Dosimetric effect of intensity-modulated radiation therapy for postoperative non-small cell lung cancer with and without air cavity in the planning target volume
Introduction
Lung cancer is a malignant tumor that has the highest incidence and the largest number of deaths worldwide.1 In the United States, it is expected that there will be 220,000 new cases of lung cancer and 130,000 deaths by 2020.2 Clinically, the preferred treatment for lung cancer is surgery, however, surgery alone cannot effectively treat advanced lung cancer that has large lesions and local metastasis. Radiotherapy is another major method for the treatment of malignant tumors, and more than 70% of patients require radiotherapy.3 The combination of surgery and radiotherapy is an important and effective method for the treatment of lung cancer patients. Several reports have shown that postoperative radiotherapy for patients with N2 non-small cell lung cancer (NSCLC) can optimize local control and improve survival.4,5
Intensity-modulated radiation therapy (IMRT) is one of the main radiotherapeutic techniques that can improve the target dose, while sparing the organs at risk (OARs).6,7 However, when treating lung cancer with IMRT, normal tissues are covered by low-dose areas that have large volumes and that are prone to unexpected hot spots, and that result in an increased risk of secondary cancer.8,9 This is specifically observed with planning target volume (PTV) that is within a certain volume of air cavity, due to the electron disequilibrium region on the air-tissue interface, where the disturbance of electron flux is easy to occur, and which results in the distortion of the dose distribution and its insufficient reach of the target.10,11 These air cavities present a challenge to inverse-planning software due to its attempt in pushing the dose into the air to achieve sufficient PTV coverage. This unnecessary attempt in dose accumulation may lead to increased hot spots within the rest of the PTV and its surrounding soft tissues.
In recent years, most of the studies on the impact of the air cavity on the planned dosimetry of IMRT have focused on the parts that have relatively large air cavity volumes, such as the nasopharynx, mouth, and larynx. Liu et al.12 found that the radiation dose of the primary tumor and brain stem of OARs could be increased by the air cavity in the IMRT of nasopharyngeal carcinoma. While studying oral cancer, Lian et al.13 compared three different methods that included the air cavity into the PTV radiotherapy plan and found that the air cavity effect could easily increase the skin and optic nerve radiation doses. Asher et al.14 found that removing the air cavity from the PTV for early stage glottic cancers could lead to a more homogeneous IMRT plan. The above studies showed that the air cavity effect may influence the dose distribution in the PTV. However, it is unclear whether there is a correlation between the dosimetric parameters, the size of the air cavity, and the volume proportion of the cavity in the PTV. Meanwhile, we did not encounter relevant literature reports on the influence of the PTV tracheal air cavity on the dose distribution in the IMRT plan for NSCLC.
For NSCLC patients who need postoperative adjuvant radiotherapy, the main irradiation areas are mainly the hilar and mediastinum regions. When radiation oncologists delineate the target, the tracheal air cavity can easily be included as part of the PTV, which may affect the PTV dose distribution.
In this study, the IMRT dosimetric effect for postoperative NSCLC, with and without the air cavity in the PTV, was studied. Based on the two kinds of PTV (with or without air cavity), two-group plans for 21 patients with postoperative NSCLC were made. By comparing the dose-volume histograms (DVHs), conformity index (CI), and homogeneity index (HI), the number of segments, and Mus, that were generated by the two plans, the effects of the air cavity in the PTV of NSCLC on the dose distribution within the target and on the dose exposure of the surrounding OARs, were evaluated. Additionally, the correlation between the change value of the two plans evaluation indexes, the air cavity size, and the cavity volume proportion in the PTV, was also studied. It is hoped that the results provide a reference for radiation oncologists to decide whether the air cavity should be included when delineating PTV for postoperative NSCLC patients.
Section snippets
Patient selection
A total of 21 NSCLC patients, who were treated in our center between January and July 2020, were included in this retrospective study. All patients received postoperative radiotherapy. Each patient received 50.4 Gy in 1.8 Gy per fraction. The patient characteristics are listed in Table 1.
Position fixation and image acquisition
All patients were in the supine position and fixed with thermoplastic membrane or vacuum mold according to the requirements of the therapeutic position. SOMATOM Definition AS (Siemens Healthcare GmbH) CT scans
PTV dosimetric effect with and without air cavity on the target
Table 3 describes the dosimetric comparison of the two PTVs in Plan-1 with the PTV-0 in Plan-0, respectively. We found that Dmean, D2 and D0.2 of PTV-1 in Plan-1 were lower than those of PTV-0 in Plan-0, and that the difference was statistically significant (p < 0.05). Similar results occurred for PTV-0 in Plan-1 and PTV-0 in Plan-0. There was no significant difference in D98 between the two plans (p > 0.05). D95 was manually normalized to the prescribed dose, therefore there was no significant
Discussion
The tissue structures in the human body are complex, and during radiotherapy, the incident photon beam will encounter soft tissue, bone, air cavity and other tissues. These structures that have different densities in the transport path, while the interface between tissues that have large differences in density, will have an electron disequilibrium phenomenon, that seriously affects radiation energy deposition in the medium.10,18 When IMRT is applied to postoperative NSCLC, the electron
Conclusions
For the postoperative NSCLC treatment with IMRT, it is possible to make the target dose closer to the prescribed dose and generate fewer hot spots and a more homogeneous dose distribution by removing the PTV air cavity. The heart V30, MHD and the esophagus MED were reduced, and a larger air cavity size would lead to a lower MED and poorer conformity. Therefore, it is suggested that, when delineating PTV, radiation oncologists should decide whether to remove the PTV air cavity based on the
Authors’ Contributions
WG was involved in conceptualization, data curation, data analysis, investigation, methodology, and writing; YD was involved in conceptualization, data curation, data analysis, and methodology; HW was involved in conceptualization, data analysis and methodology; YS, HC, AF, HG, YH, YY and XF were involved in methodology, resources, supervision; HQ was involved in writing – review and editing. ZX was involved in conceptualization, data analysis, methodology, project administration, supervision,
Acknowledgments
This study was supported by grants from the Interdisciplinary Program of Shanghai Jiao Tong University (No. YG2019ZDB07).
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