In plain terms: Turkey Tail (Trametes versicolor / PSK) shows early signals in human research but the evidence is limited -- small trials, surrogate endpoints, or not yet independently replicated. It is an emerging area, not yet a proven one.
Turkey Tail (Trametes versicolor / PSK)
The evidence as the record states it
Sakamoto et al. (Cancer Immunol Immunother, 2006; PMID 16133112): meta-analysis of three centrally randomized Japanese trials, 1,094 patients after curative colorectal cancer resection; PSK with chemotherapy improved overall survival (risk ratio 0.71) and disease-free survival (0.72). Oba et al. (Cancer Immunol Immunother, 2007; PMID 17106715): eight randomized trials, 8,009 patients after gastric cancer resection; overall survival hazard ratio 0.88. Pilkington et al. (Cochrane Database Syst Rev, 2022; PMID 36445793): seven trials, 1,569 patients with colorectal cancer; low-certainty evidence of a small five-year survival gain (RR 1.08, number needed to benefit 16) and very-low-certainty evidence on adverse effects, with chemotherapy regimens that are no longer standard. Recent trials did not confirm a benefit: Miyake et al. (Surg Today, 2018; PMID 28634730), MCSGO-CCTG, 357 patients, UFT with PSK failed non-inferiority to UFT with leucovorin (3-year disease-free survival 72.1 against 82.3 percent); Okuno et al. (Cancer Chemother Pharmacol, 2018; PMID 29094178), JFMC38, stage II rectal cancer, UFT with PSK against surgery alone, disease-free survival numerically worse, closed early; Ogawa et al. (Anticancer Res, 2024; PMID 38307591), 186 patients, adding PSK to UFT with leucovorin did not change 3-year disease-free survival. Zhong et al. (Front Pharmacol, 2019; PMID 31333449): the only pooled immune-marker data (CD3 and CD4 up, CD8 and NK cells unchanged), mixed with Ganoderma products, so it does not grade PSK alone. Funding of the older Japanese trials is not stated in the indexed records. Grade C: positive older pooled survival data on outdated regimens, with the recent trials null.
Why Grade C and not Grade B
Grade C reflects early or limited human evidence -- small trials, surrogate endpoints, or preliminary findings not yet independently replicated. The evidence points in a promising direction but does not yet meet the standard for Grade B. A single well-powered, independently replicated trial with a meaningful endpoint would move this to Grade B or higher.
What this grade covers and what it does not
What would move this grade
A well-powered randomized trial with an objective, clinically meaningful primary endpoint and at least one independent replication would support Grade B or higher.