What are your skull constraints when administering cranial SRS/fSRS?

I have never used skull constraints when delivering intracranial SRS. While intuitive to think about any adjacent uninvolved tissue as an organ at risk and limit the dose to said structure, in relation to the skull bone itself the cost of "spray" dose to the skull is much less costly than the alternative, which is parenchymal brain tissue that would surround the other 270 degrees of a peripheral/cortical/dural intracranial lesion. With that thought in mind, I definitely pay attention to brain DVH criteria related to radionecrosis (typically the V12) and seek to minimize that, while being ok with the skull bone itself serving as a low-morbidity area to "dump" dose, while of course, still aiming to keep the overall volume treated as conformal as possible to the target.

The more relevant consideration (in my opinion) when treating something peripheral/dural where one might be concerned about skull dose is SRS-related alopecia. I do not have good data on DVH parameters for single- or multi-fraction SRS-related alopecia, but have definitely seen this in my clinical experience. Particularly troublesome cases in this regard are surgical cavities, as the targets are usually peripheral, large, poorly defined, and often involve a wide base of dural surface. In these cases, the above principles still apply, i.e., limit the V12 to the adjacent brain parenchyma (Patel et al., PMID 30084023 radionecrosis DVH in cavity SRS), but also pay attention to the scalp dose. In scenarios such as a vertex scalp treatment in a very bald elderly male, the inherent conformality of SRS is enough to not be concerned about problematic dermatitis (which I've never seen) so I would not contour the scalp, but in a young patient where cosmesis/focal alopecia is a concern, I would recommend contouring a 3 mm strip of adjacent scalp/skin and striving to keep high doses away from this region (i.e., <30-50% of the peripheral SRS prescription dose) to limit significant alopecia. I learned this lesson the hard way with a few young (upset) patients that developed significant alopecia that was ultimately temporary, but distressing for ~3 months.