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Practical Approach HRCTJud W Gurney MD FACR
Charles A Dobry Professor of Radiology
University of Nebraska
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HRCT Technique
Maximize spatial resolution
thin collimation
high spatial algorithm
Advantages over chest x-ray
no summation
high contrast
There is no magic with HRCT. The basic premise issimple, maximize spatial resolution by using thethinnest collimation and a high spatial frequencyalgorithm. This use to be 1.5 mm. Moderntechnology has cut the slice thickness in half andwith multi-detector CT any scan can be a highresolution scan. Even without high resolution, CThas inherent advantages over chest radiography.
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Secondary pulmonary lobuleTerminal bronchiole
Pulmonary artery
Septa
Pulmonary vein
Secondary pulmonary lobule
1 - 2.5 cm
Acinus
RB
A quick review of anatomy. One of the buildingblocks of the lung is the secondary pulmonarylobule. Lobules are relatively large: 1 to 2.5 cm, thesize of a good thumbprint. The lobule is fed by aterminal bronchiole and artery, the periphery ispartly septated. In between are the primary lobulesfed by respiratory bronchioles where gas exchangetakes place. Radiographically we can identify thelargest of the arteries and airways and the septa.Veins run in the septa. There are 2 lymphaticsystems, an axial system travels downs the arteriesand airways, terminating at the terminal bronchiolesand a peripheral system that travels along the septaand veins. The yellow and gray rings are thecentriacinar portion of the lobule.
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HRCT dried lung specimen. The larger terminalarteries and airways are clearly visible as are theoccasional septa. However, the respiratorybronchiole and acinus are not resolvable.
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In this fresh autopsy lung, the black pigmentationmarks the centriacinar portion of the lobule.Anthracotic particles (environmental soot) isdeposited in the respiratory bronchioles. Notice theoff-axis location of the respiratory bronchiolescompared to the core bronchovascular structure.
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Diffuse Lung Disease
SarcoidLangerhansIdiopathicDesquamativeCoal workersBagassosisSilicosisDrug reactionAsbestosisLymphangiticErdheim ChesterLymphangiomyomatosisChronic failureFarmer’s lungAnkylosing SpondylitisNeurofibromatosisRheumatoidScleroderma
Hard metal disease
Emphysema
PAP
Aspiration
Gauchers
Sjogrens
LIP
Mushroom workers lung
Methotrexate lung
Respiratory bronchiolitis
BOOP
Periarteritis nodosa
Lupus
Bronchiectasis
Berylliosis
Bronchiolitis obliterans
Alveolar microlithiasis
Metastatic calcification
The number of insults that may give rise topulmonary disease is large anywhere from 100-150.It might seem a hopeless task to sort out thisdifferential. Once when I was doing a locums I cameacross a report that read: “the patient has diffuseinterstitial lung disease. Please stop by my office toreview the list of 150 possibilities”
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Most common diagnoses
1. UIP
2. Silicosis
3. Sarcoid
4. Lymphangitic ca
5. Farmers lung
1. IPF
2. Farmers lung
3. Sarcoid
4. Asbestosis
5. Boop
6. Langerhansgranulomatosis
80% LondonPadley
83% VancouverMatheson
In reality, the number of problems that we deal withare a lot less. For example, this small list of diseasesaccounted for over 80% of the diagnosis at topreferral centers in England and North America.While the spectrum of disease differs slightly, suchas asbestos in England and silicosis in BritishColumbia, the number of diseases one has toconsider is quite small.
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Round up the usual suspects
SarcoidLangerhans granulomatosisIdiopathic pulmonary fibrosisLymphangitic tumorEdemaAsbestosisCollagen vascular diseasesSilicosisFarmer’s lungDrugs
More than 100 entities manifest as diffuse lung disease. Fortunately only 10-20 of these account for about 90% of all diffuse lung diseasethat is assessed by open lung biopsy.
