What type of IGRT do you use for external beam partial breast irradiation?

Cone beam CT verification if available.

If CBCT is not available or if patient position/anatomy precludes use of CBCT, you can also use orthogonal films aligned to chest wall or clips if visible. Using this technique, you should use a wider PTV margin for set up uncertainty, of 7-10 mm. I have also placed textured fiducials in the index quadrant in a number of patients and then aligned the orthogonal films or CT to the fiducials, in which case you can use a PTV margin of 5 mm.

We have noticed kV imaging and CBCT to be complimentary. It is worth noting that seromas resolve and can result in fiducial migration. This can be particularly significant if there is a delay between the treatment planning CT and the first CBCT/treatment.

I prefer daily CBCT because it captures subtle rotations and shifts that are often not detectable on orthogonal kV or MV port films. Based on the NYU/Weill Cornell experience by Dr. Formenti et al., I treat partial breast external beam cases in prone position with CBCT, isocenter set in mid-anterior chest near the sternum to avoid clearance issues. I then try to use directional shifts only, no rotational shifts, and take port film verification in at least one of the fields after the shifts before starting treatment. If there is a large discrepancy in patient positioning, she will be asked to get up and re-set, with repeat imaging as above.

Certainly, CBCT or orthogonal films are good options that are widely available in the US. We have also used a topographic set up to try to minimize the PTV margin. It's not widely available but in our experience, the best is MR-guided therapy allows us to trim the PTV to a bare minimum by gating on the surgical bed - in these cases, our external beam APBI plans look very similar to our single-entry brachytherapy APBI plans.