Utility of ctDNA to Manage Adjuvant Therapy in CRC Based on Results of the GALAXY Study - Episode 5

GALAXY: DFS by ctDNA Status at Weeks 4 and 12 Post Surgery


GI oncologists discuss DFS data from the dynamics analysis cohort of the GALAXY study.


Mark Lewis, MD: There is a lot to take in on this slide. I’ll tell you what my takeaway is, Dr Cohen. I think what’s most fascinating, albeit in a very small group, the positive to negative meaning that that initial positive signal can actually be overcome and we might see on the next slide why. In fact, Dr [Masahito] Kotaka [MD, PhD], when he was summarizing GALAXY at ASCO GI [American Society of Clinical Oncology 2022 Gastrointestinal Cancers Symposium] called out the dynamism here. He is basically saying your positive signal at 4 weeks doesn’t necessarily have to be a persistently negative prognosis, that’s something we might actually be able to affect. Really this gets into the whole clinical rationale behind adjuvant chemotherapy. I will be very honest with you, when I was first training as an oncologist and the concept of adjuvant therapy was explained to me, I thought this is going to be a really hard sell to patients. Often what will happen is they come to my clinic and they have been told by a colorectal surgeon, often very well meaning, “We got it all.” Meaning clean resection margins in the operating room, but then we know that there can be MRD [minimal residual disease]. It’s just been that until this point these assays have not been readily available [where] we could measure the MRD and know who we were most likely to benefit with adjuvant chemotherapy and even now I am making some leaps of logic. It’s fascinating to me to see that there is positive to negative group between the 4- and 12-week time points and that then those people that do convert to negative, they have a 100% 6-month disease-free survival. I just think that is absolutely remarkable.

Stacey A. Cohen, MD: My take away from that is always whenever you see something that is so perfect you always question the data a little bit. So what also went through my head is, let’s think about the time point, this is 6 months. If you look at the other observational studies they would note a lead time of about 8 to 9 months from the time of detection of ctDNA [circulating tumor DNA] until recurrence. We also don’t know what the imaging surveillance schedule was here. Yes, I am not terribly surprised that a negative patient would be disease-free at 6 months after surgery but I would be very interested in what was done. Were extra CT scans done for that patient because they were initially positive? Have they just not yet had a scan? It would be very acceptable not to scan them for a year. On what basis are we considering them disease-free? I will be interested as this data matures and there (are) presumably more events in the positive to negative group. I think it’s interesting, but I am going to be a little bit more cynical about it.

Mark Lewis, MD: Great point. Every oncologist is at heart an optimist, but you're quite right to withhold enthusiasm until we see more data.

Transcript edited for clarity.