DeRemee Chest 92, 1987
Combining these 2 lists and my own practicalexperience, these 10 processes account for over 90%of what I see in my daily practice. The theme herecomes from the famous scene in Casablanca afterRic has shot the German general. Louie, the Frenchpolice captain, has to make a decision as to whichside he will choose. When his lieutenants comerunning he utters the line; “ the general has beenshot, round up the usual suspects”. This has becomea running joke among the pulmonologists I workwith as this is our approach. When we have a newpatient with interstitial lung disease we look for theusual suspects before considering a more exoticdisorder.
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Sarcoidosis
Common systemic disease unknown etiology
May follow treatment lymphoma
Noncaseating granulomas
Most good prognosis, resolves within 2 years
We will start with sarcoidosis, one of thecommonest of the interstitial lung diseases. Wedon’t know what causes sarcoid, however, onecurious bit of evidence is that sarcoid may followtreatment of lymphoma. It is important to recognizethis possibility as the development of interstitiallung disease and adenopathy in a patient with ahistory of lymphoma is usually equated withrecurrence and not a new disease. The pathologicfinding is the noncaseating granuloma that isprimarily located in the lung lymphatics. Most withsarcoid do well, only a small minority go on todevelop end-stage disease.
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Sarcoidosis
95% abnormal chest radiograph
Symmetric hilar adenopathy
Lung disease (<50%), often worsens with nodalregression
Most have an abnormal chest radiograph. Classicfindings of symmetric hilar adenopathy may befound alone or associated with lung disease. One ofthe distinguishing features in sarcoid is theparadoxical response that as the lung diseaseworsens the nodal enlargement usually regresses.
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Sarcoidosis
Alveolar sarcoid
Airspace masses
Air bronchograms
Usually upper lobes
Hilar adenopathy
The least common form of lung disease is alveolarsarcoid. Typically, there are several large air spacemasses that contain air bronchograms. Usually themasses are larger or more extensive in the upperlung zones. Alveolar sarcoid is usually associatedwith adenopathy as in this case. In addition, thealveolar sarcoid pattern is at high risk forspontaneous pneumothorax.
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Sarcoidosis
Ground glass opacities
Centriacinar nodules
Follow lymphatics (septa,bronchi, blood vessels)
The typical features of sarcoid at HRCT vary.Micronodules are typically located alongbronchovascular bundles, fissures and pleuramirroring the distribution of lymphatics in the lung.
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SarcoidosisFibrosis cuts a swath through lung
“Sherman’s march to the sea”
More advanced fibrosis follows the bronchi andarteries and cuts a swath through the lung from thehilum to the lung periphery. Remember, if you seesomething which looks like General Sherman’smarch to the sea, think sarcoid.
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Langerhans granulomatosis
Probable allergic reaction to cigarette smoke
Granulomas Langerhans cell
25% present spontaneous pneumothorax
2/3rds spontaneously resolve
Next on our list is langerhans cell granulomatosis orhistiocytosis. The etiology is thought to be anallergic reaction to some constituent of cigarettesmoke. The granulomas in histiocytosis contain thelangerhans cell, an antigen processing cell normallyfound in the lung and reticuloendothelial system.Usually affecting young adults, 25% of patients withlangerhans histiocytosis present with a spontaneouspneumothorax. The natural history is unknown butmost spontaneously resolve.
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Langerhans granulomatosis
Mid to upper lobe lung disease
Nodules and cysts
Increased lung volumes
Burned out disease looks like emphysema
The findings of langerhans typically involve the midto upper lung. As we will see shortly, the findingsare combinations of either nodules or cysts. Theearliest findings are thought to be nodules whicheventually evolve into cysts. In contrast to mostinterstitial lung diseases, the lung volumes are eithernormal or increased. Burned out disease may beindistinguishable from emphysema.
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Langerhans granulomatosis
Spontaneous ptx
Biapical coarsereticulonodular pattern
Increased lung volumes
Here is a typical example. Biapical disease with acoarse cystic pattern. The lung volumes are normalto slightly increased. A chest tube on the right wasplaced for a spontaneous pneumothorax.
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Langerhans granulomatosis
Centriacinar micronodules
Respiratory bronchioles
Lung periphery spared
Just like sarcoid, a common pattern at HRCT arecentriacinar micronodules. In contrast to sarcoid,however, the periphery is spared. Judge the lungalong the chest wall and fissues. If there are only ahandful of subpleural nodules then consider theperiphery to be spared.
