When Fibrosis Masks Malignancy: Dual-Tracer PET/CT Evaluation of a Lung Lesion in Scarred Lungs

A 54-year-old woman presented with pleuritic chest pain and was referred for molecular imaging for evaluation of suspected lung malignancy. She had a history of recurrent respiratory infections, and an outside contrast-enhanced computed tomography (CT) had demonstrated fibrotic changes in the right lung. Prior bronchoscopy was negative for tuberculosis and fungal infection.

With a high index of suspicion of malignancy, an¹⁸F-Fluorodeoxyglucose (FDG) PET /CT scan was performed. The MIP (Maximum Intensity Projection) image showed abnormal FDG uptake in both lungs (right > left) [Fig 1A]. The fused trans-axial image demonstrated a heterogeneously enhancing soft-tissue mass in the right lung lower lobe, approximately measuring 6.3 × 4.3 cm with intense FDG uptake (SUVmax 14.52) and central necrotic areas [highlighted by the black arrow in Fig 1]. Multiple areas of cavitation and bronchiectasis were noted in both lungs, associated with superimposed ground-glass opacities showing increased FDG uptake, more pronounced on the right. There was marked volume loss of the right lung with ipsilateral mediastinal shift, consistent with cicatrisation, along with compensatory hyperinflation of the left lung [Fig 1B and 1C]. A few discrete sub-centimetric nodules were seen in the left lung. Mediastinal lymph nodes demonstrated low-grade FDG uptake. No abnormal tracer uptake was noted in the liver, adrenal glands, skeleton, or brain.

Close

Fig 1: (A) Maximum Intensity Projection image of the ¹⁸F-FDG PET/CT and (B) trans-axial CT and (C) fused images of the bilateral lung masses, with the biopsy-proven malignant right lung lower lobe mass highlighted by the black arrows in B and C. FDG: Fluorodeoxyglucose; PET/CT: Positron emission tomography

Export to PPT

Subsequently, a ⁶⁸Ga-fibroblast activation protein inhibitor (FAPi) PET/CT scan was done; MIP [Fig 2A], axial CT [Fig 2B - arrow] and fused trans-axial image demonstrated intense tracer uptake (SUVmax 18.61) in the right lung lower-lobe mass [Fig 2C, highlighted by the black arrow]. Increased FAPi uptake was also seen in the bilateral fibrotic lung parenchyma, reflecting active fibroblast involvement [Fig 2A-C]. The mediastinal nodes showed no significant FAPi uptake, suggestive of low probability of malignant involvement. Physiological background uptake was minimal, allowing clearer delineation of disease extent.

Close

Fig 2: (A) Maximum Intensity Projection image of the ⁶⁸Ga-FAPi PET/CT and (B) trans-axial CT and (C) fused images of the bilateral lung masses, with the biopsy-proven malignant right lung lower lobe mass highlighted by the black arrows in B and C.

Export to PPT

A biopsy done from the right lung lower lobe mass confirmed a diagnosis of non-small cell lung carcinoma.

Lung carcinoma arising in association with pulmonary scarring, referred to as scar carcinoma, has been described for several decades and remains a diagnostic challenge.^[1,2]Pulmonary scars may result from prior infections, infarction, bronchiectasis, or chronic inflammatory lung disease, and malignancies arising in these regions are often peripheral and radiologically mimic benign fibrotic lesions, leading to delayed diagnosis.^[3] The most common histological type is adenocarcinoma. Histo-cytopathological studies suggest that the fibrotic component in such tumours frequently represents an active desmoplastic stromal reaction rather than an inert pre-existing scar, which may complicate lesion measurement, sampling, and staging.^[2,4]

¹⁸F-FDG PET/CT is well established for staging lung cancer but may demonstrate increased uptake in inflammatory and fibrotic lung tissue, limiting specificity in scarred lungs.^[5]

⁶⁸Ga-FAPi PET/CT targets cancer-associated fibroblasts in the tumour micro-environment and shows improved lesionto-background contrast and better detection of nodal and metastatic disease in lung cancers.^[6]

In the present case, FAPi PET/CT complemented FDG imaging by better delineating tumour extent within the fibrotic lung and identifying the nature of mediastinal nodes as likely benign. This case illustrates the potential value of dual-tracer PET/CT in evaluating lung malignancies arising in fibrotic lungs, where conventional imaging findings may be equivocal.