How do you approach the choice between 3-fx and 5-fx postop SRS for a resected brain metastasis for intermediate cavity volume?

In my practice, I generally use single-fraction SRS for lesions/cavities smaller than 2 cm in maximum dimension or ~4 cc in volume to 18 - 20 Gy, but this is a relatively rare occurrence given the fact that resected brain metastases tend to be larger.

For cavities larger than 2 cm (or about 4 cc volume), I use 27 Gy in 3 fractions as my standard dose regimen for cavities up to 4 cm maximum dimension or about 30 to 33 cc volume. The best data for this approach comes from the Stanford Phase I/2 trial (Klebaner et al., PMID 40089071), where the late radiation necrosis rate for cavities up to 4 cm / 33.5 cc was 33% (2 of 6 patients) at the dose of 30 Gy in 3 fractions vs. 0% at the 27 Gy in 3 fractions dose level.

30 Gy in 5 fractions is an isoef...

Yes, decreasing the dose per fraction is a great way to increase normal tissue safety, with a small tradeoff of increasing a few more fractions. Increasing from 1 fraction to multiple fractions has enabled effective yet safe short-course radiation treatments. How few fractions we can do safely is dependent on several factors; foremost are volume of normal tissue exposure and location in the brain or other normal tissues collaterally irradiated. Other factors to consider are systemic therapies that may increase risk of inflammation or injury acutely or delayed, age, co-morbidities with increased radiation sensitivity, and disease prognosis. Following the practice of A071801 is a great start, but if there are ever concerns for safety, decreas...