MARI is used in breast cancer treatment, especially when patients receive neoadjuvant systemic therapy (NST).
Before the therapy, a tumor-positive lymph node in the axilla (armpit) is identified and marked with a small radioactive seed.  After therapy, that same marked node is removed using a gamma-detection probe. 
They also remove other nodes (axillary lymph node dissection) in studies to check whether the marked node’s response reflects what happened in other lymph nodes. 
MARI helps tailor further axillary treatment.
If the MARI-node is cancer-free after therapy, doctors might avoid more extensive lymph node surgery, reducing complications like lymphedema. 
The procedure had a false-negative rate of ~7%, as in some cases the MARI-node was clear, but other lymph nodes still had cancer.
A 3-year follow-up study showed very good recurrence-free outcomes when using the MARI-protocol that tailors further treatment based on MARI-node status + PET/CT before therapy. 
It requires precise placement of the seed before therapy. 
Even though it lowers the need for full lymph node dissection in many patients, there’s still a risk of undertreatment if the marked node clears but other nodes still have disease.