The second point to consider is that as a matter of public policy, most legal systems provide some form of protection against allegations of defamation for complainants who take their concerns through appropriate channels. This is because otherwise there would be a very chilling effect on the ability of members of the public to raise concerns, particularly where a defendant may be able to access much greater resources than the complainant.
Additionally, in criminal cases, the decision to prosecute rests with the prosecuting authority rather than the complainant. In England and Wales, for example, this rests with the Crown Prosecution Service, who will weigh up the issues before deciding to proceed with a case. This includes assessing whether there is sufficient evidence, whether the evidence is reliable and credible, and whether a prosecution is in the public interest.
Finally, even if there were no other hurdles, and it was possible to consider an action in an individual case, it would be an unattractive case, which would be liable to attract adverse publicity, and in the event of success, given the financial position of most complainants, a doctor (or their MDO had they agreed to undertake the matter) would be unlikely to recover their costs, let alone any damages actually awarded.
Realistically speaking therefore, it is unlikely that we will see cases of this sort being brought.