Lung cancer screening: nodule identification and characterization

Abstract

The accurate identification and characterization of small pulmonary nodules at low-dose CT is an essential requirement for the implementation of effective lung cancer screening. Individual reader detection performance is influenced by nodule characteristics and technical CT parameters but can be improved by training, the application of CT techniques, and by computer-aided techniques. However, the evaluation of nodule detection in lung cancer screening trials differs from the assessment of individual readers as it incorporates multiple readers, their inter-observer variability, reporting thresholds, and reflects the program accuracy in identifying lung cancer. Understanding detection and interpretation errors in screening trials aids in the implementation of lung cancer screening in clinical practice. Indeed, as CT screening moves to ever lower radiation doses, radiologists must be cognisant of new technical challenges in nodule assessment. Screen detected lung cancers demonstrate distinct morphological features from incidentally or symptomatically detected lung cancers. Hence characterization of screen detected nodules requires an awareness of emerging concepts in early lung cancer appearances and their impact on radiological assessment and malignancy prediction models. Ultimately many nodules remain indeterminate, but further imaging evaluation can be appropriate with judicious utilization of contrast enhanced CT or MRI techniques or functional evaluation by PET-CT.

Keywords: Positron emission tomography-computed tomography (PET-CT); computer-aided detection (CAD); dynamic contrast CT; dynamic contrast magnetic resonance imaging (MRI); early lung cancer; lung cancer screening; maximum intensity projections (MIPs); missed nodules; nodule characterization; nodule detection; nodule enhancement study; reader sensitivity; risk models; screening sensitivity.

Figure 1

Simulated subsolid middle lobe nodule (A) is characterized as partial volume averaging of plate atelectasis on sagittal reconstructions (B).

Figure 2

Inconspicuous central nodule. (A) Missed central 9-mm irregular nodule (arrow) due to non-differentiation from central vessels, a common human reader deficiency; (B) significant growth is noted in the lesion 1 year later with new mediastinal nodes due to progressive adenocarcinoma.

Figure 3

Two-millimeter axial images (A,C) and corresponding 10-mm MIP images (B,D) in the same patient demonstrate the increased conspicuity of a peripheral solid 2–3 mm nodule at MIP but the reduced conspicuity of a 12-mm ground-glass nodule. MIP, maximum intensity projection.

Figure 4

Typical appearances of a right lower pulmonary hamartoma demonstrating both popcorn calcification and macroscopic fat.

Figure 5

Central pulmonary lesion demonstrates appearances suggestive of popcorn calcification but without fat (A). ¹⁸F-FDG-PET-CT demonstrates moderately increased metabolic activity (B). Biopsy demonstrated a typical carcinoid, indicating that popcorn type calcification may not be entirely specific for benign lesions.

Figure 6

Small left lower lobe nodule demonstrates central calcification on soft kernel reconstructions consistent with a benign nodule (A). Sharp reconstruction kernel misrepresents calcium distribution, suggesting additional peripheral calcification (B).

Figure 7

Broad based smooth nodule with obtuse margins to the adjacent right major fissure shows stability at baseline, 1 and 2 years (A,B,C) consistent with a benign perifissural lymph node.

Figure 8

Small nodule adjacent to the left major fissure demonstrates acute margins to the fissure and does not satisfy criteria for an intrapulmonary lymph node (A). Follow-up at 3 months demonstrates rapid growth of this peripheral pulmonary penile cancer metastasis (B).

Figure 9

Spiculated upper lobe nodule with tags to the pleural surface demonstrates multiple features of malignancy at baseline (A), however, regression at 3 (B) and then 6 months (C) highlights that small benign inflammatory lesions can simulate early cancers.

Figure 10

Screening identifies small cancers that may have limited concerning features at baseline. Left upper lobe nodule measures 5.8 mm at baseline, a borderline positive screen according to Lung-RADS with only 1–2% risk of malignancy (A). Follow-up at 6 months demonstrates growth to 7.5 mm (C). The nodule was detected and assessed by CAD on both occasions (B,D), supporting progression to resection with final diagnosis of a non-mucinous adenocarcinoma. CAD, computer-aided detection.

Figure 11

Baseline image (A) demonstrates minor thickening and nodularity of a peripheral cystic airspace. In view of concern of this morphology, earlier follow-up at 3 months was performed (B). The lesion demonstrated progressive thickening and nodularity around the retracting cystic space a recognised early presentation of lung cancer. Unusually, biopsy demonstrated a non-keratinising squamous cell carcinoma cell type.