The number of studies showing mushrooms are an effective treatment to stop or slow down many forms for cancer, is quite large.
If you seek to purchase mushroom products in capsule or powder form, make sure they are from the "fruiting bodies" of the mushroom. Most products are not.
Here is an article from the National Center for Biotechnology Information.
Title: Recent developments in mushrooms as anti-cancer therapeutics: a review
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3339609/
Below are quotes from the review. Each quote is associated with one or more studies listed in the review.
"The mounting evidences from various research groups across the globe, regarding anti-tumor application of mushroom extracts unarguably make it a fast-track research area worth mass attention."
With regards to cancer, it states that some mushrooms have "validated anti-cancer properties."
"Mushrooms are known to complement chemotherapy and radiation therapy by countering the side-effects of cancer, such as nausea, bone marrow suppression, anemia, and lowered resistance."
"The active components in mushrooms responsible for conferring anti-cancer potential are lentinan, krestin, hispolon, lectin, calcaelin, illudin S, psilocybin, Hericium polysaccharide A and B (HPA and HPB), ganoderic acid, schizophyllan, laccase, etc. (Fig. 2). Polysaccharides are the best known and most potent mushroom-derived substances with anti-tumor and immunomodulating properties."
"The purpose of the present review is to summarize the available information and to reflect the current status of this research area with a view for future direction."
"Phellinus is a genus of mushrooms belonging to the family Hymenochaetaceae. Phellinuslinteus has anti-tumor, immunomodulating and anti-metastasis properties..."
"...potential anti-tumor agent in breast and bladder cancers."
"fruiting bodies of the mushroomPleurotuscitrinopileatus. The lectin exerted potent anti-tumor activity in mice bearing sarcoma 180, and caused approximately 80% inhibition of tumor growth when administered intraperitoneally at 5 mg/kg daily for 20 days."
"Agaricusblazei Murrill extracts have immunomodulatory, anti-carcinogenic and anti-mutagenic properties as studied by its action on clastogenicity induced by cyclophosphamide (CP) in mice (Delmanto et al. 2001)."
"The broth fraction of A. blazei, when examined on the growth of human prostate cancer inhibited cell proliferation in both androgen-dependent and androgen-independent prostate cancer cell lines."
"The turkey tail mushroom or cloud mushroom,Trametesversicolor orCoriolusversicolor has been studied to have anti-tumor property against many types of cancers."
"a proprietary dietary supplement prepared from extracts ofT. versicolor reduces the growth of hormone responsive prostate cancer LNCaP cell growth."
"Ganoderma, commonly known as Lingzhi or Reishi, also called mushroom of immortality, belonging to family Ganodermataceae has been traditionally administered throughout Asia for centuries as a cancer treatment"
"Ganodermalucidum exhibits anti-cancer effect alone or in combination with chemotherapy and radiotherapy"
"...GA-Me may be a novel promising agent for the treatment of human colon carcinoma cells by mitochondrial pathway manipulation."
"Hericium erinaceus or commonly known as Lions mane has attracted great attention owing to its anti-tumor and immunomodulatory effect"
"The ß-glucan rich hot-water obtained by boiling and microwaving, when injected daily for 2 weeks, significantly reduced tumor weights by 38 and 41%, respectively."
"Cordyceps militaris shrunk tumors and increased mouse lifespan"
"...l-theanine has synergistic effect on the anti-tumor activities of doxorubicin, anthracyclines, cisplatin, and irinotecan. Consequently, the modulating effect of theanine on the efficacy of anti-tumor agents is expected to be applicable in clinical cancer chemotherapy."
"Flammulinavelutipes is an edible mushroom, commonly known as winter mushroom, velvet foot or enoki, falling under Physalacriaceae family. A stable hemagglutinin was isolated from the fruiting bodies of this mushroom, which inhibits proliferation of leukemia L1210 cells. The degree of cytotoxicity on ER- breast cancer cells was very high, whereas the ER- breast cancer cells are inhibited by about 99%..."
"Inonotusobliquus (Chaga mushroom) belonging to Aphyllophoromycetodeae, one of the widely known medicinal mushrooms, has been used to treat various cancers in Russia and most of Baltic countries for many centuries." "The ethanolic extract of sclerotium and fruiting body of I. obliquus elicited significant anti-tumor activity"
"The shaggy ink cap mushroom, Coprinuscomatus belonging to Agaricomycetideae was evaluated for its anti-cancer potential." "The results promise that the mushroom extract can be an effective therapy for malignant estrogen-independent breast cancer "
"Aqueous extract of the mycelia of Funliatrogii extract shows good anti-tumor toxicity against a range of tumor cell lines"
"The Japanese mushroom Lactariusflavidulus mycelial culture has anti-cancer properties."
"Clitocybealexandri was found a very potent inhibitor of the growth of lung, breast, colon, and gastric cancer cell lines"
"Piptoporusbetulinus, commonly known as the birch polypore belonging to Fomitopsidaceae family has been studied in vitro for its anti-cancer activity. The fraction prepared from dried fruiting bodies was subjected to anti-cancer evaluation in human lung carcinoma (A549), colon adenocarcinoma (HT-29) and rat glioma (C6) cell cultures. P. betulinus fraction elicited anti-cancer effects that were attributed to decreased tumor cell proliferation, motility and the induction of morphological changes."
"Antrodia (camphor tree mushroom) is a genus of mushrooms in the famly Fomitopsidaceae. These mushrooms are highly valued in Taiwan." This particular mushroon had the largest number of studies cited.
"Antroquinonol induces anti-cancer activity in human pancreatic cancers through an inhibitory effect"
"A Korean wild mushroom Polyozellusmultiplex checks cell proliferation in stomach cancer"
"Mushroom products are set to create a revolution in therapeutic strategies in curbing various forms of cancers. Several companies are dedicated to prepare anti-cancer formulae from mushroom extracts using state-of-the-art technology and their products are gradually being recognized globally. Zhejiang Fangge Pharmaceutical & Healthcare Products Co. Ltd., a large pharmaceutical company in China specializes in development and marketing of mushroom extracts for anti-cancer uses. The enterprise manufactures and exports polysaccharides from mushrooms G. frondosa, L. edodes, G. lucidum, A. blazei, Cordyceps sinensis and H. erinaceus. FineCo Ltd., an emerging Korean company develops effective drugs, including anti-cancer formulations from medicinal mushrooms. Its products are now sold in Japan, Hong Kong, Australia, America, and Europe, for use in clinical test on cancer patients. A pioneering mushroom company Mushroom Wisdom, based in USA is engaged in formulating supplements for inhibiting cancer. USA based Aloha medicinal Inc. manufactures an array of medicinal mushroom products including the cancer-inhibiting Ganoderma capsules. In near future, more companies are expected to join the flourishing market of cancer therapeutic manufacture from mushrooms. Better insight into the mechanisms underlying biological action of mushroom will accelerate commercial production of pharmaceuticals for cancer therapy."
"Medicinal mushrooms represent a growing segment of today’s pharmaceutical industry owing to the plethora of useful bioactive compounds. While they have a long history of use across diverse cultures, they are backed up by reasonable scientific investigation now. The mycologists around the world, firmly believe that a greater knowledge of mushroom can ameliorate many forms of cancers at various stages."