Microwave thermal ablation is a promising emerging treatment for early-stage lung cancer. Applicator design optimisation and treatment planning rely on accurate knowledge of dielectric tissue properties. Limited dielectric data are available in the literature for human lung tissue and pulmonary tumours. In this work, neoplastic and non-neoplastic lung dielectric properties are characterised and correlated with gross and histological morphology. Fifty-six surgical specimens were obtained from twelve patients undergoing lung resection for lung cancer in University Hospital of Galway, Ireland. Dielectric spectroscopy in the microwave frequency range (500 MHz–8.5 GHz) was performed on the ex vivo lung specimens with the open-ended coaxial probe technique (in the Department of Pathology). Dielectric data were analysed and correlated with the tissue histology. The dielectric properties of twelve lung tumours (67% non-small cell carcinoma (NSCC)) and uninvolved lung parenchyma were obtained. The values obtained from the neoplastic lung specimens (relative permittivity: 52.0 ± 5.4, effective conductivity: 1.9 ± 0.2 S/m, at 2.45 GHz) were on average twice the value of the non-neoplastic lung specimens (relative permittivity: 28.3 ± 6.7, effective conductivity: 1.0 ± 0.3 S/m, at 2.45 GHz). Dense fibrosis was comparable with tumour tissue (relative permittivity 49.3 ± 4.6, effective conductivity: 1.8 ± 0.1 S/m, at 2.45 GHz).

Background and Clinical Significance: Pulmonary crystal-storing histiocytosis (CSH) without an associated haematological malignancy, lymphoproliferative disorder, plasma cell disorder, or other identifiable underlying condition is exceptionally rare. Long-term radiological data from published cases remain limited, and this case, contextualised by a narrative review of the literature, may expand the recognised imaging spectrum of localised pulmonary CSH. Case Presentation: We report the case of a 59-year-old man with incidental multiple pulmonary lesions identified on CT thorax imaging. The lesions demonstrated an unusual combination of cystic change, cavitation, and surrounding ground-glass opacities. His medical history was significant for bipolar disorder treated with lithium and a 50-pack-year smoking history. Interval imaging showed progression, prompting further investigation and ultimately right upper lobectomy. Histopathological analysis confirmed pulmonary CSH; lesional cells contained crystalloid material and showed CD68 and PAS positivity, with dual kappa and lambda expression on immunohistochemistry. Markers for other differential diagnoses, including Congo red, birefringence, Langerin, and CD1a were negative. Following diagnosis and resection, serial imaging demonstrated fluctuating yet slowly progressive pulmonary abnormalities. No lymphoproliferative or plasma cell disorder has emerged after more than seven years of post-diagnostic surveillance and more than ten years since the initial imaging abnormality. Conclusions: This case demonstrates that the radiological spectrum of localised pulmonary CSH may include cystic, cavitary and ground-glass abnormalities, with subsequent fluctuating yet slowly progressive post-resection evolution. Pulmonary CSH should be considered in the differential diagnosis of unexplained or atypical pulmonary nodules, particularly when histiocyte-rich pathology with intracytoplasmic crystalloid material is identified. Long-term multidisciplinary surveillance is warranted.