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Comparison of Dosimetric Analysis of Volumetric Modulated Arc Therapy Versus Intensity Modulated Radiation Therapy for Anal Cancer

B Chiang1*, K Hibbitts1 , H Ortega1 , T Herman1, S Ahmad1(1) University of Oklahoma HSC, Oklahoma City, OK


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Volumetric modulated arc therapy (VMAT), an extension of intensity modulated radiation therapy (IMRT), employs modifications in gantry rotation speed, machine dose rate and multi-leaf collimator motion to deliver a 3D dose distribution in rotational mode while using less treatment time than conventional IMRT. This study compared VMAT to IMRT for patients with anal carcinoma.

Methods: Eleven patients with anal carcinoma previously treated with IMRT were retrospectively selected for this study. Each patient received a total dose of 57.60 to 63 Gy in 1.8 Gy per fraction. For each patient, a single-isocenter double-arc or double-isocenter double-arc VMAT treatment plan was generated using Varian’s Eclipse RapidArc treatment planning system with the same CT image sets and optimization constraints used for the corresponding clinical IMRT treatment plan. For dosimetry evaluation, the dose-volume histograms (DVH) for planning target volumes (PTV) and organs at risk (hips, bladder, and bowel) were used. For efficiency evaluation, monitor units (MU) and beam on times (BOT) were recorded.

Results: Compared to IMRT, VMAT plans showed insignificant differences in both target volume coverage (mean homogeneity index of 5.9% (IMRT) vs. 6.4% (VMAT) with (p=0.41); mean conformality index of 1.28 (IMRT) vs. 1.20 (VMAT) with (p=0.28)) and normal tissue sparing (mean hips, bladder and bowel doses of 31.42, 35.96 and 17.63 Gy (IMRT), versus 33.12, 38.06 and 18.23 Gy (VMAT) with p equals 0.51, 0.09, 0.15, respectively). VMAT required fewer mean MU and shorter BOT per fraction (1443 MU, 2.41 minutes) when compared to IMRT (2351 MU, 3.92 minutes) with p< 0.001.

Conclusion: For radiation therapy treatment of anal carcinoma, IMRT and VMAT can achieve similar PTV coverage and normal tissue sparing. However, the benefit of VMAT is fewer MU and shorter BOT, which may decrease damage from secondary radiation; and the treatment delivery uncertainty due to intrafraction tumor motion.


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