Relationship between clinical factors and metabolic parameters
The median values of SUVmax, SUVmean, MTV and TLG in 73 LAPC patients were 6.9 (P25: 5.85, P75: 9.85), 4.0 (P25: 3.2, P75: 5.6), 12.2 (P25: 7.0, P75: 22.5) cm3, and 49.3 (P25: 31.6, P75: 86.0) g, respectively. SUVmax, SUVmean and TLG of patients with lymph node metastasis were 8.6 (P25: 5.9, P75: 11.2), 4.9 (P25: 3.3, P75: 5.7), and 67.8 (P25: 37.9, P75: 117.1) g, respectively, which were significantly higher than those without lymph node metastasis (6.7 (P25: 5.6, P75: 8.1), 3.7 (P25: 3.1, P75: 4.4), and 45.6 (P25: 27.9, P75: 64.4) g, respectively (P = 0.029, 0.048 and 0.007, respectively). Moreover, a longer tumor diameter was associated with higher TLG (P = 0.035). No significant differences were found between MTV and clinical factors (all P > 0.05, Table 1).
Seventy three LAPC patients (46 males, 27 females) from January 2012 to January 2016 were enrolled in this study, the basic characteristics and pre-treatment conditions were summarized in Table 2. The median follow-up duration was 37 months (range: 12–55 months). For OS, there were 6 cases with censored data, including 2 patients lost follow-ups and 4 patients who were alive. And the rate of lost follow-ups was 2.7%. The median PFS time was 9.7 months (95% CI: 9.1–10.3), only one case presented with censored data in PFS, who showed no evidence of disease progression by the end of follow-up. The 1 year and 2 years PFS rate was 30.1 and 4.1%, respectively. Meanwhile, the median OS period was 14.4 months (95% CI: 13.3–15.5). The 1-year and 2-year OS rates were 78.1 and 11.2%, respectively.
Survival analysis and univariate analysis
The log-rank test was used to analyze metabolic parameters, including MTV and TLG (Fig. 1, Table 3), and we found MTV and TLG were prognostic factors of OS and PFS. The median overall survival time was 15.6 (95% CI: 14.1–17.2) months and 13.2 (95% CI: 11.6–14.8) months for patients with MTV ≤ 12.2 and MTV > 12.2 respectively (P = 0.036, HR:1.669,95% CI: 1.025–2.716). The median overall survival time was 15.9 (95% CI: 14.9–16.9) months for patients with TLG ≤ 49.3, and 12.9 (95% CI: 12.1–13.7) months for patients with TLG > 49.3, respectively (P = 0.001, HR: 2.320,95% CI: 1.413–3.809).
The median PFS time of patients with MTV ≤ 12.2 and MTV > 12.2 was 10.4 (95% CI:9.1–11.7)months and 8.4(95% CI:6.5–10.3)months (P = 0.048, HR: 1.601, 95% CI: 0.998–2.567), respectively. The median PFS time of patients with TLG ≤ 49.3 and TLG > 49.3 was 11.7(95% CI:8.0–15.4) months and 7.9(95% CI: 7.0–8.9) months, respectively (P = 0.001, HR: 2.424 1,95% CI: 1.495–3.939).
As for clinical factors, Lymph node metastasis, longest diameter of tumor, radiotherapy dose and chemotherapy were significant OS-related prognostic factors (P = 0.029, 0.045, 0.001 and 0.006, respectively). Patients with lymph node metastases, longer tumor diameters or lower radiotherapy dose tended to have shorter OS. Meanwhile, LAPC patients treated without chemotherapy also showed a shorter OS duration. In addition, radiotherapy dose and chemotherapy were significantly associated with PFS (P = 0.002 and 0.030, respectively). LAPC patients received higher radiotherapy dose or combined with chemotherapy demonstrated a longer PFS time (Table 3).
Multivariate analysis of prognostic factor
Multivariate analysis performed by Cox proportional hazards models showed that TLG, radiotherapy dose, and chemotherapy were independent prognostic factors for OS and PFS.
The hazard ratio (HR) of TLG, radiotherapy dose and chemotherapy upon OS was 2.145 (P = 0.003, 95% CI: 1.292–3.560), 0.480 (P = 0.004, 95% CI: 0.289–0.796) and 0.471 (P = 0.010, 95% CI: 0.267–0.833), respectively. The HR of TLG, radiotherapy dose and chemotherapy upon PFS was 2.307 (P = 0.001, 95% CI: 1.406–3.787), 0.591 (P = 0.033, 95% CI: 0.364–0.960) and 0.572 (P = 0.040, 95% CI: 0.335–0.976), respectively. All P < 0.05 (Table 4).
Among clinical factors, radiotherapy dose and chemotherapy were founed as indepent prognostic factors. Mann-Whitney U test was used verify the distribution difference of clinical factors between two groups of patients with or without chemotherapy, and between two groups with high or low radiotherapy dose (Table 5). The statistical results indicated that there was no statistical difference in the distribution of clinical factors between the chemotherapy group (23 cases) and the non-chemotherapy group (50 cases). The negative results also showed between the low radiotherapy dose group (37 cases) and the high radiotherapy dose group (36 cases). According to statistical data, the selection bias possibly caused by retrospective study could be reduced.
Typical cases of PET/CT images were presented in Figs. 2 and 3.