Solitary Pulmonary Nodules



Solitary Pulmonary Nodules


Todd M. Blodgett, MD

Alex Ryan, MD

Carl Fuhrman, MD





Key Facts


Terminology



  • Solitary pulmonary nodule (SPN)



    • Opacity in the lung parenchyma measuring up to 3 cm with no associated mediastinal adenopathy or atelectasis


Imaging Findings



  • Risk of malignancy highest when nodule has SUV > 2.5 and spiculated morphology


  • PET/CT superior to CT or PET alone for overall accuracy


  • Nodules with internal calcifications generally benign


  • Central calcification is characteristic of benign nodules


  • Dual-time point imaging may be helpful in differentiating benign from malignant pulmonary nodules


Top Differential Diagnoses



  • Metastasis


  • Infection


  • Granulomatous Disease


  • Benign Lesions


  • Pulmonary Infarct


Pathology



  • Granuloma is the most common entity accounting for SPN


Diagnostic Checklist



  • Continued CT follow-up in PET-negative SPN


  • If nodule has features of BAC, a negative PET does not rule out malignancy


  • Predictive value of stability in size over time of a SPN is only 65%






Coronal PET (A) shows increased FDG activity in a right lower lobe pulmonary nodule image with areas of fat image and calcification image on axial CT (B, C), compatible with a hamartoma.






Axial CECT (B) shows an 8 mm spiculated adenocarcinoma image, which demonstrates only mildly increased FDG activity on coronal PET (A) and axial fused PET/CT (C).


TERMINOLOGY


Abbreviations and Synonyms



  • Solitary pulmonary nodule (SPN)


  • Bronchioloalveolar carcinoma (BAC)


Definitions



  • Opacity in the lung parenchyma measuring up to 3 cm



    • Usually no associated mediastinal adenopathy or atelectasis


IMAGING FINDINGS


General Features



  • Best diagnostic clue



    • High suspicion for malignancy



      • Any detectable FDG activity higher than background (> mediastinal blood pool) for SPN < 1.5 cm


      • SUV > 2.5 in any nodule


      • Spiculated morphology, particularly with a history of smoking


    • Low suspicion for malignancy



      • Round nodule with dense calcification and uniform morphology


      • FDG uptake equal to background activity


  • Location



    • No regional pattern for benign nodules


    • 2/3 of primary lung tumors arise in upper lobes


    • SPN from extrapulmonary primary most often located in outer 1/3 of lower lobes


  • Size



    • Definition: Nodule < 3.0 cm < mass


    • Larger SPN more likely malignant



      • Over 85% are cancer when larger than 2.0 cm


    • Growth rate



      • 26% increase in diameter corresponds to a doubling of the nodule’s volume


      • Time to 26% increase in diameter = one doubling time


      • Most cancer doubling times: ˜ 30-200 day range


      • Nodule dimension stability: > 2 years highly suggestive that nodule is benign


      • Increase in size seen within 30 days suggestive of infection, infarction, lymphoma, fast-growing metastases




  • Morphology



    • Benign characteristics



      • Margin: Well-circumscribed with smooth borders


      • Density: Fat or water density


      • Calcification: Common


      • Cavitation: Wall thickness < 5 mm


      • Enhancement: Usually minimal


      • Ground-glass opacity (suggests inflammation)


      • Air-fluid level (abscess)


      • Satellite nodules: Common in granulomatous lesions


    • Malignant characteristics



      • Margin: Irregular, lobulated, ill-defined with spiculated borders


      • Density: Soft tissue density


      • Calcification: 10% demonstrate calcification that is usually peripheral and stippled


      • Cavitation: Present in 80% of cavitary lung cancers (e.g., squamous cell carcinoma)


      • BAC may appear entirely as ground-glass opacity


      • Enhancement: More prominent


      • Spiculation highly specific for malignancy


      • Up to 20% of smooth nodules with sharp margins are malignant (e.g., carcinoid)


      • Air bronchogram: Present in 25-65% of cancers


      • Pseudocavitation: Common to malignancies such as BAC


      • Wall thickness > 1.5 cm strongly suggestive of malignancy


Imaging Recommendations



  • Best imaging tool: PET/CT demonstrates superior accuracy to CT or PET alone


  • Protocol advice



    • Dual-time point imaging may be helpful in differentiating benign from malignant pulmonary nodules



