Pseudoprogression of a cutaneous lung cancer metastasis

A 38-year-old man presented to the Rocket Force Characteristic Medical Center (Beijing, China) in September, 2020, with limping, dizziness, and bone pain. PET/CT scan suggested lung cancer with bone and brain metastases, and a small cutaneous metastasis located on the lateral chest wall. Iliac bone lesion biopsy revealed poorly differentiated adenosquamous carcinoma originating from the lungs with negative PD-L1 expression (using the 22C3 assay). Next-generation sequencing of fresh biopsy tissue showed BRIP1 germline mutation and a high tumour mutational burden; mutations were detected in genes including TP53, SMARCA4, and ATM, but not in EGFR, ALK, ROS1, or KRAS. There were no mutations detected in the immune-related genes, such as JAK1, STK11, and MDM2. After two cycles of chemotherapy combined with anti-PD-1 antibody administered intravenously in 21-day cycles (nab-paclitaxel 125 mg/m2 on days 1 and 8, plus cisplatin 75 mg/m2 on day 1, plus tislelizumab 200 mg on day 1), the cutaneous lesion unexpectedly enlarged with development of a large red plaque around it (figure). Since the nodule texture was firm, there was no pain and no pus points appeared during enlargement, and CT scan showed a soft-tissue density lesion with no liquid density area, infection was ruled out. Needle biopsy findings from the nodule indicated metastatic poorly differentiated adenocarcinoma with infiltrating lymphocytes, but no tumour cells were found in the cutting biopsy specimen from the plaque. Next-generation sequencing of the skin lesion detected 18 of the 22 gene mutations found in the bone lesion (with abundance >1%). Immunohistochemistry and next-generation sequencing indicated SMARCA4 deficiency in both lesions, which might be the cause of the heterogeneity and poor differentiation; however, the diagnostic criteria for SMARCA4-deficient undifferentiated tumour was not met.