Lung Adenosquamous Carcinoma Arising From a Long-Standing Bronchogenic Cyst

What happened, in summary

A 74-year-old woman who had never smoked had lived with an intrapulmonary bronchogenic cyst since age 15. The cyst had been stable for decades, but routine follow-up eventually showed that it had enlarged and developed a solid component. Persistent low sodium levels also prompted further investigation.

A needle sample showed both glandular and squamous cancer features. Molecular testing found a KRAS G12D mutation, and PD-L1 expression was high at 60%. Testing for EGFR, BRAF, ROS1, and ALK alterations was negative. PET/CT showed a suspicious nearby lymph node, but sampling of mediastinal nodes did not find cancer.

She underwent left lower lobectomy with mediastinal lymph-node dissection. The surgical specimen confirmed adenosquamous carcinoma arising directly beside the long-standing bronchogenic cyst, with transition from the benign cyst lining into the malignant tumor. Twenty-two regional lymph nodes were negative.

After surgery, she received 4 cycles of cisplatin and docetaxel followed by 2 cycles of atezolizumab. Immunotherapy had to be stopped because she developed immune-mediated colitis. Despite this complication, she remained in remission 2.4 years after surgery. The malignant transformation occurred only after the cyst changed at age 74, so the decades-long history belonged to the benign cyst rather than to the lung cancer.