Recently, many research studies have found that the PNI has an important role in cancer development and prognosis. However, to our knowledge, no studies have investigated the prognostic value of pretreatment PNI in ESCC patients who received radical RT with or without synchronous chemotherapy. We demonstrated the significant predictive value of PNI in ESCC patients who received radical CRT or RT. Our results showed that a high PNI (< 47.975 vs. ≥49.975) was significantly associated with decreased tumour length, T-stage and synchronous chemotherapy. Moreover, the low-PNI group was significantly associated with shorter OS in ESCC patients treated with radical CRT or RT, and pretreatment PNI was shown by multivariate analysis to be an independent prognostic factor.
The PNI, which is based on the serum albumin level and lymphocyte count, was originally proposed to assess risk and prognosis of patients undergoing gastrointestinal surgery . In recent years, an increasing number of studies have used PNI to evaluate the prognosis of patients with malignant tumours and have found that it has nothing to do with the location and origin of the tumour . Thereafter, the PNI can be used to assess the prognosis of tumours as follows: (I) Serum albumin pretreatment is of great value in tumour nutrition, and the PNI can contribute not only to understanding the nutritional status of patients but also be used to determine the albumin level, which is useful because the albumin level has been shown to have a negative correlation with cancer risk , so survival and prognosis of tumour patients can be predicted. (II) Tumour-related inflammation and malnutrition can inhibit albumin synthesis, whereas hypoalbuminemia can reflect the degree of inflammation and may negatively affect patient survival [17, 18]. (III) Low lymphocyte counts in cancer patients indicate an immunosuppressed state and can lead to poor prognosis . (IV) In the process of tumour development, cancer cells can avoid and suppress the T-cell immune response in various ways [20, 21].
In recent years, several studies have begun to analyse the relationship between the PNI and prognosis in patients with oesophageal cancer. However, there is some controversy regarding the PNI in oesophageal cancer research. Han et al.  retrospectively analysed 206 patients with ESCC after esophagectomy and found that a high PNI had a positive effect on OS in patients with ECSS, but the PNI was not found to be an independent prognostic factor, which was not consistent with our finding that the PNI was significantly associated with the OS (P = 0.002) and therefore the prognosis of oesophageal cancer. Our multivariate analysis also showed that multiple variables were significantly associated with OS in oesophageal cancer, which was similar to the findings of Sun et al. . The default PNI cutoff value is 50, which may deviate from the optimal PNI value for oesophageal cancer and influence the outcome of prognostic factors.
In addition, a study conducted by Nakatani et al.  in 66 patients with ESCC who received neoadjuvant chemotherapy found that patients with a PNI ≥ 45 had OS better than those with a PNI < 45 at 3 and 5 years (66.9%, 51.2% vs .33.3%, 0%, respectively), those with a PNI ≥ 45 had 3- and 5-year recurrence-free survival (RFS) rates better than those with a PNI < 45 (60.7%, 41.8% vs. 33.3%, 0%, respectively), and the preoperative PNI was an independent prognostic factor for OS and RFS. Hirahara et al.  conducted a retrospective study in 169 patients who underwent radical oesophageal cancer surgery and found that a PNI < 49.2 was an independent poor predictor of CSS and OS. In another retrospective study in 106 patients with cervical oesophageal cancer who underwent radiotherapy, Dai et al.  found significantly lower OS rates in the PNI < 48.15 group than in the PNI ≥ 48.15 group (P = 0.004). In the present study, the ROC curve was used to calculate the optimal PNI cutoff value, which was 47.975. The 5-year OS was significantly better in the ESCC patients with a pretreatment PNI ≥ 47.975 than in those with a PNI < 47.975, and PNI was an independent prognostic factor in the multivariate analysis. We also analysed the change in PNI during the entire treatment process. The lowest PNI value in the treatment may be affected by various interventions. Oesophageal cancer is one of the most common cancers that cause malnutrition, and chemoradiation can lead to decreased appetite, obvious gastrointestinal reactions and myelosuppression. Inhibition may lead to a decrease in the PNI value.
In this study, we also found that PNI, tumour length and concurrent chemotherapy were independent prognostic factors in ESCC. The prognosis of ESCC tumours < 5 cm in length is significantly better than those ≥ 5 cm in length, which is consistent with the results of oesophageal cancer treated with proton radiation in another study . In this study, the survival rate was better for patients who received concurrent chemotherapy than for those who did not (5-year OS: 32.4% vs. 0%, respectively). Chemotherapy has been previously shown to be associated with a favourable prognosis for oesophageal cancer .
This study had several limitations. First, this was a single-centre retrospective study with a small sample size. We were not able to demonstrate the effectiveness of PNI for predicting the prognosis of ESCC patients treated with CRT because of our sample size, so a large-sample prospective study is needed. Second, according to the PNI value we recorded throughout the treatment, but after the treatment, the PNI value showed a significant decline due to various factors, we had not found its value for prognosis. Finally, the AUC was relatively low, so a more sensitive prediction model needs to be constructed.