1p/19q codeletion

What is measured: whole-arm loss of 1p and 19q, the product of an unbalanced translocation t(1;19)(q10;p10). How: FISH with 1p36 and 19q13 probes (partial deletions can mimic it and are not the same lesion), copy-number from methylation or SNP arrays, or next-generation sequencing panels that call copy number; WHO 2021 requires an IDH mutation plus 1p/19q codeletion for oligodendroglioma, grade 2 or 3, while an IDH-mutant astrocytoma is 1p/19q intact and usually shows ATRX loss and TP53 mutation, with CDKN2A/B homozygous deletion making it grade 4. TERT promoter mutation is almost universal in oligodendroglioma, with CIC and FUBP1 mutations. What a positive result changes: radiotherapy followed by PCV (procarbazine, lomustine, vincristine) roughly doubled median survival in the RTOG 9402 and EORTC 26951 trials (about 14 years against 7 with radiotherapy alone) and remains the standard for grade 3 and high-risk grade 2 disease; vorasidenib is an option for residual or recurrent grade 2 IDH-mutant glioma whether codeleted or not (INDIGO); younger patients with a gross total resection can be watched; a non-codeleted result sends the patient down the astrocytoma path of radiotherapy with temozolomide (CATNON). Where it matters: oligodendroglioma, IDH-mutant astrocytoma and glioblastoma work-up.