The 41-year-old was diagnosed with the disease in March following the discovery of a tumour.
The nine-times Irish champion jockey has just completed his course of chemotherapy.
With that stage of his recovery finished, Smullen is now likely to undergo surgery.
Smullen said: "Thankfully everything is going according to plan.
"We're through the chemotherapy now and on to the next step.
"That will hopefully be followed by surgery and then hopefully that will be the end of it.
"I'd like to thank everybody again for their support and well wishes.
"That has been a big factor in me getting on so well and getting this far."
The rider's big-race wins include the 2016 Derby on board Harzand and the 2000 Guineas aboard Refuse To Bend in 2003.
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