The pathological challenge of establishing a precise diagnosis for pulmonary tumour thrombotic microangiopathy: identification of new diagnostic criteria

Aims Pulmonary tumour thrombotic microangiopathy (PTTM) is a fatal disease of patients with cancer that causes progressive pulmonary hypertension (PH). Its pathology is characterised by fibrocellular intimal proliferation and thrombosis caused by tumour emboli in microscopic pulmonary arteries. However, such PTTM‐like lesions often appear incidentally. We sought to identify features that distinguished PTTM from incidental pulmonary tumour emboli, and to gain an overall picture of PTTM morphology in terms of its pathogenesis. Methods and results Twenty‐five PTTM cases were classified into two groups: (i) a definite group (n = 14), clinically diagnosed with PH; and (ii) a suspicious group (n = 11) with respiratory symptoms but without a clinical evidence of PH. As a control group, autopsy cases with PTTM‐like lesions lacking progressive respiratory symptoms were selected (n = 7). PTTM‐like lesions in these groups were studied and a diagnostic guide for PTTM formulated as follows: PTTM‐like lesions with >17 affected vessels observed in a 1‐cm2 area of lung specimen, and the absence of pulmonary metastatic nodules. PTTM due to gastric cancers was shown to have a significantly shorter course and larger arterial involvement than cases with non‐gastric cancers. Serial sections revealed a PTTM lesion to be a longitudinal obstruction that accumulated in microscopic pulmonary arteries and that showed a proximal extension via supernumerary arteries. Conclusion We suggest novel pathological diagnostic characteristics for PTTM deduced from a study of 25 autopsy cases. This includes PTTM‐like lesions with >17 affected vessels in a 1‐cm2 area of lung specimen and the absence of pulmonary metastatic nodules.