Center to Advance Palliative Care

Unless the cancer is exquisitely sensitive such as Hodgkins or testicular or small cell lung cancer, this is highly unlikely to be of any benefit. To the patient, that is.
It may be important to the family that "everything was done" especially if this is a new diagnosis. I have given chemo to rare patients on ventilators with chemo responsive cancers with the expectation that IF they saw benefit it would happen quickly, and they could come off. If not, stop.
This is far likely doctors who are unable to give up, or patients unable to give up, or a combination. (Like marriage, there seems to be assortative pairing of oncologists and these do-anything patients.)

Or doctors who don't have the words and tools to help people with these decisions.

As a palliative care person, you may be able to give them this "out" or teach them what to say. No one likes giving chemo under these circumstances. But it's easier than being yelled at for days by angry relatives....

And there might be a 1% chance of response and getting off the vent and living for a few weeks or months. If we value autonomy (and don't have to reckon with the waste of resources) then even chemo on the ventilator might be on the table for discussion. I would hope to dissuade people from doing it.

There are excellent guidelines for solid tumors at NCCN.org, National Comprehensive Cancer Center Network website. Free, after registration as a HCP. You can show people, "after 2 or 3 (depends) progressions on chemo, or performance status >/= 3, consider switch to palliative care."

Hope that is helpful.

Tom Smith
oncologist and pall care person