Dosimetric comparison of volumetric modulated arc therapy and intensity modulated radiation therapy for anal cancer

Aim: 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 three-dimensional dose distribution. This study compared VMAT to IMRT for patients with anal carcinoma. Materials and Methods: Sixteen patients previously treated with IMRT were retrospectively selected. Each patient received a total dose of 57 center dot 6-63 center dot 0 Gy in 1 center dot 8 Gy fractions. A single- or double-isocenter multi-arc VMAT treatment plan was generated using Eclipse RapidArc system with the same computed tomography image sets and optimisation constraints used for IMRT. Dose-volume histograms (DVHs) for planning target volumes (PTVs) and organs at risk (OARs), and monitor units (MUs) and beam on times (BOTs) were used for comparison. Results: IMRT and VMAT plans showed insignificant differences in PTV homogeneity and conformity and sparing hips and bowel. VMAT required fewer mean MU and shorter BOT per plan (1,597 MU, 2 center dot 66 min) compared to IMRT (2,571 MU, 4 center dot 29 min) with p < 0 center dot 0001. Conclusions: Fewer MU and shorter BOT for VMAT may decrease the damage from secondary radiation and treatment delivery uncertainty due to intra-fraction tumour motion, leading to higher machine throughput and improving patient comfort, with less treatment time.