmx40cm rectangular phantom was divided into 29 small 1cmx1cmx5cm subtargets along the MLC leaf motion direction. Various sizes of new targets consisting of 5 to 29 subtargets were generated. By prescribing two different doses to adjacent subtargets, the modulation index (MI), defined as the ratio of the nominal dose of the low dose subtargets to that of the high dose subtargets, was varied from 0.2 to 1.0. IMRT plans consisting of simple AP and PA beams were optimized for various combinations of target sizes and MI with 10X HIFFF, 6X HIFFF and 6X flattened beams, respectively. MLC leaf sequences and MUs for delivering 2Gy/fraction were computed using the maximum MU rates of 2400, 1400 and 600 MU/min, respectively.

Results: For any combination of target size and MI the 10X HIFFF beam required the highest number of MUs while 6X flattened beam required the lowest. For HIFFF beams, the MU increased with the target size in linear quadratic fashion while decreased approximately linearly with MI. For all tested conditions, 10X HIFFF beam required the shortest delivery time, while 6X beam required 1.4 to 3.7 times as long.

Conclusion: Despite its requiring the most MUs among the three beams investigated, the 10X HIFFF beam takes the shortest time to deliver an IMRT plan, regardless of target size and MI. 10X HIFFF beam is the choice of beam for optimal delivery time when increased leaf leakage dose is not a concern.

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