Improvement in imaging diagnosis technique and modalities for solitary pulmonary nodules: from ground-glass opacity nodules to part-solid and solid nodules

SUMMARY

With advances in CT technology and the popularity of low-dose CT as a device for lung cancer screening, the detection rate of sub-solid pulmonary nodules as well as solid nodules has been increased. Distinguishing solid from sub-solid features is an essential step in the CT evaluation of solitary pulmonary nodules (SPNs) because strategies for nodule characterization and guidelines for management are different for each category. In addition to conventional CT parameters, numerous novel concepts and modalities have been developed. Although there is currently no single effective method for differentiating malignant from benign nodules, growth rate measurement using volumetry, evaluation of tumor vascularity on dynamic helical CT, dual-energy CT and MRI and physiologic evaluation with PET/CT can all be useful for nodule characterization. New techniques such as tomosynthesis can improve detection over radiography alone. The purpose of this article is to enhance our understanding of the evidence-based strategies involved in diagnosing SPNs.

KEYWORDS:

• Chest imaging
• CT
• PET/CT
• MRI
• tomosynthesis
• biopsy
• solitary pulmonary nodule

Acknowledgements

The authors would like to thank Myung Jin Chung (Department of Radiology, Samsung Medical Center, Sungkyunkwan University School of Medicine) for the provision of images.

Financial and competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Key issues

• Advances in CT technology have enabled increased detection rates of sub-solid nodules, increasing our awareness of the importance of these sub-solid nodules, based on the CT correlation of part-solid nodules with the spectrum (in terms of malignancy grade and prognosis) of peripheral adenocarcinomas of the lung.

• Discrimination of solid and sub-solid nodules is an essential step in CT evaluation because strategies for nodule characterization and guidelines for management are different between the categories.

• For adequate evaluation of sub-solid nodules, thin-section scanning (usually less than 1.5 mm in section thickness) and reconstruction should be performed.

• Factors such as size, contour, calcification, presence of fat, multiplicity, and cavity formation are universally adopted parameters in the evaluation of SPNs.

• In the evaluation of non-solid GGOs, size is a crucial factor in distinguishing benign from malignant nodules.

• Although malignant nodules grow faster than benign nodules, malignant sub-solid or non-solid nodules generally grow slowly, necessitating a longer follow-up period of more than 2 years.

• The evaluation of tumor enhancement with dynamic helical CT, especially with the aid of wash-in and wash-out characteristics, proved useful in the differentiation of malignant and benign SPNs. State-of-the-art techniques for nodule perfusion characteristic evaluation is the dynamic first-pass contrast-enhanced perfusion area-detector CT technology.

• PET/CT is an established modality for lung cancer staging and shows prominent sensitivity and accuracy for SPN characterization. More sophisticated methods of PET/CT imaging have been developed.

• MRI has the advantage of being a radiation-free imaging modality; however, the size-threshold problem for nodule detection is still higher compared with that of CT. PET/MRI can be used with enhanced soft-tissue contrast and with significantly reduced radiation exposure.

• For tissue confirmation, percutaneous transthoracic needle aspiration or biopsy can be attempted with high diagnostic yield. VATS with various localization methods can be utilized for small lung nodule resection.