Yesterday I made superficial review of Massachusetts case
law regarding standards of medical care. My review was
superficial because of a greenhouse effect in the law library
which makes work there on hot days almost unbearable.
But I shall go back and do more, if you think it desirable.

The photocopies that I made suggested that Massachusetts
sets the standard of care as that of the "average" physician.
I found no references to standards of subspecialists,
i.e. nephrologists, as distinct from standards for internists.
I would like to know how the plaintiff's expert can testify
to his knowledge of the practice of the average physician,
unless he claims that he himself is average.

Seredipitously, because it happened to be on the same
page, I found a case "on all fours" with yours, which
is worth further scrutiny.

     U.S. District Court, Mass. 1985. Staff members of a U.S.
     Public Health Service Hospital were negligent in not
     providing for further examination of a two-centimeter
     node on plaintiff's neck, given history plaintiff noted
     in the hospital record (sic); however, plaintiff failed 
     to prove that the negligence, and the resulting delay
     in plaintiff's being treated for cancer, was a proximate
     cause of his undergoing more extensive surgery with its
     attendant greater negative impact on plaintiff.
                                Santos v. U.S. F.Supp. 417

The necrotic lymph node on the ultrasound demonstrates the
existence of metastases at the time of the allegedly negligent
failure to diagnose, hence no proof that delay impaired
effectiveness of therapy.

I have some photocopies if an when you want to look at them,
and I can look up some cases, if you think it would help you.