cfDNA and CTC Testing after Adjuvant Treatment (Guardant360)

hoosier238

Background: My mother was diagnosed with Stage 3A breast cancer almost 2 years ago. The tumor was ER+/PR-/HER2+ and grade 3, so we had 6x TCHP in the neoadjuvant setting, following by a mastectomy, radiation, and 8x HP in the adjuvant setting (we were lucky that our onco was able to get Perjeta approved for the adjuvant setting). Neoadjuvant treatment led to shrinkage but not PCR. Surgery showed 2 positive nodes (which were ER-/PR+/HER2-). Surgery also showed ECE, LVI, and elevated Ki60 levels (we don't have baseline Ki60 from the biopsy). Since completing adjuvant treatment, we have been on an AI and twice-yearly denosumab.

Recently: We have visited a new onco to discuss pain management (the AI has caused lots of pain) and use of neratinib. Given that the surgical specimens (main tumor and lymph nodes) showed that the cancer was heterogeneous, the doctor recommended using cfDNA and genetic testing to figure out the best course of action. One test he ordered was Guardant360. Based on the results of the Guardant360 test, he may order a reanalysis of the surgical specimen.

My understanding is that the Guardant360 test will (1) measure how much, if any, micrometastatic cancer remains; and (2) indicate the genetic features of the micrometasteses.

Question: Have others had experience using the Guardant360 test in this way? What other cfDNA, CTC, or other tests should we consider (eg FoundationACT, OncotypeSEQ, or Myriad MyRisk)? I feel like I don't really know all the options in this area, so I don't know if Guardant360 is the gold standard or just one option out of many. In general, I understand that different cfDNA and CTC tests can give widely different results.