      • Malignant nodules gain intensity between hour 1 and hour 2


      • Benign nodules decrease in intensity


Radiographic Findings



  • Chest X-rays (CXR) helpful for determining time course of nodule development



    • Little change over 2 years or longer is strongly suggestive of a benign process


  • 1-2 SPN detected per 1,000 CXR, routine screening radiographs



    • CXR has low sensitivity for detection of subcentimeter noncalcified nodules


CT Findings



  • Indications



    • Accurate localization of nodule (intra-/extrapulmonary)


    • Detection of additional unsuspected nodules


    • Characterization of margin, density, and calcification patterns


    • Assessment of extrapulmonary involvement (lymph nodes, pleura, chest wall, liver, adrenals, etc.)


  • Malignant morphology



    • CT may misclassify 25-40% of nodules as benign based on morphologic characteristics



      • BAC and lymphoma, for example, often appear benign


    • Coarse spiculation and bronchovascular bundle thickening around tumor



      • More common in presence of vessel invasion &/or lymph node metastasis


    • Heterogeneous internal composition


    • Hazy or indistinct margins


    • Peripheral spiculation with halo


    • Pleural retraction adjacent to tumor


    • Necrosis


    • Extension to bronchi or pulmonary veins


  • SPN calcification characteristics



    • Malignant



      • Generally not calcified


      • Nodules with eccentric calcification cannot be classified as benign


      • Bone cancer, soft tissue sarcoma, and mucinous adenocarcinoma metastases may calcify



      • 1/3 of carcinoid tumors calcify


      • Colon and ovarian metastases may show psammomatous calcification


      • Internal hemorrhage may simulate calcification (melanoma and choriocarcinoma)


    • Benign



      • Central, laminated, popcorn, diffuse


      • Diffuse calcifications > 300 HU through nodule


      • > 1/2 granulomas are calcified


      • 1/3 of hamartomas have popcorn calcification


  • Ground-glass opacity (GGO)



    • GGO nodules are lower density than solid nodules and do not obscure lung parenchyma


    • 20% of lung nodules demonstrate this density



      • 34% of these are malignant


    • More difficult to distinguish malignant from benign disease based on morphology



      • Much higher incidence of malignancy among ground-glass and mixed opacity nodules


    • Bronchoalveolar carcinoma often demonstrates this density



      • Also adenocarcinoma with BAC features


    • Adenocarcinoma > 2 cm with > 50% GGO has low risk of lymph node metastasis and vessel invasion


  • Enhancement



    • Malignant nodules often hypervascular and highly enhancing


    • Generally, > 25 HU = malignant, and < 15 HU = benign


    • Insensitive for subcentimeter, cavitary, or necrotic nodules


  • Fat



    • Malignant: Liposarcoma, renal cell carcinoma metastases (uncommon)


    • Benign: Hamartoma, lipoid pneumonia


  • Air bronchograms



    • Caused by small airway distortion


    • More typical of malignant than benign nodules



      • Seen in 30% of malignant nodules and 6% of benign nodules


    • As much as 55% of BAC shows bubble-like lucencies = pseudocavitation


Nuclear Medicine Findings



  • PET



    • Significant overlap in FDG activity between benign and malignant nodules


    • SUV > 2.5 has sensitivity/specificity 90-100%, 69-95% for detection of malignancy


    • Detection depends largely on size



      • Lower resolution limit 6-8 mm


      • Partial volume averaging of small nodules can produce falsely low SUV


    • Bronchioloalveolar carcinoma has multifocal form that is often detected with FDG PET



      • Overall, BAC tends to have lower FDG uptake than other pulmonary malignancies


    • False positives



      • Focal hypermetabolic uptake unrelated to malignancy


      • Most common include infection, inflammatory reaction, granulomata, hamartoma


    • False negatives



      • Malignant subcentimeter nodules may not be detected on FDG PET


      • Hypometabolic tumors: BAC, carcinoid


      • Temporary decrease in FDG uptake of active lesions post-therapy (“stunned tumor”)


      • Ground-glass nodules often false negative due to size and association with BAC


    • PET provides prognostic information for malignant nodules

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Sep 22, 2016 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Solitary Pulmonary Nodules

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