The under-treatment of chemotherapy patients in NSW hospitals will be subject to a parliamentary inquiry amid warnings that improper treatment could be found to extend to hundreds more patients.

The state government and its embattled Health Minister, Jillian Skinner, have resisted calls for a special commission of inquiry into the under-dosing scandal after the release last week of a report that found more than 100 cancer patients at St Vincents Hospital had received off-protocol doses of medication from Dr John Grygiel.

But the upper house agreed on Thursday to instead initiate a select committee to inquire into the treatment of patients of Dr Grygiel and that of another Sydney oncologist, operating at Sutherland and St George Hospitals.

Opposition health spokesman Walt Secord said he had received briefings that indicated Dr Grygiel had charge of as many as 300 patients a year in central-western NSW for decades, but information about those treatments was lacking.