Slow down.

Published results of B-39 and RAPID bring accelerated partial breast irradiation (APBI) back into the spotlight so let's pause to reflect on the much more widely available and readily implementable external beam techniques. First up, target volumes. This was the main difference between the trials. In RAPID the CTV was a 1 cm expansion of the lumpectomy cavity excluding the chest wall, muscles, and 5 mm of subcutaneous tissue. For B39, it was 1.5 cm with the same exclusions.The PTV in both trials was a 1 cm expansion and was used for beam arrangement. The “dose evaluation volume” (DEV) in RAPID and the PTV_EVAL in B39 used for DVH analysis was the PTV with the same exclusions as the CTV. Ok, beam arrangements. In RAPID there were four non-coplanar fields: a pair of medial and lateral tangents and a pair of anterior/superior and posterior/inferior beams using couch kicks. Anything was allowed in B39 but similar arrangements to RAPID were encouraged. In both, 3.85 Gy was prescribed to isocenter of the PTV and was delivered twice daily for 10 fractions. The contralateral breast, lung, and heart were excluded from each of the beam’s eye views. In RAPID, < 25-35% of the breast could get 95% of the prescription while in B39 it was 100%. In both, < 50-60% of the breast could get 50% of the prescription. TBL: APBI via external beam is easily implementable. | Vicini & Whelan, Lancet 2019