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

K Hibbitts*, B Chiang , H ortega , T Herman , S Ahmad , University of Oklahoma Health Sciences Center, Oklahoma City, OK


(Sunday, 7/14/2019)  

Room: ePoster Forums

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: Sixteen patients with anal carcinoma previously treated with step-and-shoot IMRT were retrospectively selected for this study. Each patient received a total dose of 57.60 to 63 Gy in 1.8 Gy fractions. For each patient, a single-isocenter multi-arc or double-isocenter multi-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 used.

Results: Compared to IMRT, VMAT plans showed insignificant differences in both target volume coverage [mean homogeneity index of 5.98% (IMRT) vs. 6.40% (VMAT) with (p=0.34) and mean conformality index of 1.24 (IMRT) vs. 1.16 (VMAT) with (p=0.14)] and normal tissue sparing [mean hips, bladder and bowel doses of 32.15, 37.12 and 19.20 Gy (IMRT), vs. 34.22, 39.84 and 20.01 Gy (VMAT) with (p= 0.26, 0.03, 0.28, respectively)]. VMAT required fewer mean MU and shorter BOT per plan (1597 MU, 2.66 minutes) when compared to IMRT (2571 MU, 4.29 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 both damage from secondary radiation and treatment delivery uncertainty due to intrafraction tumor motion.


Dosimetry, Treatment Planning


Not Applicable / None Entered.

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