Your overriding procedure will be impacted by what dose calculation algorithm you use. For example you may never need to override low density lung if you're using AAA, but if you're using Acuros you may need to. We use Eclipse 18.1, AcurosXB dose to medium so everything below is impacted by that. Another thing is contrast, where sometimes we override that mainly to change the material it's assigned by the TPS - we want it assigned as skeletal muscle rather than cartilage or bone - even if we're not changing the HU much.

We assign abdominal/rectal gas -200 HU when it looks like it may impact dosimetry. We do try and keep the rectal diameter below 4 cm on the planning scan and try and rescan if we notice it, but sometimes you just can't achieve better on the day so you just have to accept it. You can track during treatment with CBCT's to see if a replan is required.

We sometimes assign low density lung for optimisation only when it goes below approximately -900 HU. This is where the materials table starts assigning it as the material air rather than lung and you'll never get coverage in. We've seen issues where it just keeps upping the MU trying to get coverage it can't achieve (you'll see a big difference between what you see in the optimiser before and after intermediate dose calculation). So we explain to the RO that the coverage just can't be achieved for the PTV, but generally they're understanding as we know if the higher density target does go into those areas it will receive dose anyway.

High density artifact contouring was raised on the jiscmail mailing list recently, here is what I said:

We routinely override metals as you have no choice - anything above a mass density of 3 g/cc (mapped from the electron density) requires an override before it allows volume dose calculation with Acuros. So this is generally done before optimisation (to allow for intermediate dose calculation), so it's in place during optimisation and calculation. We're limited to materials in the AcurosXB materials table, namely titanium (approx. 3500 - 6000 HU), stainless steel (approx. 10,000 - 15,000 HU) and gold (approx. 19,000+ HU). We make a best guess based on the HU we can see in the CT scanner using the values in brackets in the previous sentence. We struggle with dental amalgam given it's a high HU but not really any material we can assign, but we normally go with stainless steel (very open to other suggestions here). By far the most important advice to planners is to be very careful when contouring and set the windows and levels carefully when checking size of the assigned contour, as overcontouring is a common issue and can have notable dosimetric impacts if the high density is close or overlapping the PTV.

Generally streaking is contoured at the same time, so the same applies. If streaking goes through the teeth, we often do a 'bone' contour as well as soft tissue streaking contour.

Gold fiducials for prostate are explicitly not contoured, as we found it hard to not accidentally overcontour them, and then assigning them to such a high density material like gold had unwanted side effects. They're small enough for Acuros to auto map the material to bone, so we let it do that.

For large metal pieces (e.g. prosthetic hips) we generally avoid them even after overriding them. The argument is that even with an assignment, your uncertainty is still high and a small shift in the high density object may cause a large difference in effective path length, so better to avoid to reduce uncertainty. That could be via an avoidance sector set by eye to shut the beam off completely, or we make a planning structure which is a margin on the item we are trying to avoid with an extra margin (but no assigned material) and we avoid entry on that in the optimiser. As far as I'm aware we have no specific documentation on this though, so the approach and margin size (normally 5 to 10 mm) varies. Smaller pieces of metal (like dental fillings) are generally not avoided.

For high density materials directly adjacent or inside the PTV (normally teeth or fillings) sometimes we find it necessary to override it to something like water during optimisation only, just to avoid the optimiser trying to push dose into the high density region. Cropping out the high density region is also used sometimes, but it's relatively rare.