Mike Brandon, 31, from Bristol, is being treated in Philadelphia after his British consultant said all NHS routes were "exhausted".
Kate Brandon said it had been "by far the hardest three weeks of our lives."
She said her husband had spent the last week in intensive care but was now back on the "wonderful oncology unit".
Mr Brandon was diagnosed with acute lymphoblastic leukaemia two years ago.
In May, his wife raised enough money using a social media campaign to fund his taking part  in "revolutionary" cancer trials at the University of Pennsylvania.
Mrs Brandon said: "We were told that paradoxically the sicker that people become during this treatment, the more likely they are to have a good outcome.
"This knowledge has helped carry us through this excruciatingly difficult time. Boy, did our boy get sick."
Mr Brandon said: "There are plenty of things about a leukaemia diagnosis that are tough: the constant worry about things that may never happen, the strain of the things that have definitely happened, and the stress it places on those closest to me.
"When my leukaemia relapsed in March, we were told that it was time to plan our last days together, but Kate point blank refused. We would fight on, and we have."

The members of the jury retired to consider their verdict on Friday morning. However, the case has now been adjourned until after the weekend.
The judge, Lord Burns, had earlier gone through the options available in terms of the charges against Rachel and Nyomi Fee.
He told members to consider the evidence dispassionately and without speculation.
Rachel, 31, and Nyomi Fee, 29, deny murdering the two-year-old in a house near Glenrothes on 22 March 2014 and blaming his death on another child.
The women, who are both originally from Ryton, Tyne and Wear, deny all the charges against them.
The trial has been running for seven weeks at the High Court in Livingston.

Steve Williams, the British ambassador to Bulgaria, was appointed on Tuesday and will take up the post in April.
Both islands agreed to set up a Brussels office last year, to ensure their interests were represented in the European Union.
Mr Williams said he has a lot to learn ahead of taking up his new job.
He said: "I'll be very keen to spend some quality time on the islands to get to know people, to get to know the issues.
"Once the office is up and running properly and I'm established it will be really important to come back regularly."
Guernsey's Chief Minister Lyndon Trott described the role of the office as "vital" in how the Channel Islands "engage with Brussels and the European Union over the coming years".
The appointment of a deputy director to assist Mr Williams in his new role is expected to be announced shortly.

First Greater Manchester plans to increase the price of onboard tickets by 10p in January but freeze the cost of those bought via its mTickets app.
It said it wanted to encourage customers to buy tickets in advance on smartphones to speed up journey times.
Passenger group TravelWatch NorthWest said it may reduce delays but penalised those without the technology.
A spokesman for the bus operator said about 15,000 passengers already use the app and its research showed boarding times would be 75% faster if all passengers used it.
He said the company were confident freezing ticket prices on the app would encourage more customers to pay that way.
TravelWatch NorthWest's John Moorhouse said while he understood it was "an incentive to use the app, a lot don't have the internet and rely on buying [tickets] on the bus".
He also said it was "unfortunate" for the 22% of passengers who do not travel with smart phones.
The majority of single onboard tickets will see a 10p increase from 2 January.
A man with a rare type of leukaemia whose wife raised £400,000 to cover his medical costs in the USA is "well under way" with his cancer treatment.
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The jury in the Liam Fee murder trial has been sent home until Monday.
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The man chosen to head up the Channel Islands' new Brussels office has admitted he has little first hand knowledge of the islands.
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Passengers buying tickets onboard some Greater Manchester buses are to pay more than those using a smartphone app.