No, the claims about Theta Healing have been thoroughly debunked
The claims that it can instantly cure cancer or facilitate the growing back of lost limbs should be an indication of its validity. If anyone was able to grow back a lost limb, this would have been widely noted.
Even if you consider the above claims to be plausible, perhaps her claim that god speaks to and guides her (Vianna Stibal, creator of Theta healing) directly are an indication that her methods are not based on science, nor on any proven method.
Skeptoid has a good explanation on Theta healing:
Theta healing is just another random "roll-the-dice and invent a New
Age healing technique" based on some imaginary energy field. This one
was invented in 1995 by a self-described "intuitive reader" who
believed that she could project theta waves from her brain which would
instantly heal the patient. Theta healing claims to immediately and
completely cure cancer, severe injuries, and even psychological and
In fact, a theta wave is not an energy field or a type of energy at
all. It's simply the name of one type of waveform on an
electroencephalogram. When an EEG shows a sine wave with a period of
between 4 and 8 Hertz, neurologists designate that as a theta pattern.
This waveform is observed when the subject is asleep or at rest, and
in some types of learning involving short term memory.
Probably what happened is that this "intuitive reader" heard the term
"theta wave", misinterpreted it to mean a type of mystical energy
field generated by the brain; and then erroneously put two and two
together when one of her customers reported some kind of positive
experience. If you look at any web page about theta healing, you'll
see all kinds of vaguely scientific sounding words thrown out there in
meaningless disarray. There's nothing plausible about it and no
description specific enough to test, but unfortunately, unscientific
laypeople are all too often ill-equipped to recognize that such
technobabble on a web page is baseless.
The BBC, certainly a reputable organization, did a comprehensive debunking:
Professor Ernst says such claims are "irresponsible, even criminal".
He believes that the ThetaHealing group try to distinguish themselves
from the other 20,000 faith healers in the UK by applying a "veneer of
science", but says "it's still nonsense".
Lastly, a columnist on the ABC Australia website has a good summary of why it is all a crock:
I did some more digging on Theta Healing: apparently it involves
putting the patient's brain into a theta-state, operating at four to
seven cycles per second. The practitioner then uses a highly dubious
and often-debunked practice of applied kinesiology, where muscle
response is tested to determine the patient's problem.
Moving on: Once the Theta healer has ascertained the medical issue,
the big guns come out - DNA Activation. According to the website, the
"master cell" of the "pinel (sic) gland" is activated, which
apparently switches on bits and pieces of junk DNA, allowing the
healer to treat disease, make you rich and get rid of wrinkles.