Esophageal cancer, which mainly occurs in patients aged from 50 to 80 years, is the 11th most common type of cancer in Korea, accounting for approximately 1.7% of all cancers. An estimated 1,600 new patients are diagnosed with esophageal cancer every year [1]. Treatment options for esophageal cancer are surgery, chemotherapy, or radiotherapy [2 -8]. Radiotherapy for inoperable or unresectable esophageal cancer plays a crucial role in achieving optimal tumor control along with reducing normal tissue complications. However, a planning target volume (PTV) is a long, tube-like structure that connects the throat and stomach, as shown in Fig. 1, which creates challenges when planning a treatment. As there are several organs at risk (OARs) in the vicinity of the esophageal cancer, which include the lungs, heart, liver, spinal cord, and stomach, incorrect treatment may cause serious side effects such as radiation pneumonitis and fibrosis in the lungs as well as pericardial effusion, pericarditis, and heart failure [9].

Several studies have compared various radiotherapy techniques for esophageal cancer, including 3-dimensional conformal radiation therapy (3D CRT), intensity modulated radiation therapy (IMRT), and volumetric modulated arc therapy (VMAT) [2,9 -12]. These studies have reported that VMAT plans provide better dose conformity to the PTV while reducing the doses affecting OARs, compared to 3D CRT and IMRT plans [2,9 -12]. Furthermore, hybrid IMRT plans that combine static and IMRT beams, and hybrid VMAT plans that combine static beams and VMAT arcs concurrently, have been proposed by several studies to reduce the dose volume applied to normal tissues [9,13 -15]. These techniques were found to be superior in dosimetric outcomes when compared to conventional IMRT and VMAT plans.

Li et al. [16] proposed noncoplanar IMRT plans to reduce doses to surrounding OARs while improving PTV coverage for esophageal cancer. They showed that compared to coplanar IMRT plans, noncoplanar IMRT plans reduce the low dose exposure to the lungs, while providing a similar PTV coverage. The disadvantage of noncoplanar IMRT plans is that higher doses are delivered to the heart and spinal cord, making it inadvisable to use noncoplanar IMRT plans for patients with potential cardiovascular and neurological problems. Martini et al. [17] studied noncoplanar VMAT (ncVMAT) plans for patients with esophageal cancer who underwent either definitive or preoperative radiotherapy. Martini et al. [17] evaluated the feasibility, dosimetric results, and clinical outcomes. They showed that, in patients treated with ncVMAT plans using two coplanar and one noncoplanar arc beams, the side effects observed included esophagitis, anorexia, fatigue, nausea, and vomiting. The dosimetric results showed adequate PTV coverage and sparing of normal tissue. They concluded that the use of ncVMAT plans was a feasible, safe, and effective strategy for delivering optimal dose distributions for esophageal cancer patients. Although dosimetric results for ncVMAT plans were investigated in that study, it was a simple feasibility study to confirm the clinical outcome of ncVMAT plans for esophageal cancer, and dosimetric comparisons to other plans were not performed. Therefore, in this study, we compared ncVMAT plans to cVMAT plans by evaluating the dosimetric quality for esophageal cancer with a long, tube-like PTV.

In this study, we compared the potential dosimetric improvements of the ncVMAT plans with those of the cVMAT plans, for the long, tube-like PTV of esophageal cancer. The cVMAT plans with three coplanar full arc beams and the ncVMAT plans with two coplanar full arc beams and one noncoplanar partial beam were used for planning. For the noncoplanar beam, the couch rotation angles were adjusted to 315°±5° to avoid collision between the patient on the couch and the gantry. For dosimetric evaluation, the DV parameters of the PTV and OARs and total MUs for each VMAT plan were obtained.

To date, this is the first study that attempted to evaluate the dosimetric superiority of ncVMAT plans in patients with long, tube-like esophageal targets for a large group of patients (n=20). In a similar study, Li et al. [16] investigated the dose distributions around esophageal cancer and the surrounding OARs using coplanar IMRT and noncoplanar IMRT. They reported that the conformity and mean dose of the PTV were comparable between the coplanar IMRT and noncoplanar IMRT plans while the values of V_5%, V_10%, V_20%, and the mean dose for lungs using the noncoplanar IMRT plans decreased by 16.3, 10.6, 5.8, and 248.7 cm³, respectively, compared to those for the coplanar IMRT plans. The results of the present study are consistent with those found in earlier studies. The ncVMAT plans reduce the doses to OARs such as the lungs, heart, and spinal cord, as shown in this study. We found that the maximum doses to both the spinal cord and heart were dramatically reduced in the ncVMAT plans. For representative patient cases (patients 4 and 7) in this study, when using the ncVMAT plans, the maximum dose differences of 6.4 and 3.0 Gy were observed in volumes in the spinal cord and heart, respectively. We also found that low doses to the lungs and liver were significantly reduced for the ncVMAT plans. However, the values of V_105% of the PTV and V_100% of the whole body were increased for the ncVMAT plans. Nevertheless, these findings show some potential benefit of using the ncVMAT plans.

Except for the spinal cord and heart, there were minor improvements in dose distribution from the ncVMAT plans over the cVMAT plans in this study. This is mainly because the noncoplanar arc beam used in this study was limited. When using large noncoplanar angles, the probability of collision between the patient and the gantry increased. Theoretically, the 4π technique could provide significant OAR sparing compared to both the cVMAT and ncVMAT plans and then allow substantial dose escalation to the PTV [22,23]. Woods et al. [22] demonstrated that the 4π technique reduced 50% of the prescription doses by ~23%, compared to both cVMAT and ncVMAT plans. However, this technique significantly increases both the treatment time and the probability of collision between the patient and the gantry. In the clinic, we need to determine the noncoplanar arc beam configuration for effective treatment that represents a compromise to account for the combination of the gantry rotation angle and the couch angle.

Noncoplanar techniques have been widely examined for spherical targets for brain tumor radiotherapy, lung stereotactic ablative radiotherapy (SABR), and liver SABR patients, showing dosimetric benefits to either the PTV or the OARs [9,22,24 -27]. However, there are few dosimetric studies for large, irregularly shaped PTVs such as head, neck, and esophageal cancer. Gayen et al. [28] compared the dosimetric plan quality of the ncVMAT and cVMAT plans for head and neck malignancy and then demonstrated that ncVMAT plans provide better PTV coverage and OAR sparing, compared to cVMAT plans. Nevertheless, treatment of large, irregularly shaped head, neck, and esophageal cancer with noncoplanar beams is challenging due to the complicated positional relationship between the PTVs and the surrounding OARs. Further studies to prove the superiority of ncVMAT plans over cVMAT plans according to the size and shape of the PTV will be performed in the future.

The use of ncVMAT plans with two coplanar full arc beams and one noncoplanar partial beam provides better PTV coverage and OARs sparing, compared to that of cVMAT plans with three full arc beams for long, tube-like esophageal cancer. Furthermore, this technique can be adopted in the clinic for large, irregularly shaped targets with caution.

The study was approved by the Institutional Review Board of the Veterans Health Service Medical Center (IRB No. 2020-07-011).