Somatostatin receptor PET response assessment framework for patients with neuroendocrine tumours (V1.0): a modified Delphi consensus from the European Neuroendocrine Tumor Society (endorsed by EANM and NANETS)

Somatostatin receptor PET imaging is integral to the management of patients with neuroendocrine tumours (NETs), yet standardised criteria for therapy response assessment with the use of this modality are not available. This Policy Review reports the development of the European Neuroendocrine Tumor Society somatostatin receptor PET response assessment framework, established through a structured modified Delphi process coordinated by the European Neuroendocrine Tumor Society. 34 international experts from nuclear medicine, radiology, oncology, endocrinology, surgery, and related disciplines participated in four iterative rounds evaluating 76 statements, with consensus defined as at least 75% agreement. The framework proposes response categorisation based primarily on volumetric changes in somatostatin receptor-expressing target lesions, complemented by assessment of new lesions, rather than reliance on standardised uptake value-based metrics. Partial response is defined by at least 40% reduction in target lesion volume without new lesions, whereas progressive disease is defined by at least 40% volume increase of target lesions or the emergence of new lesions. Complete response requires absence of pathological tracer uptake, and a category of unconfirmed progressive disease is introduced for equivocal cases warranting short-interval reassessment. Although not yet validated against survival outcomes, this expert-derived framework (SSTR-PeRForm) provides a pragmatic foundation for harmonising somatostatin receptor PET-based response assessment in clinical trials and routine practice and represents a key step towards outcome-based validation.