Far-left party Podemos and associations defending public health have said the donation via Amancio Ortega's foundation is inappropriate because his clothing company Inditex, which owns Zara and other brands, is accused of tax avoidance -- a claim the group denies.
Ortega should "not be demonstrating his philanthropy but his desire to contribute to public finances in a way that is proportional to his profits," the Federation of Associations for the Defence of Public Health Services said in a statement earlier this month.
Luisa Lores, spokeswoman for the federation, told AFP it was also a "way to avoid paying taxes" as part of the donation is tax deductible.
But this reaction has been met with incredulity by others -- including cancer patients -- who say any donation is welcome in a country where the public health system suffered from huge spending cuts during the financial crisis.
"I need to live, and there are thousands of people like me fighting cancer, we get up every day with side effects, we get up every day trying to smile, to fight for life," Tina Fuertes, a cancer patient, said Wednesday on Spanish television.
"It hurts, I don't understand, this doesn't make sense," she said of the controversy.
Podemos chief Pablo Iglesias has also hit out at Ortega, one of the world's richest men, accusing him of carrying out a marketing ploy and deploring that "philanthropy should be seen as a mechanism to finance public health" in Spain.
The donation represents eight percent of last year's public health budget in Spain, where more than 200,000 cancers are diagnosed annually, according to Ortega's foundation.
Hospitals in Galicia in the north and Andalusia in the south have already received various pieces of equipment.
The foundation told AFP it would continue implementing the programme in other hospitals.