A Lung Nodule in a Never-Smoker: Probability, Pitfalls, and the Next Test

The pre-test probability is intermediate rather than low, despite her never-smoking status. A part-solid nodule with a solid component of 6 mm carries a higher malignant potential than a pure ground-glass nodule of the same total size; solid components over 5 mm are associated with invasive adenocarcinoma and a substantially increased risk. The total size of 16 mm places it in a category where malignancy rates are meaningful, and the absence of prior imaging means growth cannot be assessed. Her remote stage I breast cancer is a relevant but weaker risk factor; breast metastases to the lung are usually multiple and peripheral, and a solitary part-solid nodule 7 years after stage I disease is more likely a primary lung lesion, particularly adenocarcinoma spectrum. Her age and the part-solid morphology support a primary lung neoplasm. Overall I would place the probability in the intermediate range (roughly 30-60%), high enough to warrant tissue or functional assessment rather than simple observation.

PET-CT performs poorly for part-solid and ground-glass lesions because the ground-glass component has low metabolic activity and the small solid component (6 mm) is close to the spatial resolution limit of PET, producing false negatives. A negative PET therefore cannot exclude malignancy in this nodule, and a positive result could reflect inflammation or the known breast primary. The solid component is below the size at which PET sensitivity is reliable, and the part-solid morphology is precisely the setting where PET is least useful. I would not rely on PET to rule out malignancy. If staging information is needed to exclude distant breast recurrence, a staging CT or PET could be justified for that separate question, but it should not be used to decide whether the nodule needs tissue diagnosis. The more informative next step is a dedicated thin-section CT and a multidisciplinary review, with consideration of biopsy or resection depending on the interval change and the patient's fitness.

First, obtain thin-section CT of the chest, ideally with the same technique, to characterise the solid component and assess for additional nodules; this determines whether the lesion is solitary. Second, review the original breast cancer staging and histology to understand the risk of pulmonary metastasis and whether prior chest imaging exists; a solitary part-solid lesion 7 years after stage I disease is more likely primary lung cancer. Third, discuss at a multidisciplinary meeting. Given the 6 mm solid component and intermediate probability, options are (a) interval CT at 3 months to assess growth, which is reasonable if the patient prefers to avoid intervention and the solid component is stable, or (b) tissue diagnosis by CT-guided biopsy or surgical resection, which is preferred if the solid component is growing or if the patient is fit. PET-CT should not be used to exclude malignancy. I would also address the tamoxifen history and the resolved chest pain, and document a clear follow-up plan so the nodule is not lost to follow-up.