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Langerhans granulomatosis
Star-fished shapednodules
One feature which I find useful is the shape of thelarger nodules. The may have 3-5 arms or tentacles,similar to the shape of a starfish.
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Langerhans granulomatosis
Nodules cavitate
Aggregate into bizarreshapes
As the nodules get larger they cavitate andeventually aggregate into very bizarre shapes. Thisis distinctive and nearly pathognomonic forlangerhans histiocytosis. This example comes fromthe chest x-ray that we saw earlier. Note that thecystic pattern was not as evident on the chestradiograph, the value of eliminatingsuperimposition.
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Respiratory vs lymphatic nodules
Sarcoidosis Langerhans
In both sarcoid and langerhans cell histiocytosis, oneof the earliest findings are the tiny micronodules. Insarcoid, the granulomatous nodules are locatedwithin the lymphatics and will involve both the axialand peripheral lymphatic systems. This wlll lead tomultiple subpleural nodules along the chest wall andfissures. Langerhans cell granulomatous is a diseaseof the airways. Particles in cigarette smoke depositin the respiratory bronchioles where they incite agranulmatous response. The periphery of the lobulewill be spared and the number of subpleural noduleswill be few. This leads to the concept of arespiratory nodule vs a lymphatic nodule. I find thisconcept useful in formulating a differentialdiagnosis.
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Any patternAny patternDrugs
Nodules, (respiratory)
Nodules, (lymphatic)
Farmers lung
Silicosis
Environment dusts
Reticular, honeycominglLower lobe (periphery)Collagen vasculardisease
Reticular, honeycomgingLower lobe (periphery)Asbestosis
Smooth septalBatwing, dependent lungChronic edema
Irregular septalSpares lobe or lungLymphangitic tumor
Reticular, honeycombingLower lobe (periphery)IPF
Nodules (respiratory)
Bizarre shaped cysts
Upper lobeLangerhans
Nodules (lymphatic)
Fibrotic swath
Upper lobeSarcoid
PatternDistribution
Our first two diseases share many similarities bothin distribution and pattern. However, particularattention should be directed as to whether theperiphery of the lung is involved. Sarcoid has atendency to cut a swath through the lung andhistiocytosis to evolve into bizarre shaped cysts.
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Idiopathic pulmonary fibrosis
Unknown repetitive insult to alveolar wall
Collagen deposition and fibrosis
Gradual onset dyspnea and dry cough
Variable outcome, median survival 5 yrs
Next is IPF. The concept that IPF is an alveolitis isnow discredited with the belief now that there is arepetitive insult to the alveolar wall that results incollagen deposition and fibrosis. Patients usuallyhave the gradual onset of nonspecific symptoms.The prodromal period is unknown, once diagnosedpatients have a median survival of about 5 years.
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Idiopathic pulmonary fibrosis
Peripheral basilar lungdisease
Reticular lines andhoneycombing
Decreased lung volumes
In contrast to our first two disorders, the distributionof disease is predominantly in the periphery of thelower lobes. The pattern is reticular not nodular.Honeycombing or end stage lung is common.Typically the lung volumes are decreased andparallel the restrictive pulmonary function tests.
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Idiopathic pulmonary fibrosis
Ground glass opacities
Irregular lines, septalthickening
Honeycombing
Traction bronchiectasis
Ground glass opacities in IPF are nonspecific. Theseopacities may be due to atelectasis or fibrosis whichis below the resolution of the CT scanner. Thelobule may be crisscrossed by a network of finelines. One important characteristic is the presence oftraction bronchiectasis which must be present beforeI can confidently diagnosis IPF.
