CNS5 Neuroradiology: High-Yield Concepts for FRCR Part 2A

Essential Concepts in WHO 2021 CNS Tumor Classification for the FRCR

The 2021 WHO Classification of Tumors of the Central Nervous System (CNS5) marks a paradigm shift from purely histological diagnosis to an integrated approach combining histology and molecular genetics. Understanding these changes is critical for the FRCR Part 2A, as the exam now demands familiarity with molecular drivers and modern grading nomenclature. The transition to CNS5 has fundamentally altered the diagnostic lexicon in neuroradiology.

1. Integrated Diagnosis and Molecular Grading

The cornerstone of CNS5 is the integrated diagnosis, where the final tumor report merges morphological assessment with molecular testing. A significant change is the abandonment of Roman numerals (e.g., Grade II) in favor of Arabic numerals (e.g., Grade 2), aligning CNS oncology with broader oncological standards. Molecular markers can now independently upstage tumors; for instance, the presence of CDKN2A/B homozygous deletion in an IDH-mutant astrocytoma mandates a Grade 4 classification, regardless of histological appearance. This represents a major departure from previous iterations where necrosis or high mitotic activity were the primary requirements for higher grades. Exam tip: Always look for molecular markers like CDKN2A/B deletion, TERT promoter mutation, or EGFR amplification in the clinical history to determine the grade, even if the radiologic or histologic appearance seems indolent.

2. The Redefinition of Glioblastoma

The term 'Glioblastoma, IDH-mutant' is officially deprecated. Glioblastoma is now strictly defined as an IDH-wildtype, diffuse, astrocytic glioma. Tumors formerly classified as IDH-mutant glioblastoma are now referred to as 'Astrocytoma, IDH-mutant, grade 4'. This reflects the distinct biological behavior and significantly better prognosis associated with IDH mutation compared to IDH-wildtype glioblastoma. The shift ensures that the diagnosis 'Glioblastoma' is reserved for the most aggressive, biologically distinct entities. Exam tip: If the stem mentions an IDH mutation, do not choose glioblastoma as your answer; look for astrocytoma, grade 4.

3. Molecular Signatures in Pediatric and Adult Gliomas

Many tumors are now classified by their specific molecular drivers, such as the ZFTA fusion in supratentorial ependymomas. Understanding these drivers is essential, as they often correlate with specific patient demographics and clinical outcomes. The classification now differentiates between various diffuse pediatric-type gliomas based on histone H3 status and other epigenetic modifications. Recognizing these subtypes is essential for interpreting contemporary neuroradiology literature and exam vignettes, which frequently highlight these specific molecular profiles.

Comparison of Grading and Diagnostic Shifts

4. Response Assessment and the Role of RAPNO

Radiologists must be familiar with the RAPNO (Response Assessment in Pediatric Neurooncology) criteria when evaluating treatment response in pediatric populations. These criteria address the limitations of older MacDonald or RANO criteria, specifically regarding pseudo-progression and the unique appearance of pediatric tumors. Recognizing these patterns avoids the common pitfall of misidentifying treatment-related inflammation as tumor progression. Given that pediatric brain tumors often present with unique enhancement patterns, adhering to RAPNO is essential for the FRCR exam when assessing post-surgical or post-radiotherapy imaging. Exam tip: Be cautious of interpreting post-treatment imaging changes in the first 3 months as progression without considering therapy-induced changes or RAPNO criteria.

Key Takeaway: The CNS5 classification prioritizes molecular status over pure histology. Candidates must move away from obsolete terminology like Roman numerals and IDH-mutant glioblastoma, focusing instead on how specific molecular markers (CDKN2A/B, IDH status, ZFTA fusions) define the diagnostic entity and histologic grade. Mastering these updates is vital for accuracy in the Part 2A examination.