Dendriform pulmonary ossification in a case of usual interstitial pneumonia

Clinical History

A 66-year-old male patient was evaluated for known IPF (idiopathic pulmonary fibrosis), diagnosed 1 year ago. Pulmonary function testing demonstrated restrictive disease with forced expiratory volume in 1 second of 60% of predicted value, vital capacity of 58% and diffusing capacity for carbon monoxyde of 38%.

Imaging Findings

Chest CT showed the typical pattern of UIP, revealing subpleural reticulation, cystic honeycombing with predominant basal and subpleural distribution and bronchiectasis (Fig. 1, 2). Other findings were fine irregularities of the pleural surfaces suggesting interface sign (Fig. 2b). After evaluation through mediastinal window multiple calcifications with predominant branching and reticular appearance were noted within the fibrotic areas, with predominance in the right lung (Fig. 3); other tiny calcifications with subpleural predominance were also present (Fig. 3e, 3g). In accordance with the literature, a further evaluation was made through "osteoporosis" window (width 818, level 273), that confirmed the calcific nature of the findings (Fig. 4). The diagnosis of dendriform pulmonary ossification in the setting of a known UIP pattern was made.

Discussion

Dendriform Pulmonary ossification (DPO) is the less common form of diffuse pulmonary ossification, a rare condition that includes also the more frequent nodular pattern, usually affects men between 40 and 60 years, and is characterized by the presence of mature heterotopic bone in the lung parenchyma [1, 2].
DPO is characterized by branching mature bone within the interstitium, maybe related to chronic tissue injury resulting in enzyme activation and induction of osteogenic factors [3].
The presence of diffuse pulmonary ossification itself is not related to any symptoms, and the clinical features lead to the eventual associated respiratory diseases. In fact, although DPO may be idiopathic, it also may be present in many respiratory diseases, such as COPD, adult respiratory distress syndrome and OP; however, the main association described in the literature is idiopathic pulmonary fibrosis (IPF) [4].
IPF represents the clinical syndrome associated with the morphologic pattern of UIP, that is characterized on chest CT by reduction of lung volumes, subpleural reticular opacities, macrocystic honeycombing, and traction bronchiectasis, the extent of which increases from the apex to the bases of the lungs [5].
DPO appear on chest radiograph like fine, branching reticular opacities, usually peripheral and basal. When associated to fibrosis, on chest CT DPO consists of dense reticular and nodular opacities within the area of fibrosis. Moreover, Kim et al suggested the importance of the use of an "osteoporosis" window, to recognize true calcification from high attenuation “pseudocalcification” [1].
Mandeloff proposed that 9% of patients with pulmonary fibrosis present DPO, which is related to the chronicity and/or severity of the disease [1]. Other authors suggested that the presence of DPO may be considered to be a helpful factor in distinguishing between Usual Interstitial Pneumonia (UIP) and Non-Specific Interstitial Pneumonia (NSIP), because the first one may have DPO as opposed to the second one [6]. However, because of its rarity, DPO is not considered to be a diagnostic criterium for UIP diagnosis.
In our case, the patient already had the diagnosis of UIP on the basis of radiological features, and the presence of DPO was an additional finding on follow-up chest CT. In particular, the presence of reticular abnormalities, honeycombing with basal and subpleural distribution (associated with bronchiectasis), and the absence of features listed as inconsistent with UIP pattern had allowed the diagnosis of UIP without need of biopsy, in agreement with the diagnostic criteria proposed by American Thoracic Society [7].

Differential Diagnosis List

Final Diagnosis

Dendriform pulmonary ossification in a case of usual interstitial pneumonia

URL:	https://www.eurorad.org/case/13793
DOI:	10.1594/EURORAD/CASE.13793
ISSN:	1563-4086