Dr. Shah suggested cessation because you are PCRU and have been for several years. You had very quick response on Gleevec.
There is near zero risk to trying cessation first to see if you have a functional cure (i.e. 'treatment free remission'). You would have to have a PCR test taken once a month initially to catch any PCR uptick. If you are to fail cessation, you will likely fail within the first six months. But each month that you continue to be below MMR (i.e. PCR < 0.1%), increases your odds of success.
If your PCR does rise above PCRU, you can continue off Gleevec and see if an upward trend develops. Many cessation patients have initial increases above PCRU, but then levels fall back again all the while staying below MMR. If you are comfortable tracking your CML this way, you could go a very long time without any TKI and most important - no more side effects during this time (although withdrawal symptoms are reported).
If your PCR rises and continues to trend upward, you can restart your TKI on a lower dose and continue monitoring. In this way you can find what lowest dose possible is sufficient to maintain response.
In my own case - I tested cessation before becoming PCRU. I was at PCR < 0.01% which is technically "detected", but no longer an accurate read (could be false positive). So I tried cessation. I was on 20 mg Sprycel (1/5th normal dose). I was able to stay off Sprycel for nine full months (nine WONDERFUL months). My PCR did go above 0.01% but never went above 0.1%. It was vacillating between 0.01 and 0.08%, but with an overall - very slow trend upward. My doctor wanted me to continue cessation to see if I stay in this range (below MMR). I was tempted, but decided to resume 20mg Sprycel just to prove to myself that I can push my PCR back down and how long that would take. Sure enough, my PCR plummeted within a month on resumed Spyrcel and in two months I was back below 0.01%. Lately, I have even gone "undetected" for the first time (PCRU) - I will test cessation again if I stay PCRU for another PCR test.
No one has progressed with advanced disease who tried cessation and lost PCRU. All patients who restarted their TKI regained their prior response quickly. Cessation is safe to try. But there is a psychological barrier that has to be crossed when doing so. Many patients are uncomfortable trying cessation. Near zero risk is not the same as zero risk.
Regarding your immune system "kicking in". CML disease - in fact - most cancers occur because our T-cells fail to activate in sufficient numbers to attack cancer cells. As CML cells are part of our "self", they put out proteins (cytokines) which increases the number of a special T-cell (regulatory T-cell, aka Treg) to shut down other T-cells and not attack (natural killer T-cells). It is a natural mechanism to prevent our own killer T-cells from attacking our healthy cells. Autoimmune diseases are killer T-cells run amok.
Cancer is really a problem in bad cell population control. It takes a lot of CML cells to produce enough cytokines to shut down our normal T-cell attack response. There is a theory that when we first developed CML, it was because something triggered a massive stem cell transformation creating the philadelphia chromosome of our normal blood stem cell into leukemic ones - we know radiation does this - exposure to certain chemicals can do this. One spontaneous CML cell creation is not enough. This large population of cells are then able to maintain and expand by shutting down our killer T-cell response.
The reverse may be true. By reducing this damaged population using TKI's - enough CML cells are destroyed that those remaining are unable to put out enough cytokines to shut down our normal killer T-cell response. Our bodies are able once again to defend against CML without a TKI. It's just a theory. But - those patients who have successfully tried cessation and succeeded - they no longer show CML. Something is protecting them without a TKI present. We simply don't know if it takes the eradication of every single CML cell. My own opinion is that eradication is not required give what we now know about T-cells and how they work. Regaining an immune response is what is required.
Diagnosed 11 May 2011 (100% FiSH, 155% PCR)
with b2a2 BCR-ABL fusion transcript coding for the 210kDa BCR-ABL protein
Sprycel: 20 mg per day - taken at lights out with Quercetin and/or Magnesium Taurate
6-8 grams Curcumin C3 complex.
2015 PCR: < 0.01% (M.D. Anderson scale)
2016 PCR: < 0.01% (M.D. Anderson scale)
March 2017 PCR: 0.01% (M.D. Anderson scale)
June 2017 PCR: "undetected"
September 2017 PCR: "undetected"