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Any patternAny patternDrugs
Nodules, (respiratory)
Nodules, (lymphatic)
Farmers lung
Silicosis
Environment dusts
Reticular, honeycominglLower lobe (periphery)Collagen vasculardisease
Reticular, honeycomgingLower lobe (periphery)Asbestosis
Smooth septalBatwing, dependent lungChronic edema
Irregular septalSpares lobe or lungLymphangitic tumor
Reticular, honeycombing
Traction bronchiectasis
Lower lobe (periphery)IPF
Nodules (respiratory)
Bizarre shaped cysts
Upper lobeLangerhans
Nodules (lymphatic)
Fibrotic swath
Upper lobeSarcoid
PatternDistribution
IPF is distinctive from the previous disorders,particularly with regards to distribution and pattern.
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Lymphangitic tumor
Carcinoma lung, breast, and GI tract
Permeation of lymphatics by neoplasia
Progressive dyspnea and cough
Prognosis poor
Lymphangitic spread of tumor may arise from manytumors, especially adenocarcinomas. Tumor spreadsto the lymphatics either hematogenously orretrograde from hilar and mediastinal lymph nodes.Lymphangitic spread is a late finding in the naturalhistory of tumor spread and usually portends a poorprognosis.
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Lymphangitic tumor
Not diffuse, usually spareslobe or lung
Lung volumes preserved
Adenopathy or pleuraleffusions
Lymphangitic tumor is usually not diffuse but tendsto spare lobes or even a whole lung. In this example,the left lower lobe was completely spared. Evenwith extensive permeation of lymphatics, lungvolumes are preserved. Patients may or may nothave adenopathy or small pleural effusions.
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Lymphangitic tumor
Nodular thickening axial(bronchovascular) andperipheral (septal) lymphatics
Lymphangitic spread may involve either the axial orperipheral lymphatics or both. In general, axialinvolvement is more common. In this example,lymphangitic spread from colon carcinoma involvedthe right middle lobe only. A characteristic featureof lymphangitic tumor is irregular or nodularthickening of the septa as is noted in this case.
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Edema
Occult edema not uncommon
Extravascular water: ground glass opacities,septal thickening
Orthopnea and dyspnea
Enlarged heart
It might seem surprising that edema is included inthe differential of diffuse lung disease. However, itis surprising how many patients get referred to apulmonary specialist with edema. Most specialiststreat new referrals with a diuretic prior to furtherworkup just in case that their end stage lung maydisappear. The findings of edema: ground glassopacities and septal thickening are identical to thoseof diffuse lung disease. Patients may not give ahistory of orthopena and dyspnea and while anenlarged heart is often found, some may not haverecognizable cardiomegaly.
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Edema
May be chronic, labeled asdiffuse lung disease
Most pulmonologists treatreferrals with ILD withdiuretics before furtherinvestigation
Here is just such a problematic case. Moderatecardiomegaly and mild diffuse interstitial thickeninghad not changed over multiple films obtained overseveral months.
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Edema
Gravitational gradient
Smooth septal thickening
Spares lobules (mosaicperfusion)
One clue to edema is the gravitational distribution.Since many of these patients are upright, groundglass opacities and septal thickening are more severein the lung bases than in the upper lobes. Anothercurious unexplained findings is sparing of lobules asis seen in this patient. From animal experiments, atany given time there are some lobules that are notperfused due to a local autoregulatory mechanism.Starling forces would not affect lobules that are notperfused.
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SeptaLymphangitic tumor Edema
Both lymphangitic tumor and edema involve thesepta. In lymphangitic tumor the septa areirregularly thickened and beaded whereas in chronicedema the septa are smoothly thickened.
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Any patternAny patternDrugs
Nodules, (respiratory)
Nodules, (lymphatic)
Farmers lung
Silicosis
Environment dusts
Reticular, honeycominglLower lobe (periphery)Collagen vasculardisease
Reticular, honeycomgingLower lobe (periphery)Asbestosis
Smooth septalBatwing, dependent lungChronic edema
Irregular septalSpares lobe or lungLymphangitic tumor
Reticular, honeycombing
Traction bronchiectasis
Lower lobe (periphery)IPF
Nodules (respiratory)
Bizarre shaped cysts
Upper lobeLangerhans
Nodules (lymphatic)
Fibrotic swath
Upper lobeSarcoid
PatternDistribution
The distribution of our last two entities is unique. Inaddition there are subtle differences in the septa,irregular and beaded in lymphangitic tumor anduniform and smooth in chronic edema.
