mCRC complete response to second line treatment -- what to do next?

Hi all,

First time poster here. It's great to see such a lively community here! I wish I found this earlier!

Our family is a little lost on what we should do. Here's the history:

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My father was diagnosed with Stage 3b colon cancer (in the sigmoid) a few years ago. He had surgery, 12 rounds of adjuvant chemo (FOLFOX), and then the cancer came back in the liver after ~10 months.

The recurrence was verified with increased CEA (which seems to be a great indicator for my dad), an MRI, a CT, and a PET scan. The PET scan also showed a small blip in the lungs, but the MRI and CT didn't show anything there. My dad had a partial hepatectomy, and the CEA dropped back to normal levels for a while.

After ~4 months, CEA was still normal, but a CT scan showed a small mass near the retriperitoneum. PET scan showed 2 blips in the lungs (similar to before, these didn't show up in CT) and one lymph node in tthe peritoneal area (but may have also been just on the outside of the liver). Given that the CEA was still normal though, we opted to wait and see what happened.

After another month, CT showed no change. Our first Signatera test came back at positive, but very small (0.05). We decided to just wait and see what happened.

After another month, CT showed a new mass in the liver. The retriperitoneum still did not change in size. Signatera went up to 7. PET showed the same two spots in the lungs and the new mass in the liver (but curiously, the retripertinoeum stopped lighting up). CEA also went up. Our surgeon said it was operable, but the surgeon and oncologist thought it would be better to start with chemo.

We started on FOLOFIRI + Cetuximab (no NRAS or BRAF mutations). When we started, CEA was up to 50 and Signatera was up to 60. We went through 7 rounds of this chemo, and every week the CEA and Signatera dropped. Around the 5th round, Signatera came back negative. And by the 7th round, the CEA dropped to 1.8. The 8th and final round dropped CEA to 1.3 (so it's still dropping). We had a CT scan and a PET scan, and both show a complete response (no sign of any disease).

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Should we continue the FOLFIRI treatment since CEA is still dropping? Is there a chance that it'll cure it if we keep going?

I know we're lucky that we had such a good response to this line of chemo. From what I've read, it seems to be fairly rare. We just finished the 8th round of the chemo, and we're planning to start maintenance chemo after this (5FU + cetuximab).

I'm curious if anyone has had a similar experience before? We're not really sure how to feel... we're hoping the cancer's gone, but worry that it's likely to come back. If anyone has been through something similar, I'd love to hear how it went!

Thanks in advance!