Programmed death-ligand 1 expression in carcinoma of unknown primary

We examined the expression of programmed death-ligand 1 (PD-L1) in carcinoma of unknown primary (CUP) and its potential implications. Tissue microarrays were constructed for 72 CUP cases (histologic subtypes: 22 adenocarcinoma, 15 poorly differentiated carcinoma, 19 squamous cell carcinoma, and 14 undifferentiated carcinoma; clinical subtype: favorable type 17 [23.6%], unfavorable type 55 [76.4%]), with immunohistochemical staining performed for PD-L1 (22C3, SP142, SP263, and 28 - 8), CK7, and CK20 to determine the association between staining results and clinicopathological parameters. In CUP, the PD-L1 positivity rate was 5.6-48.6% (tumor cells [TC] or tumor proportion score [TPS]: 5.6-36.1%, immune cell score [IC]: 8.3-48.6%, combined positive score [CPS]: 16.7%) using different cutoff values for 22C3 (TPS >= 1%, CPS >= 10), SP142 (TC >= 50%, IC >= 10%), SP263, and 28 - 8 (TC and IC >= 1%). PD-L1 SP142 TC and PD-L1 SP263 IC showed the lowest (5.6%) and highest (48.6%) positivity rates, respectively. The PD-L1 positivity rate did not significantly differ based on the histologic subtype, clinical subtype, or CK7/CK20 across clones. Considering TC kappa >= 1%, TC kappa >= 50%, IC kappa >= 1%, and IC kappa >= 10%, the PD-L1 positivity rate was TC = 4.2-36.1% and IC = 9.7-48.6%; the overall agreement between antibodies ranged from 69.4 to 93.1%, showing fair or better agreement (kappa >= 0.21). In CUP, PD-L1 positivity varied depending on antibodies and scoring systems, with no difference observed according to histologic or clinical subtypes. Carcinoma of unknown primary (CUP) refers to a heterogeneous collection of cancers where metastatic growth is observed, but the origin of the primary tumor remains unidentified. The type of primary cancer is critical for establishing the treatment strategy in metastatic carcinoma, presenting a considerable challenge in CUP. Patients with programmed death-ligand 1 (PD-L1)-positive tumors are well-known to benefit from targeted therapy against PD-L1. However, the expression of PD-L1 in CUP remains poorly explored. The present study demonstrated that PD-L1 was expressed in CUP with varying positivity rates depending on the antibody and scoring system employed. There was no difference in PD-L1 expression based on histological or clinical subtypes. Based on PD-L1 expression, immune checkpoint inhibitors could afford an effective treatment strategy in CUP.