34
Asbestosis
Pneumoconiosis from fibrous silicates
Latent period 20 - 30 years
Fibrosis + asbestos bodies = asbestosis
High proportion die of lung cancer
Asbestos has been widely used in industry becauseof it’s advantageous fire-retardant properties ininsulation. Asbestosis is only 1 manifestation ofasbestos exposure which includes pleural plaques,diffuse pleural thickening, and mesothelioma. Thepathologic signature in asbestosis is the asbestos orferrugenous body. The asbestos body is an asbestosfiber surrounded by dead macrophages which stainfor iron. Asbestos is carcinogenic and combinedwith smoking, the risk of developing lung cancer ismultiplicative, that is higher than the risk of eitheragent alone.
35
Asbestosis
Peripheral and basilar
Irregular reticular opacites
Small lung volumes
25% asbestos plaques
Identical to IPF, asbestosis primarily involves theperiphery of the lower lobes. The pattern is alsoidentical, irregular reticular opacities andhoneycombing. One clue is the asbestos plaque,however, plaques are seen in only 25% of those withasbestosis.
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Asbestosis
Irregular short septal lines
Long parenchymal bands
Centriacinar nodules
Honeycombing
At HRCT, short irregular lines extend from thecentriacinar region to the lung periphery. Longerthicker parenchymal bands may be found in moresevere cases. End-stage honeycombing when severe.
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Collagen vascular disease
Generalized connective tissue disease
Cellular or fibrotic (NSIP or UIP)
Dyspnea
Variable prognosis
Diseases such as rheumatoid arthritis andscleroderma are also associated with diffuseinterstitial thickening. Pathologically, there may be aUIP or NSIP pattern, indistinguishable from IPF.Dsypnea is a common complaint. Prognosis isvariable and life-expectancy is longer with NSIPthan UIP.
38
Collagen vascular disease
Peripheral basilar
Honeycombing
Decreased lung volumes
Dilated esophagus(scleroderma)
In this example, the peripheral basilar interstitialthickening is identical to IPF. However, the tip-off isthe dilated esophagus in this patient withscleroderma.
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Collagen vascular disease
Wide specturm: ground glass,micronodules, honeycombing
NSIP: uniform pattern
Again, at HRCT ground glass opacities are commonbut nonspecific. The key is the distribution and thepresence of intralobular lines or honeycombing.NSIP is a newer pathologic designation, common inthe collagen vascular diseases. NSIP may be eithercellular or fibrotic, a wide gamut. The key in NSIPis that the pathologic fields will be the same from allsampled sites. That implies that the insult occurredat the same time. In contrast, in UIP the biopsyspecimens are inhomogenous with some cellularsections and other noncellular or fibrotic.
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Any patternAny patternDrugs
Nodules, (respiratory)
Nodules, (lymphatic)
Farmers lung
Silicosis
Environment dusts
Reticular, honeycombingLower lobe (periphery)Collagen vasculardisease
Reticular, honeycombingLower lobe (periphery)Asbestosis
Smooth septalBatwing, dependent lungChronic edema
Irregular septalSpares lobe or lungLymphangitic tumor
Reticular, honeycombing
Traction bronchiectasis
Lower lobe (periphery)IPF
Nodules (respiratory)
Bizarre shaped cysts
Upper lobeLangerhans
Nodules (lymphatic)
Fibrotic swath
Upper lobeSarcoid
PatternDistribution
For our last 2 disorders, the distribution and patternare identical to IPF. We must be attentive toadditional clues: asbestos plaques or skeletal oresophageal changes in collagen vascular diseases forhelp.
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Environmental dusts
Inorganic - organic dusts
Silica, coal, farmer’s lung
Granulomas, collagen nodules, dust macules
Finally let’s discuss the dusts. Inorganic dusts suchas coal or silica or the organic dusts such as Farmerslung. The pathologic response will vary dependingon the particular dust.
