How do you contour a resected brain metastases for fractionated partial brain radiation?

When treating a resection cavity in the brain after surgical excision of the tumor, I contour the resection cavity on a Gd-contrast enhanced volumetric MRI. For most large metastases, an average cavity volume reduction of 15% to 43% has been reported in several published studies within 3-4 weeks after the surgery, compared to the MRI obtained within 48 hrs from surgery. Larger tumor cavities (> 3 cm) are associated with greater reduction in the 2nd postoperative MRI Minniti et al., PMID 33858474. The CTV expansion will depend on the pathology being treated with fractionated radiation; for example, if it is large metastases, I use a 3 mm expansion on the cavity, and for pretreatment tumor volume in contact with dura any meningeal attachment,...

If you're just giving 30 in 10, I don't think you have to sweat the details much, after all, you could give the whole brain 30 in 10. If your equipment is limited, you could try a wedged pair or 3 field plan to spare structures like the lens, salivary glands, and hippocampus, if possible. Using 2d or 3d techniques, I'd probably give 1 cm of margin to the contrast-enhancing cavity and would not chase it out to the skull unless I had a specific reason to suspect the skull's involvement.

OTOH, if you have the capability to treat using VMAT or IMRT, why not give 8 Gy x 3 QOD to the cavity with a 2-3 mm margin? The patient would be done more quickly and the local control should be higher.

I agree with @Dr. First Last's excellent summary.

And @Dr. First Last brings a good point - no need to worry about margins as much or OAR for 30 in 10.

If this was a resected brain metastasis, either offer HA-WBRT (using whole brain as per the initial Patchell study) or use post-operative SRS/SRT. In this case, the margins and doses are somewhat per institution, as some would not place any PTV margin and some use up to 2-3 mm. We use 21-24 Gy in 3 fractions to resected cavities </= 3 cm and 25-30 Gy in 5 fractions for cavities > 3 cm or those adjacent to the brainstem or optic structures. 25 Gy in 5 may not have a high BED, but is well tolerated by most brain parenchyma so can be used to a relatively large volume.