Relationship between the consolidation to maximum tumor diameter ratio and outcomes following stereotactic body radiotherapy for stage I non-small-cell lung cancer
Introduction
Stereotactic body radiotherapy (SBRT) is used to treat medically inoperable patients with stage I non-small cell lung cancer (NSCLC) and achieves good local control and survival rates. Previous studies have reported relationships between several factors, including tumor size [1], biologically effective dose (BED) [2], and operability [3], [4], and outcomes following SBRT.
Recently, associations between thin-section computed tomography (TSCT) findings and pathological characteristics have been reported. The Japan Clinical Oncology Group (JCOG) 0201 study [5] reported that the ratio of the maximum diameter of consolidation to the maximum tumor diameter (consolidation/tumor ratio, CTR) (Fig. 1), measured radiologically, correlated with pathological findings in clinical stage IA lung cancer patients. They found that a CTR < 0.5 corresponded well with pathological non-invasiveness (the specificity and sensitivity were 96.4% and 30.4%, respectively), although their predetermined criterion (the lower limit of the 95% confidence interval [CI] for a specificity of 97%) was not statistically confirmed. The cutoff value was also able to define patients with an excellent prognosis before surgery [6]. According to this study, 5-year overall and relapse-free survival rates for patients meeting this criterion were 96.7% and 95.9%, respectively. Based on these results, JCOG initiated a prospective study [7] evaluating the validity of sublobar resection for patients with stage IA NSCLC.
To the best of our knowledge, no studies have focused on the relationship between the CTR and outcomes following SBRT. Therefore, we retrospectively investigated the CTR’s ability to predict the outcomes of NSCLC patients who had received SBRT.
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Patients
Between April 2005 and March 2014, 237 patients diagnosed clinically or pathologically with stage I NSCLC were treated with SBRT, receiving 48 Gy in 4 fractions. Regarding patients without a pathological diagnosis, our multidisciplinary tumor board had decided to treat them with SBRT by reviewing radiological findings and identifying characteristic findings and tumor growth on CT imaging and/or a characteristic maximum standardized uptake value (SUV max) on 18F-fluorodeoxyglucose positron
Baseline patient characteristics
Table 1 summarizes the clinical characteristics of the three groups. Regarding pathologically proven tumors, the pathological diagnosis of all tumors for patients in the CTR < 0.5 and CTR 0.5–<1 groups was adenocarcinoma. The median PTV D95 of the CTR < 0.5 group was 1.2–1.5 Gy lower than that of the other groups. Statistically significant differences in sex (P = 0.040), pathology (P < 0.001), SUV max (P < 0.0001), and PTV D95 (P = 0.001) were observed between the three groups. The other characteristics did
Discussion
Our study showed that the post-SBRT LC rate was high for patients with a CTR < 1; no patients in the CTR < 0.5 group developed local recurrence. To date, few clinical reports have examined the relationship between radiological tumor characteristics and outcomes after SBRT. Badiyan et al. [9]. reported that the 3-year local control rate was 100% for biopsy-proven or radiographically diagnosed bronchioloalveolar carcinoma, visualized as nodules with a GGO on CT. In a retrospective multicenter study
Conclusions
The CTR effectively predicts DFS after SBRT in NSCLC patients. Longer follow-up is required to determine whether local control rates for tumors with a low CTR remain excellent over the long term.
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