PD-L1 Expression in Papillary Thyroid Cancer

Abstract

Abstract There is limited data on prevalence of PD-L1 expression in papillary thyroid cancer (PTC). Anti- PD-L1 therapy is now being trialled for head and neck cancers including thyroid cancer. Whether presence of PD-L1 staining in PTC might result in a better response to anti-PD-L1 therapy is uncertain. We evaluated the frequency of tumoral PD-L1 expression using standard antibody (SP263) and clinicopathologic factors including mutation status - BRAF, NRAS, KRAS using OncoFocus Mass Array System in an Australian PTC cohort with low or high risk of recurrence /treatment response as per 2015 ATA guidelines. The cohort had 128 patients; 66% were females; 40% had both multifocal and bilateral PTC; 47% had low recurrence risk, of whom 92% had excellent treatment response; 34% had high recurrence risk, of whom 37% had structural incomplete treatment response. There was nil (0%) PD-L1 staining in 59%, 1% staining in 20%, 5-10% staining in 11%, >10% staining in rest of the cohort; 30%, 40% and 70% staining was seen in 2% each. PD-L1 in immune cells was positive in 62%. There was no significant association between PD-L1 tumour status and age, gender, size of tumour, multifocality, bilaterality, both multifocality and bilaterality, lymphovascular invasion, perineural invasion, extrathyroidal extension (ETE), histological variants, risk stratification, treatment response, metastases or mutation status. Among PD-L1 negative vs positive patients, BRAFV600E was present in 64.9% vs 67.5%, NRAS in 8.8% vs 0% and nil mutation in 26 % vs 33 % p=0.15. The staining thresholds were chosen according to existing literature. When analysed according to the percentage of PD-L1 staining (0%, 1-10% and >=10% thresholds), there was no significant association between PD-L1 staining category and other features, except for ETE (35% vs 65% vs 27%; p=0.007), a relationship that remained significant in a multivariable-adjusted model (OR 9.9, p=0.04). PD-L1 staining thresholds of <5% and >=5% were significantly associated with the size of tumour (24.7±15.4 vs 33.3±21.8; p=0.02) and histological variants (57 % vs 61% for classic variants and 27% vs 11% for follicular variants, p=0.03). This is the first study to our knowledge that analysed PD-L1 staining based on the risk stratification categories. The majority of PTC showed negative or low staining intensity for PD-L1 expression. There was no significant association between PD-L1 staining, clinicopathologic characteristics and mutation status of the PTC. Relatively less ETE was seen in the group that showed >=10% and also >=5% PD-L1 staining as compared to the group that showed 1% <10% and 1% to <5% staining which is an interesting observation and needs further investigation. The significant variation in reported PD-L1 staining in current literature reiterates that the method is yet to be optimised. Moreover the relationship between PD-L1 staining and PTC remains to be further investigated.