The introduction of immunotherapy, particularly immune cell therapies, have transformed the therapeutic landscape in recent years. Cell therapies are now finally reaching patients in growing numbers, with many more set to come through the pipeline in the next few years. In 2017, Novartis's Kymriah ® received FDA approval for patients up to 25 years with acute lymphoblastic leukemia (ALL) that has either relapsed or is refractory. This approval followed the near extraordinary results of the case

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... of Emily Whitehead, a 7-year-old girl who had been fighting ALL for more than a year, whose cancer went in to remission following treatment. Emily is now 6 years cancer-free. In the months that followed Kymriah's ® approval, two other gene therapies joined the market: Yescarta ® , a CAR T-cell therapy marketed by Gilead to treat certain types of large B-cell lymphoma; and Luxturna ® , Spark Therapeutics' gene therapy for a rare form of inherited vision loss. Although these three therapies would be the first, they will certainly not be the last, with over 900 cell and gene therapy-related clinical trials in planning phase or ongoing, according to www.ClinicalTrials.gov. Following the success of Kymriah ® , all eyes now turn towards large patient groups with solid tumor cancers and whether the success of CAR-Ts can be reproduced in these patients. However, solid tumors represent a much greater challenge. These tumors incorporate mechanisms designed to keep T cells out, due to the tumor microenvironment where any cell and gene therapy would need to incorporate a strategy to overcome the tumor's blocking mechanism. To date, a strategy for this simply does not exist, and before any such strategy can be comprehensively evaluated in vivo, a means to track these cells in situ and subsequently monitor their success is fundamental. This strategy Center (Radboud UMC), De-