Simoncini has been struck off by the medical authorities in his country and successfully convicted of manslaughter and defrauding of his patients, apparently sentenced to 4 years in 2006.
Is there anything to back up his claims? Well let's be charitable and suggest that what he actually means is the cancer is caused by fungi and that if we open up a tumour we will find a fungus within - though what he actually states is that the cancer IS the fungus. The fact that he presents no data, no pictures of fungi within a tumour must mean that it is technically a challenge to do so? Or that the thousands of histologists who do this every day are missing something?
In fact it is very easy to remove a tumour, thinly slice it, stain it and look at the cells within and here is a website of a pathologist doing exactly that. No sign of fungi there and plenty of evidence of human cells undergoing changes related to them turning cancerous.
The tumour must be fixed & stained to carry out these examinations - perhaps then that process removes all sign of the fungus from the tumour tissue and THAT will explain why histologists & pathologists don't see it? Well no that isn't the case either - this webpage clearly demonstrates that fungi (in this case yeast) are easy to fix, stain and see under the microscope. There are many more pictures here of Aspergillus (another type of fungus) fixed and stain in sections of lung tissue.
There is no obvious medical or scientific reason why Simoncini could not carry out similar work to back up his assertions. It would do his case a huge amount of good (and earn him a huge income) if he would adopt this approach, but as far as we can reasonable find, he doesn't. Likewise it would be straightforward for him to publish the results of his efforts at treating patients based on his hypothesis, but again there is no sign of that happening. Perhaps he would care to leave a comment below enlightening us?
Make your own conclusions.