Background: The majority (90%) of anal cancers are human papillomavirus (HPV)-driven, identified using immunochemistry for p16. Compared with HPV- patients, those with HPV+ disease generally show improved survival, although relapse rates around 25% indicate a need for further stratification of this group.

Methods: Using two cohorts of anal cancer, previously characterised for p16, we assessed the prognostic value of tumour-infiltrating lymphocytes (TILs).

Results: Tumour-infiltrating lymphocyte scores were used to stratify p16+ cases, where tumours with absent/low levels of TIL had a relapse-free rate of 63%, as opposed to 92% with high levels of TIL (log rank P=0.006).

Conclusions: Assessment of TIL adds to p16 status in the prognosis of anal cancer following chemo-radiotherapy and provides evidence of the clinical importance of the immune response.