Sir,

A 55-year-old woman, with no history of smoking, was referred to our department for further evaluation of a subcutaneous mass on the right chest wall. She had lived opposite a stone shop since childhood, which resulted in repeated exposure to asbestos. At the age of 43, she was diagnosed with malignant pleural mesothelioma (MPM) via biopsy and underwent systemic chemotherapy with multiple regimens, achieving temporary remission. At the age of 46, she developed right pleural effusion; however, pleural fluid cytology and thoracoscopic examination revealed no malignant findings, and she was followed conservatively. At the age of 52, pleurodesis was performed for symptomatic relief. The patient subsequently underwent thoracentesis every 6 months for drainage. One month before presentation, pleural fluid cytology revealed histopathological features suggestive of MPM. Around the same time, a subcutaneous mass appeared on the right chest wall, precisely corresponding to a previous chest drain insertion site, leading to the referral. On examination, the mass measured 6 × 6 cm and was mobile (Fig. 1a). Incisional biopsy revealed tubular proliferation of cells with eosinophilic cytoplasm and prominent nucleoli (Figs. 1b and 1c), exhibiting morphology similar to that of pleural tissue. Immunohistochemically, the tumor cells were positive for calretinin (Fig. 1d), D2-40, and Wilms tumor-1 (WT-1) (Fig. 1e), but negative for carcinoembryonic antigen (CEA) (Fig. 1f) and thyroid transcription factor-1 (TTF-1) (Fig. 1g), with loss of BRCA1-associated protein 1 (BAP1) expression. Based on these findings, a diagnosis of cutaneous metastasis from MPM was made. Subsequently, the residual nodule was surgically removed, and combination immunotherapy with nivolumab and ipilimumab was started.

Figure 1: a) Clinical appearance showing subcutaneous mass on the chest. (b and c) Hematoxylin and eosin staining shows tubular proliferation of cells with eosinophilic cytoplasm and prominent nucleoli (b: ×40, c: ×200). (d and e) The tumor cells were positive for calretinin and WT-1 (d: ×100, e: ×100). (f and g) The tumor cells were negative for CEA and TTF-1 (f: ×100, g: ×100).

In our patient, the lesion developed precisely along the previous drainage tract, suggesting a tract-related mechanism rather than random dissemination. Cutaneous metastasis of MPM is rare [1,2], with several reported cases occurring at injured sites such as catheter or needle insertion tracks [3–5]. The possible mechanisms of skin metastasis affecting the injures sites include direct implantation and hematogenous spread. During thoracentesis or chest tube management, pleural effusion containing viable mesothelioma cells may contaminate injured tissue and proliferate along the tract. In addition, circulating tumor cells in advanced disease may preferentially lodge at sites rendered permissive by recent injury. Tracts created by drains or biopsies provide activated endothelium and newly formed capillaries, lowering the threshold for tumor engraftment. The wound-healing response further promotes tumor growth, angiogenesis, and immune evasion through mediators such as platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and interleukin-6 (IL-6) [6]. Pleurodesis has also been identified as a risk factor for chest wall metastasis, likely through enhanced local inflammation and fibrosis that increase susceptibility to tumor implantation [7]. In summary, drainage-site metastasis in MPM appears to result from a convergence of direct implantation and hematogenous dissemination, amplified by wound-healing processes and procedural interventions. Careful follow-up is warranted for patients who have undergone surgical procedures.

Written informed consent for publication of the case and accompanying images was obtained from the patient.

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