a pulmonary nodule is defined as a spherical well - circumscribed radiographic opacity that is surrounded completely by aerated lung .
lesions that are larger than 3  cm are described as lung masses . as size has been shown to be important for classification of nodules ( see below ) the term 
micronodules is usually applied to very small nodules ( < 7  mm , < 5  mm ) which are almost always multiple and diffuse .
if only one pulmonary nodule is detected , it is described as a solitary pulmonary nodule ( spn ) .
if a nodule is detected in an examination performed for other reasons than a search for pulmonary nodules , it is called an  incidental nodule , .
most of the information on the aetiology and the natural course of incidentally found small pulmonary nodules is derived from studies on lung cancer screening with unenhanced low - radiation dose computed tomography ( low - dose ct ) in asymptomatic smokers and to a lesser extent non - smokers and workers exposed to asbestos . in these studies , at least one non - calcified pulmonary nodule was found in up to 66% of subjects , depending on the imaging technique ( e.g. multidetector - computed tomography ( ct ) versus single - detector ct ) .
it was also shown that > 95% of these nodules were 10  mm in diameter and of these > 95% were benign .
therefore , in order to avoid a high rate of invasive procedures for benign lesions in small nodules , algorithms were proposed that are based on mostly non - invasive strategies and , in particular , follow - up with low - dose ct . during follow - up changes in size ( or even better volume ) are recorded .
nodules that decrease in size or resolve are obviously regarded as benign and no further follow - up is required .
nodules that increase in size are likely malignant , particularly , if the growth rate is typical of malignant growth .
most malignant tumors have been shown to double their volume ( i.e. increase their diameter in spherical lesions by 26% ) between 30 and 400 days .
if malignancy is suspected , invasive procedures are usually performed to obtain a histological diagnosis ( bronchoscopic , percutaneous or thoracoscopic biopsy ) .
in addition , positron emission tomography ( pet)-ct may be useful for small nodules of intermediate size ( 610  mm ) , as many malignant nodules show increased uptake of [ f]fluorodeoxyglucose ( fdg ) , whereas most benign nodules do not .
pet - ct , however , is often negative in well - differentiated adenocarcinoma and bronchioloalveolar carcinoma ( bac ) and may be positive in inflammatory nodules ( e.g. sarcoid , tuberculosis ) . 

figure 1three non - solid nodules ( ground glass opacities ) in the posterior segment of the right upper lobe .
three non - solid nodules ( ground glass opacities ) in the posterior segment of the right upper lobe .
if the density of a nodule is lower than soft tissue attenuation , it does not obscure adjacent or transgressing vessels .
part - solid nodules are characterised by a mixture of ground glass attenuation and solid components .
benign non - solid nodules are due to inflammation ( eosinophilic pneumonia , cryptogenic organizing pneumonia ) , focal haemorrhage ( pulmonary endometriosis , pulmonary trauma , post biopsy ) , and focal interstitial fibrosis .
persistent non - solid lesions , in contrast , mostly represent neoplasms , predominantly malignant ones . in a study by nakata et al .
54% of persistent ggos represented bronchioloalveolar carcinoma , 26% adenocarcinoma with mixed bronchioloalveolar carcinoma components and 21% atypical adenomatous hyperplasia .
the proportion of malignant lesions differs between solid , part - solid and non - solid lesions . in a study by henschke et al .
7% of solid , 63% of part - solid and 18% of non - solid lesions were malignant .
the majority of part - solid and non - solid malignant lesions were bronchioloalveolar carcinomas and adenocarcinomas with bronchioloalveolar features .
noguchi et al . have described a classification of adenocarcinomas based on their histology which is reflected by their prognosis and this is also reflected by their thin - section ct morphology .
types a and b represent bronchioloalveolar carcinomas , are characterised by a 100% 5-year survival rate and appear as non - solid lesions at thin - section ct , whereas types c , d , e and f represent more aggressive bacs or adenocarcinomas , exhibit part - solid or solid appearances with a less favourable prognosis . 

peripheral ill - defined lesion with mixed density ; the more peripheral component exhibits ground glass attenuation , whereas the more central part exhibits soft tissue ( solid ) attenuation .
peripheral ill - defined lesion with mixed density ; the more peripheral component exhibits ground glass attenuation , whereas the more central part exhibits soft tissue ( solid ) attenuation . another non - solid lesion which was just recently defined is atypical adenomatous hyperplasia ( aah ) .
this lesion is recognized as a separate entity which in itself is not malignant but may be a precursor of adenocarcinoma .
it is characterised by proliferation of atypical cuboidal or columnar epithelial cells along the alveoli and respiratory bronchioles without invasion of the stroma or adjacent structures . at ct
most of the recommendations for the management of solid nodules , also apply to non - solid and part - solid lesions .
for example , the risk of malignancy should be estimated , previous imaging should be reviewed , and if growth is detected biopsy should be performed .
there are , however , some recommendations unique to non - solid or part - solid nodules : 
  if such a nodule is malignant the underlying lesion is likely to be bronchioloalveolar carcinoma or adenocarcinoma with bronchioloalveolar features. these may grow more slowly than other malignant tumors .
therefore , the recommendation of follow - up after 3 , 6 , 12 and 24 months and no further follow - up when no growth is demonstrated in 2 years should not be applied to non - solid and part - solid lesions .
these require longer follow - up to exclude growth for several years . because of the cumulative radiation exposure , dose levels during follow - up should be as low as possible. as bronchioloalveolar carcinoma and adenocarcinoma with bronchioloalveolar features may be negative at pet , examination with pet - ct should not be performed routinely as a negative result does not exclude malignancy .
 if such a nodule is malignant the underlying lesion is likely to be bronchioloalveolar carcinoma or adenocarcinoma with bronchioloalveolar features .
therefore , the recommendation of follow - up after 3 , 6 , 12 and 24 months and no further follow - up when no growth is demonstrated in 2 years should not be applied to non - solid and part - solid lesions .
these require longer follow - up to exclude growth for several years . because of the cumulative radiation exposure , dose levels during follow - up should be as low as possible .  as bronchioloalveolar carcinoma and adenocarcinoma with bronchioloalveolar features may be negative at pet , examination with pet - ct should not be performed routinely as a negative result does not exclude malignancy .
non - solid pulmonary nodules are more likely malignant than solid nodules . if malignant they are usually due to atypical adenomatous hyperplasia and bronchioloalveolar carcinoma .
these lesions tend to grow more slowly than other malignant pulmonary lesions and are often not hypermetabolic at fdg - pet .
therefore , follow - up to exclude growth in non - solid nodules needs to be longer than in solid lesions and pet - ct is not appropriate to rule out malignancy in non - solid nodules .
 abstractthis article describes the aetiology , epidemiology and clinical significance of incidental non - solid pulmonary nodules . 
 non - solid nodules are more likely malignant . 
 if malignant , they are mostly due to atypical adenomatous hyperplasia and bronchioloalveolar carcinoma . 
 as these may be negative on positron emission tomography and slow growing , the diagnostic algorithms that are used for solid nodules have to be modified for non - solid nodules . 
Introduction
Algorithms in incidental SPN
Non-solid nodules
Algorithms in non-solid nodules
Conclusions