For APBI, do you prefer a brachytherapy or external beam technique?

I offer a variety of PBI techniques depending on situation and patient preference.

With respect to external beam, I primarily utilize a 30 Gy/5 fraction regimen delivered with VMAT (2-3 coplanar arcs). We use breath hold regardless of laterality and CBCT to reduce motion and reduce PTV expansions. The pros of this technique are that its non-invasive, qday and only 5 fx (as compared to BID 3D-CRT) and outcomes to date have shown excellent clinical and toxicity/cosmetic outcomes. The cons are you do see higher contralateral breast dose with more central tumors and that we don't have 10 year data with this technique.

Alternatively, I will offer patients 40 Gy/15 fx external beam based on IMPORT LOW with larger cavities. The pros are its well studied with respect to toxicity based on HWBI trials with the con being its still 15 fx.

Brachytherapy remains a great option for patients interested in the technique particularly with shorter courses such as those published in TRIUMPH-T. It works well for medial cancers. The cons are that it is a procedure and moving forward applicator brachytherapy may fall out of favor with new payment models.

For APBI at WashU we lean towards external beam. Over the years, the fraction of APBI patients that we elected to treat with brachytherapy has gone down from about half of our APBI volume to less than 10% largely driven by factors that @Dr. First Last lists. For patients eligible for APBI, we will often do a bedside ultrasound and assess the surgical bed for a seroma; if suitable for a single entry device we may proceed with brachytherapy using a SAVI and often deliver a TRIUMPH-T fractionation. For external beam, our preference is to use MR guided therapy via MRIdian. We plan on a MR simulation scan and contour the surgical bed then add 1 cm of breast tissue limited 5 mm from the skin and no further margin (i.e., the same target volume as brachy). We use MR imaging for each fraction and if needed use MR gating on the surgical bed during treatment delivery. All MR scans are non-contrast. After many years of 38.5 Gy/10 fractions/5 days, we switched to 30 Gy/5 over consecutive working days. Most patients don't get a skin reaction.

With guidance from @Dr. First Last, we have treated many patients this way in our community practice with confidence and good outcomes (albeit, very short follow up). BMDACC does use somewhat different CTV volumes than CCF (which are also different than Livi), but the technique is nearly identical. We do an SIB of 30/5 to a smaller volume (1cm CTV expansion), 26/5 to a larger volume (2cm CTV expansion), and 0.5cm PTV margin for both. Both volumes are edited to come back from skin, muscle, chest wall, lung. 

The plans look excellent dosimetrically, the acute toxicity is essentially non-existent, and patients are exceedingly delighted with the short course.