Pauline Pain, 58, from the Isle of Wight, became the first patient in the world to receive the radiotherapy.
Mrs Pain, who has a cancer of the blood called multiple myeloma, was able to return home after her radiotherapy to await her bone marrow transplant. A conventional total body irradiation would have meant a long stay in hospital.
She said: "The only side-effects were a little temporary tiredness and mild sickness; other than that I felt very, very well. It was incredible to be walking around knowing that something inside me was fighting the cancer, but I couldn't feel it at all.
"The beauty of it was that I had the big dose of radiotherapy in the morning and I was at home by 5pm the same evening." The two-year trial at Southampton General Hospital will involve 80 patients, half of whom will receive the new radiotherapy with chemotherapy and the other half will have chemotherapy alone.
The early results are "very encouraging", doctors said.
The radiotherapy is used to kill the cancer cells in the system before a transplant of healthy stem cells to replace the lost ones.
The same dose of traditional radiotherapy would cause severe or even fatal damage to the body, proving toxic to the liver and kidneys.
But because the new system delivers a radioisotope that attaches only to the surface of cancer cells, the healthy tissue is not affected.
The treatment is being tested on patients with multiple myeloma, but it may be extended to other cancers of the blood and bone marrow such as leukaemia. Multiple myeloma and leukaemia affect 11,000 people in the UK each year.
Dr Kim Orchard, a senior lecturer at the University of Southampton's School of Medicine who is leading the trial, said: "Radiotherapy is used to clear the bone marrow of myeloma cells before a stem-cell transplant. Current treatment uses high doses of radiation, which are delivered by X-rays, but the sensitivity of healthy organs limits the dose that can be tolerated.
"Previous attempts to use antibodies to deliver the radioactivity have been frustrated by their accumulation in the liver, lungs and kidneys, which can cause grave complications. The key to this new treatment is that the antibody accumulates only in the bone marrow.
"We hope that the trial will show a clear benefit in better and longer remissions from myeloma. If we are successful, this approach offers great promise for the treatment of a range of other blood cancers."
Dr David Grant, Scientific Consultant at Leukaemia Research, which is funding the study, said: "This trial is very exciting. One of the main reasons why stem cell transplants have been less successful in the long-term treatment of myeloma is that patients are not cleared of all the cancer cells before the transplant.
"This new radiotherapy is not only more effective and potentially cheaper than existing treatments, it is far less toxic for the patient. It also enables patients to go home immediately after the procedure, reducing time spent in hospital."