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Comparison of Volumetric Modulated Arc Therapy and Intensity Modulated Radiation Therapy for Whole Brain Hippocampal Sparing Treatment Plans Based On Radiobiological Modeling

E Kendall

E Kendall*, O Algan , S Ahmad , University of Oklahoma Health Sciences Center, Oklahoma City, OK


SU-I-GPD-T-247 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To compare radiobiological aspects of IMRT and VMAT for patients needing Hippocampal Avoidance Whole Brain Radiation Therapy following the NRG Oncology protocol NRG-CC001.

Methods: In this retrospective study we compared biophysical indices (EUD, TCP and NTCP) of VMAT and IMRT treatment plans calculating through mechanistic and empirical models for fifteen patients. Plans were made on Varian Eclipse Treatment Planning system using 3-arc VMAT and 9-Field IMRT. The PTV was defined as the whole brain excluding a uniform three-dimensional 5mm expansion of the hippocampus. Prescribed doses were 30 Gy in 10 fractions normalized to a minimum of 95% target volume receiving 100% dose. All plans used identical contour sets and were determined clinically acceptable by the radiation oncologist. A paired t-test analysis was used for statistical analysis.

Results: The average EUDs calculated using SF2 of 0.4, 0.5, and 0.6 and α/β ratios of 2, 6, and 10 for each SF2 fluctuated from 25.74-29.11 Gy for IMRT and 25.27-28.58 Gy for VMAT treatment plans. In all cases, average EUDs which increased with the increase of both α/β ratio and SF2 values were greater for IMRT than VMAT (p ≤ 0.02). Our results showed that IMRT had a 2% higher EUD than VMAT irrespective of models used for calculation. The EUDs for IMRT plans were higher for hippocampus (p=0.19) but lower for optical structure (p < 0.01) compared to VMAT plans. The ratio of average IMRT TCPs over VMAT TCPs was 1.02 (p ≤ 0.07). The average IMRT NTCP was higher than the average VMAT NTCP for the hippocampus (p = 0.13), but was lower (p < 0.01) for all optical structures irrespective of model calculation techniques used.

Conclusion: VMAT treatment plans indicated lower hippocampus EUDs and NTCPs compared to IMRT plans but these were other way around for all optical structures.

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