Diffusion-weighted imaging (DWI) is a type of magnetic resonance imaging that makes it possible to identify malignancies by measuring differences in the diffusion of water molecules among tissues [Wu LM et al. Magn Reson Imaging. 2013]. Whether DWI is superior to fluorodeoxyglucose positron emission tomography (FDG-PET) at distinguishing malignant from benign pulmonary nodules and masses (PNMs) is uncertain. Jean-Paul Sculier, MD, Institut Jules Bordet, Brussels, Belgium, discussed a poster by Katsuo Usuda, MD, PhD, Kanazawa Medical University, Kanazawa, Japan, which described a study designed to compare the diagnostic performance of DWI and FDG-PET in distinguishing malignant from benign PNMs and clarify the advantages and disadvantages of these imaging modalities [Usuda K et al. Ann Oncol. 2015]. The investigators used both modalities to assess 143 lung cancers, 17 metastatic lung tumors, and 29 benign PNMs.

Using a receiver operating characteristic curve, the optimal cutoff value (OCV) of the apparent diffusion coefficient (ADC) value for diagnosing malignancy was determined to be 1.44 × 10^-3 mm²/sec. PNMs with an ADC value equal to or less than the OCV were assessed as positive for malignancy; PNMs with an ADC value higher than the OCV or that could not be detected on DWI were assessed as negative for malignancy. On this basis, 128 true positives, 19 true negatives, 32 false negatives, and 10 false positives were identified with DWI; 112 true positives, 19 true negatives, 48 false negatives, and 10 false positives were identified with FDG-PET. The sensitivity of DWI (80%) was significantly higher than that of FDG-PET (70%) for malignant PNMs (P = .0389). Prof Sculier said that by using Bayesian analysis, he determined that the positive likelihood ratio for malignant vs benign PNMs was 12.8 for DWI and 11.2 for FDG-PET.

Interestingly, Prof Sculier said that as the percentage of bronchioloalveolar carcinoma in adenocarcinoma increased, the sensitivity of FDG-PET decreased. He also noted that under the revised 2011 international multidisciplinary lung adenocarcinoma classification system [Travis WD et al. J Thorac Oncol. 2011], the designation of bronchioloalveolar carcinoma was discontinued and replaced by 5 new classification categories.

PET scanning is not valuable when the result of a chest x-ray is suspicious for cancer, Prof Sculier stated. He noted that the Fleischner Society recommendations for the management of subsolid pulmonary nodules detected by computed tomography state that FDG-PET is of limited value, potentially misleading, and therefore not recommended for solitary pure ground-glass nodules > 5 mm and pure ground-glass nodules > 5 mm without a dominant lesion [Naidich DP et al. Radiology. 2013].

In conclusion, Prof Sculier stated that evidence suggests that PET is not a good imaging modality in the diagnosis of lung cancer. He suggested that the study authors consider repeating their assessment by DWI and FDG-PET after reclassifying the study specimens according to their radiologic presentation.