Original ArticleVolumetric-modulated Arc Therapy in Head and Neck Radiotherapy: A Planning Comparison using Simultaneous Integrated Boost for Nasopharynx and Oropharynx Carcinoma
Introduction
Radical radiotherapy for head and neck malignancy is technically challenging, requiring delivery of high tumoricidal doses to the target while constraining the dose to adjacent critical structures and organs at risk (OARs). Intensity-modulated radiotherapy (IMRT) can improve dosimetric end points when compared with parallel opposed photon fields and three-dimensional conformal radiotherapy and there is evidence that IMRT correlates with decreased toxicity, without compromising local control [1], [2], [3], [4], [5], [6]. However, although IMRT is a standard technique for head and neck radiotherapy, there are concerns regarding the high number of monitor units and extended treatment times [7].
Volumetric-modulated arc therapy (VMAT) is a form of intensity-modulated arc radiotherapy, which, since its initial investigation [8], has generated widespread interest in radiation oncology circles. The technique enables increased flexibility of dose delivery from a full range of angles (gantry rotation) with continuous modulation of beam aperture and dose rate. It thus has potential for further improved dosimetry, including target coverage, dose to normal tissues and dose homogeneity compared with IMRT. The reduced monitor units and shorter treatment time of VMAT have already been shown [9], [10], [11], [12]. This has implications for increasing departmental efficiency and patient throughput and also has the potential to increase patient compliance.
The aim of this planning study was to evaluate VMAT and static field IMRT using a simultaneous integrated boost technique. This evaluation was carried out using the prescribed dose levels and constraints to OARs based on the Radiation Therapy Oncology Group (RTOG) clinical trials RTOG 0615 and RTOG 0522 for nasopharynx and oropharynx, respectively (copies of the protocols can be found at www.rtog.org). These RTOG trial protocols define a set of accepted target coverage and OAR dose constraints allowing evaluation of plan quality against accepted reference standards. This planning study was restricted to a comparison of double-arc VMAT and IMRT, consistent with recent studies [13].
Section snippets
Materials and Methods
All cases were American Joint Committee on Cancer (AJCC) stage III or IV, apart from one case with AJCC stage IIB (T2bN1M0) nasopharyngeal carcinoma (see Table 1). Planning computed tomography data from sequential patients with locoregionally advanced head and neck cancer were selected for this study. All had undergone radical radiotherapy at our institution with IMRT during 2008–2009. Five patients had oropharyngeal primaries and five had nasopharyngeal carcinoma. The tumour and target volume
Results
Clinically acceptable plans were achieved for both IMRT and VMAT plans. The dynamic IMRT plans typically required more than three times the number of monitor units to deliver the treatment as compared with the corresponding VMAT plan (Table 3). The dynamic MLC or sliding window IMRT method used in this study traditionally requires more monitor units than other IMRT techniques, such as step and shoot. Hence, the monitor unit advantages might not be as high as when compared with these other IMRT
Discussion
VMAT can offer a potential advantage over IMRT in terms of speed of treatment and reduced monitor units. Planning studies have reported that VMAT plans are comparable with IMRT for a number of sites, including the head and neck [12], [20], [21]. Initial reports on treatment delivery using VMAT report comparable accuracy of dose delivery compared with IMRT [20], [22].
The VMAT technique was described initially using single-arc plans [8], but subsequent studies have also been published describing
Conclusions
This planning study has shown the ability of double-arc VMAT to achieve comparable plans to IMRT in head and neck cases, using a simultaneous integrated boost technique. We have also shown that VMAT is able to achieve target and OAR constraints as outlined in recent RTOG protocols for IMRT planning. Because the delivery of VMAT uses less monitor units, it offers potential advantages over sliding window IMRT, although plan quality needs to be carefully assessed to ensure improved dosimetry is
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