In a hearing that was often pointed, VA officials also withdrew their own previous estimates of the number of patients who were affected, asserting that the mistakes were far less common than previously believed.
NRC officials said they were surprised by the VA's about-face.
After 19 months and numerous on-site inspections and delays, "now you come with a new criteria" for counting botched cases, said Steven A. Reynolds, director of nuclear-materials safety for NRC's Region III, which has led the agency's investigation. "It is troubling."
The NRC demanded written testimony by Jan. 15 to back up the VA's rationale to limit sanctions. The NRC also said it would issue violations and any penalties four to six weeks after that.
The four-hour hearing yesterday was a chance for the VA to explain its troubled brachytherapy program. Between February 2002 and June 2008, a VA team gave incorrect radiation doses to 97 of 114 veterans implanted with tiny radioactive seeds to destroy their cancer.
That was the official count the VA gave up until yesterday.
Using new assessment criteria developed by a "blue-ribbon panel" of medical experts - and not yet approved for use by the VA - the agency's top radiation oncologist said that in fact 19 veterans, not 97, had gotten incorrect doses of radiation to their prostate or surrounding tissue.
The VA's original standard estimated the radiation dose delivered to the prostate. The agency's new methodology is to examine where the seeds are placed in and around the prostate.
The new criteria are less subjective, said Michael Hagan, the VA's national director of radiation oncology. They are not meant to "mitigate" the problems of the Philadelphia program, which included no quality assurance or independent oversight, he said.
Even by the new standard, about one in five veterans treated with brachytherapy got substandard care at the Philadelphia VA.
"In my opinion, these results reflect a program that had substantial problems, but not at the level characterized," Hagan said.
So far, 11 of the 114 men have had a recurrence of prostate cancer, a rate that is within the expected range for brachytherapy treatments, the VA officials told the NRC.
An additional eight men have shown signs of a possible return of the cancer, and Hagan said he would not be surprised if the number of those whose cancer returns rises in the next few years.
Prostate brachytherapy involves implanting dozens of tiny radioactive seeds into the acorn-size gland to kill cancerous cells over several months. It is an effective treatment when done correctly.
Records show that the Philadelphia VA's program was deeply flawed from its earliest patients, and that doctors and officials repeatedly missed chances to correct it.
On Feb. 3, 2003, for example, the brachytherapy team implanted its ninth patient, planning to put 74 radioactive seeds into his prostate. But a routine check after the procedure showed that 40 of the seeds landed instead in the bladder.
In another case in 2005, 45 of the 90 seeds implanted in an 86-year-old veteran were put in his bladder and had to be extracted.
$58 million in claims
Some seeds ended up near the patient's rectum, and the patient reported significant pain in urination, records showed. He was one of eight men the VA sent to Seattle last year for a reimplantation.
Both cases were reported to the NRC, which did not deem them to be violations.
So far, 31 veterans or wives have filed claims totaling $58 million against the VA, according to records obtained last month by The Inquirer through a Freedom of Information Act request.
Before Hagan's introduction of a new analysis of the treatments yesterday, the VA's own analysis of the implants found that 63 were underdosed and that 35 got too much radiation to tissue near their prostates.
The mistakes led to internal investigations, congressional scrutiny, the NRC probe, and one by the VA's inspector general.
So far one physician has accepted a three-day suspension. A radiation safety official received a letter of reprimand.
Several lawmakers who have investigated the cases said that the VA responses were weak and that the agency acted only after prominent newspaper articles appeared in the summer, detailing radiation overdoses and underdoses.
Last month, the NRC cited the VA for eight apparent violations, including the failure to train doctors and other staff on how to identify bad implants, lacking procedures to ensure safe implants and not reporting mishaps as quickly or fully as required.
The so-called predecisional enforcement conference yesterday grew contentious when Joel Maslow, chairman of the Philadelphia VA's radiation safety committee, addressed the eight apparent violations the NRC had found.
Maslow referred to inspections by the NRC of two problematic cases in 2003 and 2005. He quoted from the 2003 NRC report after the first incident, which said "no violations of [NRC] requirements were identified."
He stated that the Philadelphia VA had the required procedures in place and had trained doctors and other staff as required, and thus did not violate the NRC regulation.
Maslow acknowledged that the staff appeared not to follow the written policies and that the program lacked rigorous quality assurance or peer reviews that might have prevented the problems.
He noted that the medical center has taken "significant measures" to make sure no similar failures occur in the future.
Philadelphia Medical Center director Richard Citron said he was glad for the opportunity to testify before the NRC as "we approach closure on this very difficult situation."
"This incident did indeed occur at the Philadelphia VA and I want to again state that I apologize for any harm we may have caused any patient."
Philadelphia VA Timeline
February 2002: First prostate-cancer patient is treated. Six of the first eight patients get less than 80 percent of the prescribed radiation - the NRC standard for reporting medical events.
February 2003: The ninth patient had more than half the seeds land in his bladder while receiving less than 17 percent of the correct dose.
October 2005: A patient, 86, gets half the seeds in his bladder during brachytherapy.
May 2008: Another dosing error triggers a full review.
June 2008: The program is shut down. Director Gary Kao stops treating patients at the Philadelphia VA and the University of Pennsylvania.
June 2009: Articles in the New York Times and The Inquirer detail a troubled program. Kao takes a leave from Penn research post. The first congressional hearing is held.
August 2009: Radiation oncologist Richard Whittington is suspended for three days.
October 2009: Radiation safety officer Mary E. Moore gets a letter of reprimand.
November 2009: NRC cites the Philadelphia VA for eight apparent violations for misusing radioactive materials.
December 2009: NRC holds predecisional hearing. VA maintains the number of maltreated vets is far lower than believed.