Tomotherapy is currently the most advanced radiotherapy device. It uses a radiotherapy system that has the same source of treatment (6 MV) as impact-guided CT (3.5 MV). It has high imaging accuracy and automatically corrects positioning errors before radiotherapy, which makes the target area conformability and dose distribution more reasonable. TOMO integrates three-dimensional conformal radiation therapy, intensity-modulated radiation therapy, image-guided radiation therapy, dose-guided radiation therapy, adaptive radiation therapy, and other radiotherapy technologies in one, could be clinically used for a variety of tumors throughout the body, especially for frequently occurring tumors and tumors adjacent to important organs and tissues, which improves the accuracy of the target area while reducing the occurrence of complications (11, 12). All patients in this study used TOMO to treat lung metastases, and this has a certain research value.
Studies have shown that the survival rate of certain patients with oligometastases can be improved through local treatment, such as surgery, ablation therapy, and radiation therapy (13–17). Yamamoto et al. performed stereotactic body radiotherapy (SBRT) on patients with colorectal cancer and oligometastatic lung tumors. The 3-year LC, OS, and PFS rates were 64.9%, 63.4%, and 34.9%, respectively (18). The RADIOSTEREO-CAMPTO study was a prospective multicenter phase 2 trial that observed SBRT (40–48 Gy/4 times) in the treatment of inoperable colorectal cancer with liver and/or lung oligometastases combined with intravenous irinotecan, in which pulmonary oligometastases were found in 12 patients with tumors. In that study, the 1- and 2-year survival rates without local (distal) progression and OS were 84.2% (38.4%), 67.4% (21.3%), and 97.5% and 75.5%, respectively(19). Ricco et al. reported SBRT treatment in patients with pulmonary oligometastases of different primary tumors, using a dose of 48–54 Gy divided into 3–5 fractions, and the 1-year OS and LC rates were 74.1% and 80.4%, respectively (20). Lardinois et al. retrospectively analyzed 100 patients with head and neck squamous cell carcinoma with lung metastases, and the median OS and recurrence-free survival rates were 21 and 7 months, respectively (21). This study found that the median OS, LC, and PFS rates were 24.9, 25.9, and 11.8 months, respectively, and these were similar to the results of related studies.
In most studies, the risk factors related to OS, LC, and PFS include functional status, lesion diameter, primary tumor type, number of metastases, history of local metastases, and the number of metastatic organs throughout the body. Lardinois et al. found that the survival time of patients with single lung metastasis was significantly prolonged, and the local control rate of the site of metastasis was better than those of patients with multiple lung metastases (P < 0.001) (21). Yamamoto et al. reported that functional status PS was an independent prognostic factor for OS (PS 1 vs. PS 0, p = 0.0 2; PS 2–3 vs. PS 0, p = 0.0 4) (22). Chai et al. discussed the therapeutic effects of SBRT on pulmonary metastatic tumors. Univariate analysis showed that age ≥ 63 years, primary colorectal cancer, BED10 < 85.2 Gy, adenocarcinomas, PTV min BED10 < 76.6 Gy, and GTV ≥ 8.8 cc were significantly correlated with local recurrence-free survival (LRFS)(23). This study found that patients who were younger than 57 years of age (p = 0.037, HR = 3.35, 95% CI = 1.08–10.41) and those whose metastatic organs were the lungs (p = 0.046, HR = 3.15, 95% CI = 1.02–9.76) had a better overall survival time, which is consistent with the findings of previous reports.
Studies have reported the impact of the primary tumor type on the survival rate of patients with pulmonary oligometastases. Takeda et al. found that the primary tumor type was an important factor that affected the survival rate of patients with lung metastases. Comparing patients with colorectal cancer and those with non-colorectal cancer revealed that the 1-year LC rates were 80% and 72%, respectively, the 2-year LC rates were 94% and 94%, respectively, and the 3-year non-local progression rates were 39% and 83%, respectively (24). Wang et al. compared the effects of SBRT treatment on early-stage non-small cell lung cancer and colorectal cancer with less severe metastatic lung cancer. The 1- and 3-year LRFS rates of patients with colorectal cancer were 80.6% and 100%, respectively, and for those with non-small cell lung cancer, they were 68.6% and 97.2%, respectively (25). However, Duijm et al. in a study of patients with inoperable lung metastases with SBRT, found that the 2-year-, 3-year-, and 5-year OS rates were 63%, 47%, and 30%, respectively, while the 2-year-, 3-year-, and 5-year PFS rates were 36%, 25%, and 16%, respectively. Further analysis found that compared with patients with other histological types, patients whose primary tumor was colorectal cancer had a median OS rate of 39.2 months, and their OS was relatively good (p = 0.018, HR = 0.64, 95% CI = 0.44–0.92) (26). In this study, the median OS of patients with colorectal cancer as the primary tumor was 10.2 months, while the median OS rate of patients with non-colorectal cancer was 29.4 months. The difference was statistically significant at p < 0.05, which may be because non-colorectal cancer patients included head and neck tumors and breast cancer patients with good prognosis; however, its relationship with LC and PFS was not found. This was a retrospective study, with certain limitations. Combined with the results of previous studies, the impact of the primary tumor type on the survival of patients with pulmonary oligometastases needs to be determined by a well-designed prospective study. The possible reason was that the older the patient, the more metastatic organs, the worse the physiological function, and the poor tolerance of treatment.
Sharma et al. showed that BED10 ≥ 100 Gy can improve the OS and LC of patients with tumors (27). Similarly, Kang et al. found that the SBRT dose was correlated with OS (28). Considering that the patient in this study had distant metastasis and the possibility of side effects as well as potential toxic effects, the median BED10 in this study was 76.8 Gy (range, 56–96 Gy), and this was lower than that reported in related studies. However, no relationship was found between radiation dose and survival rate.
Regarding the adverse reaction report, a study by Osti et al. reported that 34.4% of patients had grade 2 fibrosis, 7.4% of patients had grade 3 fibrosis, 1.3% had rib fractures, and one toxic death occurred after treatment (29). Le et al. reported that 19% of patients had pneumothorax, 12.7% had grade 2–3 pneumonia, and three deaths related to side effects occurred after treatment(30). The toxic reactions in this study included radiation pneumonia (10 patients had grade 1 radiation pneumonitis; six patients had grade 2 radiation pneumonitis), bone marrow suppression (four patients had grade 1 bone marrow suppression, six patients had grade 2 bone marrow suppression), Grade 1 radiation dermatitis in two patients had, and seven out of 10 patients had myelosuppression and received systemic simultaneous treatment, and these may be related to the side effects of chemotherapy. There were no grade 3 or 4 toxic reactions, and no toxicity-related deaths.