Why do we tend to boost grade 3 DCIS, but not grade 1 invasive disease?  

For the same reason that a surgeon has to re-excise a 1 mm margin in a pure DCIS case, but that same surgeon does not re-excise a 1 mm margin on the DCIS that is associated with an invasive component...because guidelines.

I agree with @Sushil Beriwal. In addition, the calculus for a treatment decision is based on the expected value, which is mathematically a function of both the risk (probability) of an event as well as the significance of the event if it occurred. Biomarker status was not reported in the question, b...

The risk of local recurrence is a function of grade, age, phenotype, and margin status. More the risk factors, the higher the risk of recurrence, and the rationale for ASTRO's recommendation for boost https://www.astro.org/Patient-Care-and-Research/Clinical-Practice-Statements/Whole-Breast-Irra...