The goal of adjuvant systemic therapy after locoregional treatment of cancer is to increase the probability of cure by eradicating micro-metastases. Adjuvant treatment adds side-effects and can impair health-related quality of life (HRQOL); this must be balanced against a reduction in the probability of recurrence, which has a substantial negative effect on HRQOL. Since many adjuvant therapies lead to small gains in relapse-free or overall survival, the severity and duration of treatment-related side-effects and their impact on patient functioning and overall HRQOL are relevant to clinical decision making. To facilitate understanding by oncologists and patients, we recommend reporting in publications the proportion of patients receiving adjuvant therapy who experience a prespecified deterioration in a relevant and validated HRQOL scale, and the duration of this deterioration during and after treatment completion, with appropriate handling of missing data and intercurrent events. HRQOL assessment should include symptoms, functioning domains, and overall HRQOL. Between-group comparisons should address key HRQOL outcomes (as defined by both oncologists and patients) supplemented by descriptive outcomes across remaining HRQOL domains. Once the proportion of patients with specific adverse changes in symptoms is identified, it is possible to determine if specific patient group(s) are at particular risk for these effects. HRQOL information should be included in the primary report of trials evaluating adjuvant therapy as it is key to informed decisions about treatment choice, and updated long-term evaluation is also important to capture any lasting deficits in HRQOL.

The gut microbiome is increasingly recognised as a clinically modifiable determinant of efficacy and toxicity across systemic cancer therapies, with the most mature evidence in patients receiving immune checkpoint inhibitors (ICIs). The composition and function of intestinal microbes can influence treatment outcomes by affecting immune priming, antigen presentation, host-microbial co-metabolism, drug biotransformation, and epithelial barrier integrity. In melanoma, non-small-cell lung cancer, and renal cell carcinoma, higher microbial diversity, ecological stability, and enrichment of immunostimulatory pathways have been associated with improved ICI response and survival. By contrast, antibiotics and proton pump inhibitors are linked to inferior outcomes because they disrupt the microbiome. Microbiome-directed interventions, including faecal microbiota transplantation, dietary modifications, prebiotics, probiotics, and live biotherapeutic products, are undergoing clinical testing but remain at the investigational stage. This Review synthesises current evidence linking the intestinal microbiome to anticancer therapy outcomes, evaluates strategies and challenges for therapeutic modulation of the microbiome, and introduces emerging insights into the mycobiome.