Thymic health and immunotherapy outcomes in patients with cancer

Although immunotherapy has revolutionized cancer treatment, many patients still experience limited benefit, highlighting the urgent need for improved biomarkers1. Although immunotherapy is founded on unleashing T cells2, most existing biomarkers remain tumour-centric and mainly overlook host immune competence. The thymus is a key immune organ that is crucial for T cell maturation, and we hypothesized that thymic functionality is associated with immunotherapy outcomes3. Here we show that thymic health, a radiographic measure of thymic functionality, is strongly associated with immunotherapy outcomes across several cancer types. Using a deep-learning framework applied to routine computed tomography images, we quantified thymic health in a pan-cancer cohort of 3,476 patients receiving immune checkpoint inhibitors. In patients with non-small cell lung cancer, higher thymic health was associated with reduced risks of progression and all-cause mortality. These associations remained significant across clinically relevant levels of programmed death ligand 1 (PD-L1) and tumour mutation burden. In the prospective TRACERx lung cancer study, thymic health was positively associated with T cell receptor diversity and T cell receptor excision circles, and correlated with immune-system signalling pathways, supporting radiographic thymic health as a proxy for thymic activity and adaptive immune competence. Analysis across patients with melanoma, breast cancer or renal cancer demonstrated pan-cancer relevance. Together, these findings identify thymic health as a previously unrecognized, tumour-agnostic determinant of immunotherapy efficacy, with potential implications for patient stratification, treatment timing and the development of immune-rejuvenating strategies in precision immuno-oncology.