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Differences Between 7MV-Flattening Filter Free and 6MV-Flat Intensity Modulated Radiotherapy Plans at Different Fractionation and Dose Rates

J Gray

J Gray*, E Ekwelundu, J Farr, St. Jude Childrens Research Hospital, Memphis, TN

SU-E-T-704 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The purpose of the investigation is two-fold. To determine clinical use sensitivity of the high and low dose rate modes of flattening filter free (FFF) inverse modulation radiotherapy (IMRT) treatment plans compared to flattened beam plans. Secondly, do the FFF plans have a conformity advantage?

Methods: Planning comparisons were performed on C-shape (AAPM TG-119) and mock head and neck (H&N) cases for high and low dose rate 7MV FFF modes, 2000 (7UF2000) and 500 (7UF500) monitor units (MU) per minute, respectively, as well as for the 6MV-Flat 300MU/min option. The 7UF2000 beam is limited to a minimum segment dose of 10MU. Plans utilized 200cGy and 500cGy fractionation schemes. Plans were compared based on target dose conformity index(CI) and heterogeneity, critical structure dose, integral dose, MU, segments, fractionation and the dose falloff at critical structures.

Results: Data collected indicate that there is minimal difference between the three techniques in the 200cGy C-shape plans. The high dose rate 7UF2000 was hotter for both cases than other techniques in conventional fractionation. For both fractionation levels in the C-shape plans the 7UF500 technique provides a lower critical structure dose, however, the 7UF2000 200cGy level achieves the opposite. The dose falloff is worse in both cases at low fractionation for 7UF2000, whereas, at high fractionation dose falloff is similar. The CI for the 7UF2000 technique at 200cGy fractionation is less than other techniques.

Conclusion: A clinically relevant finding was the potential of reduced conformity for the 7UF2000 technique compared to 7UF500 and 6MV-Flat with conventional fractionation. The minimum deliverable MU/segment limitation results in a coarse granularity that impedes the optimizer in achieving adequate target coverage. Conversely, the 7UF500 results were comparable with those of the 6MV-Flat. The potential high conformity of FFF beams over like energy flattened beams was equivocal. Further investigations are ongoing.

Funding Support, Disclosures, and Conflict of Interest: Funding support provided by the American Lebanese Syrian Associated Charities.

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