Endocrine Abstracts

Introduction: Pituitary tumours may express somatostatin receptors (SSTR), which are a potential drug target to achieve tumour reduction and control of hormone secretion. There is a well-established role for somatostatin analogues in the management of acromegaly and Cushing’s disease. The recent 2022 WHO classification of pituitary tumours provides a comprehensive approach to the diagnosis of pituitary tumours, allowing identification of additional morphological subtypes. Our aim is to assess SSTR expression in a large cohort of pituitary tumours and to determine patterns of expression in the rarer morphological subtypes identified under this new classification system.

Methodology: We conducted a retrospective study of pituitary tumours archived at the Institute of Clinical Pathology and Medical research at Westmead Hospital from 2011 to 2018. Immunohistochemistry was performed on each tumour to evaluate for expression of pituitary hormones, transcription factors (PIT1, TPIT, SF1), co-factors (GATA3, oestrogen receptor), somatostatin receptors (SSTR2A, SSTR5) and cytokeratin (CK 8/18). After reclassification according to the 2022 WHO classification of pituitary tumours, expression of SSTR 2 and 5 in each morphological subtype was assessed.

Preliminary Results: 208 of 284 available pituitary tumours have been assessed and classified according to the 2022 WHO classification. SSTR2 expression was positive in 72.2% of PIT1 tumours, 43.9% gonadotroph (SF1) tumours and 33.3% corticotroph (TPIT) tumours. The corresponding expression of SSTR5 was 69.4% of PIT1 tumours, 1% gonadotroph (SF1) tumours and 18.2% corticotroph (TPIT) tumours. Among corticotroph tumours, expression of SSTR2 and SSTR5 was higher in Crooke cell tumours. In the PIT1 group, 100% of mature plurihormonal tumours and immature PIT1 lineage tumours expressed both SSTR2 and SSTR5. Densely granulated (DG) somatotroph tumours preferentially expressed SSTR2 (100%) over SSTR5 (33.3%), whereas their sparsely granulated (SG) counterpart showed high expression of both SSTR2 (87.5%) and SSTR5 (100%). SG lactotroph tumours showed no immunoreactivity for either form of SSTR; while 50% of DG lactotroph tumours expressed either SSTR2 or 5. We identified 5 plurihormonal tumours which co-expressed PIT1 and SF1 and all were positive for both SSTR.

Conclusion: This is the first study to systematically assess SSTR expression in all subtypes of the 2022 WHO classification. We have demonstrated that mature plurihormonal PIT1, immature PIT1, SG somatotroph and plurihormonal PIT1+SF1 highly express both types of SSTR. Our findings may be useful in predicting therapeutic response to somatostatin analogues in the future.