Intensity modulated radiation therapy versus volumetric intensity modulated arc therapy

The advanced developments in external beam radiation therapy (EBRT) over the past few decades have improved dose conformity to the target while minimizing dose to the surrounding organs at risk (OAR). Intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) are two commonly used EBRT techniques to treat cancer. In sliding window (SW) or dynamic IMRT, each radiation beam is modulated by continuously moving multileaf collimators (MLC), whereas in step-and-shoot (SS) or static IMRT, the MLC divide each radiation beam into a set of smaller segments of differing MLC shape, and the radiation beam is switched off between the segments. The modulation of beam intensity within each treatment field leads to construction of conformal dose distributions around the target volume. However, the delivery of a modulated IMRT plan takes longer than the delivery of a nonmodulated three-dimensional (3D) plan due to increased number of monitor units (MU). In contrast, the VMAT can decrease the treatment delivery time as VMAT has more beam entry angles, which likely contributes to the lower number of MU needed compared with the IMRT plan. In the VMAT, one or multiple arcs are used for the treatment, and the delivery technique allows the simultaneous variation in gantry rotation speed, dose rate, and MLC leaf positions. Recently, there has been increased interest in treating cancer using VMAT. Several authors have done the treatment planning studies comparing IMRT versus VMAT for different tumour sites, but the findings from one study are conflicting with those of another study in some cases. For example, current literature comparing VMAT and IMRT for a lung tumour shows that both techniques could provide comparable target coverage and dose conformity. However, the OAR results in the case of lung tumour are contradictory among different studies. Rao et al. showed that the relative volume of normal lung receiving 20 Gy (V20) was higher in the VMAT plans than in the IMRT plans. In contrast, Verbakel et al. showed that the VMAT and IMRT plans achieved comparable V20 of normal lung. The planning studies of prostate cancer have produced inconsistent results too. Yoo et al. reported lower doses to the OAR in the IMRT plans than in the VMAT plans, but Ost et al. showed that VMAT was better at reducing rectal dose compared to IMRT. Furthermore, the planning techniques within the VMAT have shown inconsistent results as well. For prostate cancer, in comparison to the single-arc technique (SA), Sze et al. reported that the double-arc technique (DA) produced higher bladder dose, whereas Yoo et al. showed that the DA produced lower doses to the bladder. Guckenberger et al. showed that the DA yielded higher rectal dose, whereas Sze et al. reported lower rectal doses with the DA when compared to the SA. The inconsistency in the results among different planning studies may have been due to difference in selection of beam parameters, dose calculation algorithm, plan optimization technique, and delivery technique of the treatment machine. In comparison to the VMAT plan with one arc, the VMAT plan with multiple arcs has more control points that give higher degree of freedom for possible MLC positions. This could result in higher degree of modulation and better plan quality, especially for a complex-shaped target volume. However, a higher degree of modulation generally increases the planning time due to longer plan optimization and dose calculation processes. Thus, treatment planning personnel may be required to make a compromise between planning time and plan quality depending on the physician requirements and available planning resources. The dosimetric results of the OAR can also be affected by the design of the treatment machine as dose to the OAR is dependent on