1. Volumetric Modulated Arc Therapy (VMAT) versus Intensity Modulated Radiation Therapy (IMRT) for Anal Carcinoma
Heather Ortega, BSRT(T), CMD, Kerry Hibbitts, MS, DABR, Bing-Hao Chiang, MS, Terence Herman, MD, Salahuddin Ahmad, PhD, DABR
Department of Radiation Oncology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma
Results
Purpose
Homogeneity Index and Conformity Index
From Table 1, compared to IMRT, VMAT plans showed insignificant differences in target volume coverage as characterized by mean homogeneity index (HI) [6.1% (IMRT) vs. 6.6% (VMAT) with (p=0.31)] and mean conformity index (CI) [1.26 (IMRT) vs (VMAT) with (p=0.19)].
Organs at risk
From Table 2, VMAT also showed insignificant differences compared to IMRT in normal tissue sparing (mean hips, bladder and bowel doses of 33.37, and Gy (IMRT), versus 35.10, and Gy (VMAT) with p = 0.41, 0.06, 0.50, respectively).
Monitor Units and Delivery time
From Table 3, VMAT required fewer mean total MU and shorter BOT per fraction (1420 MU, 2.37 minutes) when compared to IMRT (2362 MU, 3.94 minutes) with p<
Intensity modulated radiation therapy (IMRT) is a treatment technique that employs multi-leaf collimator motion to modulate the beam intensity, allowing for highly conformal 3D dose distributions. Volumetric modulated arc therapy (VMAT), an extension of IMRT, also employs modifications in both gantry rotation speed and machine dose rate, in addition to multi-leaf collimator motion, to deliver a 3D dose distribution in rotational mode while using less treatment time than conventional IMRT. The objective of the present study is to compare VMAT to IMRT plans for patients with anal carcinoma.
Materials and Methods
Fourteen patients with anal carcinoma previously treated with step-and-shoot IMRT were retrospectively selected for this study. Each patient received a total dose of to 63 Gy in 1.8 Gy fractions. 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. Dose-volume histograms (DVH) for planning target volumes (PTV) and organs at risk (OAR) [hips, bladder, and bowel] were generated for dosimetric evaluation and comparison. For efficiency comparison, total monitor units (MU) and beam on times (BOT) per fraction were evaluated.
Conclusion
For radiation therapy treatment of anal carcinoma, IMRT and VMAT plans 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 reduce the treatment delivery uncertainty due to intra-fraction tumor motion.