Course of chemotherapy is not rare, let alone very rare!

And you are still missing the point that you can use chemotherapt treatment within my expression to give "a course of chemotherapy treatment".

And also the meaning differs.

"She has finished her chemotherapy treatment" - That's it, it is done, she won't be having anymore, no more is planned.

"She has finished her course of chemotherapy"
"She has finished her course of chemotherapy treatment" - A course of treatment does not have to be the whole thing. There could be more planned, another course.

With cancer treatment they can do it in stages:

e.g. have one course, then have a break, then have another course of therapy.

So just because someone course of chemotherapy treatment is over it doesn't mean their chemotherapy treatment as a whole is over.

Below are quotes from ENGLISH articles about Kylie:

"she had frozen her eggs before starting a course of chemotherapy."

"[He] said a course of chemotherapy usually lasted four to six months and was followed by a course of radiotherapy. " - ooh, and course of radiotherapy. From her OFFICIAL site. And kids look at that. Surely if it was some rare unusual expression they would dumb it down for the kids?

"He said the characteristic of the tumour determined the course of treatment." - still from the OFFICIAL site.

"Kylie, 37, suffered several surgery and a course of chemotherapy"

"as well as helping doctors to decide what course her treatment will take."

"Women then face a six to eight-month course of chemotherapy in three to four weekly cycles."

"Kylie was diagnosed with breast cancer in May, and has undergone a series of operations and a course of chemotherapy."

And don't go and post examples with just "chemotherapy treament" in them, because I know it is there, but in this context it doesn't sound good, it doesn't convey the meaning as effectively.

I enjoy your logic. "I ain't heard it so it's wrong".