Pseudo-regression followed by regrowth in peripheral lung squamous cell carcinoma: a case report

Background Current pulmonary nodule management guidelines emphasize growth kinetics, as a persistently enlarging solid nodule strongly suggests malignancy. However, untreated non-small cell lung cancer (NSCLC), particularly squamous cell carcinoma, can occasionally exhibit pseudo-regression. This rare phenomenon may lead to clinical misjudgment and delayed diagnosis. Case presentation A 73-year-old male presented with a solid nodule (20 mm × 15 mm) in the left lower lobe, featuring spiculation, lobulation, and cavitation. The patient declined biopsy. At the 7-month follow-up, the nodule had shrunk significantly (10 mm × 9 mm) with a more regular morphology. However, over the subsequent 13 months, the lesion regrew (15 mm × 12 mm) with increasingly malignant features. Surgical resection ultimately confirmed a moderately differentiated squamous cell carcinoma (pT1cN0M0, stage IA3). Conclusion Even when a pulmonary nodule with initially suspicious imaging features shows volumetric reduction on follow-up, clinicians should remain vigilant for “pseudo-regression.” The observed fluctuation is consistent with pseudo-regression, which likely results from intratumoral necrosis and cavitation drainage rather than true spontaneous resolution. Strict adherence to surveillance protocols is essential to prevent diagnostic delays.