OBJECTIVE: Programmed cell death protein 1 (PD-1) and DNA mismatch repair (MMR) deficiency play an important role in tumour progression and response to treatment.Both markers have been studied in some ocular tumours but little is known about these markers in orbital tumours. This pilot study reports on PD-L1 expression and MMR mutations using next generation sequencing (NGS) in specific orbital tumours.
METHODS: We reviewed surgical specimens from patients with rhabdomyosarcoma, adenoid cystic carcinoma (ACC), pleomorphic adenoma (PA) and biopsy negative tissue from orbital tumours used as a control. immunohistochemistry (IHC) was performed on Formalin fixed paraffin embedded tissue using a PD-L1 antibody. DNA was extracted for targeted gene panel NGS of the MMR genes PMS2, MLH1, MSH6 and MSH2.
RESULTS: The study included 17 orbital specimens. Scattered membrane PD-L1 staining was noted in 3/6 rhabdomyosarcoma specimens without an accompanying lymphocytic infiltrate. PD-L1 immunostaining was absent in 3/3 ACC, and 5/6 PA specimens. PD-L1 immunostaining was not detected in 2/2 control specimens. 4/17 samples shared the same pathogenic mutation in the MLH1 gene, including 3/6 rhabdomyosarcoma and 1/3 ACC samples. 1/6 PA samples had a mutation in MSH6.
CONCLUSIONS: Our study demonstrated scattered, non-quantifiable or absent PD-L1 staining in a limited sample of orbital tumours suggesting that PD-1/PD-L1 inhibitor therapy may not be useful in treatment of malignant orbital tumours (rhabdomyosarcoma and ACC) when refractory to conventional therapy. Our pilot study suggest that PD-L1/MMR axis might not play a major role in the pathogenesis of primary orbital tumour.

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BACKGROUND: Only 15%-20% of newly diagnosed pancreatic ductal adenocarcinoma (PDAC) cases are surgically operable. Cytology samples may be the only source for guiding clinical management. Current research indicates that mismatch repair (MMR) status could be valuable for implementing novel treatment modalities. In this study, immunohistochemical (IHC) MMR protein expression on cytology cell blocks was compared with that of the correlating surgical resection specimens.
METHODS: A retrospective review of 120 pancreatic solid tumor needle biopsies from 2016 to 2019 was performed, and 15 experimental cases were selected comprising of cellular (>100 tumor cells and >100 benign positive control cells that include benign ductal cells and/or lymphocytes) alcohol-prefixed formalin-fixed cytology cell blocks (CB), and corresponding subsequent surgical resections (Surg). The cases include 13 PDACs (87%), 1 (6.5%) low grade neuroendocrine tumor (PNET), and 1 (6.5%) acinar cell carcinoma (ACC). A routine four-panel (MLH1, MSH2, MSH6, and PMS2) MMR IHC testing was performed. The percentage of protein expression was evaluated and compared between individual CB-Surg pairs.
RESULTS: About 8 of the 15 (53.3%) cytology cases showed matching protein expressivity with the surgical specimens for all four MMR markers (Table 1). The remaining 7 pairs (46.7%) appeared to have a partial concordance, including 6 values for which a surgical marker showed less expression, and 13 values for which a cytological marker showed less expression. [Table: see text] CONCLUSION: Cytology CB MMR protein panel testing could be a useful consideration for inoperable patients who would benefit from medical therapy such as immune checkpoint inhibition. However, the cellularity and overall quality of the CB is expected to be paramount in obtaining satisfactory IHC MMR results. The MMR results could be more confidently trusted when the staining is intact, but when not retained, they should be interpreted with caution in a low cellularity sample to avoid mistakenly identifying MMR-deficient tumors.