Dosimetric and Efficiency Comparisons between Volumetric Modulated Arc Therapy and IMRT for Cervical-Thoracic Esophageal Cancers

Purpose: This study aims to evaluate the treatment plans of Volumetric-mo-dulated arc therapy (VMAT) and intensity-modulated radiation therapy (IMRT) techniques for cervical-thoracic esophageal cancers. Methods and Materials: Sixty patients were retrospectively identified. Several parameters were evaluated based on target conformity and dose-volume histograms of organs at risk (lung, spinal cord, and heart). A phantom for time comparison was also assessed for each plan. Results: The IMRT plans (5f-IMRT: V95% = 99.4 ± 0.3, 7f-IMRT: V95% = 99.8 ± 0.1) results in better PTV coverage than RA plans (Single-arc: V95% = 95.8 ± 3.2, Double-arc: V95% = 95.4 ± 2.3). The target dose conformity of the 5f-IMRT plan was inferior to all plans (CI = 70.4 ± 7.1). The Single-arc plan achieved the best conformity (CI = 72.5 ± 4.6), whereas the Double-arc plan (CI = 72.1 Conclusion: For similar PTV parameters, VMAT delivers a lower dose to organs at risk than IMRT in a shorter time, and this has warranted clinical im-plementation.


Introduction
Chemo-radiation has emerged as a standard method in the treatment of cervical-thoracic esophageal cancer [1] [2]. However, because of complexity of anatomic structures in head and neck, especially in cervical-thoracic esophagus, radiotherapy presents a particularly difficult treatment planning problem [3].
In recent years, volumetric-modulated arc therapy (VMAT) has become more and more important in the treatment of several tumor types and has been shown to provide significant dosimetric and delivery benefits over intensity-modulated radiation therapy (IMRT) [4]- [9]. The term intensity-modulated radiation therapy (IMRT) refers to a radiation therapy technique in which a nonuniform fluence is delivered to the patient from any given position of the treatment beam to optimize the composite dose distribution. The delivery of a rotational cone beam with variable shape and intensity is commonly called volumetric-modulated arc therapy (VMAT). In a VMAT treatment, the gantry moves continuously, with the MLC leaves and dose rate varying throughout the arc. The TPS computes the dose by sampling the delivery at a number of discrete gantry angles [10]. So far, there have been no published reports on the advantages of VMAT use in cervical-thoracic esophageal patients compared with IMRT. Volumetric arc modulation using VMAT enables IMRT-like dose distributions to be delivered using a single and/or multiple rotations of the gantry. Recent studies report the efficacy of RapidArc (RA) over conventional IMRT in terms of PTV conformity and reduction in organs at risk (OAR) doses. These studies have further demonstrated the ability of VMAT to reduce the number of monitor units and overall treatment times when compared with IMRT. The purpose of this study is to investigate whether VMAT has the capability to deliver plans that are superior to IMRT for cervical-thoracic esophageal cancers [11] [12] [13].

Methods and Materials
Sixty patients with pathologically proven primary and secondary cervical-thoracic esophageal squamous carcinoma were selected for the study, the median age was 65. Patients were immobilized in supine position using a thermoplastic mask system with active fixation of light points and scanned with a Philips Brilliant spiral CT (Philips Brilliant, Cleveland, OH) according to standard procedures with 3 mm slice spacing.

Result
The dose distributions for the four plans were shown for one patient in Figure 1.
In Tables 1-5, an overview of the numerical findings from average DVH analysis on OARs and PTV are reported as mean values ± standard deviation to assess for the relative inter-patient variability. The significant differences between delivery methods for OARs and PTV are also reported in the     Heart VMAT plans were superior in sparing the heart in terms of D mean and D 35% .
The mean dose to heart for VMAT plans was better than IMRT plans by a marginal percentage variation (1% -2%). Similar results were for D 35% heart doses.
However, there was no statistical significance between these four plans, except that Single-arc plan showed a significant difference in heart mean dose when compared with 7f-IMRT plan. Details were in Table 4.

Monitor Units and Delivery Time
As displayed in Table 5

Discussion
Radiation technique develops from conventional radiotherapy to conformal radiotherapy, and then to IMRT. IMRT reduce the dose to lung parenchyma thus facilitating tumor dose escalation, which might improve local control and improves the uniformity of tumor irradiation and reduces the dose to lung. However, the disadvantage of IMRT is the higher number of MU and longer treatment time. The relative long treatment time will increase patients' discomfort.
The prolongation of the fraction time will spare tumors with a fast DNA healing result in inefficient therapeutic benefits. And longer treatment times increase the likelihood of patient movement during treatment, which can potentially lead to PTV miss. VMAT, first raised by Yu in 1995 [14], was capable for intensity-modulated radiation delivery during gantry rotation with dynamic multi-leaf collimator (MLC) motion, variable dose rates (DR) and gantry speed modulation. Previous studies [4]- [9] showed VMAT can generate similar or better dose distributions including target volumes and OARs and achieve a reduction in treatment time and a reduction in monitor units (MU And shorter treatment times reduce the likelihood of patient movement during treatment, which can potentially lead to PTV miss. In addition, shorter treatment times improved the utilization of machine.

Conclusion
For the treatment of cervical-thoracic esophageal cancer, VMAT is capable of delivering plans with better OAR sparing and without compromising PTV pa-Journal of Cancer Therapy rameters. In addition, VMAT technique can significantly reduce the number of monitor units required and overall treatment time. In a word, the VMAT technology can be an alternative method for cervical-thoracic esophageal cancer.

Funding
The study was partially supported by a grant from Guangdong Province innovation school project No. 2018KQNCX128, Xiamen Medical and Health Guidance Project No. 3502Z20209111.