Top Line: When it comes to accelerated treatment schedules for breast cancer, it can’t get any faster than one fraction (ok, except maybe a FLASH fraction). 
The Study: Single fraction accelerated partial breast irradiation, “APBI,” has largely come in the form of intra-op radiation (IORT) to this point. However, IORT wasn’t an option on the big APBI trials. Furthermore, there’s the whole ordeal of having to have an IORT device. Plus the concern over higher rates of tumor recurrence. We’ve seen it done pre-op, and now we have an early phase study of single-fraction external beam APBI for women >50 years of age with T1 or Tis (<2 cm) node-negative breast cancer after lumpectomy. Targets were (1) the surgical cavity limited to >5 mm from skin, and (2) a 1 cm margin from the cavity excluding 5 mm from skin and chest wall musculature. The cavity was prescribed a minimum of 15 Gy x 1 with a max dose of 22 Gy, while the 1 cm expansion received a minimum of 5 Gy x 1 (although the median D95% was actually ~10 Gy). In an interesting tech juxtaposition, most patients were treated on a Co-60-based unit with MRI guidance. At a median follow-up just over 3 years, among the 50 patients enrolled, one (2%) developed an ipsilateral breast tumor recurrence, which was actually a new in situ lesion in a different quadrant. Another developed an isolated axillary recurrence. Importantly, there were no grade 3+ toxicity events. One-third of patients had grade 1 skin erythema, and an eyebrow-raising 100% had good-to-excellent cosmesis.  
TBL: You heard it here first, single-fraction external beam breaStBRT using MRI guidance is on it’s way to larger trials. | Kennedy, Int J Radiat Oncol Biol Phys 2020


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