Risk assessment in relation to the detection of small pulmonary nodules

Abstract

The National Lung Cancer Screening trial (NLST) demonstrated that individuals assigned to the LDCT screening arm had a 20% lower mortality than those who were assigned to the conventional chest radiography. The NLST was thoroughly analyzed by the US Preventive Task Force on CT Screening and they recommended that lung cancer screening should be implemented. A number of other countries have also recommended implementation, whilst others are awaiting the outcome of the NELSON Trial. However, recommendations for the management of CT screen detected nodules have only recently had any clarity. The management of CT detected nodules in the NLST was based on the identification and reporting of 4 mm diameter nodules found on the CT screens but there was no NLST radiology protocol in place for the management of nodules. The use of volumetric analysis is not routinely used in the USA and there is still a reliance on utilising the CT nodule diameter as the management parameter. The first pulmonary risk model was developed by the Canadians, utilising data sets from the Pan-Canadian Early detection of Lung cancer (PanCan) and validated in the chemoprevention trial dataset at the British Columbian Agency. This Canadian model, known as the Brock Model, is currently available and has been integrated into the British Thoracic Society guidelines on the management of pulmonary nodules. The American College of Radiology setup a Lung Cancer Screening Committee subgroup on Lung-RADS, to standardize lung cancer screening CT reporting and provide management recommendations. However, it has been recommended that the Lung-RADS system should be revised as the system as it has never been studied in a prospective fashion. The NELSON trial introduced a third screening test, the "indeterminate" screening test result, this was done with the aim to reduce the false-positives CT screening results and also utilized by the UKLS trial successfully. On comparing the radiological CT screen volumetric and diameter based protocols in the NELSON trial, the sensitivity and negative predictive value appeared to be comparable, however a higher specificity and positive predictive value was found for the volume-based protocols, thus confirming the advantage of utilising the volumetric approach over diameter The British Thoracic Society (BTS) has undertaken an in-depth piece of work developing guidelines on the management of pulmonary nodules, utilising the wealth of data published by the NELSON team and support the use of volumetric analysis for the management of pulmonary nodules.

Keywords: Lung cancer; management; pulmonary nodules; risk prediction.

Figure 1

Comparison of the diameter and volume of CT screen detected nodules. (A) A volume growth of 26%, defined as growth by NELSON criteria, is hardly appreciable by diameter measurement (8% diameter increase which is NO growth by current criteria); (B) a 25% diameter increase i.e., threshold for the current growth definition reflects almost a doubling in volume (95%). It reflects the insensitivity for growth of diameter measurement. Reproduce from reference (3).

Figure 2

Contour plot of the effect of the combined effect of nodule volume and volume doubling time on 2-year lung cancer probability. The risk isolines represent the percentage of NELSON participants that will be diagnosed with lung cancer within 2 years according to the volume of their largest nodule and volume doubling time of the fastest growing nodule in the 50–500 mm³ range. Reproduce from reference (21).

Figure 3

Management of pulmonary nodules. Redrawn from reference (23).