Uptake of 89Zr-APOMAB by the tumour was most obvious in the days following the final anti-PD-1 treatment (day 6) with %ID/g increasing from 25.5 ± 4.4 to 33.1 ± 6.5 on day 10, which corresponded to a reduction in tumour size (Fig. 4F).We hypothesize that this finding relates to residualization of 89Zr radiometal in tumour-associated macrophages that have engulfed 89Zr-APOMAB-bound dead tumour cells as described [23]. Here, PDCD1 is linked to neoplasm.