The National Labor Relations Board office in Philadelphia will hear arguments this month on whether 500 southern New Jersey doctors in private practice, organized by the United Food and Commercial Workers, are employees and can be part of a union. Federal law bars independent contractors from banding together to set prices and terms of work, but it does allow unions for independent contractors -- such as actors, musicians, athletes, and cab and truck drivers -- whose income and working conditions are primarily dictated by others. The doctors argue that they fall into the latter category, as de facto employees, because AmeriHealth -- a health maintenance organization that is a subsidiary of Blue Cross/Blue Shield -- governs many of their decisions on patient care. A decision in favor of the physicians would trigger a nationwide rush of physicians joining unions. Only about 3% of doctors are in unions today, nearly all of them hospital employees.