This is the first report to examine DPO in patients with DLDs who underwent SLB. The findings of this study were almost consistent with the previously reported common features of DPO patients, including slow progression, the location of the lesions, and the co-existence of fibrosis; however, we further found that inflammation and hemorrhage were observed in patients diagnosed with idiopathic DPO.
In this study, of the 313 cases with DLDs that involved SLBs, three cases were definitely diagnosed with DPO, representing an average incidence of 0.96%. Diffuse pulmonary ossification has been reported to be present in 0.16–0.40% of autopsies [1, 9]. However, limited to alive DLDs, the incidence of DPO may be rather high. In this study, pulmonary ossifications were located in both lower lobes in all five cases. Similarly, pulmonary ossifications were located in the lower lobes in 36 of 42 cases diagnosed with DPO in a previous study . In our study, pulmonary function tests showed no remarkable impairment at the baseline and significant decline during the follow-up period. None of the DPO patients met the criteria for progressive fibrosing interstitial lung disease  or had died at the time of review. DPO is typically considered to be indolent or slowly progressive, inducing a gradual decline in the pulmonary function [5, 12, 13]. On the other hand, a DPO case that showed progressive restrictive ventilatory impairment was reported . Nevertheless, the risk factors for progression in DPO remain unclear.
Dendriform-type ossifications are frequently accompanied by interstitial fibrosis [9, 15, 16], and their association has been suggested. As a factor causing ossification, transforming growth factor-beta, which is strongly implicated in idiopathic pulmonary fibrosis and other fibrotic pulmonary diseases, stimulates osteoblast and chondrocyte proliferation . In this study, all five DPO cases showed interstitial fibrosis in lung tissues. Furthermore, on histopathologic review, ossifications were identified in one (0.6%) of the 174 cases with UIP and one (0.9%) of the 112 cases with NSIP, out of 313 cases with DLDs. Travis et al. reported that ossifications were identified during pathologic examination of lung biopsy specimens in 12 of 56 (21%) cases of UIP and in two of 22 (9%) cases of fibrotic NSIP . Kim et al. reported that ossifications were identified during pathologic examination of lung biopsy specimens in five of 75 (6.7%) cases of UIP and not identified in 44 cases of NSIP . The frequency of ossifications in UIP was lower in this study than in the previous studies. This might be explained by the following reasons. First, the difference in patient characteristics might have influenced the findings. Second, false-negative biopsy results may have occurred due to sampling errors.
In this study, case 4 showed demonstrable hiatal hernia and case 5 showed GERD. In a previous report, 39 of 52 DPO cases without UIP (75%) showed obstructive sleep apnea, GERD, or a chronic neurologic disorder, and DPO was suggested to be associated with recurrent acid aspiration . Local acidity and hypoxemia can cause differentiation of pulmonary fibroblasts and possibly macrophages into osteoblasts, and microscopic and chronic low-level acid aspiration could therefore be expected to cause DPO .
As mentioned above, DPO has been reported to be often associated with usual interstitial pneumonia (UIP) or chronic aspiration of gastric acid. However, DPO is believed to be a multifactorial disease and causes other than UIP or acid aspiration should underlie the mechanism of DPO. Three cases (cases 1, 2, and 3), which were considered to be idiopathic due to the lack of any underlying disease, showed neither UIP nor any risk factor of acid aspiration. In case 1, in addition to ossifications compatible with DPO, fibrosis and slight inflammatory cell infiltration were observed in the lung tissues. The ossifications in the fibrosis and slight inflammatory cell infiltration were considered to be relatively recent because they co-existed with osteoblasts. This case suggested that fibrosis, which typically results from chronic inflammation, and inflammation might be related to new bone formations. Furthermore, cases 2 and 3 also showed temporary consolidation, suspected to be caused by inflammation or hemorrhage. Namely, all three DPO cases without UIP or risk factors of acid aspiration did show consolidation suspected to be caused by inflammation or hemorrhage in common. We, therefore, speculated that inflammation or hemorrhage associated with fibrosis could contribute to bone formations in DPO. Inflammation itself is suggested to be involved in the pathogenesis of bone formation by promoting the transformation of cultured fibroblasts into osteoblasts [19, 20]. With respect to hemorrhage, organization of intra-alveolar hemorrhage associated with chronic passive hyperemia and phagocytosis of intra-alveolar hemosiderin deposits are suggested to be involved in the pathogenesis of bone formations . In two cases, DPO was reported to occur secondarily from diffuse alveolar hemorrhage [22, 23]. In addition, in vascular Ehlers-Danlos syndrome, which is caused by mutations of type III collagen (COL3A1) gene, laceration of the lung, vascular disruption, and alveolar injury as a result of tissue fragility caused acute hematoma, hemorrhage, and organizing pneumonia [24, 25]. Subsequently, they often formed fibrous nodules, representing abnormal wound healing with abnormal collagen deposition, and ossifications developed in the background of the fibrous nodules. Asymptomatic mild inflammation or hemorrhage might have been overlooked in previous DPO cases, especially in idiopathic cases.
This study had several limitations. First, the sample size was small. Second, histological specimens corresponding to consolidation suspected to be caused by inflammation or hemorrhage could not be obtained in cases 2 and 3. Third, in the three cases, localized consolidation could not explain the diffuse ossifications in the whole lungs. However, unrecognized repeated inflammation and hemorrhage might have occurred diffusely. In this study, we could not reveal the factors causing inflammation or hemorrhage.