42
Silicosis
Upper lobe distribution
Nodules, PMF
Adenopathy, egg-shellcalcification
Similar to sarcoid and langerhans cell histiocytosis,silica and the other round dust particles such as coalconcentrate in the upper lung zones. The pattern isnodular, and the nodules may aggregate into largermasses known as PMF. Particles are removed bylymphatics and thus the nodules are distributed inboth the axial and peripheral lymphatic system. Themediastinal nodes may be enlarged. Particularlywith silica, the enlarged nodes may exhibit egg-shellcalcification.
43
Silicosis
Micronodules, lymphaticdistribution
Dorsal distribution
May calcify
HRCT, the nodules are diffuse, identical to that seenin sarcoid. In some, the profusion of nodules may begreater in the dorsal aspect of the lung. Occasionallythe nodules may calcify.
44
Farmer’s lung
Often normal
Mid lung distribution
Spares costophrenic angle
Miliary nodules
Farmers lung is our prototypical reaction to organicdusts. Numerous dusts have been shown to cause ahypersensitivity reaction, but Farmer’s lung, due to areaction to the Thermophillic actinomyces organismis the most common. Up to 90% of chest x-rays maybe normal. When abnormal, a fine nodular pattern inthe mid-lung spares the costophrenic angle.
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Farmer’s lung
Centriacinar nodules, respiratory
mid-lungs
Spares costophrenic angle
HRCT may also be normal in up to 50%.Nonspecific ground glass opacities are common.The most characteristic findings include centriacinarnodules often admixed with areas of air trapping.
46
chronic farmers lung
idiopathic pulmonary fibrosis
The main differential for chronic farmers lung is IPF.One helpful distinction is involvement of thesubpleural costophrenic lung. IPF is usually mostsevere in the costophrenic angles, whereas this areaof lung is usually less involved in chronic farmerslung.
47
Any patternAny patternDrugs
Nodules, (lymphatic)
Nodules, (respiratory)
Inorganic: upper lobe
Organic: spare cp angle
Environment dusts
Reticular, honeycombingLower lobe (periphery)Collagen vasculardisease
Reticular, honeycombingLower lobe (periphery)Asbestosis
Smooth septalBatwing, dependent lungChronic edema
Irregular septalSpares lobe or lungLymphangitic tumor
Reticular, honeycombing
Traction bronchiectasis
Lower lobe (periphery)IPF
Nodules (respiratory)
Bizarre shaped cysts
Upper lobeLangerhans
Nodules (lymphatic)
Fibrotic swath
Upper lobeSarcoid
PatternDistribution
The environmental dusts are similar to sarcoid, bothin pattern and distribution. Farmers lung mostcommonly involves the mid-lung, and spares thecostophrenic angle.
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Drugs
Often overlooked
Numerous pathologic responses
DAD, hypersensitivity, UIP
May lead to respiratory failure
Lastly, consider drug reaction. These are oftenoverlooked. The number of drugs which canadversely affect the lung is immense. Nearly anypathologic response can be due to drug toxicity. Ifthe offending drug is not removed, it may causerespiratory failure.
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Drugs
Macrodantin: UIP pattern
Add to differential for anydiffuse lung disease
In this patient with macrodanton toxicity, the patternis indistinguishable from IPF.
50
Any patternAny patternDrugs
Nodules, (lymphatic)
Nodules, (respiratory)
Inorganic: upper lobe
Organic: spare cp angle
Environment dusts
Reticular, honeycombingLower lobe (periphery)Collagen vasculardisease
Reticular, honeycombingLower lobe (periphery)Asbestosis
Smooth septalBatwing, dependent lungChronic edema
Irregular septalSpares lobe or lungLymphangitic tumor
Reticular, honeycombing
Traction bronchiectasis
Lower lobe (periphery)IPF
Nodules (respiratory)
Bizarre shaped cysts
Upper lobeLangerhans
Nodules (lymphatic)
Fibrotic swath
Upper lobeSarcoid
PatternDistribution
Our final pattern is that of drug reaction which mayproduce any pattern or distribution that we’ve seenso far. I always add drug reaction into the mix ofany differential diagnosis for diffuse lung disease.
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