Those cases in which centrilobular nodules are present in patients in the absence of tree-in-bud opacities constitute the last part of this CT scan algorithm (Table 1). Included in this category are a variety of diseases or “mixed” entities that have in common localization to the centrilobular portion of the secondary lobule. This includes diseases that primarily affect the centrilobular bronchiole, as well as those that are either primarily peribronchiolar or perivascular in origin. Most often, this group of diseases results in a pattern of diffuse, poorly defined ground-glass nodules, which are typically the result of a primarily peribronchiolar distribution. The classic example of this appearance is subacute HP (Fig 12)., This diagnosis is frequently first suggested on the basis of CT scan findings and is usually established by a combination of exposure history, clinical symptoms of a flu-like illness, the presence of specific serum antibodies when those data are available, increased numbers of lymphocytes and neutrophils in BAL fluid, and, when feasible, clinically significant improvement in symptoms when the patient is removed from the offending environmental agent.
The differential diagnosis encompasses a number of important disease entities, most importantly including RB, lymphocytic interstitial pneumonitis (LIP), and LCH. RB/RB-interstitial lung disease (ILD) are smoking-related disorders that may also result in poorly defined centrilobular nodules., RB typically results in far fewer ground-glass nodules than the number in patients with subacute HP and generally display a distinctly upper lobe predominance. RB-ILD is associated with widespread ground-glass attenuation and reticular opacities, which are findings that are not seen in patients with subacute HP. http://birthcontroltab.com/
Also included in the differential diagnosis of individuals with diffuse centilobular nodules are diseases related to bronchiolar lymphatics. This includes mucosa-associated lymphoid tissue lymphoma (malto-mas) and, in particular, LIP.
Figure 12. Centrilobular disease: subacute HP. A magnified HRCT scan section through the right upper lobe shows innumerable poorly defined, hazy ground-glass nodules throughout the lung (arrows). In addition to a uniform distribution, none of these nodules lies adjacent to the visualized pleural surfaces. Note as well the lack of any tree-in-bud opacities that would suggest the presence of mucoid impacted airways (compare with Fig 11). Few entities besides subacute HP result in this pattern of ill-defined nodules. Differential diagnoses includes RB, typically causing fewer nodules restricted to the upper lobes in known smokers, and LIP, which is usually associated with either Sjogren syndrome or AIDS.