Ancillary testing in paediatric non-CNS solid tumours: a real-world comparison with WHO criteria.

AIMS: Paediatric tumour pathology poses diagnostic challenges due to morphological variation, overlap and the rarity and diversity of neoplasms, for which the WHO provides a comprehensive diagnostic framework. In clinical practice, however, diagnostic reasoning starts with an unknown tumour and a broad differential diagnosis which may conflict with theoretical requirements. This study evaluates the contribution of ancillary techniques to the diagnostic process of non-central nervous system (CNS) solid paediatric tumours. METHODS: Clinicopathological data were retrospectively obtained from all patients diagnosed between January 2019 and December 2021 in a national single paediatric centre. Contribution of ancillary techniques was categorised into four groups: (1) morphology only, (2) morphology and immunohistochemistry (IHC), (3) morphology and molecular diagnostics or (4) morphology, IHC and molecular diagnostics. RESULTS: In total, 309 patients were included. Morphology alone was sufficient in 58.9% of cases, additional IHC was required in 15.2%, additional molecular techniques in 1.3% and all three modalities in 24.6%. Morphology alone was sufficient for most neuroblastic, renal, bone and digestive system tumours. IHC was required for undifferentiated neuroblastomas and frequently for germ cell and ovarian tumours. The combined use of morphology, IHC and molecular techniques was necessary to diagnose soft tissue tumours and undifferentiated small round cell sarcomas. CONCLUSIONS: The majority of paediatric, solid, non-CNS, tumours can be diagnosed based on morphology alone, whereas ancillary techniques are required to resolve remaining cases. These findings highlight that current tumour classification does not always match daily clinical practice.