The present study aimed to identify the risk factors associated with tumor recurrence after limited resection for small-sized NSCLC. Evaluation of various CT features, such as the mCT values, C/T ratio, solid tumor size, whole tumor size, and SUV can be helpful in predicting the potential for tumor recurrence. In particular, we initially demonstrated that the mCT value of the GGO lesion is a sensitive marker for predicting tumor recurrence.
Although lymphatic or blood vessel invasion is reportedly a strong predictor for postoperative tumor recurrence in completely resected stage I non-small cell carcinoma 15–18 , it is difficult to preoperatively identify patients with stage I adenocarcinoma who will have lymphatic or vessel invasion. The preoperative ability to biologically distinguish aggressive tumors from indolent tumors is extremely important for judging whether patients are suitable for sublobar resection. A considerable effort has been exerted to preoperatively distinguish non-invasive versus invasive cancer. In the clinical setting, pathological examination was more important when selecting cases for limited resection. We previously reported that the evaluation of mCT value is useful in predicting less invasive lung cancer.12 There have been no studies regarding the mCT value in lung cancer and recurrence, which is of great significance for treatment decisions. In the present study, we had selected tumor recurrence as the endpoint.
With recent advances in diagnostic imaging technologies, GGO lesions are increasingly detected using HRCT scans. 19,20 GGO is defined as a shadow that is completely occupied by a hazy area of increased attenuation in the lung with preserved bronchial and vascular lesion margins when assessed using HRCT.1 In a clinical setting, several types of GGO can be encountered. It is difficult to measure the size of the solid part of the tumor when the nodule comprises a heterogeneous mixture of GGO and solid tumor. Suzuki et al.6 classified peripheral small-sized adenocarcinoma into six categories, and reported that the classification was significantly associated with pathologic prognostic factors.
Several authors have classified small lung lesions into nonsolid (pure) GGO, partly solid (mixed) GGO, and solid types. However, it is sometimes difficult to differentiate between pure and mixed GGO, and between high-density GGO and solid tumors, because no definite radiological criteria exist to distinguish these differences. Some authors used quantitative densitometric methodologies to evaluate GGO lesions.11,18−20 Although the one-dimensional quantitative mCT value can be slightly affected by the densities of vessels or bronchi within the tumor, this calculation method is straightforward, and can similarly estimate pure GGO and mixed GGO.
Our study indicated that the predictive ability of the mCT value for tumor recurrence was higher for small-sized tumors and tumors showing extensive GGO. For solid-predominant tumors, SUV demonstrated higher predictive ability than did the mCT value. The mCT value is useful for tumors that are mainly homogeneous in density, or those that are too dense to be called pure GGO but are not pure solid tumor. The mCT value adds a diagnostic value to the C/T ratio for tumors for which the size of solid portion is difficult to measure. The ROC area under the curve value of the mCT value was the highest, excluding the pure GGO and pure solid (i.e. 0 < C/T ratio < 100) group (Table 3). The GGO legion is commonly seen with adenocarcinomas, while most squamous cell carcinomas or other histology groups show pure solid lesion. That is the reason why the diagnostic power of mCT, SUV, and C/T ratio is seen in the adenocarcinoma group.
This study has several limitations. First, it was performed retrospectively. Second, concerning the type of surgery, limited resection was selected for high-risk patients; however, lobectomy is more desirable. Third, the cut-off values of the ROC curves that dichotomized the two groups could be an arbitrary value and not available universally. Fourth, the follow-up duration was rather short, even though the mean interval between the two groups, with and without recurrence, showed no differences (p = 0.52).