A comparative study of five patients with breast cancer and ten patients with NSCLC [37] looked at both xCT and CD44 expression, finding a significant correlation of 18F-FSPG SUVmax at 60 min p.i. with both xCT (p = 0.68, p < 0.01) and CD44 expression (p = 0.77, p < 0.01). Here, CD44 is linked to breast cancer.