How would you approach a patient over the age of 40 with a sub-totally resected frontal oligodendroglioma, WHO Grade II, 1p/19q co-deleted, IDH mutant, with imaging concerning for second site in the pontomedullary junction?

The pontomedullary junction is not usually amenable to a biopsy (unless the lesion is exophytic); as such, there are 2 possibilities: a) the second lesion is related to the one that has undergone subtotal resection, or b) the lesion is of a different nature. Statistically, it is more likely to be a second oligodendroglioma. The patient is in the high risk category (older than 40 years of age and subtotally resected) and I would therefore treat the known primary with chemo RT per RTOG 9802 using 54 Gy at 1.8 Gy/fraction.

For the second lesion at the pontomedullary junction, one can use a lower dose of RT (50.4 Gy in 28 fractions), and depending on the relative location of the 2 lesions, I might use 2 isocenters.

If treating the full thickness of the pontomedullary junction is daunting, then I would treat the primary lesion as above and the secondary lesion using Vorasidenib, an FDA-approved drug for IDH mutated low grade gliomas (Mellinghoff et al., PMID 37272516). In this phase III RCT, the time to the next intervention was significantly improved in the vorasidenib group as compared with the placebo group (hazard ratio, 0.26; 95% CI, 0.15 to 0.43; P<0.001).