From October 2000 to December 2007, a total of 14,040 lung MRI examinations were performed in 11,766 consecutive individuals, who were asymptomatic and had no prior cancer history in the lung or elsewhere. The mean age was 50.4 years (range, 11-94 years), and 56.8% of examinees were male. Never-smokers accounted for 89.2% and smokers for 10.8%. The median number of pack-years of smoking was 22.5 (range, 2-90). No patients had a history of asbestos exposure. Characteristics of the examinees are shown in Table . Of the 11,766 examinees, 10,160 (86.3%) underwent lung MRI once (only the prevalence examination), 1172 (10%) twice, and 434 (3.7%) three times or more. Of the 14,040 lung MRI studies, 5037 were performed without the use of a contrast agent and 9003 with a contrast agent. Table shows the interindividual comparison of lung MRI examinations. Contrasted MRI disclosed significantly more lung nodules, both benign and suspicious, than noncontrasted MRI (p < 0.001).
Characteristics of individuals undergoing lung magnetic resonance imaging examination
Interindividual comparison of noncontrasted and contrasted lung magnetic resonance imaginga
A total of 559 people (4.8%) were found to have suspicious lung nodules. Of the 559, 376 (67.3%) had nodules considered to be benign during follow-up and 138 (24.7%) were lost to follow-up. A total of 49 primary lung cancers were diagnosed in 46 examinees: 41 prevalence cancers and 8 incidence cancers. The rate of primary lung cancers detected by MRI was 0.4% (49 of 11,766). The detection rate was 0.9% (11 of 1266) for smokers, and 0.4% (38 of 10,500) for never-smokers (Table ). There was no significant difference of cancer detection rate between smokers and never-smokers (p = 0.11).
Distribution of detected primary lung cancers by magnetic resonance imaging
Of the 49 lung cancers, the mean age of the individual at diagnosis was 60.8 years (range, 36-87 years), and 25 (51.0%) were in males. Eleven cases were in people who had smoked, and the median number of pack-years of smoking was 35 (range, 10-80). Characteristics of individuals with detected primary lung cancers are shown in Table . Suspected lung cancers were subjected to various procedures for proper histopathological documentation. Thoracotomy (including video-assisted thoracoscopic surgery) was performed for 40 cancers, lobectomy for 33 cancers, wedge resection for 6, and segmentectomy for 1. Three cancers were histologically proved by bronchoscopic biopsy, 1 by CT-guided lung biopsy, 1 by sputum cytology, 1 by biopsy at supraclavicular lymph node, and 1 by endoscopic biopsy at metastatic duodenal lesion. Two individuals refused any procedures or therapies and later developed rib metastases or progressive pulmonary nodules. Forty cancers had pathological staging, and 9 had clinical staging. Twenty-five cancers were TNM stage Ia disease (51.0%), 12 stage Ib disease (24.5%), 4 stage IIIa (8.2%), 2 stage IIIb (4.1%), and 6 stage IV disease (12.2%). The mean size of detected lung cancers was 1.98 cm (median, 1.5 cm; range, 0.5-8.2 cm). Thirty-six cancers (73.5%) had a primary tumor measuring ≤3 cm, and 29 (59.2%) ≤2 cm. Histological types were as follows: 38 adenocarcinoma (77.6%), 5 bronchioloalveolar carcinoma (10.2%), 2 large cell carcinoma (4.1%), 1 squamous cell carcinoma (2.0%), and 1 carcinoid tumorlet (2.0%). There were 3 female individuals with synchronous double primary lung cancers. One biopsy proved to be non-small cell lung carcinoma with no primary tumor origin in the lungs when retrospectively and prospectively assessed. Two cases did not have tissue proof because the individuals refused any procedures.
Of the 8 incidence cancers, 5 were diagnosed at the second MRI examination, with a mean interval of 3.3 years (range, 2-5.5 years). Two cancers were diagnosed at the third annual MRI examination. The comparison of prevalence cancers and incidence cancers is shown in Table . The mean size of the incidence cancers was 1.8 cm, compared with 2.0 cm for the prevalence cancers.
Comparison between prevalence lung cancers and incidence lung cancers by magnetic resonance imaging examination
Follow-up (duration, 2-9 years) for the 49 cancers showed 8 deaths due to lung cancer, 2 stage IIIa cancers, 2 stage IIIb cancers, and 4 stage IV cancers. One stage Ib individual developed recurrent tumors 2 years later and underwent pulmonary resection again. Thirty-three individuals (67.3%) were alive and free of disease at the time of this writing.
Eight false-positive individuals whose standard-dose CT also suggested malignancies underwent invasive procedures: 2 CT-guided biopsies and 6 thoracotomies (including video-assisted thoracoscopic surgery). Infectious diseases were histologically confirmed in these 8 people, with cryptococcal infections in 6.