$Objective:$ The purpose of this UK study was to evaluate interfraction reproducibility and body image score when using ultraviolet (UV) tattoos (not visible in ambient lighting) for external references during breast/chest wall radiotherapy and compare with conventional dark ink.

$Methods:$ In this non-blinded, single-centre, parallel group, randomized control trial, patients were allocated to receive either conventional dark ink or UV ink tattoos using computer-generated random blocks. Participant assignment was not masked. Systematic ($\sum$) and random ($\sigma$) setup errors were determined using electronic portal images. Body image questionnaires were completed at pre-treatment, 1 month and 6 months to determine the impact of tattoo type on body image. The primary end point was to determine that UV tattoo random error ($\sigma$$\mathrm{setup}$) was no less accurate than with conventional dark ink tattoos, $i.e.$ <2.8 mm.

$Results:$ 46 patients were randomized to receive conventional dark or UV ink tattoos. 45 patients completed treatment (UV: $n$ = 23, dark: $n$ = 22). $\sigma$$\mathrm{setup}$ for the UV tattoo group was <2.8 mm in the $u$ and $v$ directions ($p$ = 0.001 and $p$ = 0.009, respectively). A larger proportion of patients reported improvement in body image score in the UV tattoo group compared with the dark ink group at 1 month [56% (13/23) $vs$ 14% (3/22), respectively] and 6 months [52% (11/21) $vs$ 38% (8/21), respectively].

$Conclusion:$ UV tattoos were associated with interfraction setup reproducibility comparable with conventional dark ink. Patients reported a more favourable change in body image score up to 6 months following treatment.

$Advances\; in\; knowledge:$ This study is the first to evaluate UV tattoo external references in a randomized control trial.