• GTV: Surgical bed, including clips (if available)
• CTV: Uniform 1-1.5 cm expansion from GTV, with CTV subtracted from muscle and bone
• PTV: Uniform 0.5-1 cm margin (with daily CBCT), subtracted 5 mm from skin surface for PTV_eval structure

NOTE: Goal is total 2cm from GTV to PTV

• Maximal dose to 2% of PTV: <33.6Gy (ideal <32.1 Gy)
• Uninvolved breast: V15Gy<50% (ideal <25%)
• Contralateral breast: Max dose <2Gy (ideal <1Gy)
• Ipsilateral lung: V10Gy<20%
• Contralateral lung: V5Gy<10%
• Heart: V5Gy<10% (ideal V3Gy<10%)

NOTE: Optimal PBI candidates have a PTV to total breast volume ratio of <25%, though there was no upper limit on Florence trial.

• ELIGIBILITY: UK recently reviewed their eligibility criteria, including women >50yo, ER+, <3cm (Taylor et al Clin Oncol 2020), though NSABP B-39 also included  a large number of TNBC.

• LOCAL CONTROL: This 30 Gy in 5 fraction PBI regimen was non-inferior to whole breast irradiation at 10-year follow-up (abstract only) in Florence trial.

• SIDE EFFECTS: Acute toxicity is less with APBI and cosmetic outcome is similar at 5-year follow up using Florence regimen (every other day). Correspondence with the trial PI suggests that preliminary evidence suggests daily treatment is equally safe, if needed. NSABP B-39 and Canadian RAPID using a BID regimen which should be used with caution due to concerns about cosmesis.

• SECOND MALIGNANCY: VMAT using 2 coplanar partial arcs is the preferred approach based on dose homogeneity. Second malignancy rates were very low in Florence trial (7 total at 10 years) and contralateral breast cancers <1% vs 3% with whole breast (not significant).