In Japan, 常勤医師　is clearly defined under 健康保険法 (Health Insurance Act) as those who work more than 8 hr/day, 32 hr/wk, 4 days/wk.

So, under the Japanese regulations, 非常勤医師 can also be defined clearly as those who work less hours than the above, i.e. it's work hours that defines 非常勤医師 in Japan.

Meanwhile, what the word "locum" suggests is not necessarily defined by working hours per week outside Japan (see the first link). In these countries, a locum may work as long as a 常勤医師 in Japan.

Now, Mairi says the word 非常勤医師 appears in a clinical trial document and the sub-investigator (=治験分担医師) is a 非常勤医師. Not knowing to whom the document is submitted, I think "part-time physician" is more accurate to describe the status of the sub-investigator. It may even be safer to use "part-time physician" not to get the health authorities confused (which is, of course, the last thing your client wants to see).

Although the sub-investigator may get hurt seeing he/she was described "part-time", there seems to be lots of doctors who are called "part-time physicians" (vs "full-time physicians") as the second link shows.

Cheers,